Part III · Regulation and Integration · Estimated reading time 135 minutes · Prerequisites: Chapters 11, 12, 13
In This Chapter
- Learning Objectives
- 14.1 What the Autonomic Nervous System Is For
- 14.2 Sympathetic Anatomy: The Thoracolumbar Division
- 14.3 Parasympathetic Anatomy: The Craniosacral Division
- 14.4 Neurotransmitters and Receptors
- 14.5 Effects on Every Organ System
- 14.6 Autonomic Regulation: Tone, Dual Innervation, and Balance
- 14.7 Autonomic Reflexes and Central Control
- 14.8 Heart Rate Variability: Reading the Balance Directly
- 14.9 Advanced Topic · Autonomic Dysfunction and the Logic of Autonomic Pharmacology
- Chapter Summary
- Case File 14 · Resolution
- Systems Integration Case File · Entry 14
- Review
- Key Terms
14. The Autonomic Nervous System
Involuntary Control of the Internal Environment
Case File 14 — "The Heart Has No Off Switch"
This entry is dated earlier than the last one. The printout below was made on hospital day four, six months before anyone thought to ask Amara Osei to take off her shoes — which is the first thing worth noticing about it, because the autonomic half of her neuropathy was measurable half a year before the sensory half was noticed.
Amara, 45, is on continuous telemetry in a step-down unit. She feels, in her words, "fine, apart from everything." Her nurse practitioner is not looking at how she feels. She is looking at a report most patients never see: a 24-hour analysis of the intervals between Amara's heartbeats.
| Measurement | Amara's value | Usual for a healthy 45-year-old |
|---|---|---|
| Resting heart rate (supine, awake) | 88 beats/min | 55–75 |
| SDNN — 24-h standard deviation of normal beat intervals | 42 ms | 120–160 ms |
| RMSSD — beat-to-beat variability | 18 ms | 25–50 ms |
| Respiratory sinus arrhythmia (HR swing with each breath) | 3 beats/min | 8–20 beats/min |
| Heart rate response to standing (30:15 ratio) | 1.02 | > 1.04 |
| Blood pressure, day 4, pre-treatment | 152/94 mm Hg | < 120/80 |
| Fasting glucose (Chapter 2) | 138 mg/dL (7.7 mmol/L) | 70–99 |
Every beat on the strip is a normal sinus beat. There is no arrhythmia, no ectopy, no block. The rhythm is, in the most literal sense, regular — and that is exactly the problem.
The team starts metoprolol succinate, 50 mg once daily. Metoprolol is a beta-1 selective adrenergic antagonist: a drug that blocks one receptor subtype found in high density on the heart and in far lower density on the lungs and on most blood vessels.
Three days later:
| Measurement | Day 4 (before) | Day 7 (after) |
|---|---|---|
| Resting heart rate | 88 beats/min | 64 beats/min |
| Blood pressure | 152/94 mm Hg | 138/84 mm Hg |
| Reported symptoms | none | "a bit tired on the stairs" |
Three questions to hold on to.
- What is heart rate variability actually measuring? Amara's heart is beating regularly — the monitor shows normal sinus rhythm — so why is less variability a bad sign rather than the sign of a well-regulated machine?
- There is no nerve from the brain that tells the heart when to beat. Cut every nerve to the heart and it keeps beating; a transplanted heart, entirely denervated, beats for decades. The heart has no "off switch" from the nervous system. So how can a drug slow it?
- Metoprolol blocks a receptor concentrated in the heart. Blood pressure is generated by the resistance of thousands of miles of blood vessels. Why does blocking a cardiac receptor lower a vascular number — and by 14 mm Hg systolic in three days?
Learning Objectives
By the end of this chapter you should be able to:
- Compare the somatic and autonomic motor divisions across every axis — neurons in the pathway, cell body location, axon class and speed, ending, neurotransmitter, receptor, effector, direction of effect, degree of conscious control, and the consequence of denervation.
- Describe the sympathetic (thoracolumbar) outflow: lateral horn of T1–L2, white and gray rami communicantes, the sympathetic chain, and the three possible fates of a preganglionic fiber that enters it.
- Name the splanchnic nerves, their spinal origins, their prevertebral ganglia, and the organs each supplies.
- Explain why the adrenal medulla behaves like a sympathetic ganglion, and connect that behavior to its neural crest origin and to the epinephrine it uniquely makes.
- Describe the parasympathetic (craniosacral) outflow: cranial nerves III, VII, IX, and X with their four cranial ganglia, and sacral segments S2–S4, and account for the extraordinary distribution of the vagus.
- Predict, from ganglion position and divergence ratio, why sympathetic responses are diffuse and parasympathetic responses are discrete.
- Trace the synthesis, release, and termination of acetylcholine and of norepinephrine, and explain why cholinergic effects last milliseconds and adrenergic effects last seconds.
- State which cholinergic (nicotinic N_N and N_M, muscarinic M1–M5) or adrenergic (α1, α2, β1, β2, β3) receptor sits on each major effector, name its G protein, and predict the effect of activating or blocking it.
- Read a full autonomic effects table and use it in both directions — from drug to effect and from effect back to receptor.
- Explain autonomic tone, dual innervation, antagonistic versus cooperative effects, and why most organs sit at an intermediate state held between two continuous opposing drives.
- Trace the baroreceptor reflex as a complete five-component loop, and describe the chemoreceptor, micturition, defecation, and sexual reflexes.
- Describe the hierarchy of central autonomic control from spinal cord through brainstem to hypothalamus, limbic system, and cortex.
- Define heart rate variability, distinguish time-domain from frequency-domain measures, and explain why low HRV predicts mortality after myocardial infarction.
- Explain orthostatic hypotension, diabetic autonomic neuropathy, Horner syndrome, autonomic dysreflexia, and Raynaud phenomenon as failures of named components of named loops.
- Predict the effects of agonists and antagonists at each receptor class, and explain why a β1-selective blocker differs from a non-selective one and why atropine and organophosphates are pharmacological mirror images.
14.1 What the Autonomic Nervous System Is For
The autonomic nervous system (ANS) is the motor division that controls the internal environment. Its job description is homeostasis: it adjusts heart rate, vascular tone, airway calibre, gut motility, glandular secretion, pupil size, bladder function, thermoregulation, and metabolic fuel mobilization, mostly without consulting you. It is the efferent arm of nearly every homeostatic loop in this book, and by the end of Chapter 33 you will have met it inside the cardiovascular, respiratory, digestive, urinary, endocrine, reproductive, and thermoregulatory chapters, doing the same job with the same eight receptors every time.
The traditional name for it is the visceral motor system, which is more accurate than "autonomic." Autonomic means self-governing, and it is not: it is governed continuously by the hypothalamus, the brainstem, and the limbic system, and it can be trained, conditioned, and — as anyone who has been embarrassed into blushing knows — provoked by a thought.
The comparison that explains everything else
Chapter 13 followed the sensory division and the somatic motor division. This chapter takes the other branch. Before any anatomy, put the two motor divisions side by side, because almost every peculiarity of the autonomic system is a consequence of the one structural difference in row two.
| Axis | Somatic motor | Autonomic motor |
|---|---|---|
| Effector | Skeletal muscle only | Cardiac muscle, smooth muscle, glands, adipose tissue, some lymphoid tissue |
| Neurons in the pathway | One — cell body in CNS, axon runs uninterrupted to the target | Two — a preganglionic neuron in the CNS synapses on a postganglionic neuron whose cell body lies in a peripheral ganglion |
| Cell body location | Ventral horn of the spinal cord; cranial motor nuclei | Preganglionic: lateral horn T1–L2, sacral S2–S4, or brainstem nuclei. Postganglionic: chain, prevertebral, or terminal ganglion |
| Axon class and speed | Thick, heavily myelinated A-alpha, 13–20 µm, 80–120 m/s | Preganglionic: thin, lightly myelinated B, 1–3 µm, 3–15 m/s. Postganglionic: unmyelinated C, 0.2–1.5 µm, 0.5–2 m/s |
| Nerve ending | Neuromuscular junction — a specialized synapse with junctional folds, a 30–50 nm cleft, and a safety factor of 3–4 | Varicosities — beads along the axon releasing transmitter into a cleft 20–2,000 nm wide, with no specialized postsynaptic membrane |
| Transmitter at the target | Acetylcholine, always | ACh (all parasympathetic, plus sympathetic fibers to sweat glands) or norepinephrine (almost all other sympathetic) |
| Receptor at the target | Nicotinic (N_M), always — a ligand-gated cation channel | Muscarinic M1–M5 or adrenergic α1, α2, β1, β2, β3 — all G-protein-coupled |
| Direction of effect | Always excitatory. There is no inhibitory neuromuscular junction in human skeletal muscle | Excitatory or inhibitory, decided by which receptor the target cell expresses |
| Where inhibition happens | Inside the CNS, by silencing the motor neuron | At the periphery, by the receptor itself |
| Tone | None. A resting muscle's motor neurons are silent | Continuous. Both divisions fire at rest; regulation is modulation of a baseline |
| Divergence | Low — one motor neuron supplies 10–1,000 fibers of one muscle | Sympathetic 1:10–20 across several ganglia; parasympathetic 1:1–3 |
| Voluntary control | Direct and conscious | Indirect at best — through breathing, posture, thought, and training |
| Effect of denervation | Flaccid paralysis and atrophy; the muscle does nothing | The effector keeps working, loses regulation, and develops denervation hypersensitivity — it over-responds to circulating agonists |
| Effect of denervating the heart | (not applicable) | Rate rises to about 100/min, because a restraint was removed, not a command |
ONE NEURON versus TWO — the structural difference that decides the rest
SOMATIC MOTOR PATHWAY AUTONOMIC MOTOR PATHWAY
═══════════════════════════════════════════════════════════════════════════
┌───────────────────┐ ┌───────────────────┐
│ CNS │ │ CNS │
│ ┌──────────┐ │ │ ┌──────────┐ │
│ │ VENTRAL │ │ │ │ LATERAL │ │
│ │ HORN │ │ │ │ HORN │ │
│ │ (alpha │ │ │ │ (pregang-│ │
│ │ motor │ │ │ │ lionic) │ │
│ │ neuron) │ │ │ │ │ │
│ └────┬─────┘ │ │ └────┬─────┘ │
└───────┼───────────┘ └───────┼───────────┘
│ │
│ A-alpha 13-20 µm │ B fiber 1-3 µm
│ THICK MYELIN │ thin myelin
│ 80-120 m/s │ 3-15 m/s
│ ▼
│ ╭──────────────╮
│ │ GANGLION │ ◄── ACh on
│ │ (peripheral)│ NICOTINIC
│ NO synapse │ multipolar │ (N_N)
│ along the way │ cell body │
│ ╰──────┬───────╯
│ │ C fiber 0.2-1.5 µm
│ │ NO MYELIN
│ │ 0.5-2 m/s
▼ ▼
╔═══════════════════╗ ╔═══════════════════════╗
║ NEUROMUSCULAR ║ ║ VARICOSITIES ║
║ JUNCTION ║ ║ ○──○──○──○──○──○ ║
║ ┌─┐ 30-50 nm ║ ║ ░ ░ ░ ░ ░ ║
║ │ │ cleft ║ ║ 20 - 2000 nm ║
║ └┬┘ ▓▓▓▓▓▓▓▓ ║ ║ no folds, no trough ║
║ junctional folds ║ ║ volume transmission ║
║ 10,000 receptors ║ ║ 1 axon → HUNDREDS ║
║ per µm² ║ ║ of cells ║
╚═════════╤═════════╝ ╚══════════╤════════════╝
▼ ▼
SKELETAL MUSCLE FIBER CARDIAC / SMOOTH MUSCLE
ACh → NICOTINIC (N_M) GLAND / ADIPOSE
► ALWAYS CONTRACTS ACh → MUSCARINIC or
► safety factor 3-4 NE → ADRENERGIC
► one-to-one, guaranteed ► EXCITES *or* INHIBITS
► graded, diffuse, slow
DENERVATE IT: DENERVATE IT:
► flaccid paralysis, atrophy ► keeps working, loses control,
becomes HYPERSENSITIVE
═══════════════════════════════════════════════════════════════════════════
THE POINT: the extra synapse adds an extra RECEPTOR, and receptors
come in subtypes. Subtypes are unevenly distributed across organs.
That unevenness is the entire foundation of selective pharmacology.
Figure 14.1 — The somatic and autonomic motor pathways compared, from cell body to effector.
Described: Two pathways drawn side by side from the central nervous system to their effectors. On the left, the somatic motor pathway begins with an alpha motor neuron in the ventral horn whose single thick, heavily myelinated A-alpha axon, thirteen to twenty micrometres across and conducting at eighty to one hundred twenty metres per second, runs all the way to the target without synapsing. It ends in a neuromuscular junction with a thirty to fifty nanometre cleft, junctional folds carrying about ten thousand nicotinic receptors per square micrometre, and a safety factor of three to four, so transmission is guaranteed one to one; acetylcholine acting on the muscle nicotinic receptor always causes contraction, and denervation produces flaccid paralysis and atrophy. On the right, the autonomic pathway begins with a preganglionic neuron in the lateral horn whose thin, lightly myelinated B fiber, one to three micrometres across and conducting at three to fifteen metres per second, runs to a peripheral ganglion. There it releases acetylcholine onto neuronal nicotinic receptors on a multipolar postganglionic cell body, whose unmyelinated C fiber, 0.2 to 1.5 micrometres across and conducting at half to two metres per second, continues to the effector. It ends not in a junction but in a chain of varicosities releasing transmitter across gaps of twenty to two thousand nanometres, with no junctional folds and no specialized postsynaptic membrane, so one axon influences hundreds of cells by volume transmission. Acetylcholine acting on muscarinic receptors or norepinephrine acting on adrenergic receptors may excite or inhibit cardiac muscle, smooth muscle, glands, or adipose tissue, and denervation leaves the effector working but unregulated and hypersensitive to circulating agonists. A closing statement makes the argument explicit: the extra synapse adds an extra receptor, receptors come in subtypes, subtypes are distributed unevenly across organs, and that unevenness is the foundation of selective pharmacology.
Reading the table properly
Four rows repay a second look, because each contains an idea that students routinely get backwards.
"Always excitatory" versus "excitatory or inhibitory." A somatic motor neuron cannot relax a muscle. Every somatic ending uses one transmitter on one receptor — acetylcholine on a nicotinic cation channel — and opening a cation channel can only depolarize. Somatic inhibition therefore has to happen upstream, inside the CNS, by silencing the motor neuron (§13.6). The autonomic system achieves inhibition at the target, because the same transmitter acting on a different receptor subtype produces opposite intracellular consequences: G_s raises cyclic AMP, G_i lowers it, G_q raises intracellular calcium. This is the single design decision from which most of pharmacology follows, and it is the reason a drug can slow Amara's heart without paralysing her diaphragm.
"Tone." A resting skeletal muscle receives no motor traffic at all; its motor neurons are silent, and switching them on is how movement begins. Autonomic effectors never experience silence. Vascular smooth muscle receives roughly one sympathetic impulse per second at rest; the sinoatrial node receives continuous vagal discharge. Regulation is therefore achieved by turning a running signal up or down, which is faster and more precise than starting one from zero. Hold this; §14.6 is built on it.
"Divergence." A somatic motor unit is a private line: one axon, one muscle, a precisely delimited set of fibers. A sympathetic preganglionic axon synapses on 10 to 20 postganglionic neurons distributed across several chain ganglia, and those postganglionic axons then branch across many organs. The sympathetic system is built as a public address system; the parasympathetic system, with a divergence of 1:1–3 and ganglia sitting inside the target organ, is built as a set of private telephone lines. You can salivate without emptying your bladder. You cannot become frightened in only one organ.
"Effect of denervation." Cut the somatic nerve to a muscle and the muscle does nothing. Cut the autonomic nerve to the heart and the heart beats faster than before, because what was removed was a restraint rather than a command. This is the anatomical seed of Case File Question 2, and it is the reason a transplanted heart runs at about 100 beats per minute. Denervated smooth muscle and glands also up-regulate their receptors and become hypersensitive to circulating agonists — which is why a patient with Horner syndrome has a pupil that dilates dramatically to a dilute adrenergic eye drop that does nothing to a normal eye (§14.9).
Predict This
A patient's heart is completely denervated — every sympathetic and every vagal fiber to it has been cut, as happens in cardiac transplantation.
Before reading on, commit to two predictions. (a) What will the resting heart rate be: about 50, about 70, or about 100? (b) When this patient stands up quickly, how fast will the heart rate rise — within one beat, within five seconds, or after 30 to 60 seconds?
(Answer: (a) about 100. The sinoatrial node is autorhythmic: it depolarizes spontaneously and needs no command. Its unmodulated intrinsic rate is roughly 95–110 per minute, and a normal resting rate of 70 is that intrinsic rate minus about 30 beats of continuous vagal restraint. Remove the vagus and the restraint goes with it. (b) After 30 to 60 seconds — and this is the clinically important half of the answer. Both fast mechanisms, vagal withdrawal and direct sympathetic drive, require nerves that no longer exist. The only route left is circulating catecholamines released from the adrenal medulla, which must be secreted into the blood, circulate, and reach cardiac beta-1 receptors. Transplant recipients are therefore taught to stand slowly and to warm up and cool down gradually, because their heart rate response to any demand is delayed by half a minute.)
Check Your Understanding 14.1
- The autonomic system uses two neurons where the somatic system uses one. Name three functional capabilities that the extra synapse buys, and one cost.
- A drug blocks all nicotinic receptors in the body. Predict the consequences, and explain why this drug class is almost never used, whereas drugs blocking one muscarinic or one adrenergic subtype are used constantly.
Show answers
- (a) Inhibition at the periphery. The second synapse introduces a second receptor, and because receptor subtypes couple to different G proteins, the same transmitter can excite one organ and inhibit another. A one-neuron system with one receptor can only excite. (b) Divergence and amplification. One preganglionic neuron contacts 10–20 postganglionic neurons, so a small central command produces a body-wide response — which is what a threat response needs. (c) Selective pharmacology. Different organs express different subtypes, so a drug can be aimed at one organ. The cost is speed and precision: two synapses, thin and unmyelinated axons, and diffuse volume transmission mean autonomic responses take hundreds of milliseconds to seconds and cannot be aimed at a single cell. That trade is acceptable because no viscus needs millisecond timing, and it is unacceptable for skeletal muscle, which does.
- Blocking all nicotinic receptors blocks the N_M receptor at every neuromuscular junction — producing complete flaccid paralysis, including the diaphragm — and the N_N receptor at every autonomic ganglion of both divisions, plus the adrenal medulla. The autonomic effects are chaotic rather than directional, because each organ simply loses whichever division dominated it: the heart speeds up (vagal tone lost), blood pressure collapses (sympathetic vasomotor tone lost), the gut stops, the bladder is retained, and the pupils fix mid-position. Ganglionic blockers such as hexamethonium were once used for hypertension and were abandoned for exactly this reason. Subtype-selective drugs work because subtypes are distributed unevenly: β1 is concentrated in heart and kidney, β2 in bronchi and skeletal muscle arterioles, M3 on glands and smooth muscle. Selectivity is possible only where nature has already been selective.
14.2 Sympathetic Anatomy: The Thoracolumbar Division
The sympathetic division is built for mobilization. Its classical caricature — fight, flight, fright — is true as far as it goes, but it understates the system badly. Sympathetic outflow rises when you stand up, when you exercise, when you eat a large meal, when you are cold, when your blood pressure falls for any reason, and when you are asked a difficult question in front of people. It is not an emergency system. It is a continuously graded system that has an emergency setting at the top of its range.
Its defining structural feature is the two-neuron chain:
preganglionic neuron (cell body in CNS) → autonomic ganglion → postganglionic neuron (cell body in ganglion) → effector
and everything else about the division follows from where the ganglion sits.
Origin: the lateral horn of T1 to L2
Sympathetic preganglionic cell bodies occupy the lateral horn — the small lateral wing of gray matter present only in spinal segments T1 through L2 (Chapter 12). This is the entire origin of the system. There is no cervical sympathetic outflow, no sacral sympathetic outflow, and yet the sympathetic system supplies the eye, the salivary glands, the skin of the scalp, and the vessels of the toes. §14.2 exists mostly to explain how a fourteen-segment outflow serves a thirty-one-segment body.
The route out is fixed and worth memorizing as a sequence:
- The preganglionic axon leaves the cord in the ventral root, exactly like a somatic motor axon, because it is a motor axon.
- It travels a centimetre or two in the spinal nerve.
- It immediately branches off through a white ramus communicans into the sympathetic chain. The ramus is white because these axons are myelinated (B fibers). White rami exist only at T1–L2, because those are the only segments that have preganglionic cell bodies.
- Whatever happens next, any postganglionic axon that needs to rejoin a spinal nerve does so through a gray ramus communicans — gray because postganglionic axons are unmyelinated C fibers. Gray rami exist at every spinal level, all thirty-one, because every dermatome and every limb needs sympathetic supply to its blood vessels, sweat glands, and arrector pili.
That asymmetry — white rami at 14 levels, gray rami at 31 — is the whole trick, and Figure 14.2 draws it.
The sympathetic chain and the three fates
The sympathetic trunk (chain) is a paired string of roughly 22–23 ganglia running the whole length of the vertebral column from the base of the skull to the coccyx, lying just anterolateral to the vertebral bodies. It is often called the paravertebral chain for that position. The count is not uniform: the cervical region has three ganglia (superior, middle, and inferior, the last usually fused with the first thoracic ganglion to form the stellate ganglion), the thoracic region has eleven or twelve, the lumbar four or five, and the sacral four or five, with a single midline ganglion impar at the coccyx.
A preganglionic fiber entering the chain has exactly three options, and every sympathetic pathway in the body is one of them.
THE THREE FATES OF A SYMPATHETIC PREGANGLIONIC FIBER
(entering the chain at, say, T5)
SPINAL CORD CHAIN (paravertebral) TARGET
═══════════════════════════════════════════════════════════════════════
╭─── superior cervical g. ────► HEAD
│ (C1-C2 level) eye · lids ·
┌────────────┐ │ salivary ·
│ LATERAL │ ╔══════╪══════╗ ② ASCEND scalp sweat
│ HORN │ ║ ▲ │ ║ or DESCEND
│ T1 - L2 │ ║ │ ●──────╫──────────────► the chain, synapse
│ │ ║ │ ║ at a DIFFERENT level
│ ●────────╫──────╫──● T5 g. ║
│ pregang. │ WHITE║ │ ║ ① SYNAPSE HERE, at the level of
│ cell body │ ramus║ │ ●───────╫───► entry. Postgang. axon returns
└────────────┘ (T1- ║ │ \ GRAY ║ via the GRAY RAMUS to the spinal
L2 ║ │ \ramus║ nerve ► SKIN of that dermatome:
only)║ ▼ \ ║ sweat glands · arrector pili ·
║ │ \ ║ cutaneous vessels
║ ● \ ║
║ │ ▼ ║ ┌──────────────────────┐
╚══╪══════════╝ │ ALL 31 spinal nerves │
│ │ have a GRAY ramus. │
│ ③ PASS STRAIGHT │ Only T1-L2 have a │
│ THROUGH — no │ WHITE ramus. │
│ synapse in chain └──────────────────────┘
▼
SPLANCHNIC NERVE
│
▼
╭───────────────────────╮
│ PREVERTEBRAL GANGLION │ celiac · superior mesenteric ·
│ (on the AORTA) │ aorticorenal · inferior mesenteric
╰───────────┬───────────╯
▼
ABDOMINAL AND PELVIC VISCERA
═══════════════════════════════════════════════════════════════════════
SPECIAL CASE — the ADRENAL MEDULLA:
greater splanchnic fibers pass through the chain AND through the
celiac ganglion WITHOUT synapsing, and end directly on CHROMAFFIN
CELLS. Those cells ARE the postganglionic neuron — they simply never
grew an axon, and secrete into BLOOD instead: 80% EPI · 20% NE.
GEOMETRY: short PREganglionic · long POSTganglionic
DIVERGENCE: 1 pre : 10-20 post ⇒ DIFFUSE, body-wide response
Figure 14.2 — The three fates of a sympathetic preganglionic fiber entering the chain.
Described: A diagram tracing a sympathetic preganglionic fiber from the lateral horn of a thoracic segment, here T5, into the paravertebral chain, and then along three alternative routes. The fiber leaves the cord in the ventral root and enters the chain through a white ramus communicans, which exists only at levels T1 to L2 because only those segments contain preganglionic cell bodies. Fate one: the fiber synapses in the chain ganglion at its own level, and the postganglionic axon returns to the spinal nerve through a gray ramus communicans to supply the sweat glands, arrector pili muscles, and cutaneous blood vessels of that dermatome; gray rami exist at all thirty-one spinal levels. Fate two: the fiber ascends or descends within the chain before synapsing at a different level, which is how fibers from T1 and T2 reach the superior cervical ganglion at the C1 to C2 level to supply the eye, eyelid, salivary glands, and scalp sweat glands, and how fibers from L1 and L2 descend to reach the lower limb. Fate three: the fiber passes straight through the chain without synapsing, forming a splanchnic nerve, and travels to a prevertebral ganglion lying on the front of the aorta — celiac, superior mesenteric, aorticorenal, or inferior mesenteric — where it synapses before supplying abdominal and pelvic viscera. A special case is shown separately: greater splanchnic fibers that pass through both the chain and the celiac ganglion without synapsing and end directly on chromaffin cells of the adrenal medulla, which are postganglionic neurons that never grew axons and secrete eighty percent epinephrine and twenty percent norepinephrine into the blood. A footer notes the sympathetic geometry of short preganglionic and long postganglionic fibers with a divergence ratio of one to ten or twenty, producing a diffuse, body-wide response.
Fate 1 — synapse at the level of entry. The postganglionic cell body sits in the chain ganglion of that same segment, and its axon returns to the spinal nerve by the gray ramus and travels out with it. Its targets are the three structures that every patch of skin needs: sweat glands, arrector pili muscles, and cutaneous blood vessels. This is why sympathetic supply to skin follows the ordinary dermatomal distribution, and why a spinal or epidural block produces a warm, dry, vasodilated segment of skin along with the numbness.
Fate 2 — ascend or descend within the chain, then synapse. This is how a T1–L2 outflow reaches the whole body. Fibers from T1–T2 ascend the chain to the superior cervical ganglion at the level of C1–C2 and from there supply the entire head: the pupillary dilator, the superior tarsal muscle of the eyelid, the sweat glands and vessels of the face and scalp, and the salivary glands. That long, exposed path is why Horner syndrome can be produced by a lesion anywhere from the hypothalamus to the orbit (§14.9). Fibers from L1–L2 descend to the lumbar and sacral chain ganglia to supply the lower limbs.
Fate 3 — pass straight through and form a splanchnic nerve. These fibers do not synapse in the chain at all. They emerge medially as the splanchnic nerves and travel to a prevertebral (collateral) ganglion lying on the anterior surface of the abdominal aorta, named for the artery it surrounds.
| Splanchnic nerve | Spinal origin | Prevertebral ganglion | Supplies |
|---|---|---|---|
| Greater | T5–T9 | Celiac | Stomach, liver, spleen, gallbladder, pancreas, proximal duodenum |
| Lesser | T10–T11 | Superior mesenteric, aorticorenal | Distal duodenum, jejunum, ileum, proximal colon, kidney |
| Least | T12 | Renal plexus | Kidney |
| Lumbar | L1–L2 | Inferior mesenteric, hypogastric plexus | Distal colon, rectum, bladder, reproductive organs |
Two clinical consequences of that table are worth carrying. First, visceral pain afferents travel back along these same nerves, which is why pain from the stomach, gallbladder, and proximal duodenum is referred to the epigastrium (T5–T9 dermatomes), pain from the small bowel and proximal colon to the periumbilical region (T10 dermatome), and pain from the distal colon and pelvic organs to the suprapubic region (L1 dermatome) — the classic migration of appendicitis pain from umbilicus to right iliac fossa is the switch from a visceral T10 referral to direct parietal peritoneal irritation. Second, a celiac plexus block — local anesthetic or alcohol injected around the celiac ganglion — relieves the otherwise intractable pain of pancreatic cancer, because every afferent from that organ passes through one small anatomical bottleneck.
Why sympathetic responses are diffuse
Three features compound.
Geometry. The ganglion is close to the cord and far from the target, so the sympathetic system has short preganglionic and long postganglionic fibers. A long postganglionic axon has room to branch widely across several organs on its way.
Divergence. Each preganglionic fiber synapses on 10 to 20 postganglionic neurons, often distributed across several adjacent chain ganglia. One central command becomes a regional response.
The hormonal channel. The adrenal medulla broadcasts catecholamines into the blood, reaching every capillary bed in the body, including tissues with almost no sympathetic innervation at all.
The result is that the sympathetic system is very good at producing coordinated, whole-body state changes and very bad at doing one thing in one place. That is the correct design for a system whose job is to reconfigure an entire organism for a different metabolic mode.
The adrenal medulla: a ganglion that forgot to grow axons
Some greater splanchnic preganglionic fibers pass through the celiac ganglion without synapsing and run straight into the adrenal medulla, where they synapse on chromaffin cells. Those cells are modified postganglionic sympathetic neurons that never developed axons. Instead of releasing transmitter onto a nearby effector, they release it into the bloodstream: about 80 percent epinephrine and 20 percent norepinephrine, functioning as a hormone.
This gives the sympathetic system a second, slower, longer-lasting broadcast channel with different kinetics from the neural one. Neural sympathetic signals act in under a second and stop within seconds of the nerve falling silent, because norepinephrine is removed from the cleft by reuptake. Circulating catecholamines take 20–30 seconds to build and persist for one to three minutes after release stops, because they must be cleared by uptake and by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) in liver and kidney. The lingering shakiness after a near-miss in traffic is the hormonal tail outlasting the neural signal.
The hormonal channel also has a different receptor profile, which matters more than the timing. Norepinephrine released from nerve terminals has strong α1, α2, and β1 activity and almost no β2 activity. Epinephrine from the medulla has high affinity for β2. So the adrenal contribution reaches targets the nerves effectively cannot: bronchial smooth muscle, whose β2 receptors have almost no sympathetic innervation; skeletal muscle arterioles; the liver's glycogenolytic machinery. Hold that fact — §14.4 and Case File Question 3 both need it.
Development · Neural Crest, the Adrenal Medulla, and Hirschsprung Disease
Adult anatomy is often incomprehensible until you know where the tissue came from, and the adrenal gland is the best example in this book. It is two organs in one capsule, from two different germ layers, and developmentally they have nothing to do with each other.
The adrenal cortex arises from mesoderm, from coelomic epithelium near the developing gonad — which is why it makes steroid hormones out of cholesterol exactly as the gonad does, and why a defect in one cortical enzyme (21-hydroxylase deficiency) shunts precursors into androgen synthesis and virilizes a female fetus (Chapter 16).
The adrenal medulla arises from the neural crest — the transient population of cells that delaminates from the dorsal neural tube and migrates throughout the embryo. Neural crest gives rise to every ganglion in the peripheral nervous system: sympathetic chain ganglia, prevertebral ganglia, parasympathetic terminal ganglia, the entire enteric nervous system, and the dorsal root ganglia of Chapter 13 — plus Schwann cells, melanocytes, the meninges' outer layer, and much of the facial skeleton. Crest cells destined to become sympathetic neurons migrate ventrally alongside the aorta; most stop and form chain ganglia, but one group is captured by the developing cortical tissue and migrates inside it.
Everything peculiar about the medulla follows:
- It is innervated by preganglionic fibers, not postganglionic ones, because it is the postganglionic element.
- Its cells respond to acetylcholine acting on nicotinic receptors — the pharmacology of a ganglionic synapse, not of a gland — which is why nicotine causes a catecholamine surge and why a smoker's first cigarette of the day raises blood pressure.
- It synthesizes catecholamines by the same pathway as a sympathetic neuron, with one extra enzyme: phenylethanolamine-N-methyltransferase (PNMT), which methylates norepinephrine to epinephrine. PNMT is induced by high local cortisol, and the medulla sits inside the cortex, bathed in cortical venous blood at concentrations up to a hundred times systemic. The medulla makes epinephrine because it lives inside the cortex. Move the same cells elsewhere — as happens in extra-adrenal chromaffin tissue — and they make norepinephrine only.
- Tumours of these cells (pheochromocytoma) release catecholamines in paroxysms, causing episodic hypertension, headache, palpitations, and drenching sweats. Because the tumour secretes continuously into vesicles and metabolizes internally, the best screening test is plasma or urinary metanephrines — the metabolites — rather than the catecholamines themselves.
The same origin story explains two other diseases. Neuroblastoma, the commonest extracranial solid tumour of childhood, arises from primitive sympathetic neuroblasts and can appear anywhere along the migration route from neck to pelvis, most often in the adrenal medulla itself. And Hirschsprung disease is a failure of neural crest migration: crest cells colonize the gut craniocaudally, and if they stop short, the distal segment is left aganglionic — no submucosal plexus, no myenteric plexus. The affected bowel is permanently contracted, not dilated, because the enteric inhibitory neurons that would relax it never arrived; the proximal, normally innervated bowel dilates behind the obstruction. A newborn who fails to pass meconium in 48 hours has an anatomy problem that began as a migration problem in week seven, and the diagnostic test is the absent rectoanal inhibitory reflex (§14.7) — the one reflex that requires the missing neurons.
Histology · Telling an Autonomic Ganglion from a Dorsal Root Ganglion
Both are clusters of neuron cell bodies outside the CNS, and on low power they look similar. Four features separate them, and each is a direct readout of function.
Cell shape and process count. Dorsal root ganglion neurons are pseudounipolar: large, perfectly round somata with a single process that bifurcates. Autonomic ganglion neurons are multipolar, with visible dendrites and an irregular, angular outline. Round means no integration; angular means dendrites, and dendrites mean synapses.
Nucleus position. DRG neurons have a large, central, pale nucleus with a prominent nucleolus, sitting exactly in the middle of the cell. Autonomic ganglion neurons frequently have an eccentric nucleus pushed to one side, and many are binucleate.
Cell size and uniformity. DRG somata are large — up to 100 µm — and fall into a bimodal distribution: big cells giving rise to large myelinated A-beta fibers, small cells giving rise to C fibers. Autonomic ganglion cells are smaller, 20–45 µm, and much more uniform, because they all do the same job.
Satellite cells. Both are surrounded by satellite glia, but in a DRG they form a complete, regular, tightly packed ring around each soma — a striking picket fence at high power. In autonomic ganglia the satellite sheath is looser and interrupted, precisely because presynaptic axons must penetrate it to reach the dendrites.
Two more clues. DRG neurons are arranged in clusters at the periphery of the ganglion with axon bundles running through the middle, whereas autonomic ganglion cells are scattered throughout. And a synaptic ganglion contains a visible neuropil — a felt-work of interweaving fine processes between the somata — which a DRG does not, because a DRG contains no synapses at all.
One further specimen is worth seeking out: the adrenal medulla stained with chromium salts, the reaction that gave chromaffin cells their name. The cells are large, polyhedral, arranged in cords around wide venous sinusoids, and stain brown as their catecholamines oxidize. Look for the preganglionic axons entering directly and for the occasional true ganglion cell scattered among the chromaffin cells — the medulla's origin showing through.
Check Your Understanding 14.2
- Sympathetic outflow exists only from T1 to L2, yet the sympathetic system supplies the face and the feet. Explain the anatomy that makes this possible, and name the ganglion involved for the face.
- Why does every spinal nerve have a gray ramus communicans but only fourteen pairs have a white ramus?
- A patient has a stab wound that severs the greater splanchnic nerve on one side. Which ganglion loses its input, which organs are affected, and why is the deficit hard to detect clinically?
Show answers
- Preganglionic fibers entering the sympathetic chain can ascend or descend within it before synapsing (fate 2). Fibers from T1–T2 ascend to the superior cervical ganglion at the level of C1–C2 and supply the whole head; fibers from L1–L2 descend to lumbar and sacral chain ganglia to reach the lower limb. The chain is a distribution network that lets a fourteen-segment outflow serve a thirty-one-segment body — and the length of the path to the head is why a lesion at the lung apex, in the neck, or in the carotid sheath all produce the same Horner syndrome.
- Because the two rami carry different traffic in different directions. A white ramus carries preganglionic axons into the chain, and preganglionic cell bodies exist only in the lateral horn of T1–L2 — so only those levels have anything to contribute. A gray ramus carries postganglionic axons out of the chain to rejoin the spinal nerve, and every spinal nerve in the body needs them, because every dermatome has sweat glands, arrector pili, and cutaneous vessels requiring sympathetic supply. White is myelinated preganglionic input at 14 levels; gray is unmyelinated postganglionic output at all 31.
- The celiac ganglion loses its preganglionic input from T5–T9 on that side, affecting the stomach, liver, spleen, gallbladder, pancreas, and proximal duodenum. The deficit is hard to detect for three reasons: the abdominal viscera are innervated bilaterally, so the contralateral greater splanchnic nerve continues to supply the midline organs; the enteric nervous system runs gut motility largely autonomously and merely receives modulation from the sympathetic system; and the adrenal medulla's circulating catecholamines reach the same organs by blood regardless of what the nerves are doing. What may be noticeable is loss of visceral pain from those organs on that side, since the afferents travel with the same nerve — which is precisely the basis of therapeutic celiac plexus blockade.
14.3 Parasympathetic Anatomy: The Craniosacral Division
If the sympathetic division is a public address system, the parasympathetic division is a switchboard of private lines. It is built for specificity: it can raise gastric secretion without slowing the heart, constrict a pupil without emptying a bladder, and produce an erection without any other visible change in the body. Its anatomy is the reason.
Origin: cranial and sacral, and nothing in between
Parasympathetic preganglionic cell bodies occupy two widely separated territories, which is why the division is called craniosacral:
- Brainstem nuclei associated with cranial nerves III, VII, IX, and X — the "1-9-7-3" rule of Chapter 13.
- The lateral gray matter of sacral segments S2–S4, whose axons leave as the pelvic splanchnic nerves (nervi erigentes).
There is no thoracic and no lumbar parasympathetic outflow. More importantly, there is no parasympathetic supply to the skin, to sweat glands, to arrector pili, or to the vast majority of blood vessels. That asymmetry is not a detail; it is one of the two or three most consequential facts in this chapter. Vascular tone is a sympathetic monologue, and every drug that dilates an artery therefore works either by reducing sympathetic drive, by blocking the sympathetic receptor, or by acting on the vessel directly — never by "increasing parasympathetic tone," because there is nothing there to increase.
Ganglia: terminal and intramural
Parasympathetic ganglia sit on or inside the wall of the target organ. Those on the organ are terminal ganglia; those embedded within its wall — as throughout the gut, where they form the myenteric and submucosal plexuses — are intramural ganglia. Four named ganglia in the head are the exception, being discrete enough to be dissected and named:
| Origin | Nucleus | Ganglion | Target |
|---|---|---|---|
| CN III | Edinger-Westphal | Ciliary | Pupillary sphincter (constriction); ciliary muscle (accommodation) |
| CN VII | Superior salivatory | Pterygopalatine; submandibular | Lacrimal and nasal glands; submandibular and sublingual salivary glands |
| CN IX | Inferior salivatory | Otic | Parotid gland |
| CN X | Dorsal motor nucleus; nucleus ambiguus | Terminal and intramural ganglia in the organ walls | Heart, lungs, esophagus, stomach, liver, pancreas, small intestine, colon to the splenic flexure |
| S2–S4 | Sacral parasympathetic nucleus | Terminal and intramural ganglia in the organ walls | Distal colon, rectum, bladder, reproductive organs |
Because the ganglion sits at the target, the geometry is the exact mirror of the sympathetic: long preganglionic, short postganglionic. And because each preganglionic fiber contacts only one to three postganglionic neurons, parasympathetic effects are discrete and organ-specific. A postganglionic axon a few hundred micrometres long cannot reach anything except the organ it is already inside.
The vagus: eighty percent afferent, seventy-five percent of the outflow
Vagus means "wandering," and it does. From the medulla it leaves the skull through the jugular foramen, descends the neck within the carotid sheath between the internal jugular vein and the common carotid artery, enters the thorax, and there the two vagi behave asymmetrically because the embryonic aortic arches did. The left vagus gives off a recurrent laryngeal branch that hooks under the arch of the aorta; the right gives off one that hooks under the right subclavian artery. Both then ascend in the tracheo-oesophageal groove to the larynx. This is why a left-sided lung tumour, an enlarged left atrium, or an aortic arch aneurysm causes hoarseness and the right side rarely does: the left recurrent laryngeal nerve takes a detour of fifteen centimetres through the mediastinum for no adult reason whatsoever.
Below that, the vagi form a plexus on the oesophagus, rotate with it during development so that the left vagus becomes the anterior trunk and the right becomes the posterior trunk, pass through the diaphragm with the oesophagus, and distribute to the stomach, liver, pancreas, small intestine, and colon as far as the splenic flexure.
Three quantitative facts about the vagus are worth memorizing.
It carries roughly 75 percent of all parasympathetic fibers in the body. "Vagal tone" and "parasympathetic tone" are, for most practical purposes, the same phrase.
Roughly 80 percent of its own fibers are afferent, not efferent. The vagus is mostly a sensory nerve. It reports blood pressure from the aortic arch, lung inflation from pulmonary stretch receptors, gastric distension, intestinal chemistry, and inflammatory signals from the gut. This vast afferent stream is why vagal reflexes are so fast and so powerful, and why a distended stomach can slow a heart.
It is the longest autonomic nerve in the body, which is why it is the first to fail in a length-dependent autonomic neuropathy — Amara's SDNN of 42 ms is a measurement of the distal end of the longest C fibers she has.
The splenic flexure and why three systems change there
The vagus supplies the gut to the splenic flexure; the pelvic splanchnic nerves supply everything beyond it. That boundary also marks the change in arterial supply from the superior to the inferior mesenteric artery, and the change in embryological origin from midgut to hindgut. Three independent systems switch at the same landmark, which is not a coincidence: nerves, vessels, and gut tube all follow the same developmental segmentation, and the adult boundary is the fossil record of it. Clinically, the splenic flexure is a watershed — the region most vulnerable to ischaemic colitis when perfusion falls, because it lies at the far end of two arterial territories at once.
SYMPATHETIC ("thoracolumbar") │ PARASYMPATHETIC ("craniosacral")
mobilize · expend · redistribute │ conserve · digest · restore
════════════════════════════════════════╪═══════════════════════════════════
ORIGIN lateral horn T1 ─ L2 │ ORIGIN CN III, VII, IX, X
│ + sacral cord S2 ─ S4
────────────────────────────────────────┼───────────────────────────────────
EYE ◄─ T1-T2 ► superior cervical g. │ EYE ◄─ CN III ► ciliary g.
├ pupil DILATES (alpha-1) │ ├ pupil CONSTRICTS (M3)
└ lid held up, sup. tarsal (alpha-1) │ └ lens THICKENS (M3)
────────────────────────────────────────┼───────────────────────────────────
HEAD GLANDS ◄─ superior cervical g. │ HEAD GLANDS
├ thick, scanty, protein-rich saliva │ ├ TEARS ◄─ CN VII ► ptery-
└ SWEAT — by ACETYLCHOLINE on M3 ! │ │ gopalatine g.
(the one cholinergic sympathetic │ ├ watery saliva ◄─ CN VII ►
pathway in the body) │ │ submandibular g.
────────────────────────────────────────┤ └ parotid ◄─ CN IX ► otic g.
LUNG │ ───────────────────────────────
└ bronchoDILATE (beta-2 — reached │ LUNG ◄─ CN X
mostly by CIRCULATING EPINEPHRINE) │ └ bronchoCONSTRICT + mucus (M3)
────────────────────────────────────────┼───────────────────────────────────
HEART ◄─ T1-T5 ► cardiac plexus │ HEART ◄─ CN X (VAGUS)
├ RATE ↑ (beta-1, SA node) │ ├ RATE ↓ (M2, SA node)
├ CONDUCTION ↑ (beta-1, AV node) │ ├ CONDUCTION ↓ (M2, AV node)
└ CONTRACTILITY ↑ (beta-1, ventricle) │ └ ventricle: almost NO effect
────────────────────────────────────────┼───────────────────────────────────
VESSELS ◄── the ONLY control │ VESSELS ── NO SUPPLY AT ALL
├ skin, gut, kidney CONSTRICT (a-1) │ └ (exception: erectile tissue,
└ skeletal muscle, coronary DILATE │ via nitric oxide)
(beta-2 + local metabolites) │
────────────────────────────────────────┼───────────────────────────────────
GUT ◄─ greater/lesser/lumbar │ GUT ◄─ CN X to SPLENIC FLEXURE,
splanchnics ► celiac, SMG, IMG │ then S2-S4 pelvic splanchnic
├ motility ↓ secretion ↓ (a-2, b-2) │ ├ motility ↑ secretion ↑ (M3)
└ sphincters CONTRACT (alpha-1) │ └ sphincters RELAX (M3)
────────────────────────────────────────┼───────────────────────────────────
LIVER glycogenolysis ↑ (beta-2) │ LIVER glycogen synthesis ↑
ADIPOSE lipolysis ↑ (beta-3) │
KIDNEY RENIN ↑ (beta-1) ◄──── DO NOT FORGET THIS ONE
────────────────────────────────────────┼───────────────────────────────────
BLADDER = STORAGE │ BLADDER ◄─ S2-S4 = VOIDING
├ detrusor RELAXES (beta-3) │ ├ detrusor CONTRACTS (M3)
└ internal sphincter CONTRACTS (a-1) │ └ internal sphincter RELAXES
────────────────────────────────────────┼───────────────────────────────────
GENITALS emission/ejaculation (a-1) │ GENITALS ERECTION (nitric oxide)
"SHOOT" │ "POINT"
────────────────────────────────────────┼───────────────────────────────────
ADRENAL MEDULLA ◄─ PREganglionic │ ADRENAL — no parasympathetic
fibers direct; EPI 80% / NE 20% ► BLOOD│
════════════════════════════════════════╪═══════════════════════════════════
GANGLIA chain (paravertebral, 22-23) │ TERMINAL / INTRAMURAL — on or in
+ prevertebral (celiac, SMG, │ the wall of the target organ;
aorticorenal, IMG) │ 4 named cranial ganglia
FIBERS short PRE / long POST │ long PRE / short POST
DIVERGE 1 pre : 10-20 post │ 1 pre : 1-3 post
RESPONSE DIFFUSE, body-wide │ DISCRETE, organ-specific
Figure 14.3 — Sympathetic and parasympathetic outflow compared: origins, ganglia, targets, and receptors.
Described: A two-column whole-body comparison. The left column is the sympathetic or thoracolumbar division arising from the lateral horn of T1 to L2; the right is the parasympathetic or craniosacral division arising from cranial nerves three, seven, nine and ten and sacral segments two to four. Organ by organ: the sympathetic dilates the pupil and holds the eyelid up through alpha-1 receptors, using fibers from T1 and T2 that relay in the superior cervical ganglion, while cranial nerve three relaying in the ciliary ganglion constricts the pupil and thickens the lens through M3 receptors. Sympathetic supply to head glands produces thick, scanty, protein-rich saliva and, uniquely, sweating mediated by acetylcholine acting on M3 receptors — the one cholinergic sympathetic pathway in the body; parasympathetic supply produces tears through cranial nerve seven relaying in the pterygopalatine ganglion, watery saliva through cranial nerve seven relaying in the submandibular ganglion, and parotid secretion through cranial nerve nine relaying in the otic ganglion. In the lung, beta-2 receptors reached mostly by circulating epinephrine dilate bronchi, while the vagus constricts them and increases mucus through M3. In the heart, sympathetic fibers from T1 to T5 travelling through the cardiac plexus raise rate at the sinoatrial node, conduction at the atrioventricular node, and ventricular contractility through beta-1 receptors, while the vagus slows rate and conduction through M2 receptors and has almost no effect on the ventricle. Blood vessels receive no parasympathetic supply at all except erectile tissue, so sympathetic tone is their only neural control: alpha-1 constricts skin, gut, and kidney vessels while beta-2 and local metabolites dilate skeletal muscle and coronary vessels. In the gut, splanchnic nerves relaying in the celiac, superior mesenteric, and inferior mesenteric ganglia reduce motility and secretion through alpha-2 and beta-2 and contract sphincters through alpha-1, while the vagus to the splenic flexure and then the pelvic splanchnic nerves do the reverse through M3. The sympathetic system also drives hepatic glycogenolysis through beta-2, adipose lipolysis through beta-3, and renin release from the kidney through beta-1. In the bladder, sympathetic beta-3 relaxes the detrusor and alpha-1 contracts the internal sphincter for storage, while sacral parasympathetic fibers contract the detrusor and relax the sphincter for voiding. In the genitals the parasympathetic system produces erection through nitric oxide and the sympathetic system produces emission and ejaculation through alpha-1. The adrenal medulla receives preganglionic sympathetic fibers directly and releases eighty percent epinephrine and twenty percent norepinephrine into the blood, with no parasympathetic supply. A summary block contrasts the anatomy: sympathetic chain ganglia numbering twenty-two to twenty-three plus prevertebral ganglia, short preganglionic and long postganglionic fibers, one to ten or twenty divergence, and diffuse body-wide responses; against parasympathetic terminal and intramural ganglia in the organ wall plus four named cranial ganglia, long preganglionic and short postganglionic fibers, one to one-to-three divergence, and discrete organ-specific responses.
Histology · Varicosities, and Why Autonomic Effects Are Diffuse
Put a stretched sheet of ileal smooth muscle or a whole-mount of iris under the microscope, stain it for tyrosine hydroxylase or for acetylcholinesterase, and you will not see anything resembling a neuromuscular junction. What you see is a network — a loose plexus of very fine, unmyelinated axons wandering across the tissue, each one beaded at intervals of 3–5 µm with swellings 0.5–2 µm across. Those beads are varicosities, and a single postganglionic axon carries between 10,000 and 30,000 of them.
Four structural features distinguish a varicosity from a synapse, and each has a functional consequence.
No specialized postsynaptic membrane. There is no trough, no junctional fold, no receptor cluster, and no basal lamina specialization. Receptors are distributed over the whole surface of the target cell rather than concentrated opposite a release site. The consequence is that any cell expressing the receptor can respond, whether or not an axon happens to run past it.
Enormous and variable cleft width. In densely innervated tissue such as the vas deferens or the iris the gap can be as narrow as 20 nm; in a large artery or the ventricular myocardium it may be 1,000–2,000 nm — a hundredfold difference. Transmitter therefore reaches its targets by diffusion through interstitial fluid, not by crossing a defined cleft. This is volume transmission, and it takes tens to hundreds of milliseconds rather than the sub-millisecond timing of a neuromuscular junction.
En passant release. The axon does not terminate on a cell; it passes by, releasing from successive varicosities as the impulse travels. One axon can influence hundreds of muscle cells along its route, and no cell has a private line.
Release is probabilistic. At a neuromuscular junction, one action potential reliably releases enough acetylcholine to fire the muscle fiber. At a varicosity, the probability that a given impulse releases a vesicle is often below 0.1. Individual varicosities are unreliable; the tissue response is reliable because thousands of them are summed.
Why build it this way? Because the effector is not built like skeletal muscle either. Single-unit smooth muscle cells are electrically coupled by gap junctions into a syncytium that contracts as a sheet, and cardiac muscle likewise. There is no advantage in addressing one cell, and considerable advantage in bathing a whole region. The varicosity is the correct ending for a tissue that acts together — and the reason a drug that reaches the interstitial fluid by any route, including the bloodstream, works just as well as the nerve does.
Check Your Understanding 14.3
- Why are parasympathetic effects organ-specific while sympathetic effects are body-wide? Give two structural reasons and one hormonal one.
- A patient develops hoarseness after a chest X-ray shows a mass in the left hilum. Explain the anatomy, and state why a right hilar mass of the same size would be less likely to do this.
- Blood vessels have essentially no parasympathetic innervation. What does that imply about how the body produces vasodilation, and name three distinct mechanisms it uses instead.
Show answers
- (a) Ganglion position. Parasympathetic ganglia sit on or within the wall of a single target organ, so their short postganglionic axons physically cannot reach anything else; sympathetic ganglia sit near the vertebral column, and their long postganglionic axons branch widely on the way to their targets. (b) Divergence ratio. Each parasympathetic preganglionic fiber contacts only one to three postganglionic neurons; each sympathetic preganglionic fiber contacts ten to twenty, distributed across several adjacent ganglia. (c) The hormonal channel. The adrenal medulla broadcasts epinephrine into the blood, reaching every perfused tissue including those with no sympathetic innervation; there is no parasympathetic equivalent — no gland secretes acetylcholine into the circulation, and any that did would be defeated by plasma cholinesterase within seconds.
- The left recurrent laryngeal nerve, a branch of the left vagus, hooks under the arch of the aorta and ascends in the tracheo-oesophageal groove to supply all the intrinsic muscles of the larynx except cricothyroid. Its course therefore takes it through the left hilum and aortopulmonary window, where a tumour, enlarged lymph nodes, an enlarged left atrium, or an aortic aneurysm can compress it — producing a paralysed left vocal cord and a breathy hoarseness. The right recurrent laryngeal nerve hooks under the right subclavian artery, much higher in the root of the neck, so it never enters the mediastinum and is out of reach of a hilar mass. The asymmetry is developmental: both nerves originally hooked under the sixth aortic arch, and on the right that arch and the fifth regress, letting the nerve ride up to the fourth arch derivative, while on the left the sixth arch persists as the ductus arteriosus and holds the nerve down.
- It implies that there is no vasodilator nerve for most of the circulation, so dilation cannot be commanded — it can only be produced by removing constriction or by acting on the vessel directly. Three mechanisms: (a) withdrawal of sympathetic tone — reducing the roughly 1 Hz baseline alpha-1 discharge, which is the principal moment-to-moment mechanism and the reason baroreflex-mediated dilation is possible at all; (b) local metabolic and endothelial control — adenosine, potassium, carbon dioxide, hydrogen ions, and shear-stress-induced nitric oxide from endothelium acting directly on smooth muscle, which is what matches flow to demand in working muscle and myocardium; (c) circulating epinephrine on beta-2 receptors in skeletal muscle and coronary arterioles, the one adrenergic mechanism that dilates rather than constricts. A defensible fourth is the parasympathetic exception itself — nitric-oxide-mediated dilation of the helicine arteries in erectile tissue, the only place a true vasodilator nerve exists.
14.4 Neurotransmitters and Receptors
This is the section that answers the Case File, and the section that makes modern pharmacology comprehensible. It rests on a single idea, which you should state to yourself before reading further:
The effect of a neurotransmitter is determined by the receptor, not by the transmitter.
Norepinephrine constricts one vessel and dilates another. It speeds the heart and slows the gut. It does all of this with one molecule, because the target cells express different receptors. A transmitter is a key; the receptor is the lock; and it is the lock that decides what opening it does.
Which fiber releases what
The rule has four clauses and two exceptions, and it is worth learning as a single sentence.
All preganglionic fibers of both divisions release acetylcholine. All parasympathetic postganglionic fibers release acetylcholine. Almost all sympathetic postganglionic fibers release norepinephrine — except those to thermoregulatory sweat glands, which release acetylcholine, and those to renal vascular smooth muscle, which release dopamine as well.
Fibers releasing acetylcholine are cholinergic; fibers releasing norepinephrine are adrenergic. The sweat gland exception matters enormously in practice: sweating is anatomically sympathetic but pharmacologically cholinergic, which is why atropine stops sweating and why anticholinergic drugs cause hyperthermia in hot weather.
Acetylcholine: synthesis, release, termination
Synthesis. Choline is taken into the nerve terminal by a sodium-dependent high-affinity choline transporter — the rate-limiting step of the whole pathway, and the target of the research drug hemicholinium. Inside, choline acetyltransferase (ChAT) transfers an acetyl group from acetyl-CoA to choline. ChAT is the definitive marker of a cholinergic neuron.
Storage. The vesicular acetylcholine transporter (VAChT) pumps ACh into synaptic vesicles using a proton gradient, concentrating it several hundredfold. Vesamicol blocks this step.
Release. An action potential opens voltage-gated N-type calcium channels in the terminal; calcium entry triggers SNARE-mediated vesicle fusion exactly as at any chemical synapse (Chapter 11). Botulinum toxin cleaves SNARE proteins and abolishes release — which is why botulism produces flaccid paralysis with dry mouth and fixed dilated pupils, both cholinergic failures at once, and why therapeutic botulinum injection treats focal dystonia, spasticity, and hyperhidrosis.
Termination. ACh is destroyed in the cleft by acetylcholinesterase (AChE), one of the fastest enzymes known: a single molecule hydrolyses about 25,000 molecules of ACh per second, and the transmitter's lifetime in the cleft is well under a millisecond. The choline released is recaptured by the terminal and reused.
That last point has a large consequence. Because termination is enzymatic destruction at the site, cholinergic effects are brief and locally confined — the transmitter never travels far because it does not survive long enough. It also explains why acetylcholine cannot function as a circulating hormone: plasma cholinesterase would destroy it before it left the vein.
Norepinephrine: synthesis, release, termination
Synthesis. Tyrosine → DOPA (by tyrosine hydroxylase, the rate-limiting step and the point of feedback control) → dopamine (by DOPA decarboxylase) → transported into the vesicle → norepinephrine (by dopamine beta-hydroxylase, which acts inside the vesicle). In the adrenal medulla only, one further step converts norepinephrine to epinephrine by PNMT, in the cytoplasm, after the norepinephrine leaves the vesicle — a cortisol-dependent step (§14.2).
Storage. The vesicular monoamine transporter (VMAT) loads catecholamines into dense-core vesicles. Reserpine blocks VMAT, so transmitter leaks into the cytoplasm and is destroyed, depleting the terminal — the mechanism of an old and effective antihypertensive with an unacceptable rate of depression.
Release. Calcium-triggered exocytosis, as for ACh, and modulated by presynaptic autoreceptors: released norepinephrine acts back on alpha-2 receptors on its own terminal to inhibit further release. This negative feedback is a genuinely elegant piece of design — the transmitter regulates its own output — and it is the mechanism of clonidine, an alpha-2 agonist that lowers blood pressure by reducing norepinephrine release, and of dexmedetomidine, which sedates by the same route.
Termination. This is where the two transmitters differ most. Norepinephrine is not destroyed in the cleft. About 80 percent is transported back into the nerve terminal by the norepinephrine transporter (NET, "uptake-1") and reused; a smaller fraction is taken up by surrounding cells ("uptake-2") or diffuses into the blood. Only after reuptake is it degraded, by monoamine oxidase (MAO) on the outer mitochondrial membrane inside the neuron and by catechol-O-methyltransferase (COMT) in the extraneuronal tissue and liver.
Three drug classes fall directly out of that paragraph. Cocaine and tricyclic antidepressants block NET, leaving norepinephrine in the cleft — which is why cocaine causes tachycardia, hypertension, vasoconstriction, and coronary spasm. Amphetamines reverse NET, pumping stored transmitter out of the terminal. MAO inhibitors prevent intraneuronal degradation, which is why a patient taking one who eats aged cheese — rich in tyramine, which displaces stored norepinephrine — can have a hypertensive crisis.
And the physiological consequence: because norepinephrine is recycled rather than destroyed, its effects last seconds rather than the milliseconds of acetylcholine. Combine that with the second-messenger kinetics of §14.6 and you have the reason vagal control of the heart is fast and sympathetic control is slow.
Cotransmitters
The two-transmitter scheme is a useful simplification, not the truth. Autonomic terminals co-release: ATP and neuropeptide Y with norepinephrine, and vasoactive intestinal peptide (VIP) and nitric oxide with acetylcholine. ATP mediates the fast initial phase of smooth muscle contraction in the vas deferens and some arteries; neuropeptide Y produces slow, long-lasting vasoconstriction and is the reason sympathetic vasoconstriction outlasts the nerve traffic; VIP mediates much glandular secretion; and nitric oxide is the transmitter of penile erection and of much gastrointestinal relaxation, released by a distinct population of nitrergic neurons. The neurons that fail to arrive in Hirschsprung disease are largely these.
The receptors
WHICH RECEPTOR SITS ON WHICH TARGET — and what happens when it is hit
╔══ CHOLINERGIC (bound by ACETYLCHOLINE) ══════════════════════════════════╗
║ ║
║ NICOTINIC (N) — LIGAND-GATED CATION CHANNEL — always EXCITATORY, fast ║
║ N_M skeletal muscle motor end plate ......... CONTRACTION ║
║ N_N ALL autonomic ganglia (both divisions) .. postganglionic firing ║
║ adrenal medulla chromaffin cells ........ EPI/NE into BLOOD ║
║ CNS ║
║ ║
║ MUSCARINIC (M) — G-PROTEIN COUPLED — excite OR inhibit, slow (ms-s) ║
║ M1 Gq gastric parietal cells · CNS · ganglia .. ACID secretion ║
║ M2 Gi HEART: SA node, AV node, atria ......... RATE ↓ CONDUCTION ↓ ║
║ + presynaptic autoreceptor .............. ACh release ↓ ║
║ M3 Gq smooth muscle: bronchi, gut, detrusor, ║
║ iris sphincter, ciliary muscle ......... CONTRACT ║
║ Gq glands: salivary, lacrimal, SWEAT ...... SECRETE ║
║ Gq vascular ENDOTHELIUM (no nerves!) ...... NITRIC OXIDE ► DILATE║
║ M4 Gi CNS (striatum) ║
║ M5 Gq CNS (dopaminergic midbrain), cerebral vessels ║
║ ODD numbers = Gq (excite) · EVEN numbers = Gi (inhibit) ║
╚══════════════════════════════════════════════════════════════════════════╝
╔══ ADRENERGIC (bound by NOREPINEPHRINE and EPINEPHRINE) ══════════════════╗
║ ║
║ ALPHA-1 Gq → ↑ IP3, ↑ Ca²⁺ "SQUEEZE" ║
║ vascular smooth muscle — skin, gut, kidney ... VASOCONSTRICT ► BP ↑ ║
║ iris radial (dilator) muscle ................. pupil DILATES ║
║ bladder internal sphincter, prostate ......... CONTRACT (storage) ║
║ superior tarsal muscle (eyelid) .............. lid held up ║
║ liver ........................................ glycogenolysis ║
║ ║
║ ALPHA-2 Gi → ↓ cAMP "BRAKE ON THE NERVE ITSELF" ║
║ PREsynaptic sympathetic terminals ............ autoinhibits NE release║
║ brainstem vasomotor centre ................... ↓ central symp outflow ║
║ pancreatic beta cells ........................ ↓ INSULIN ║
║ platelets .................................... AGGREGATION ║
║ ║
║ BETA-1 Gs → ↑ cAMP "1 HEART" ║
║ HEART: SA node ....... ↑ RATE (chronotropy) ║
║ HEART: AV node ....... ↑ CONDUCTION (dromotropy) ║
║ HEART: ventricle ..... ↑ FORCE (inotropy) ║
║ HEART: relaxation .... ↑ rate of relaxation (lusitropy) ║
║ KIDNEY juxtaglomerular cells .... ↑ RENIN ► angiotensin II ► BP ↑ ║
║ ◄── DO NOT FORGET THIS ONE ║
║ ║
║ BETA-2 Gs → ↑ cAMP "2 LUNGS" ║
║ bronchial smooth muscle .............. RELAX ► bronchoDILATE ║
║ skeletal muscle + coronary arterioles DILATE ║
║ uterus ............................... RELAX (tocolysis) ║
║ liver ................................ GLYCOGENOLYSIS ► glucose ↑ ║
║ skeletal muscle cells ................ K⁺ INTO cells ► serum K⁺ ↓ ║
║ mast cells ........................... stabilize, ↓ mediators ║
║ ► REACHED MAINLY BY CIRCULATING EPINEPHRINE, not by nerves ║
║ ║
║ BETA-3 Gs → ↑ cAMP "3 = FAT + BLADDER" ║
║ adipose tissue ....................... LIPOLYSIS ║
║ bladder detrusor ..................... RELAX (storage) ║
╚══════════════════════════════════════════════════════════════════════════╝
AFFINITY — which is why "selective" is possible at all:
NOREPINEPHRINE : alpha-1 = alpha-2 > beta-1 >> beta-2 (almost none)
EPINEPHRINE : beta-2 > beta-1 ≈ alpha-1 = alpha-2 (all of them)
ISOPRENALINE : beta-1 = beta-2 >> alpha (pure beta agonist)
PHENYLEPHRINE : alpha-1 only
METOPROLOL : blocks beta-1 >> beta-2 (~30:1 selective)
PROPRANOLOL : blocks beta-1 = beta-2 (non-selective)
ATROPINE : blocks M1-M5, NOT nicotinic
Figure 14.4 — The autonomic receptor map: which receptor sits on which effector, its G protein, and the effect of activating it.
Described: A two-panel receptor chart. The cholinergic panel lists two families. Nicotinic receptors are ligand-gated cation channels, always excitatory and fast; the muscle subtype N_M sits at the skeletal neuromuscular junction and causes contraction, and the neuronal subtype N_N sits in all autonomic ganglia of both divisions, on adrenal medullary chromaffin cells, and in the central nervous system, causing postganglionic firing and catecholamine release into the blood. Muscarinic receptors are G-protein coupled and slower and may excite or inhibit: M1 coupled to Gq sits on gastric parietal cells, in the central nervous system and on ganglia and drives acid secretion; M2 coupled to Gi sits on the sinoatrial node, atrioventricular node and atria, lowering heart rate and slowing conduction, and also acts as a presynaptic autoreceptor reducing further acetylcholine release; M3 coupled to Gq sits on smooth muscle of bronchi, gut, bladder detrusor, iris sphincter and ciliary muscle causing contraction, on salivary, lacrimal and sweat glands causing secretion, and on vascular endothelium, which has no cholinergic nerves at all, where it releases nitric oxide and dilates; M4 and M5 are largely central. Odd-numbered muscarinic receptors couple to Gq and excite, even-numbered couple to Gi and inhibit. The adrenergic panel lists five subtypes. Alpha-1, coupled to Gq and raising inositol trisphosphate and calcium, sits on vascular smooth muscle of skin, gut and kidney causing vasoconstriction and raising blood pressure, on the radial dilator muscle of the iris dilating the pupil, on the bladder internal sphincter and prostate causing contraction for storage, on the superior tarsal muscle holding the eyelid up, and on the liver. Alpha-2, coupled to Gi and lowering cyclic AMP, sits presynaptically on sympathetic terminals where it autoinhibits norepinephrine release, in the brainstem vasomotor centre where it reduces central sympathetic outflow, on pancreatic beta cells where it reduces insulin, and on platelets where it promotes aggregation. Beta-1, coupled to Gs and raising cyclic AMP, is the heart receptor: it raises rate at the sinoatrial node, conduction at the atrioventricular node, force in the ventricles and the rate of relaxation, and it also sits on juxtaglomerular cells of the kidney where it releases renin, which through angiotensin II raises blood pressure. Beta-2, also Gs-coupled, relaxes bronchial smooth muscle, dilates skeletal muscle and coronary arterioles, relaxes the uterus, drives hepatic glycogenolysis raising glucose, drives potassium into skeletal muscle cells lowering serum potassium, and stabilizes mast cells; it is reached mainly by circulating epinephrine rather than by nerves. Beta-3, also Gs-coupled, drives lipolysis in adipose tissue and relaxes the bladder detrusor for storage. A footer gives drug and transmitter affinities: norepinephrine acts on alpha-1 and alpha-2 more than beta-1 and has almost no beta-2 activity; epinephrine acts on all of them with greatest potency at beta-2; isoprenaline is a pure beta agonist; phenylephrine is alpha-1 only; metoprolol blocks beta-1 about thirty times more than beta-2; propranolol blocks both equally; and atropine blocks all muscarinic receptors but no nicotinic ones.
Three ideas hidden in the receptor map
Nicotinic receptors are channels; the others are not. A nicotinic receptor is the ion channel — five subunits around a central pore that opens within microseconds of binding two acetylcholine molecules. That is why ganglionic and neuromuscular transmission is fast and always excitatory: a cation channel can only depolarize. Every other autonomic receptor is a G-protein-coupled receptor working through a second messenger, which introduces delay (tens of milliseconds to seconds), amplification (one receptor activates many G proteins, one adenylyl cyclase makes many cyclic AMP molecules), and the possibility of inhibition.
M2 is the one that acts almost as fast as a channel. In the sinoatrial node, the beta-gamma subunit released from G_i binds directly to a G-protein-gated inward-rectifier potassium channel (GIRK) and opens it — a membrane-delimited pathway with no diffusible second messenger. Potassium leaves, the cell hyperpolarizes, and the pacemaker potential takes longer to reach threshold. Because no cascade is involved, the effect appears and disappears within a single cardiac cycle. Beta-1 signalling, by contrast, must run G_s → adenylyl cyclase → cyclic AMP → protein kinase A, which takes about five seconds to become significant and 20–30 seconds to plateau. The vagus can change one heartbeat; the sympathetic system cannot. That asymmetry is the physical basis of heart rate variability, and §14.8 is built on it.
The alpha-2 receptor is not on the effector. Most autonomic receptors sit on target cells; alpha-2 sits mainly on the nerve terminal itself and in the brainstem. It is a control receptor rather than an effector receptor, and drugs acting on it therefore behave paradoxically: an alpha-2 agonist such as clonidine lowers blood pressure, because it stimulates a receptor whose job is to reduce sympathetic output. Students who memorize "agonist = more sympathetic effect" get this one wrong every time. The correct rule is: find out what the receptor does, then decide.
Check Your Understanding 14.4
- Acetylcholine is destroyed in the cleft in under a millisecond; norepinephrine is recycled over seconds. Give one physiological consequence and one pharmacological consequence of that difference.
- A patient in anaphylaxis is given intramuscular epinephrine. List four of its actions and name the receptor responsible for each. Then explain why a pure alpha-1 agonist would be a dangerous substitute.
- Clonidine is an alpha-2 agonist and it lowers blood pressure. Explain why this is not a contradiction.
Show answers
- Physiological: cholinergic transmission can carry fast, precisely timed, discrete signals — the vagus can shorten or lengthen a single R-R interval, and the neuromuscular junction can fire a muscle at 50 Hz without transmitter accumulating. Adrenergic transmission cannot; its effects build over seconds and decay over seconds, which suits a system regulating a background state. Pharmacological: because acetylcholine is terminated by an enzyme, inhibiting that enzyme is a way to amplify all cholinergic transmission at once — the basis of neostigmine in myasthenia gravis and of organophosphate poisoning. Because norepinephrine is terminated by a transporter, blocking that transporter amplifies adrenergic transmission — the basis of cocaine's cardiovascular toxicity and of tricyclic antidepressant action.
- (a) Vasoconstriction of skin and splanchnic vessels, raising blood pressure and reducing mucosal and laryngeal oedema — alpha-1. (b) Bronchodilation, relieving wheeze and stridor — beta-2. (c) Increased heart rate and contractility, restoring cardiac output — beta-1. (d) Mast cell stabilization, reducing further mediator release — beta-2. Epinephrine is chosen precisely because it hits all of these. A pure alpha-1 agonist such as phenylephrine would raise the blood pressure while doing nothing for the bronchospasm, nothing for cardiac output, and nothing to stop ongoing mediator release — a patient with a good blood pressure who cannot breathe.
- Because the alpha-2 receptor's job is inhibition of noradrenergic transmission, not stimulation of an effector. Clonidine agonizes alpha-2 receptors in the brainstem vasomotor centre, reducing central sympathetic outflow, and presynaptically on sympathetic terminals, reducing norepinephrine release per impulse. The net result is less alpha-1 stimulation of vessels and less beta-1 stimulation of the heart, so pressure falls. The general lesson is that "agonist" and "antagonist" describe an action at a receptor, not a direction of physiological effect; you must know what the receptor does before you can predict the outcome. A corollary worth remembering clinically: abrupt withdrawal of clonidine causes rebound hypertension, because the suppressed sympathetic system has up-regulated and is suddenly released.
14.5 Effects on Every Organ System
Here is the table the rest of the book will keep sending you back to. Read it once now for the patterns, then use it as a reference. Every entry names the receptor, because the receptor is what a drug acts on and what a disease destroys.
| Target | Sympathetic effect (receptor) | Parasympathetic effect (receptor) |
|---|---|---|
| Iris — radial (dilator) muscle | Contracts: mydriasis, pupil dilates (α1) | No innervation |
| Iris — sphincter muscle | No innervation | Contracts: miosis, pupil constricts (M3) |
| Ciliary muscle | Slight relaxation, far vision (β2) | Contracts: accommodation for near (M3) |
| Superior tarsal (Müller) muscle | Contracts: holds the upper lid up (α1) | No innervation |
| Lacrimal gland | Slight | Copious tears (M3) |
| Salivary glands | Thick, scanty, protein-rich (α1) | Copious, watery, enzyme-rich (M3) |
| Sweat glands — thermoregulatory | Sweating — ACh on M3 (sympathetic cholinergic) | No innervation |
| Sweat glands — apocrine, emotional | Sweating (α1) | No innervation |
| Arrector pili | Contract: piloerection (α1) | No innervation |
| Heart — SA node | Rate increases (β1) | Rate decreases (M2) |
| Heart — AV node | Conduction speeds (β1) | Conduction slows (M2) |
| Heart — atria | Contractility increases (β1) | Contractility decreases (M2) |
| Heart — ventricles | Contractility and relaxation rate increase (β1) | Minimal — sparse vagal supply |
| Coronary arteries | Dilate (β2 plus metabolic autoregulation) | Slight dilation |
| Bronchial smooth muscle | Dilates (β2, mainly hormonal) | Constricts (M3) |
| Bronchial glands | Reduced, thicker secretion (α1) | Increased secretion (M3) |
| Vessels — skin, mucosa | Constrict (α1) | No innervation |
| Vessels — splanchnic, renal | Constrict (α1) | No innervation |
| Vessels — skeletal muscle | Dilate (β2) / constrict (α1) | No innervation |
| Vessels — cerebral | Minimal; locally autoregulated | No innervation |
| Vessels — erectile tissue | Constrict (α1): detumescence | Dilate (nitric oxide): erection |
| Veins | Constrict (α1): raises venous return | No innervation |
| GI motility and tone | Decrease (α2 presynaptic, β2) | Increase (M3) |
| GI sphincters | Contract (α1) | Relax (M3) |
| GI secretion | Decrease | Increase (M3) |
| Gastric parietal cells | — | Acid secretion (M1, plus vagal gastrin release) |
| Gallbladder | Relaxes (β2) | Contracts (M3) |
| Liver | Glycogenolysis, gluconeogenesis (β2, α1) | Glycogen synthesis |
| Pancreas — exocrine | Decreased secretion (α) | Increased secretion (M3) |
| Pancreas — beta cells | Insulin decreased (α2); slight increase (β2) | Insulin increased (M3) |
| Adipose tissue | Lipolysis (β3, β1) | — |
| Kidney — juxtaglomerular cells | Renin release (β1) | — |
| Kidney — tubules | Sodium reabsorption ↑ (α1) | — |
| Bladder — detrusor | Relaxes (β3): storage | Contracts (M3): voiding |
| Bladder — internal sphincter | Contracts (α1): storage | Relaxes: voiding |
| Uterus | Contracts if pregnant (α1); relaxes (β2) | Variable |
| Male genitalia | Emission and ejaculation (α1) | Erection (nitric oxide, M3) |
| Skeletal muscle cells | K⁺ uptake, glycogenolysis, tremor (β2) | No innervation |
| Platelets | Aggregation (α2) | — |
| Mast cells | Stabilized, fewer mediators (β2) | — |
| Adrenal medulla | EPI and NE released (nicotinic, preganglionic) | — |
| Spleen capsule (limited in humans) | Contracts (α1) | — |
Reading the table properly
Four patterns do most of the clinical work, and once you see them you no longer need to memorize rows.
Pattern 1 — vessels are a sympathetic monologue. Almost no blood vessel receives parasympathetic innervation. Vascular tone is set by continuous, tonic sympathetic alpha-1 discharge, modulated up or down. There is no "vasodilator nerve" for the systemic circulation; dilation is achieved by reducing sympathetic firing, by local metabolic signals, or by circulating epinephrine reaching beta-2 receptors. It follows that anything reducing central sympathetic outflow lowers blood pressure — which is one of the three limbs of Case File Question 3.
Pattern 2 — the alpha/beta split in the vasculature is a flow-redistribution machine. Skin, gut, and kidney vessels are dominated by alpha-1 and constrict under sympathetic drive. Skeletal muscle and coronary vessels carry beta-2 and dilate. Because circulating epinephrine has its highest affinity for beta-2, an adrenal surge simultaneously constricts the splanchnic bed and dilates the muscle bed, shunting cardiac output from digestion to locomotion with only a modest change in total peripheral resistance. This is why blood pressure during exercise rises far less than cardiac output does — Chapter 19 develops it quantitatively, and Chapter 32 measures it in Nia.
Pattern 3 — beta-1 is not only cardiac. The juxtaglomerular cells of the kidney carry beta-1 receptors, and stimulating them releases renin, initiating the renin–angiotensin–aldosterone cascade: angiotensin II is a potent direct vasoconstrictor and also stimulates aldosterone, which retains sodium and water and raises blood volume (Chapters 16 and 26). A beta-1 blocker therefore acts on blood pressure through the kidney as well as the heart. Students who file beta-1 under "heart" cannot answer Case File Question 3.
Pattern 4 — the exceptions are where the exam questions live. Four entries break the general rules, and each is worth stating explicitly. Sweat glands are sympathetic but cholinergic. Vascular endothelium carries M3 receptors with no cholinergic nerves, so circulating or injected muscarinic agonists dilate vessels that no parasympathetic nerve reaches. Bronchial beta-2 receptors are reached almost entirely by hormone, not by nerve. And the adrenal medulla is innervated by preganglionic fibers acting on nicotinic receptors, not by postganglionic fibers acting on adrenergic ones.
Using the table backwards
The table is most valuable read in the reverse direction: from an observed effect to the receptor responsible. Work one example completely.
A patient is brought in confused, with a temperature of 39.4 °C, dry flushed skin, dilated pupils, a heart rate of 132, a distended bladder, and absent bowel sounds. Read the findings as a receptor list. Dilated pupils and paralysed accommodation: M3 blocked on the iris sphincter and ciliary muscle. Dry skin with hyperthermia: M3 blocked on sweat glands, removing the only evaporative cooling the body has. Tachycardia: M2 blocked at the SA node, removing vagal restraint. Urinary retention: M3 blocked on the detrusor. Absent bowel sounds: M3 blocked on gut smooth muscle. Confusion: M1 and M4 blocked centrally. Every finding is one receptor family, blocked, which names the poisoning as antimuscarinic — atropine, an antihistamine, a tricyclic, or Datura — and predicts both the treatment (physostigmine, which raises acetylcholine to compete) and the finding that would rule it out (sweating, which is muscarinic and must be absent).
That is what the table is for. Do not memorize it as a list of facts; use it as a lookup table run in whichever direction the patient presents.
Predict This
A drug is given that blocks muscarinic receptors throughout the body.
Before reading on, predict its effect on each of the following, and name the receptor in each case: heart rate, pupil size, salivation, sweating, gut motility, bladder emptying.
(Answer: Heart rate rises — M2 at the SA node is blocked, vagal restraint is removed, and the rate climbs toward the intrinsic 100. Pupil dilates and accommodation is paralysed — M3 on the iris sphincter and ciliary muscle is blocked, leaving the sympathetic alpha-1 dilator unopposed. Salivation falls — M3 on the salivary glands, giving dry mouth. Sweating falls — thermoregulatory sweat glands are sympathetic but cholinergic, using ACh on M3, so an antimuscarinic blocks them; in hot weather this causes hyperthermia. Gut motility falls — M3-mediated peristalsis blocked, causing constipation. Bladder emptying is impaired — M3-mediated detrusor contraction blocked, causing retention, which is why these drugs are hazardous in older men with prostatic obstruction. Together: dry as a bone, red as a beet, hot as a hare, blind as a bat, mad as a hatter. Note that this list is exactly the antimuscarinic toxidrome worked through above — the same table, run in the two opposite directions.)
14.6 Autonomic Regulation: Tone, Dual Innervation, and Balance
Dual innervation and the concept of tone
Most visceral organs receive fibers from both divisions — dual innervation — and both fire continuously. This is the single most misunderstood idea in autonomic physiology. The sympathetic system is not a fire alarm that is off until something happens, and the parasympathetic system is not a bedtime setting. Both run all the time, and every organ sits at an equilibrium determined by the balance between them.
That continuous background firing is autonomic tone, and the body maintains two distinct kinds.
Sympathetic (vasomotor) tone. Vascular smooth muscle receives a steady stream of sympathetic impulses at roughly one per second, holding arterioles at about half-maximal constriction. That baseline is why blood pressure exists at rest. Increasing the firing rate constricts further and raises pressure; decreasing it dilates and lowers pressure. Because vessels have no parasympathetic supply, this one dial does all of the work — which is why a spinal cord injury above T6, severing the descending pathways that drive vasomotor tone, causes catastrophic hypotension (neurogenic shock) even though the heart and vessels themselves are undamaged.
Parasympathetic (vagal) tone. The heart receives continuous vagal discharge that holds the sinoatrial node well below its intrinsic rate. Isolate a human SA node from all neural input and it fires at about 100 beats per minute. A resting heart rate of 70 is not the heart's natural rhythm; it is 100 minus roughly 30 beats of vagal restraint. This is why the fastest way to raise heart rate is not to add sympathetic drive but to withdraw vagal drive — a mechanism that works within a single beat, because acetylcholine at the M2 receptor gates a potassium channel directly with no second-messenger delay, whereas beta-1 signalling through cyclic AMP takes five seconds or more to develop (§14.4).
Amara's resting heart rate of 88 should now read differently to you. It is not a heart being driven fast. It is largely a heart that has lost its brake, sitting closer to its intrinsic rate than it should because the vagal fibers restraining it are failing.
Antagonistic, cooperative, and single innervation
Where dual innervation exists, the two divisions relate in one of two ways.
Antagonistic effects are the familiar case: heart rate, bronchial calibre, gut motility, pupil size. The organ is held at an intermediate state by opposing drives, and this buys something a single control line cannot provide — the ability to move the effector in either direction from rest, immediately, without waiting for a slow drive to build or decay. A thermostat with only a heater can warm a room but must wait for it to cool; a thermostat with a heater and a cooler can do both at once. Dual innervation buys bidirectional speed, and it is worth the metabolic cost of running two systems continuously.
Cooperative effects are less advertised but common. Salivation is stimulated by both divisions in different ways: parasympathetic drive produces the watery, enzyme-rich volume, sympathetic drive produces the thick, protein-rich component, and normal saliva requires both. Sexual function is the clearest case: parasympathetic activity produces erection through nitric oxide, sympathetic activity produces emission and ejaculation, and normal function requires them in sequence rather than in opposition.
Single innervation applies to most blood vessels, sweat glands, arrector pili, the adrenal medulla, the kidney, the liver, and adipose tissue — all sympathetic only. These are regulated purely by changing the firing rate of one system, which is slower to reverse and is the reason vascular control depends so heavily on local metabolic and endothelial mechanisms as well.
The intermediate state, and why it is the whole point
Take the three examples together and a general principle appears. A dually innervated organ almost never sits at either extreme. The heart sits at 70 rather than at 40 or 180. Arterioles sit at half constriction rather than open or shut. The pupil sits at 3–4 mm rather than pinpoint or maximally dilated. The gut sits at a moderate rate of peristalsis.
That intermediate position is not laziness. It is the operating point of a controller, and it exists so that the controller has room to move in both directions. A system already at maximum can only respond to a disturbance in one direction; a system in the middle can respond to anything. This is why the loss of one limb is so damaging even though the other still works: Amara's failing vagal supply does not merely raise her heart rate, it moves her operating point toward one end of the range and shrinks the space she has left to regulate in. Chapter 30 will call this loss of physiological reserve, and it is the unifying idea of ageing.
Exercise & Sport · Vagal Withdrawal, the Athlete's Slow Heart, and Post-Exercise Hypotension
Three exercise phenomena, all explained by tone.
Onset — vagal withdrawal, not sympathetic activation. In the first two or three seconds of exercise, heart rate rises by 20–30 beats per minute. Sympathetic activation cannot account for this: beta-1 signalling through cyclic AMP takes several seconds to develop. What happens is that central command from motor cortex, together with feedback from muscle mechanoreceptors, silences the cardioinhibitory neurons of the nucleus ambiguus, and vagal restraint is simply released. Because M2 receptors gate a potassium channel directly, removing acetylcholine changes the SA node's firing within a single beat. Sympathetic drive then takes over above roughly 100 beats per minute — which is exactly the heart's intrinsic denervated rate. The division of labour is neat: the vagus controls the range from about 50 to 100; the sympathetic system controls everything above 100. Anyone whose vagal limb has failed therefore starts exercise at a disadvantage, because the fast, free part of the response is gone.
The athlete's resting heart rate of 44. Nia Osei-Barrett, marathon runner, has a resting pulse of 44 and a VO₂max of 58.4 mL/kg/min. Two mechanisms produce it. First, markedly elevated vagal tone at rest: trained endurance athletes have higher HRV, higher RMSSD, and pronounced respiratory sinus arrhythmia, and blocking the vagus with atropine raises their rate far more than it raises an untrained person's — the classic demonstration that the low rate is neural rather than intrinsic. Second, structural remodelling: a larger, more compliant left ventricle with a greater end-diastolic volume delivers a larger stroke volume, so the same cardiac output is achieved at a lower rate. Sinus bradycardia in a trained athlete is a sign of adaptation; the same number in Amara would mean something entirely different. Context, not the value alone, is what a clinician reads.
Post-exercise hypotension. For up to two hours after a bout of moderate aerobic exercise, blood pressure sits 5–8 mm Hg systolic below the pre-exercise baseline, and in hypertensive people the fall is larger — often 10–14 mm Hg. Two mechanisms combine: sustained withdrawal of sympathetic vasoconstrictor outflow to previously active muscle, and persistent local vasodilation from metabolites and shear-stress-induced nitric oxide. The baroreflex does not correct it, because the reflex's operating point is transiently reset downward. This is not a curiosity — it is one of the reasons regular aerobic exercise lowers resting blood pressure over weeks, and it means the timing of Amara's rehabilitation sessions relative to her blood pressure measurements genuinely matters. A pressure taken 45 minutes after a walk is not the same measurement as one taken before it.
Check Your Understanding 14.6
- A patient's heart rate rises from 70 to 95 within two seconds of starting to cycle. Which autonomic change did this, and how do you know it was not the other one?
- Explain why a complete spinal cord injury at T4 causes hypotension, using the concept of tone — and predict the heart rate.
- Why does dual innervation exist at all, given that a single system could raise and lower its own firing rate?
Show answers
- Vagal withdrawal. The evidence is the time course. Parasympathetic M2 receptors open a G-protein-gated potassium channel directly, with no second-messenger step, so removing acetylcholine changes SA node firing within one to two beats. Sympathetic beta-1 receptors act through G_s, adenylyl cyclase, cyclic AMP and protein kinase A — a cascade that takes roughly five seconds to build and 20 to 30 seconds to reach full effect. A change complete within two seconds is too fast to be sympathetic. Note also that 95 is below the intrinsic rate of about 100, which is exactly the ceiling that pure vagal withdrawal can reach.
- Vascular tone is maintained by continuous tonic sympathetic discharge originating in the rostral ventrolateral medulla and descending in the spinal cord to the lateral horn of T1–L2. A complete injury at T4 severs that descending pathway for every segment below it, so the preganglionic neurons of T4–L2 lose their driving input and vasomotor tone collapses. Arterioles dilate, venous capacitance increases, venous return falls, and pressure drops — neurogenic shock. The heart rate will be slow or normal, not fast: the cardiac sympathetic outflow at T1–T4 is also lost, while the vagus travels from the medulla to the heart entirely outside the spinal cord and is intact. Hypotension with bradycardia is the opposite of every other kind of shock and is a diagnostic giveaway.
- Because a single system can only change its output as fast as that output builds and decays, and the two are not symmetric. Sympathetic drive takes 20–30 seconds to reach full effect and seconds to decay; a heart controlled by that system alone could not respond to a disturbance within a beat. Two opposing systems with an intermediate operating point allow the effector to be moved in either direction immediately, by adding drive to one limb or removing it from the other — and it allows the fast limb (the vagus, acting through a membrane-delimited pathway) to do the moment-to-moment work while the slow limb sets the background level. The cost is running two systems continuously; the benefit is bidirectional speed and a controller that always has room to move.
14.7 Autonomic Reflexes and Central Control
The hierarchy: four levels, each able to work alone
Autonomic function is organized as a stack, and each level can operate — badly — without the ones above it. That is why spinal cord injury does not abolish autonomic function; it decapitates it.
Level 1 · The spinal cord. Complete reflex circuits for defecation, micturition, erection, ejaculation, and segmental vasomotor responses live in the cord itself. After a complete cord injury these reflexes disappear during spinal shock and then return, in disordered and untimed form, over weeks — proof that the circuitry was never in the brain.
Level 2 · The brainstem. The medulla holds the cardiovascular centres — the nucleus of the solitary tract (NTS) receiving all visceral afferents, the rostral ventrolateral medulla (RVLM) generating tonic sympathetic drive, and the nucleus ambiguus and dorsal motor nucleus of the vagus generating vagal outflow — plus the respiratory centres. The pons holds the pontine micturition centre, and the midbrain the pupillary reflex circuitry. This is the level at which most homeostatic reflexes are closed.
Level 3 · The hypothalamus. The main integrator: thermoregulatory, osmotic, feeding, and stress centres, plus the suprachiasmatic nucleus carrying the circadian clock — which is where Amara's twenty years of night shift enter this chapter (Chapter 12). The hypothalamus sets the targets that the brainstem then defends, and it is the point at which autonomic, endocrine, and behavioural responses are coordinated into a single strategy.
Level 4 · Limbic system and cortex. The amygdala, insula, and anterior cingulate cortex supply emotional and anticipatory input. This is why fear raises heart rate before anything physical has happened, why a person can faint at the sight of a needle, why blushing exists, and why blood pressure measured in a clinic is not blood pressure measured at home. The insula in particular is the cortical representation of the internal body, and insular strokes produce cardiac arrhythmias and blood pressure instability with no cardiac disease at all.
The baroreceptor reflex
The baroreflex is the fastest blood pressure controller you own, and the model that every other homeostatic loop in this book is compared against.
THE BARORECEPTOR REFLEX — a complete negative feedback loop
(response time: 1-2 heartbeats)
┌────────────────────────────────────┐
STIMULUS ──────►│ BP RISES — e.g. 120 → 160 mm Hg │
└─────────────────┬──────────────────┘
▼
RECEPTOR CAROTID SINUS (afferents in CN IX, glossopharyngeal)
AORTIC ARCH (afferents in CN X, vagus)
stretch-sensitive endings in the ADVENTITIA
► fire FASTER when the wall is stretched further
► they sense WALL DEFORMATION, not pressure ◄── remember
│
▼ afferents
CENTRE NUCLEUS OF THE SOLITARY TRACT (NTS), medulla
│ │
excites ──┘ └── inhibits
▼ ▼
NUCLEUS AMBIGUUS + ROSTRAL VENTROLATERAL
DORSAL MOTOR NUCLEUS of X MEDULLA (RVLM)
= CARDIOINHIBITORY = VASOMOTOR CENTRE
│ │
▼ VAGUS ↑ ▼ SYMPATHETIC ↓
EFFECTORS ┌─────────────────────┐ ┌──────────────────────────────┐
│ SA node M2 │ │ arterioles: less alpha-1 → │
│ ► HEART RATE ↓ │ │ VASODILATE ► resistance ↓ │
│ AV node M2 │ │ VEINS: venodilate ► venous │
│ ► conduction ↓ │ │ return ↓ ► stroke volume ↓ │
└─────────────────────┘ │ heart: less beta-1 ► │
│ contractility ↓ │
│ kidney: less beta-1 ► │
│ RENIN ↓ (slow limb) │
└──────────────────────────────┘
│
▼
┌────────────────────────────────────┐
RESPONSE ──────►│ CARDIAC OUTPUT ↓ × RESISTANCE ↓ │
│ = BLOOD PRESSURE ↓ │ NEGATIVE FEEDBACK
└────────────────────────────────────┘
MIRROR IMAGE when BP FALLS (standing up, haemorrhage):
baroreceptor firing ↓ → NTS quiet → vagus WITHDRAWN + sympathetic RELEASED
→ HR ↑ , contractility ↑ , arterioles AND veins constrict → BP restored
┌──────────────────────────────────────────────────────────────────────────┐
│ WHAT THE LOOP CANNOT DO │
│ · It cannot set long-term blood pressure. Over days it RESETS to │
│ whatever pressure it is repeatedly exposed to — which is why it │
│ defends hypertension once hypertension is established, and why │
│ long-term pressure is set by the KIDNEY (Chapter 26), not here. │
│ · It cannot work if the WALL will not deform: a stiff, calcified │
│ carotid sinus reduces the stimulus without any nerve being damaged. │
└──────────────────────────────────────────────────────────────────────────┘
Figure 14.5 — The baroreceptor reflex as a complete five-component negative feedback loop.
Described: A flow diagram of the baroreceptor reflex arranged in the five components of a reflex arc. The stimulus is a rise in blood pressure, for example from 120 to 160 millimetres of mercury. The receptors are stretch-sensitive nerve endings in the adventitia of the carotid sinus, whose afferents run in cranial nerve nine, and of the aortic arch, whose afferents run in cranial nerve ten; they fire faster when the vessel wall is stretched further, and they sense wall deformation rather than pressure itself. The integration centre is the nucleus of the solitary tract in the medulla, which excites the cardioinhibitory nucleus ambiguus and dorsal motor nucleus of the vagus while inhibiting the rostral ventrolateral medulla, the vasomotor centre. The efferent limbs are therefore increased vagal output and decreased sympathetic output. At the effectors, increased vagal activity acting on M2 receptors slows the sinoatrial node and atrioventricular conduction, while reduced sympathetic activity means less alpha-1 stimulation so arterioles dilate and resistance falls, veins dilate so venous return and stroke volume fall, less beta-1 stimulation so contractility falls, and less beta-1 stimulation of renal juxtaglomerular cells so renin release falls as a slower limb. The response is a fall in both cardiac output and peripheral resistance and therefore in blood pressure — negative feedback. The mirror image is described for a fall in pressure on standing or after haemorrhage: baroreceptor firing decreases, the solitary nucleus falls quiet, the vagus is withdrawn and the sympathetic system released, heart rate and contractility rise, arterioles and veins constrict, and pressure is restored. A final panel states two things the loop cannot do: it cannot set long-term blood pressure, because over days it resets to whatever pressure it is repeatedly exposed to and therefore ends up defending established hypertension, with long-term pressure set instead by the kidney; and it cannot work if the arterial wall will not deform, so a stiff calcified carotid sinus weakens the reflex without any nerve being damaged.
Three properties of this loop are worth stating separately.
It is fast and it is proportional. Baroreceptor firing changes within a fraction of a second and the vagal response appears in one to two beats, because M2 gates a potassium channel directly. The reflex responds not only to the absolute pressure but to the rate of change of pressure, which is why it corrects a sudden drop faster than a gradual one.
It is a short-term controller only. Expose the baroreceptors to a persistently higher pressure and within one to two days they reset: firing returns to baseline at the new pressure, and the reflex now defends the new value. This is why the baroreflex does not correct chronic hypertension — it has been recalibrated to it — and why long-term arterial pressure is determined by renal pressure–natriuresis (Chapter 26) rather than by any neural reflex.
Its sensor is mechanical, so it fails mechanically. Baroreceptors measure stretch, not pressure. Age-related elastin fragmentation and collagen cross-linking stiffen the carotid sinus, so the same pressure change produces less deformation and less afferent firing. The sensor has not failed; the thing it is glued to has become less deformable (§14.9).
The chemoreceptor reflex
Peripheral chemoreceptors in the carotid bodies (CN IX) and aortic bodies (CN X) respond to a fall in arterial PO₂ below about 60 mm Hg, to a rise in PCO₂, and to a fall in pH. Their primary output is to the respiratory centres, raising ventilation (Chapter 22). Their autonomic output raises sympathetic drive: vasoconstriction, raised blood pressure, and — if ventilation is prevented from increasing — a vagally mediated bradycardia, the combination seen in the diving response and in the fetus during hypoxia.
The clinically important version of this reflex is chronic. In obstructive sleep apnea, repeated nocturnal desaturations produce repeated chemoreflex-driven sympathetic surges; over months, carotid body sensitivity increases and daytime sympathetic outflow rises even when oxygenation is normal. This is one of the mechanisms by which sleep apnea causes sustained hypertension and a non-dipping nocturnal blood pressure profile — Amara's exact finding in Chapter 12, and one of the reasons her sleep study in Chapter 22 matters to this chapter.
Micturition, defecation, and the sexual reflexes
Micturition. Bladder filling stretches the detrusor; visceral afferents in the pelvic nerve reach the sacral cord and, ascending, the pontine micturition centre. Below about 200–300 mL, sympathetic outflow keeps the detrusor relaxed (β3) and the internal sphincter closed (α1) while the somatic pudendal nerve holds the external sphincter shut. Above threshold, the pontine centre — gated by the frontal cortex, which is what "waiting" means — switches the entire pattern: sacral parasympathetic outflow contracts the detrusor (M3), the internal sphincter relaxes, and pudendal outflow is inhibited so the external sphincter opens. Note that this single act requires three different motor systems to cooperate — sympathetic, parasympathetic, and somatic — and it is the best demonstration in the body of how artificial the voluntary/involuntary divide is.
The pharmacology follows directly and is used daily: antimuscarinics and β3 agonists (mirabegron) both promote storage and treat overactive bladder; α1 blockers (tamsulosin) relax the internal sphincter and prostatic smooth muscle and improve flow in benign prostatic hyperplasia.
Defecation. Rectal distension triggers a sacral parasympathetic reflex that contracts the rectum and relaxes the internal anal sphincter (smooth muscle, involuntary), while the external anal sphincter (skeletal muscle, pudendal nerve) remains under cortical control. The rectoanal inhibitory reflex — internal sphincter relaxation on rectal distension — is mediated entirely by the enteric nervous system, and its absence is the diagnostic finding in Hirschsprung disease (§14.2), because the intramural inhibitory neurons that mediate it never arrived.
Sexual reflexes. Erection is parasympathetic (S2–S4, nitric oxide relaxing the smooth muscle of the helicine arteries so blood fills the corpora); emission and ejaculation are sympathetic (L1–L2, α1 contracting the vas deferens and seminal vesicles and closing the bladder neck to prevent retrograde ejaculation). The mnemonic point and shoot — parasympathetic points, sympathetic shoots — is crude and completely accurate. In females the same divisions produce clitoral engorgement and vaginal lubrication, and the same rhythmic orgasmic contractions. The pharmacology is again a direct readout: sildenafil inhibits phosphodiesterase-5, preventing breakdown of the cyclic GMP that nitric oxide generates, and therefore amplifies a parasympathetic signal without initiating one — which is why it requires arousal to work. And α1 blockers used for prostatic symptoms can cause retrograde ejaculation by preventing bladder neck closure.
Imaging · Testing an Autonomic Nervous System You Cannot See
There is no scan that shows autonomic nerves. Autonomic testing therefore consists of provoking a reflex and measuring the response, which is why every test below is a manoeuvre rather than a picture — with one nuclear medicine exception.
Heart rate response to deep breathing (the E:I ratio). The patient breathes at six breaths per minute — five seconds in, five out — while the ECG runs. The ratio of the longest expiratory R-R interval to the shortest inspiratory R-R interval is calculated. This is the purest available measure of cardiac vagal function, because respiratory sinus arrhythmia is almost entirely vagally mediated. A healthy adult under 50 swings by more than 15 beats per minute. Amara swings by 3.
The Valsalva manoeuvre and the Valsalva ratio. The patient blows against 40 mm Hg for 15 seconds while beat-to-beat pressure and heart rate are recorded. Four phases follow, and each tests a different limb. Phase I: pressure rises mechanically as thoracic pressure is transmitted. Phase II: venous return falls, pressure drops, and the sympathetic limb must constrict vessels and raise heart rate to arrest the fall. Phase III: release, a brief further drop. Phase IV: venous return floods back into a constricted circulation, pressure overshoots, and the vagal limb must produce a compensatory bradycardia. The Valsalva ratio — longest R-R in phase IV divided by shortest R-R in phase II — is normally above 1.21 under age 50. Absent phase II recovery indicates sympathetic adrenergic failure; absent phase IV overshoot indicates the same; a flat ratio indicates vagal failure.
Head-up tilt table testing. The patient is strapped to a table and tilted to 60–70 degrees for up to 45 minutes with continuous beat-to-beat pressure and heart rate. It distinguishes neurogenic orthostatic hypotension (progressive fall with no compensatory tachycardia), postural orthostatic tachycardia syndrome (rate rise of more than 30 beats per minute without hypotension), and vasovagal syncope (a sudden, late, combined fall in rate and pressure after a period of normal compensation) — three conditions that present identically as "I faint" and have entirely different mechanisms.
Sudomotor testing. Because sympathetic cholinergic sudomotor fibers are unmyelinated C fibers, sweating is an accessible readout of small-fiber function. QSART (quantitative sudomotor axon reflex test) applies iontophoretic acetylcholine and measures the axon-reflex sweat response; the thermoregulatory sweat test coats the patient in an indicator powder that changes colour where sweat appears, producing a whole-body map of anhidrosis. In length-dependent autonomic neuropathy the map shows a stocking of absent sweating on the feet — the autonomic mirror image of the sensory stocking in Chapter 13.
MIBG cardiac scintigraphy. The one true image. Meta-iodobenzylguanidine is a norepinephrine analogue that is taken up by the norepinephrine transporter and stored in sympathetic nerve terminals but is not metabolized. Labelled with iodine-123 and imaged, it maps functioning cardiac sympathetic innervation directly. The heart-to-mediastinum uptake ratio is reduced in diabetic cardiac autonomic neuropathy, in heart failure, and — strikingly — in Parkinson disease and Lewy body dementia, where cardiac sympathetic denervation occurs early and is used to distinguish them from multiple system atrophy, in which the postganglionic neurons survive and uptake is preserved. A picture of the nerve terminals of the heart, taken with a gamma camera, is as close as this chapter gets to seeing its own subject.
Thread 2 · Homeostasis Is the Master Concept
The baroreflex is the cleanest homeostatic loop in the human body, and it is worth mapping onto Chapter 1's abstract diagram component by component, because every other loop in this book has the same shape.
| Chapter 1's term | The baroreflex's version |
|---|---|
| Regulated variable | Mean arterial pressure |
| Sensor | Stretch endings in the carotid sinus and aortic arch |
| Afferent pathway | CN IX and CN X to the medulla |
| Control centre | Nucleus of the solitary tract, comparing input against a set point |
| Efferent pathways | Vagal (nucleus ambiguus) and sympathetic (RVLM → cord → chain) |
| Effectors | SA node, AV node, myocardium, arterioles, veins, juxtaglomerular cells |
| Feedback sign | Negative — the response opposes the disturbance |
Three refinements that Chapter 1 could not yet give you now appear.
Regulation happens by modulating a baseline, not by switching a system on. Both efferent limbs fire continuously, so a correction can be made in either direction instantly. This is the functional payoff of tone, and it is why dual innervation is worth its metabolic cost.
Set points can move. The baroreflex resets over one to two days to whatever pressure it is chronically exposed to. A homeostatic mechanism that defends the wrong value is not broken — it is defending exactly what it was told to. This idea returns in fever (Chapter 25), in chronic hypercapnia (Chapter 22), and in the osmotic set point of pregnancy (Chapter 28).
Every component can fail separately, and each failure has a name. §14.9 is nothing more than this table read one row at a time with a broken entry: a stiff sensor in ageing, a severed efferent pathway in cord injury, a dead efferent neuron in autonomic neuropathy, a blocked effector receptor in beta blockade. When you can name the broken component, you can predict the clinical picture without memorizing it.
Check Your Understanding 14.7
- During a Valsalva manoeuvre, a patient shows no rise in heart rate during phase II and no bradycardic overshoot in phase IV. Which limb or limbs are failing, and what single additional bedside test would you do next?
- A patient with a complete T4 cord injury still has reflex erections and reflex bladder emptying but no voluntary control of either. Explain, using the four-level hierarchy.
- Why does the baroreflex fail to correct chronic hypertension, when it corrects a 40 mm Hg rise within two heartbeats?
Show answers
- Both limbs. Phase II recovery is the sympathetic adrenergic limb — vasoconstriction and tachycardia defending pressure against reduced venous return — and its absence indicates sympathetic failure. Phase IV overshoot triggers a vagal bradycardia, and its absence indicates cardiovagal failure. Loss of both is generalized autonomic failure, as in advanced diabetic autonomic neuropathy, amyloidosis, or a synucleinopathy. The next bedside test is orthostatic blood pressure and heart rate — supine, then at 1 and 3 minutes standing — because a fall of ≥20 mm Hg systolic without an appropriate rise in heart rate confirms neurogenic orthostatic hypotension and immediately changes management. Heart rate response to paced deep breathing is an equally defensible answer, isolating the vagal limb.
- Because the reflex circuits for erection and micturition live in the sacral spinal cord (level 1), which is below the injury and therefore intact and still connected to its receptors and effectors. What has been lost is descending control from the pontine micturition centre (level 2) and from the cortex (level 4), which normally gate those reflexes in time and place. The result is preserved but untimed and involuntary function: reflex erections to local stimuli without psychogenic erection, and reflex bladder emptying at whatever volume the sacral reflex triggers, often against a sphincter that fails to relax in coordination (detrusor–sphincter dyssynergia) because the pontine centre that normally synchronizes them is disconnected.
- Because the reflex resets. Over one to two days of exposure to a higher pressure, baroreceptor firing returns toward its previous baseline at the new pressure — through both mechanical changes in the vessel wall and adaptation of the afferent endings — so the medulla now reads the elevated pressure as normal and defends it. The baroreflex is a short-term controller of fluctuations, not a long-term controller of level. Long-term arterial pressure is set by the relationship between arterial pressure and renal sodium excretion (Chapter 26), which has no equivalent capacity to reset, which is why every effective long-term antihypertensive strategy acts on volume, on the renin–angiotensin system, or directly on vascular smooth muscle rather than on the reflex.
14.8 Heart Rate Variability: Reading the Balance Directly
Heart rate variability (HRV) is the variation in the time between consecutive heartbeats — the R-R interval on an ECG. It is not arrhythmia. Every beat in Amara's recording is a normal sinus beat originating in the SA node. What varies is the interval, and that interval is set, beat by beat, by the balance of vagal and sympathetic input to the node.
The reason HRV is informative rather than merely curious is the kinetic asymmetry established in §14.4. Vagal acetylcholine at M2 opens a G-protein-gated potassium channel directly, with no second messenger, so it can lengthen or shorten a single interval. Sympathetic norepinephrine at β1 must run G_s → adenylyl cyclase → cyclic AMP → protein kinase A, taking about five seconds to build. It follows, with no further assumption, that fast beat-to-beat variability is essentially a readout of vagal activity, while slower variability reflects both limbs.
THE SAME RHYTHM, THE SAME DIAGNOSIS, TWO DIFFERENT NERVOUS SYSTEMS
(both strips are 100% normal sinus rhythm)
── A · HEALTHY 45-YEAR-OLD · HR 62 · SDNN 148 ms · RMSSD 41 ms ──────────
R R R R R R R R R R
│ │ │ │ │ │ │ │ │ │
─────┴──────┴─────┴────┴────┴────┴─────┴──────┴───────┴──────┴─────
R-R: 1010 960 910 870 880 930 990 1050 1090 1030 ms
└───── INSPIRATION ─────┘ └──────── EXPIRATION ──────────┘
vagus inhibited → FASTER vagus restored → SLOWER
TACHOGRAM (interval plotted beat by beat):
1100┤ ╭──╮
1000┤ ╮ ╭─╯ ╰─╮ ╭──╮ ╭─╮ RESPIRATORY
900┤ ╰──╮ ╭──╯ ╰──╮ ╭──╯ ╰───╮ ╭───╯ ╰──╮ SINUS
800┤ ╰───╯ ╰──╯ ╰──╯ ╰─ ARRHYTHMIA
└────────────────────────────────────────────────► ~4 s cycle
── B · AMARA, DAY 4 · HR 88 · SDNN 42 ms · RMSSD 18 ms ─────────────────
R R R R R R R R R R R R
│ │ │ │ │ │ │ │ │ │ │ │
─────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴─────
R-R: 684 681 686 682 679 683 685 680 684 682 681 ms
└──── inspiration ────┘ └──────── expiration ────────┘
swing = 3 beats/min · expected 8-20
TACHOGRAM:
1100┤
1000┤
900┤ "A METRONOME
800┤ IS NOT A
700┤ ────────────────────────────────────────────── HEALTHY
└────────────────────────────────────────────────► HEART"
═════════════════════════════════════════════════════════════════════════
THE MEASURES
TIME DOMAIN
SDNN standard deviation of all normal R-R intervals over 24 h
► TOTAL variability: respiratory + baroreflex + thermal
+ circadian + hormonal. Normal 120-160 ms
RMSSD root mean square of SUCCESSIVE differences
► only the VAGUS can change one interval relative to the
next, so RMSSD ≈ pure PARASYMPATHETIC index. Normal 25-50
pNN50 % of adjacent intervals differing by > 50 ms — also vagal
FREQUENCY DOMAIN
HF 0.15-0.40 Hz = the respiratory rhythm ......... PURELY VAGAL
LF 0.04-0.15 Hz = baroreflex oscillation ......... BOTH limbs
VLF < 0.04 Hz = thermoregulatory, hormonal, circadian
═════════════════════════════════════════════════════════════════════════
PROGNOSIS AFTER MYOCARDIAL INFARCTION
SDNN > 100 ms .... reference risk
SDNN 50-100 ms ... ~2x mortality
SDNN < 50 ms ..... ~4x mortality — independent of ejection
fraction and infarct size
Figure 14.6 — Two rhythm strips in normal sinus rhythm: high heart rate variability and low.
Described: Two ECG rhythm strips, both in normal sinus rhythm, drawn one above the other with their R-R intervals listed and plotted. Strip A is from a healthy forty-five-year-old with a heart rate of sixty-two, an SDNN of one hundred forty-eight milliseconds and an RMSSD of forty-one milliseconds. Its R waves are visibly unevenly spaced, and the listed intervals fall from 1010 to 870 milliseconds during inspiration, when lung stretch receptors inhibit the vagus and the heart speeds up, then rise to 1090 milliseconds during expiration, when vagal restraint is restored and the heart slows. A tachogram beneath plots those intervals beat by beat as a smooth oscillating wave with a cycle of about four seconds, labelled respiratory sinus arrhythmia. Strip B is Amara's recording on hospital day four, with a heart rate of eighty-eight, an SDNN of forty-two milliseconds and an RMSSD of eighteen milliseconds. Its R waves are evenly spaced and its intervals vary only between 679 and 686 milliseconds across the whole respiratory cycle, giving a swing of three beats per minute where eight to twenty is expected. Its tachogram is an almost perfectly flat line, annotated "a metronome is not a healthy heart." A key below defines the measures. In the time domain, SDNN is the standard deviation of all normal R-R intervals over twenty-four hours and captures total variability from respiratory, baroreflex, thermoregulatory, circadian and hormonal sources, normally one hundred twenty to one hundred sixty milliseconds; RMSSD is the root mean square of successive differences and, because only the vagus can change one interval relative to the next, is close to a pure parasympathetic index, normally twenty-five to fifty milliseconds; pNN50 is the percentage of adjacent intervals differing by more than fifty milliseconds and is also vagal. In the frequency domain, the high frequency band from 0.15 to 0.40 hertz corresponds to the respiratory rhythm and is purely vagal, the low frequency band from 0.04 to 0.15 hertz reflects baroreflex oscillation driven by both limbs, and the very low frequency band below 0.04 hertz reflects thermoregulatory, hormonal and circadian influences. A final panel gives prognosis after myocardial infarction: an SDNN above one hundred milliseconds is the reference risk, fifty to one hundred milliseconds carries roughly twice the mortality, and below fifty milliseconds roughly four times, independent of ejection fraction and infarct size.
Where the variability comes from
Four rhythms are superimposed on every normal tachogram, and each has an identifiable source.
Respiratory sinus arrhythmia (0.15–0.4 Hz). During inspiration, intrathoracic pressure falls, venous return rises, pulmonary stretch receptors fire, and central respiratory neurons directly inhibit the cardioinhibitory vagal neurons. The vagus is transiently withdrawn and the heart speeds up. During expiration vagal restraint returns and the heart slows. The magnitude of that swing is a direct, non-invasive measurement of cardiac vagal function, and it is what the six breaths per minute test in §14.7 quantifies.
Baroreflex oscillation (0.04–0.15 Hz, "Mayer waves"). Blood pressure is not constant; it oscillates at about 0.1 Hz because the baroreflex is a feedback loop with a delay, and any delayed negative feedback loop oscillates. Heart rate follows those pressure waves. This band is driven by both limbs and is the reason low-frequency power cannot be read as "sympathetic activity" without qualification.
Thermoregulatory and humoral rhythms (< 0.04 Hz). Slow drifts driven by skin blood flow regulation, the renin–angiotensin system, and circulating catecholamines.
Circadian variation. Heart rate falls and vagal tone rises during sleep, particularly in slow-wave sleep. The 24-hour SDNN captures this, which is why a shift worker with a non-dipping nocturnal profile (Chapter 12) has a reduced SDNN partly for circadian reasons that have nothing to do with nerve damage.
Why low variability is bad when stability sounds good
This is the counterintuitive core of the Case File, and it deserves to be argued rather than asserted.
A variable heart rate means the SA node is being actively modulated, moment to moment, by an intact control system responding to breathing, posture, pressure, temperature, and time of day. A metronomic heart rate means the modulation has stopped, and there are only two ways for that to happen: either the vagal efferents have died, or sympathetic drive is so high that it saturates the node and overwhelms vagal influence. Both are bad, and in most patients both are present.
Low HRV is therefore a marker of reduced regulatory reserve. State it as a control-theory proposition and it becomes obvious: a controller that cannot vary its output cannot correct a disturbance. When Amara stands quickly, bleeds, becomes dehydrated, develops a fever, or receives an antihypertensive, the responses that should defend her blood pressure are weakened, because the system has lost the range it would have used.
But the more important reason is not the lost reserve. It is what low HRV reports about the current state. Low HRV is the signature of sustained sympathetic dominance with withdrawn vagal restraint, and that state is itself harmful. Chronic catecholamine exposure promotes ventricular arrhythmia by lowering the fibrillation threshold, myocardial hypertrophy and interstitial fibrosis, platelet activation, insulin resistance, systemic inflammation, and renal sodium retention. The vagus, conversely, is anti-arrhythmic and anti-inflammatory — acetylcholine acting on macrophage α7-nicotinic receptors suppresses cytokine release, the so-called cholinergic anti-inflammatory pathway.
So the correct summary is the one to carry out of this chapter: HRV is the thermometer, not the fever. It is not the low variability that kills. Low variability is the most sensitive available readout of an autonomic state that does the killing.
The prognostic data
After myocardial infarction, an SDNN below 50 ms carries roughly a fourfold increase in mortality compared with an SDNN above 100 ms, and — this is the part that makes it clinically interesting — that association is independent of ejection fraction and infarct size. It is not a proxy for how much muscle was lost. Low HRV also predicts progression from prediabetes to diabetes, predicts mortality in heart failure and in chronic kidney disease, predicts sepsis outcome, and predicts the onset of neonatal sepsis several hours before clinical signs — the last being the basis of continuous HRV monitoring in some neonatal intensive care units.
Amara's SDNN of 42 ms therefore places her, on the day-four printout, in the highest-risk band of a measurement she and her team have never discussed, taken from a monitor she is already wearing.
Predict This
Amara is started on metoprolol and her resting heart rate falls from 88 to 64.
Before reading on, commit to a prediction: will her heart rate variability go up, down, or stay the same?
(Answer: it goes UP, often substantially — SDNN typically improves by 15–30 percent on beta blockade after myocardial infarction. This surprises most students, who expect a drug that flattens sympathetic drive to flatten variability too. The reason is that most of the fast, beat-to-beat variability is vagal, and vagal influence on the SA node had been swamped by high sympathetic tone. Remove the sympathetic drive and the vagal signal is unmasked: respiratory sinus arrhythmia reappears, RMSSD rises, and the heart becomes both slower and more variable at once. This is one of the mechanisms by which beta blockade improves survival after infarction, and it is a direct demonstration that HRV measures the balance between the two limbs rather than the activity of either one alone. Note the limit of the effect: metoprolol cannot regrow the vagal C fibers that hyperglycaemia has destroyed, so Amara's HRV will improve without normalizing.)
Exercise & Sport · Heart Rate Recovery, and What HRV-Guided Training Actually Shows
Heart rate recovery is the vagus coming back, and it is a mortality predictor. The fall in heart rate during the first minute after stopping exercise is dominated by vagal reactivation, not by sympathetic withdrawal — atropine abolishes most of it, beta blockade does not. A drop of fewer than 12 beats in the first minute after a treadmill test (or fewer than 18 after a cool-down protocol) is an independent predictor of mortality, with a hazard ratio around 2 after adjusting for exercise capacity, ejection fraction, and coronary anatomy. It measures the same thing as RMSSD — how intact the parasympathetic limb is — but requires nothing more than a stopwatch and a pulse. Amara's heart rate recovery is worth measuring at every rehabilitation session in Chapter 10, and it is a number she can be taught to track herself.
Training raises vagal tone, and the effect is dose-dependent and reversible. Eight to twelve weeks of moderate aerobic training raises RMSSD and HF power in sedentary adults, in patients after myocardial infarction, and in people with type 2 diabetes, with effect sizes that are modest individually but consistent across trials. Detraining reverses it within weeks. In cardiac rehabilitation the HRV improvement parallels but does not fully explain the mortality benefit.
HRV-guided training: what the evidence supports and what it does not. The idea is simple — measure RMSSD or a derived index each morning, train hard when it is at or above your rolling baseline, train easily when it is below. Randomized trials in endurance athletes and recreational runners show that HRV-guided programmes produce equal or slightly better improvements in performance markers than fixed pre-planned programmes, largely by preventing hard sessions from being performed in a poorly recovered state, and with fewer non-responders. That is a real but narrow claim, and three cautions belong with it.
First, the day-to-day signal is noisy: a single morning value is strongly affected by the previous evening's alcohol, meal timing, sleep, hydration, ambient temperature, and measurement posture, which is why the useful quantity is a 7-day rolling mean rather than today's number. Second, consumer devices vary: chest-strap ECG and finger photoplethysmography give usable RMSSD; wrist optical sensors during movement often do not. Third, HRV does not diagnose anything in an individual. A low morning RMSSD in a 24-year-old marathon runner such as Nia means she trained hard yesterday or slept badly; a persistently low SDNN in a 45-year-old with an HbA1c of 7.4% means something entirely different. The measurement is the same. The prior probability is not, and that is what turns a number into information.
Thread 3 · The Body Is Integrated
One number on a telemetry printout — SDNN 42 ms — is a statement about five organ systems at once, and the arrows all have directions.
Endocrine → nervous. Four years of insulin resistance and hyperglycaemia (Chapters 16, 24) damaged small unmyelinated C fibers through polyol flux, glycation, and oxidative stress. The longest of those fibers belong to the vagus.
Cardiovascular → nervous. The microvascular disease that narrowed her coronary arteries also narrowed the vasa nervorum supplying those nerves (Chapter 19), so the nerve is being starved as well as poisoned.
Nervous → cardiovascular. Loss of vagal restraint raises her resting heart rate toward the intrinsic 100 — hers is 88 — which raises myocardial oxygen demand in a heart supplied by a 90% stenosed left circumflex artery, and shortens diastole, which is when coronary perfusion actually occurs (Chapter 18). Sustained sympathetic dominance simultaneously lowers the ventricular fibrillation threshold.
Nervous → renal → cardiovascular. Sympathetic β1 receptors on juxtaglomerular cells raise renin, hence angiotensin II (vasoconstriction) and aldosterone (sodium and water retention), raising both resistance and volume — 152/94 (Chapters 16, 26). This loop is the reason a nervous system finding shows up as a kidney number.
Nervous → nervous. The same C-fiber population carries cardiac pain afferents. As they die, ischaemia stops producing the symptom that brought her to hospital in Chapter 1.
And the arrow that closes the circle: cardiovascular → nervous, six months later, when the same disease is found in her feet with a tuning fork (Chapter 13). The autonomic neuropathy was measurable first, because the vagus is longer than the sural nerve. One disease, one mechanism, seven systems, and a printout nobody had looked at.
Check Your Understanding 14.8
- Two patients both have a resting heart rate of 90. Patient A has an RMSSD of 45 ms; patient B has an RMSSD of 12 ms. What is different about them, and which is more concerning?
- Explain why RMSSD is a nearly pure index of vagal activity while SDNN is not.
- A patient's HRV rises after starting a beta blocker. Does this mean the drug increased parasympathetic activity? Answer precisely.
Show answers
- Patient A's heart is being driven — the vagal limb is intact and modulating normally (RMSSD 45 ms is within the normal 25–50 range), so the tachycardia is a response to something: fever, pain, anxiety, hypovolaemia, thyrotoxicosis, exercise just finished. Remove the cause and the rate falls. Patient B's heart has lost its brake — an RMSSD of 12 ms means vagal influence on the node is minimal, so the rate of 90 reflects a node sitting near its intrinsic rate rather than being driven above it. Patient B is far more concerning, because the tachycardia is structural rather than reactive, will not respond to treating a cause, indicates established cardiac autonomic neuropathy, and carries the fourfold mortality signal. The same number, two entirely different physiologies — which is why the variability, not the rate, is the informative measurement.
- Because of the kinetics of the two efferent limbs. Changing the length of one R-R interval relative to the immediately preceding one requires a mechanism that acts within a single cardiac cycle. Only the vagus has one: acetylcholine at M2 releases a G-protein beta-gamma subunit that binds and opens a GIRK potassium channel directly, with no diffusible second messenger, so the effect appears and disappears within a beat. Sympathetic beta-1 signalling runs through a cyclic-AMP cascade needing about five seconds — far too slow to alter one interval relative to the next. RMSSD, which is computed from successive differences, therefore isolates the only limb fast enough to produce them. SDNN, computed over 24 hours, sums all sources of variation: the same vagal component plus baroreflex oscillations, thermoregulatory drifts, hormonal rhythms, the circadian fall in rate during sleep, and physical activity. It is a broader and less specific measure — more prognostically powerful in populations, less mechanistically interpretable in an individual.
- No — it means the drug unmasked a vagal influence that was already there. Metoprolol is a competitive antagonist at beta-1 receptors; it has no action at muscarinic receptors and does not increase acetylcholine release, vagal firing, or M2 signalling. What it does is remove the sympathetic drive that was saturating the SA node and swamping the vagal modulation. With the sympathetic contribution reduced, the same vagal traffic now produces a visible effect on the pacemaker potential, so respiratory sinus arrhythmia reappears and RMSSD rises. This is worth stating carefully because it is a general principle of physiological measurement: a measured variable reflects a balance, and changing either side of a balance changes the measurement. The precise statement is that beta blockade shifts autonomic balance toward parasympathetic dominance without changing parasympathetic activity.
14.9 Advanced Topic · Autonomic Dysfunction and the Logic of Autonomic Pharmacology
Autonomic circuits fail in characteristic ways, and each failure is a natural experiment that reveals what the intact circuit was doing. The organizing question for this whole section is the one Thread 2 posed: which component of the loop is broken?
Orthostatic hypotension
Stand up and roughly 500–800 mL of blood shifts into the veins of the legs and splanchnic bed within seconds. Venous return falls, stroke volume falls, and arterial pressure at the carotid sinus drops. In an intact person the baroreflex corrects this within one or two heartbeats: vagal withdrawal raises heart rate by 10–15 beats per minute, and sympathetic outflow constricts arterioles and, more importantly, veins, restoring venous return. The pressure dips a few millimetres and recovers.
Orthostatic hypotension is defined as a fall of ≥20 mm Hg systolic or ≥10 mm Hg diastolic within three minutes of standing. The value of the definition is that the pattern of the response names the broken component.
Clinical Connection · Orthostatic Hypotension — Reading the Heart Rate to Find the Broken Limb
Two patients each drop 30 mm Hg systolic on standing. One has a volume problem; the other has a nervous system problem. The single measurement that separates them takes no equipment beyond a finger on a pulse.
Fall in pressure WITH an appropriate rise in heart rate (≥ 15–20 beats/min). The reflex is working: it has detected the fall and executed its fastest response. What is missing is something to work with. Causes are volume and venous: dehydration, blood loss, diuretics, vasodilators, prolonged bed rest, venous pooling in varicosities, a large meal (post-prandial splanchnic pooling), or heat. This is a volume/effector problem, and the treatment is volume, compression, and reviewing the drug list.
Fall in pressure WITHOUT a rise in heart rate (< 10 beats/min). The reflex itself cannot execute. This is neurogenic orthostatic hypotension, and the absent tachycardia is the diagnostic finding — read straight off the reflex arc. The differential is: autonomic neuropathy (diabetes, amyloidosis, B₁₂ deficiency, chemotherapy), a synucleinopathy (Parkinson disease, multiple system atrophy, pure autonomic failure), or a drug blocking the efferent limb — beta blockers, which prevent the tachycardia, and alpha blockers, which prevent the vasoconstriction.
Amara is at risk of acquiring both categories at once. Her SDNN of 42 ms says the reflex is already blunted; metoprolol then blocks the efferent limb pharmacologically. Her day-six orthostatic measurement is 138/84 supine falling to 126/78 standing with a heart rate rising only from 64 to 68 — a drop that does not meet the definition, but a heart rate response of 4 beats per minute that would be abnormal in anyone. That is a warning about what will happen when the next antihypertensive is added.
Practical management, in order of evidence: stop or reduce the offending drug; raise fluid and salt intake unless heart failure forbids it; physical counter-manoeuvres (leg crossing with tensing, squatting, toe raises before standing) which raise pressure 10–15 mm Hg by squeezing venous capacitance; waist-high compression stockings and an abdominal binder, which work on the splanchnic bed where most of the pooling occurs; sleeping with the head of the bed raised 10–20 degrees, which reduces nocturnal pressure natriuresis and morning hypovolaemia; and only then drugs — midodrine, a direct alpha-1 agonist, or fludrocortisone, which expands volume. Both worsen supine hypertension, which is why the standing and the supine pressure must be managed as one problem.
Diabetic autonomic neuropathy
Chronic hyperglycaemia damages peripheral nerves by the four mechanisms of §13.7 — polyol flux, advanced glycation end products, oxidative stress, and microvascular disease of the vasa nervorum. Small unmyelinated C fibers are the most vulnerable, and all autonomic postganglionic fibers are C fibers. Length-dependent damage means the longest fibers fail first, and the longest autonomic nerve in the body is the vagus.
| System | Manifestation | Broken component |
|---|---|---|
| Cardiovascular | Reduced HRV — often the earliest measurable sign | Vagal efferents to the SA node |
| Resting tachycardia (90–100) | Vagal denervation unmasks the intrinsic rate | |
| Exercise intolerance, blunted heart rate response | Loss of both vagal withdrawal and sympathetic reserve | |
| Orthostatic hypotension without tachycardia | Efferent limb of the baroreflex | |
| Silent myocardial ischaemia | Visceral afferent pain fibers | |
| Gastrointestinal | Gastroparesis, early satiety, erratic glucose | Vagal efferents to the stomach |
| Nocturnal diarrhoea alternating with constipation | Enteric and extrinsic autonomic damage | |
| Genitourinary | Bladder that does not signal fullness; retention, overflow | Afferents from detrusor stretch receptors |
| Erectile dysfunction — often the earliest symptom of all | Parasympathetic nitric oxide pathway | |
| Sudomotor | Anhidrosis of the feet, compensatory truncal sweating | Sympathetic cholinergic fibers to sweat glands |
| Pupillary | Small pupil, poor dark adaptation | Sympathetic dilator denervation |
Clinical Connection · Diabetic Autonomic Neuropathy and the Silent Myocardial Infarction
Amara's fasting glucose is 138 mg/dL (7.7 mmol/L), her SDNN is 42 ms, and her resting heart rate is 88. Those three numbers together are early cardiac autonomic neuropathy, and they carry a specific and dangerous prediction.
Cardiac pain is carried by visceral afferent fibers — unmyelinated C fibers and thinly myelinated A-delta fibers — that travel with the sympathetic nerves, pass through the sympathetic chain without synapsing, and enter the cord at T1–T5, where convergence with somatic afferents produces the referred chest, arm, and jaw pain Amara had in Chapter 1 (§13.3). These are the same small fibers that diabetes destroys. As they die, ischaemia stops producing pain.
Roughly 20–30 percent of myocardial infarctions in people with long-standing diabetes are clinically silent — no chest pain at all. The presentation instead is unexplained dyspnoea, sudden fatigue, nausea, a fall, confusion in an older patient, or nothing until an ECG done for another reason shows old Q waves. Because the warning signal is what brings people to hospital, silent infarction means later presentation, longer ischaemic time, larger infarcts, and higher mortality.
Note the cruel logic. Amara's chest pain in Chapter 1 is the thing that saved her, and it was delivered by fibers her diabetes is in the process of killing. The alarm system is being disconnected by the disease it exists to warn about.
This is also why cardiac autonomic neuropathy is an independent predictor of mortality even after adjusting for every other risk factor, and why guidelines recommend screening for it — with exactly the tests already in Amara's chart: resting heart rate, heart rate variability, heart rate response to deep breathing, and orthostatic blood pressure. The bedside version takes ninety seconds: have the patient breathe at six breaths per minute and measure the difference between maximum and minimum heart rate. A healthy adult under 50 swings by more than 15 beats per minute. Amara swings by 3.
Clinical Connection · Horner Syndrome — Three Signs That Map a Pathway
Horner syndrome is the loss of sympathetic supply to one side of the head, and its triad is a direct readout of the receptor map in §14.4.
- Ptosis, and specifically partial ptosis of 1–2 mm — loss of alpha-1 tone in the superior tarsal (Müller) muscle, a small smooth muscle that assists the levator palpebrae. The levator itself, supplied by CN III, still works, which is why the lid droops but does not close. Complete ptosis means CN III, not Horner. There may also be a slight elevation of the lower lid ("upside-down ptosis") from the same loss in the inferior tarsal muscle, narrowing the palpebral fissure from both directions.
- Miosis — loss of alpha-1 drive to the iris dilator muscle, so the intact parasympathetic sphincter acts unopposed. The pupil is small but still reacts briskly to light, because the parasympathetic pathway is untouched. The anisocoria is greatest in the dark, when the normal pupil dilates and the denervated one cannot — the single most useful confirmatory observation, and the reason the examination must be done in a dim room.
- Anhidrosis — loss of sympathetic cholinergic supply to sweat glands. Its extent localizes the lesion: sudomotor fibers to the face follow the external carotid artery, while the oculosympathetic fibers follow the internal carotid, so a lesion distal to the carotid bifurcation spares most facial sweating.
The diagnostic value of Horner syndrome comes from the absurd length of the pathway. It is a three-neuron chain: hypothalamus → down the brainstem and cervical cord to the ciliospinal centre at T1 (first-order); T1 ventral root → up the sympathetic chain to the superior cervical ganglion (second-order); then along the internal carotid artery through the cavernous sinus into the orbit (third-order). A lesion anywhere along it produces the same triad, so the accompanying findings do all the localizing. With crossed sensory loss, vertigo, and dysphagia, suspect a lateral medullary (Wallenberg) stroke — first-order. With ipsilateral arm pain and wasting of the small hand muscles, suspect a Pancoast tumour at the lung apex compressing the T1 root and the chain — second-order. With acute neck pain after trauma or a chiropractic manipulation, suspect carotid artery dissection — third-order, and a stroke emergency.
Pharmacological confirmation is a beautiful application of denervation hypersensitivity (§14.1). Apraclonidine, a weak alpha-1 agonist, has no visible effect on a normal pupil but dilates a Horner pupil dramatically, because the denervated dilator muscle has up-regulated its alpha-1 receptors. The test reverses the anisocoria, and the reversal is the diagnosis.
Autonomic dysreflexia
In a person with a complete spinal cord injury at or above T6, a noxious stimulus below the level of injury — a blocked catheter, an impacted rectum, an ingrown toenail, a pressure sore, labour — triggers a massive, unregulated sympathetic discharge from the isolated cord segments below the lesion. Arterioles in the entire splanchnic bed constrict and systolic pressure can reach 250–300 mm Hg within minutes.
The baroreceptors detect it perfectly well and the medulla responds correctly. But the compensatory sympathetic inhibition cannot descend past the lesion. Only the vagus, which leaves the brainstem and never enters the cord, can be delivered.
Clinical Connection · Autonomic Dysreflexia — A Loop With Its Descending Limb Cut
The examination in autonomic dysreflexia literally draws the level of the injury as a horizontal line across the body, and every finding is predictable from which fibers can still reach where.
- Severe hypertension with a pounding headache — unopposed sympathetic vasoconstriction below the lesion, in a splanchnic bed that holds a quarter of the blood volume.
- Bradycardia, often 40–50, coexisting with systolic pressures above 200 — intact vagal compensation delivered through CN X, which never enters the spinal cord. This combination occurs in essentially no other condition and is diagnostic. It also cannot fix anything: the vagus does not innervate resistance vessels, so no amount of cardiac slowing lowers a pressure generated by arteriolar constriction.
- Flushing, sweating, and nasal congestion ABOVE the lesion — compensatory vasodilation that the medulla can still deliver to segments above the injury.
- Pale, cold, dry skin BELOW the lesion — where the sympathetic storm is running unopposed.
It is a medical emergency: the pressures reach the range that causes intracerebral haemorrhage, seizures, retinal detachment, and death. Management is ordered by speed and cost, not by sophistication. (1) Sit the patient upright and lower the legs — free, immediate, and it uses orthostatic pooling against the very mechanism causing the problem. (2) Loosen every constrictive item — abdominal binder, leg bag straps, tight clothing, splints. (3) Find and remove the stimulus, starting with the bladder, which causes the large majority of episodes: check and unkink the catheter, or catheterize, using lidocaine gel because the instrumentation itself is a noxious stimulus. Then examine for faecal impaction, again with local anaesthetic lubricant. Short-acting antihypertensives — topical nitrate, or oral immediate-release nifedipine — are added only if the pressure remains dangerously high after the stimulus is addressed, because removing the stimulus usually resolves the episode within minutes, and a long-acting agent then produces profound hypotension in a patient who has just lost their vasoconstrictor drive.
Every patient with an injury at or above T6, and everyone who cares for them, should carry a card describing this, because the presentation — a "normal" blood pressure of 90/60 rising to 160/100 — looks unremarkable to anyone who does not know that the patient's baseline is 90/60.
Raynaud phenomenon
Cold or emotional stress triggers episodic, sharply demarcated vasospasm of the digital arteries, producing the classic triphasic colour change: white (ischaemia from arteriolar closure), then blue (deoxygenation of the static blood remaining in capillaries), then red (reactive hyperaemia on reperfusion). Fingers, and sometimes toes, nose, and ears.
The mechanism is a receptor story. Cutaneous arterioles carry alpha-2C adrenergic receptors that are normally held inactive inside the cell; cooling causes them to translocate to the cell membrane, where they become responsive to circulating and released norepinephrine. This is an elegant physiological adaptation — it is how cold produces cutaneous vasoconstriction and conserves core heat (Chapter 25) — and in Raynaud phenomenon it is exaggerated, with increased receptor number, increased sensitivity, and reduced endothelial nitric oxide opposition.
The distinction that matters clinically is between primary Raynaud disease — young, female, symmetrical, no tissue loss, normal nailfold capillaries, benign — and secondary Raynaud phenomenon, in which the vasospasm sits on top of structural vessel disease from systemic sclerosis, lupus, vasculitis, or vibration injury, and can progress to digital ulceration and gangrene. Abnormal nailfold capillaroscopy and a positive antinuclear antibody separate them. Treatment is a readout of the mechanism: keep warm (removing the trigger for receptor translocation), stop smoking and beta blockers (non-selective beta blockade removes beta-2 vasodilation and leaves alpha constriction unopposed — a direct reason to prefer a beta-1-selective agent), and use calcium channel blockers such as nifedipine, which relax vascular smooth muscle downstream of the receptor entirely.
Aging · Baroreflex Sensitivity, Orthostatic Intolerance, and Why Old People Fall
Baroreflex sensitivity is measured as the change in R-R interval per millimetre of mercury change in systolic pressure. In a healthy 25-year-old it is 15–20 ms/mm Hg. By 75 it is commonly 4–6 ms/mm Hg — a threefold to fourfold reduction. Four changes combine to produce it, and each maps onto a different component of the loop.
A stiffer sensor mounting. Baroreceptors are stretch receptors embedded in the arterial wall, so what they actually sense is wall deformation, not pressure. Age-related elastin fragmentation and collagen cross-linking stiffen the carotid sinus and aortic arch, so the same pressure change produces less stretch and less afferent firing. The sensor has not failed; the thing it is attached to has become less deformable. This is Thread 1 running in reverse, and it explains why isolated systolic hypertension and impaired baroreflex sensitivity appear together — they share a cause.
A less responsive effector. Beta-1 receptor density falls modestly with age, and post-receptor coupling to G_s and adenylyl cyclase falls substantially, so the same sympathetic drive produces a smaller chronotropic and inotropic response. Maximum heart rate falls by roughly one beat per year for this reason as much as any other.
A weaker fast limb. SDNN declines by roughly 25–40 percent between age 25 and 75, and respiratory sinus arrhythmia becomes almost undetectable in many people over 80. The vagal limb — the one that acts within a single beat — is the one that goes.
Less volume to work with. Older veins are less compliant and less responsive to alpha-1 stimulation, and the ageing kidney is less able to conserve sodium overnight, so many older adults wake mildly volume-depleted. Thirst perception is also blunted.
The clinical result is that roughly 20 percent of community-dwelling adults over 65 have demonstrable orthostatic hypotension, rising above 30 percent in nursing homes. It is a leading contributor to falls — and a fall in the osteoporotic skeleton of someone like Adwoa Mensah, 78, whose femoral neck T-score is −2.9 (Chapter 6), means a hip fracture, six months of decline, and a 20–30 percent one-year mortality.
Follow the chain: ageing arterial wall → reduced baroreceptor stretch → blunted reflex → transient cerebral hypoperfusion on standing → fall → fracture in a bone weakened by a completely separate process. Two unrelated age changes multiply into one catastrophic event. That multiplication is exactly how geriatric medicine works, and it is why treating either factor alone underperforms.
Two practical consequences. In an older patient, every antihypertensive, every diuretic, every alpha blocker, and every psychotropic should be started at low dose, and blood pressure should be measured standing as well as sitting — a well-controlled sitting pressure that becomes 95/55 on standing is not good control. And in a patient like Adwoa, a fall is a cardiovascular event until proven otherwise; "mechanical fall" is a description, not a diagnosis.
The logic of autonomic pharmacology
Almost every drug in this chapter does one of four things: agonize or antagonize a receptor, or raise or lower the amount of transmitter available to it. Once the receptor map of §14.4 is secure, the pharmacology is a lookup rather than a memory task.
| Class | Example | Site of action | Principal use | Predictable adverse effect |
|---|---|---|---|---|
| α1 agonist | Phenylephrine, midodrine | Vascular smooth muscle | Hypotension; nasal decongestion | Reflex bradycardia; supine hypertension |
| α1 antagonist | Doxazosin, tamsulosin | Vessels; prostate, bladder neck | Hypertension; benign prostatic hyperplasia | First-dose orthostatic syncope; retrograde ejaculation |
| α2 agonist | Clonidine, dexmedetomidine | Brainstem; presynaptic terminal | Hypertension; sedation; withdrawal | Sedation, dry mouth, rebound hypertension on withdrawal |
| β1-selective antagonist | Metoprolol, bisoprolol, atenolol | Heart; renal JG cells | Ischaemic heart disease, heart failure, hypertension, rate control | Fatigue, bradycardia, blunted exercise response |
| Non-selective β antagonist | Propranolol, timolol | β1 and β2 everywhere | Tremor, migraine, thyrotoxicosis, portal hypertension, glaucoma | Bronchospasm; masked hypoglycaemia; cold extremities |
| β2 agonist | Salbutamol, formoterol | Bronchial smooth muscle | Asthma, COPD | Tremor, tachycardia, hypokalaemia |
| β3 agonist | Mirabegron | Detrusor | Overactive bladder | Hypertension |
| Muscarinic antagonist | Atropine, oxybutynin, ipratropium, hyoscine | M1–M5 | Bradycardia; overactive bladder; COPD; motion sickness | The antimuscarinic toxidrome (§14.5) |
| Cholinesterase inhibitor | Neostigmine, pyridostigmine, donepezil | Raises ACh everywhere | Myasthenia gravis; reversal of neuromuscular blockade; dementia | Bradycardia, cramps, diarrhoea, bronchorrhoea |
| Ganglionic blocker (N_N) | Hexamethonium (historical) | All ganglia, both divisions | (abandoned) | Every organ loses whichever division dominated it |
Why β1-selectivity matters. Metoprolol blocks beta-1 roughly 30 times more avidly than beta-2 at usual doses; propranolol blocks both equally. In a healthy person the difference is modest. In a person with asthma or COPD it is the difference between a useful drug and a dangerous one, because blocking beta-2 on bronchial smooth muscle removes the epinephrine-mediated bronchodilation that airways depend on and leaves vagal M3 bronchoconstriction unopposed. Beta-2 blockade also blunts hepatic glycogenolysis and abolishes the tremor and palpitations that warn a person with diabetes of hypoglycaemia — masking the symptoms while impairing the recovery. Sweating, being sympathetic cholinergic, is not masked, which is why diaphoresis becomes the only remaining warning sign. For Amara, with a fasting glucose of 138 mg/dL and a trajectory toward type 2 diabetes, that selectivity is not a detail.
And selectivity is a ratio, not a switch. At 200 mg daily, metoprolol's thirtyfold preference is overwhelmed and it behaves substantially like a non-selective agent. "Cardioselective" means relatively selective at ordinary doses. Understanding that a receptor preference is a ratio is one of the more valuable things this chapter can give you.
Clinical Connection · Atropine and Organophosphates — Two Poisons, One Antidote, Two Receptors
Nothing demonstrates the cholinergic system as completely as a poison that floods it and a drug that blocks half of it.
Organophosphates — nerve agents, and many older insecticides — irreversibly inhibit acetylcholinesterase. Acetylcholine is no longer destroyed, so it accumulates at every cholinergic synapse in the body simultaneously. The resulting syndrome maps exactly onto the two receptor families, which is why it is the perfect teaching case.
- Muscarinic excess (mnemonic DUMBBELSS): Diarrhoea, Urination, Miosis (pinpoint pupils), Bronchorrhoea, Bradycardia, Emesis, Lacrimation, Salivation, Sweating. Patients drown in their own secretions; bronchorrhoea and bronchospasm are what kill them, not the bradycardia.
- Nicotinic excess: fasciculations, cramps, weakness, and finally depolarizing paralysis — the end plate is so persistently depolarized that voltage-gated sodium channels inactivate and the muscle stops responding. Because the diaphragm is skeletal muscle, this means respiratory arrest.
Atropine is a competitive muscarinic antagonist and is therefore the exact mirror image of the muscarinic half of the poisoning: it reverses every item on the DUMBBELSS list and not one item on the nicotinic list, because it does not bind nicotinic receptors at all. This is why atropine alone is insufficient treatment — it dries the secretions and restores heart rate but cannot relieve the paralysis. Pralidoxime is added to reactivate the enzyme by displacing the phosphate group, and it must be given before the enzyme–inhibitor complex "ages" into an irreversible form. Two receptor families, two drugs. A patient can be fully atropinized, dry and pink, and still die of respiratory failure.
Note also the dosing principle, which follows from the mechanism: atropine is titrated not to heart rate or pupil size but to drying of pulmonary secretions, and the doses required are enormous — tens of milligrams, against the 0.5 mg used for bradycardia — because it is competing against a cleft saturated with acetylcholine.
Atropine's ordinary clinical uses are simply the muscarinic table read backwards: symptomatic bradycardia (blocking M2 at the SA node removes vagal restraint and the rate rises); drying secretions before surgery (M3 on glands); and eye drops to dilate the pupil and paralyse accommodation for retinal examination (M3 on the iris sphincter and ciliary muscle) — the last being how Amara's retina is examined in Chapter 15. The side effects follow the same table and give the classic teaching phrase: dry as a bone, red as a beet, hot as a hare, blind as a bat, mad as a hatter — no secretions, cutaneous vasodilation, no sweating and therefore no evaporative cooling, paralysed accommodation, and central antimuscarinic delirium.
Thread 1 · Structure Determines Function
Every claim in this chapter reduces to a statement about shape or position.
Position of the ganglion. Sympathetic ganglia sit beside the vertebral column, so preganglionic fibers are short and postganglionic fibers are long and can branch across many organs — a diffuse system. Parasympathetic ganglia sit in the wall of the target organ, so postganglionic fibers are a few hundred micrometres long and cannot reach anything else — a discrete system. Nothing else about the two divisions needs to be memorized once that one geometric fact is secure.
Shape of the ending. A neuromuscular junction is a sealed, folded, receptor-dense apparatus built for guaranteed one-to-one transmission. A varicosity is a bead on a passing wire that sprays transmitter into interstitial fluid. One is a wire; the other is a sprinkler. The tissues they supply — one addressable fiber versus an electrically coupled syncytium — are exactly the tissues each design suits.
Shape of the receptor. A nicotinic receptor is a pore, so it acts in microseconds and can only excite. A G-protein-coupled receptor is a signalling switch, so it acts in milliseconds to seconds, amplifies, and can inhibit. And within the muscarinic family, M2's beta-gamma subunit opens a potassium channel directly while beta-1's cascade must build cyclic AMP — which is why the vagus can change one heartbeat and the sympathetic system cannot, which is why beat-to-beat variability is vagal, which is why RMSSD means what it means. A four-step chain of reasoning from molecular architecture to a prognostic number on a telemetry printout.
Position of the adrenal medulla. It makes epinephrine rather than norepinephrine because it sits inside the adrenal cortex, bathed in cortisol at a hundred times systemic concentration, which induces PNMT. Move the identical neural crest cells anywhere else in the body and they make norepinephrine only. The hormone is a consequence of the address.
Stiffness of an artery. A baroreceptor measures deformation. Cross-link the collagen in a carotid sinus over fifty years and the identical, undamaged nerve ending reports less. Adwoa's fall risk is, in the end, a materials-science problem.
Check Your Understanding 14.9
- A patient's blood pressure falls from 130/80 to 100/62 on standing, and their heart rate goes from 72 to 74. What does the heart rate tell you, and what does it rule out?
- Why does a patient with a T4 spinal cord injury develop bradycardia during autonomic dysreflexia when their blood pressure is 240/130 — and why does the bradycardia not fix the pressure?
- A patient with Raynaud phenomenon and migraine is offered propranolol. Explain the specific physiological objection and name a better alternative with a reason.
Show answers
- The heart rate should have risen by 10–20 beats per minute — that is the baroreflex's first and fastest response to a fall in carotid sinus stretch, delivered by vagal withdrawal within one to two beats. A rise of only 2 beats means the reflex cannot execute its efferent limb, so this is neurogenic orthostatic hypotension. It effectively rules out simple volume depletion, haemorrhage, and diuretic effect, in all of which the reflex is intact and the tachycardia is prominent. The differential becomes autonomic neuropathy (diabetes, amyloid, B₁₂), a synucleinopathy (Parkinson disease, multiple system atrophy, pure autonomic failure), or drugs blocking the limb — beta blockers preventing the tachycardia, alpha blockers preventing the vasoconstriction. In a patient on metoprolol, the drug must be excluded before the diagnosis is made.
- Because the two efferent limbs leave the CNS by different routes, and only one of them is interrupted. The baroreceptors, the solitary nucleus, and the medullary centres are intact and detect the hypertension correctly. The sympathetic inhibitory command must descend through the spinal cord to reach the lateral horn of T1–L2, and it cannot get past T4 — so the sympathetic storm below the lesion continues unopposed. The vagal command leaves the medulla in CN X and travels to the heart entirely outside the spinal cord, so it is delivered normally, producing maximal bradycardia. It cannot fix the pressure because the vagus does not innervate resistance vessels. Blood pressure is cardiac output times peripheral resistance, and the crisis is entirely a resistance problem generated by splanchnic arteriolar constriction; slowing the heart reduces one term modestly while the other is at maximum. This is the clearest demonstration in medicine that knowing where a nerve travels is not academic.
- Propranolol is non-selective and blocks beta-2 as well as beta-1. Beta-2 receptors on skeletal muscle and cutaneous arterioles mediate the only adrenergic vasodilation the circulation has; blocking them leaves alpha-1 vasoconstriction unopposed, which worsens digital vasospasm and can precipitate more frequent and more severe Raynaud attacks — and, in secondary Raynaud with structural vessel disease, digital ulceration. Better alternatives: a calcium channel blocker such as nifedipine or amlodipine, which both prevents migraine (less well than propranolol, but effectively) and directly relaxes vascular smooth muscle downstream of every adrenergic receptor, treating both conditions with one mechanism; or a migraine preventive from a different class entirely, such as amitriptyline or topiramate. If a beta blocker is genuinely required for another indication, a beta-1-selective agent such as bisoprolol is far preferable — remembering that selectivity is a ratio that is lost at high dose.
Chapter Summary
§14.1 The autonomic nervous system is the visceral motor division: it commands cardiac muscle, smooth muscle, glands, and adipose tissue, and it is the efferent arm of nearly every homeostatic loop in the body. Its defining feature is a two-neuron chain with a synapse in a peripheral ganglion, against the somatic system's single neuron. Every other difference follows: thin myelinated B preganglionic and unmyelinated C postganglionic axons instead of fast A-alpha ones; varicosities and volume transmission instead of a neuromuscular junction; acetylcholine or norepinephrine acting on eight G-protein-coupled receptor subtypes instead of ACh on one nicotinic channel; the ability to inhibit as well as excite at the periphery; continuous tone instead of silence at rest; and an effector that, when denervated, keeps working and becomes hypersensitive rather than paralysed.
§14.2 Sympathetic preganglionic cell bodies occupy the lateral horn of T1–L2 only. Their axons leave in the ventral root and enter the chain through a white ramus (myelinated, 14 levels); postganglionic axons rejoin spinal nerves through a gray ramus (unmyelinated, all 31 levels). A preganglionic fiber has three fates: synapse at its own level and supply that dermatome's sweat glands, arrector pili, and cutaneous vessels; ascend or descend the chain to reach head or limb, which is how a 14-segment outflow serves the whole body; or pass through as a splanchnic nerve to a prevertebral ganglion on the aorta. Short preganglionic, long postganglionic, and 1:10–20 divergence make sympathetic responses diffuse. The adrenal medulla is a modified sympathetic ganglion of neural crest origin whose chromaffin cells never grew axons and secrete 80 percent epinephrine into the blood, adding a slower hormonal broadcast channel with a different receptor profile.
§14.3 Parasympathetic preganglionic cell bodies sit in the brainstem nuclei of CN III, VII, IX, and X and in S2–S4. Ganglia are terminal or intramural — on or in the organ wall — so the geometry is long preganglionic and short postganglionic, and with a divergence of 1:1–3 the effects are discrete and organ-specific. There is no parasympathetic supply to skin, sweat glands, arrector pili, or most blood vessels, which makes vascular tone a sympathetic monologue. The vagus carries about 75 percent of all parasympathetic fibers, is about 80 percent afferent, is the longest autonomic nerve in the body, and supplies the gut to the splenic flexure, where nerve supply, blood supply, and embryological origin all change together.
§14.4 All preganglionic fibers and all parasympathetic postganglionic fibers release acetylcholine; almost all sympathetic postganglionic fibers release norepinephrine, except those to sweat glands. ACh is made by choline acetyltransferase and destroyed in the cleft by acetylcholinesterase within a millisecond, so cholinergic effects are brief and local. NE is made from tyrosine, released with ATP and neuropeptide Y, and terminated mainly by reuptake through NET, so adrenergic effects last seconds and reuptake blockers such as cocaine amplify them. Nicotinic receptors are cation channels — fast, always excitatory, at all ganglia, the adrenal medulla, and the neuromuscular junction. Muscarinic M1–M5 and adrenergic α1, α2, β1, β2, β3 are G-protein-coupled: M2 gates a potassium channel directly, which is why the vagus can change a single beat, while β1 must build cyclic AMP over seconds.
§14.5 The effects table names the receptor for every action, and four patterns carry most of the clinical weight. Vessels are a sympathetic monologue with no vasodilator nerve, so dilation means withdrawal of tone, local metabolites, or β2. The α1/β2 split in the vasculature is a flow-redistribution machine that shunts output from gut to muscle during exercise. β1 is renal as well as cardiac, releasing renin. And four exceptions carry the exam questions: sympathetic cholinergic sweat glands, endothelial M3 receptors with no cholinergic nerves, hormonally reached bronchial β2, and the nicotinically innervated adrenal medulla. The table is most useful read backwards, from a patient's findings to the receptor family involved.
§14.6 Both divisions fire continuously; organs sit at an intermediate operating point set by their balance. Vasomotor tone at about 1 Hz holds arterioles half-constricted and is why blood pressure exists at rest; vagal tone holds the SA node about 30 beats below its intrinsic rate of 100. Dual innervation may be antagonistic (heart, pupil, gut) or cooperative (salivation, sexual function); single sympathetic innervation governs vessels, sweat glands, arrector pili, adrenal medulla, kidney, liver, and fat. The purpose of an intermediate operating point is bidirectional speed and reserve — a controller in the middle of its range can respond to any disturbance, and losing one limb shrinks the space it has left to regulate in.
§14.7 Autonomic control is a four-level hierarchy — spinal reflex circuits, brainstem reflex centres, the hypothalamus setting targets, and limbic and cortical modulation — and each level can run without those above it, which is why cord injury disorders reflexes rather than abolishing them. The baroreflex is the model loop: carotid sinus and aortic arch stretch receptors → CN IX and X → nucleus of the solitary tract → increased vagal and decreased sympathetic outflow → slower heart, dilated arterioles and veins, less renin. It corrects within one to two beats, resets over one to two days and therefore cannot set long-term pressure, and fails mechanically when the arterial wall stiffens. Chemoreceptor, micturition, defecation, and sexual reflexes each require sympathetic, parasympathetic, and often somatic systems to cooperate.
§14.8 Heart rate variability is the beat-to-beat variation in R-R interval within normal sinus rhythm. Because only the vagus can alter one interval relative to the next, RMSSD and the high-frequency band are near-pure parasympathetic indices, while SDNN sums respiratory, baroreflex, thermoregulatory, hormonal, and circadian sources. Low HRV means either that vagal efferents have died or that sympathetic drive is saturating the node — usually both. It measures regulatory reserve, and it reports a state of sustained sympathetic dominance that is itself arrhythmogenic, hypertrophic, pro-inflammatory, and pro-thrombotic. After myocardial infarction SDNN below 50 ms carries roughly a fourfold mortality increase independent of ejection fraction. Beta blockade raises HRV by unmasking vagal influence, which is one of the mechanisms of its survival benefit.
§14.9 Every autonomic disorder is a named component of a named loop, broken. A stiff arterial wall breaks the sensor in ageing baroreflex decline. Dying C fibers break both efferent and afferent limbs in diabetic autonomic neuropathy, producing reduced HRV, resting tachycardia, gastroparesis, anhidrotic feet, and silent myocardial ischaemia. A severed descending pathway breaks the correction in autonomic dysreflexia, giving hypertension with bradycardia. One branch on one side is lost in Horner syndrome, whose triad maps a three-neuron pathway from hypothalamus to orbit. Receptor translocation with cooling produces Raynaud phenomenon. And a drug is an intentional lesion: β1-selective blockade differs from non-selective blockade because β2 mediates bronchodilation, hepatic glycogenolysis, and the warning symptoms of hypoglycaemia, while atropine and organophosphates are mirror images that between them define the two cholinergic receptor families.
The Three Threads in Chapter 14
Structure → Function. The position of a ganglion decides whether a division is diffuse or discrete. The shape of an ending — sealed junction versus passing varicosity — decides whether transmission is one-to-one or one-to-hundreds. The architecture of a receptor decides its speed: a nicotinic pore acts in microseconds and can only excite, a G-protein-coupled receptor acts in milliseconds to seconds and can inhibit, and M2's direct gating of a potassium channel is the reason beat-to-beat variability is vagal. Even the hormone the adrenal medulla makes is a consequence of the address it lives at. In every case the anatomy came first.
Homeostasis. This chapter supplied the efferent limb of nearly every loop in the book, together with the refinement that makes those loops work: regulation is achieved by modulating a continuous baseline in both directions, not by switching a system on. Tone is why the baroreflex can correct in one beat. The intermediate operating point is why an organ can be moved either way. And the loss of that range — reduced regulatory reserve — is what low HRV measures and what ageing produces.
Integration. Amara's SDNN of 42 ms is an endocrine finding (hyperglycaemia), a vascular finding (vasa nervorum disease), a nervous finding (dying vagal C fibers), a cardiac finding (raised resting rate and lowered fibrillation threshold), and a renal finding (β1-driven renin) at the same time. Metoprolol lowers her blood pressure through the heart, the kidney, and the brainstem at once. One number, one drug, and six systems — which is the whole argument of this book compressed into a telemetry printout.
Case File 14 · Resolution
Question 1 — What is heart rate variability actually measuring, and why is less of it bad?
HRV measures the millisecond-to-millisecond difference between consecutive R-R intervals in a normal sinus rhythm. Every beat in Amara's recording originates in the SA node and is entirely normal; what varies is the spacing between them.
That spacing is set by the balance of two continuous inputs to the node (§14.6). Vagal acetylcholine acting on M2 receptors releases a G-protein beta-gamma subunit that opens a G-protein-gated potassium channel directly, hyperpolarizing the node and flattening the pacemaker potential — and because there is no second-messenger step, it can change the length of a single interval. Sympathetic norepinephrine at β1 receptors raises cyclic AMP, which increases the funny current (I_f) through HCN channels and, via protein kinase A, the L-type calcium current, steepening the pacemaker potential — but through a cascade that takes about five seconds to build. So fast, beat-to-beat variability is essentially a readout of vagal activity, and RMSSD isolates it. Slower variability, captured by SDNN, adds baroreflex oscillations, thermoregulatory drift, hormonal rhythms, and the circadian swing.
A normal 45-year-old's heart is being nudged constantly: sped up on each inspiration as lung stretch receptors and central respiratory neurons inhibit the vagus, slowed on each expiration, adjusted every few seconds by the baroreflex as pressure oscillates, and shifted across the day by the circadian clock. All of that appears as variability. Amara's SDNN of 42 ms, RMSSD of 18 ms, and respiratory swing of 3 beats per minute say that the nudging has stopped.
Low HRV is bad for two linked reasons, and they are not the same reason.
First, it reports loss of regulatory reserve. A control system that cannot vary its output cannot correct a disturbance. When Amara stands quickly, bleeds, becomes dehydrated, develops a fever, or is given another antihypertensive, the responses that should defend her blood pressure are weakened — which is exactly why her orthostatic heart rate response on day six is 4 beats per minute instead of 15.
Second, and more importantly, low HRV is the signature of a harmful state. It reports sustained sympathetic dominance with withdrawn vagal restraint, and chronic catecholamine exposure lowers the ventricular fibrillation threshold, promotes myocardial hypertrophy and interstitial fibrosis, activates platelets, worsens insulin resistance, drives systemic inflammation, and retains sodium. The vagus, meanwhile, is anti-arrhythmic and anti-inflammatory. After myocardial infarction, SDNN below 50 ms is associated with roughly a fourfold increase in mortality independent of ejection fraction and infarct size.
So: HRV is the thermometer, not the fever. And in Amara specifically, it is showing early cardiac autonomic neuropathy — four years of unrecognized hyperglycaemia acting on the small unmyelinated C fibers of the longest autonomic nerve she has. Six months later the same disease will be found in her feet with a tuning fork (Chapter 13); the vagus simply failed first, because it is longer.
Question 2 — The heart has no "off switch" from the brain. So how does a drug slow it?
The premise is exactly right, and it is the key to the answer. Cardiac muscle is autorhythmic (Chapter 18): SA node cells have no stable resting potential but drift spontaneously toward threshold through the funny current, so the heart generates its own rhythm and requires no command to beat. A denervated transplanted heart beats for decades. There is no motor neuron that fires a heartbeat the way a somatic motor neuron fires a muscle twitch, and therefore nothing for a drug to switch off.
What the autonomic nervous system does instead is modulate the slope of the pacemaker potential — it changes how fast the node drifts to threshold, and therefore how long each interval lasts. The intrinsic, unmodulated rate of a human SA node is about 100 beats per minute. A resting rate of 70 is 100 minus roughly 30 beats of continuous vagal restraint. Amara's resting rate of 88 is therefore not a heart being driven fast; it is largely a heart that has lost its brake.
Metoprolol works on the other input. It is a competitive antagonist at the β1 adrenergic receptor, which on SA node cells couples through G_s to adenylyl cyclase and cyclic AMP. Cyclic AMP binds directly to the HCN channels carrying I_f and, through protein kinase A, phosphorylates L-type calcium channels; both effects steepen the pacemaker potential. Block the receptor and cyclic AMP falls, the slope flattens, threshold is reached later, and each cardiac cycle takes longer. The node still fires. It simply takes about 940 milliseconds instead of 680.
So metoprolol does not turn anything off. It removes one of two opposing modulatory influences from a self-starting oscillator, shifting the equilibrium toward the other. Three consequences follow, and all three are visible in Amara's chart:
- Rate falls from 88 to 64 — negative chronotropy.
- Contractility falls modestly and AV conduction slows — negative inotropy and dromotropy, all β1.
- HRV rises, because vagal influence on the node, previously swamped by high sympathetic tone, is unmasked. The heart becomes slower and more variable at once — which is the outcome that actually improves survival.
The general principle is worth extracting and carrying into the cardiovascular chapters: you cannot switch off an oscillator; you can only bias it. Most cardiovascular pharmacology is the art of shifting a balance rather than issuing a command.
Question 3 — Why does blocking a receptor on the heart lower a pressure generated by vessels?
Because mean arterial pressure is not generated by vessels alone. It is a product:
MAP ≈ Cardiac Output × Total Peripheral Resistance, and Cardiac Output = Heart Rate × Stroke Volume
Metoprolol attacks that equation at three separate points, two of them quick and one slow.
Limb 1 — cardiac output falls (immediate). β1 blockade lowers heart rate from 88 to 64 and reduces contractility, which lowers stroke volume somewhat. A longer diastole means better filling, which recovers part of the stroke volume, so the net fall in cardiac output is roughly 10–15 percent rather than the 27 percent the rate change alone would suggest. With resistance initially unchanged, pressure falls. This is most of the first few days' effect and most of Amara's 14 mm Hg.
Limb 2 — renin falls, so the resistance term falls too (days to weeks). This is the limb students miss, and it is why §14.5 insisted that β1 is not only a cardiac receptor. The juxtaglomerular cells of the kidney carry β1 receptors, and sympathetic stimulation of them releases renin. Renin generates angiotensin I, which ACE converts to angiotensin II — one of the most potent direct arteriolar vasoconstrictors in the body — which also stimulates aldosterone, retaining sodium and water and expanding blood volume (Chapters 16, 19, 26). Metoprolol reduces renin release by 30–60 percent. Less angiotensin II means less arteriolar constriction, so the resistance term drops; less aldosterone means less volume, so preload and stroke volume fall further. A "cardiac" drug lowers vascular resistance because one of its target receptors sits in the kidney.
Limb 3 — reduced central sympathetic outflow (weeks). Beta receptors in the CNS and on presynaptic sympathetic terminals contribute a modest, slower reduction in overall sympathetic drive, further relaxing vascular tone. This limb is real but small and is the least well characterized of the three.
Two refinements complete the picture. First, note what metoprolol does not do: it does not block α1 receptors, which are the principal vasoconstrictors. That is why beta blockers lower blood pressure less than ACE inhibitors or calcium channel blockers do in many patients, and why Amara's 138/84 is improved but not yet at target. Second, a non-selective blocker such as propranolol would additionally block β2, removing epinephrine-mediated vasodilation in skeletal muscle — which can transiently raise peripheral resistance and leaves α1 constriction unopposed. Selectivity is not only about protecting the airway; it also shapes the vascular response, and it is why cold hands are a propranolol side effect and much less a metoprolol one.
Amara's 152/94 becoming 138/84 in three days is therefore mostly Limb 1, with Limb 2 building underneath it. Her mild fatigue on stairs is the expected cost: a β1-blocked heart cannot raise rate and contractility as steeply on demand, so the cardiac output response to exertion is blunted. Her rehabilitation programme (Chapter 10) will have to work with a heart rate ceiling that is now pharmacologically 20 to 30 beats below her age-predicted maximum — which is why exercise prescription after beta blockade uses rating of perceived exertion rather than a target heart rate.
Systems Integration Case File · Entry 14
Entry 14 — The autonomic nervous system enters the file
New findings this chapter:
| Finding | Value |
|---|---|
| Resting heart rate, supine | 88 → 64 beats/min on metoprolol 50 mg daily |
| SDNN (24 h) | 42 ms (expected 120–160) |
| RMSSD | 18 ms (expected 25–50) |
| Heart rate variation at 6 breaths/min | 3 beats/min (expected > 15 under age 50) |
| Heart rate response to standing (30:15 ratio) | 1.02 (expected > 1.04) |
| Blood pressure | 152/94 → 138/84 mm Hg |
| Orthostatic BP, day 6 | 138/84 supine → 126/78 standing; HR 64 → 68 |
| Fasting glucose | 138 mg/dL (7.7 mmol/L) |
| Six months later (Chapter 13) | Absent vibration at both great toes; absent ankle jerks |
Your entry:
1 · ADD (2–3 sentences). State what the autonomic nervous system contributes to Amara's picture. Use at least three of the numbers above, name the receptor subtype metoprolol blocks, and name the two organs on which that blockade acts.
2 · CONNECT (2–3 sentences). Link the ANS to at least two systems already in your file, stating the direction of causation each time. Consider: nervous (Chapters 11, 12, 13), cardiovascular (Chapter 1's vital signs), integumentary (Chapter 5's cool, pale, diaphoretic skin), muscular (Chapter 10's rehabilitation), and the endocrine and renal systems arriving in Chapters 16 and 26.
3 · PREDICT (1–2 sentences). Name one finding you now expect in a later chapter, and say why.
Model responses — read only after writing your own
1 · ADD. Amara has early cardiac autonomic neuropathy: an SDNN of 42 ms, an RMSSD of 18 ms, and a heart rate swing of only 3 beats per minute with paced deep breathing together indicate that vagal efferents to her SA node are failing, which is why a resting rate of 88 sat close to the heart's intrinsic denervated rate of about 100 — her heart had lost its brake rather than gained an accelerator. Metoprolol blocks β1 receptors on the SA node, flattening the pacemaker potential and slowing the rate to 64, and on the renal juxtaglomerular cells, reducing renin release and therefore angiotensin II-mediated vasoconstriction and aldosterone-mediated volume retention — which is why a cardiac drug lowered her systolic pressure by 14 mm Hg. Her orthostatic heart rate response of 4 beats per minute shows the efferent limb of her baroreflex is now impaired both by disease and by the drug.
2 · CONNECT. Endocrine → nervous: four years of hyperglycaemia (fasting glucose 138 mg/dL, HbA1c 7.4%) has damaged the vasa nervorum and driven polyol and glycation pathways in peripheral nerve, killing the small unmyelinated C fibers that carry both vagal efferents and cardiac pain afferents — hyperglycaemia causes the autonomic neuropathy, not the reverse. Nervous → cardiovascular: loss of vagal restraint raised her resting heart rate and left sympathetic tone unopposed, which raises myocardial oxygen demand and shortens diastole in a heart supplied by a 90% stenosed left circumflex artery — the same imbalance that produced her chest pain in Chapter 1. Nervous → renal → cardiovascular: β1 receptors on juxtaglomerular cells connect sympathetic tone directly to renin, so her autonomic state is already raising angiotensin II and aldosterone, and therefore both resistance and volume, before Chapter 26 formally introduces the kidney. Nervous → integumentary: her cool, pale, diaphoretic skin at triage was α1-mediated cutaneous vasoconstriction plus sympathetic cholinergic sweating — one system's decision written on another system's surface. Nervous → muscular: β1 blockade caps her exercise heart rate, so her rehabilitation prescription (Chapter 10) must be written in perceived exertion rather than beats per minute.
3 · PREDICT. I expect that when Amara's coronary anatomy is defined in Chapter 18, at least one episode of ischaemia will have occurred without chest pain, because the visceral afferent C fibers that carry cardiac pain are the same population her diabetes is destroying. I also expect her renin and aldosterone to be measurable in Chapter 16, partially suppressed by metoprolol, and her eGFR to have drifted by Chapter 26, because chronic angiotensin II exposure damages the glomerulus. A third defensible prediction: her orthostatic drop will worsen as further antihypertensives are added, because a blunted baroreflex cannot compensate — and that will become a falls risk exactly as it already has for her mother Adwoa (Chapter 30).
Review
Level 1 · Recall
14.1 Sympathetic preganglionic neurons have their cell bodies in the:
a) dorsal root ganglia b) lateral horn of spinal segments T1–L2 c) ventral horn of all spinal segments d) sympathetic chain ganglia
Answer
b — the lateral horn of T1–L2, the thoracolumbar outflow. (a) Dorsal root ganglia hold sensory cell bodies and contain no synapses at all. (c) The ventral horn holds somatic motor neurons — though sympathetic preganglionic axons do exit through the ventral root alongside them. (d) The chain ganglia hold postganglionic cell bodies: the second neuron, not the first.
14.2 Which structure is a modified sympathetic ganglion whose cells release hormone into the blood?
a) celiac ganglion b) superior cervical ganglion c) adrenal medulla d) adrenal cortex
Answer
c — the adrenal medulla. Its chromaffin cells are neural-crest-derived postganglionic sympathetic neurons that never grew axons; they receive preganglionic fibers directly, respond to acetylcholine at nicotinic receptors, and secrete about 80 percent epinephrine into the blood. (a) and (b) are conventional ganglia with postganglionic axons. (d) The cortex is mesodermal and makes steroids — a different organ that happens to share a capsule, and whose cortisol induces the enzyme (PNMT) that lets the medulla make epinephrine at all.
14.3 Which receptor mediates bronchodilation, and is reached mainly by circulating epinephrine rather than by direct innervation?
a) α1 b) β1 c) β2 d) M3
Answer
c — β2. Bronchial smooth muscle carries β2 receptors but almost no sympathetic innervation, so relaxation is a hormonal effect of adrenal epinephrine — which is why epinephrine is the drug in anaphylaxis and why non-selective beta blockade is dangerous in asthma. (a) α1 is the principal vasoconstrictor. (b) β1 is cardiac and renal. (d) M3 is the vagal receptor that constricts bronchi and increases mucus.
14.4 A patient's thermoregulatory sweating is abolished by atropine. This is because sweat glands are:
a) parasympathetic, using ACh on M3 b) sympathetic, using ACh on M3 c) sympathetic, using NE on α1 d) not innervated at all
Answer
b — sympathetic, but cholinergic. Thermoregulatory sweat glands are the one important exception to the rule that sympathetic postganglionic fibers are adrenergic: they release acetylcholine onto muscarinic M3 receptors. The pathway is anatomically sympathetic and pharmacologically cholinergic, which is why an antimuscarinic drug stops sweating and can cause hyperthermia in hot weather. (a) is wrong because sweat glands receive no parasympathetic supply. (c) describes apocrine "emotional" sweating, which is α1-mediated and a minor contributor. (d) is wrong outright.
14.5 The intrinsic rate of a completely denervated human sinoatrial node is approximately:
a) 40 beats/min b) 60 beats/min c) 100 beats/min d) 140 beats/min
Answer
c — about 100 beats/min. A normal resting rate of 70 is that intrinsic rate minus roughly 30 beats of continuous vagal restraint, which is why the fastest way to raise heart rate is to withdraw the vagus rather than to add sympathetic drive, and why a transplanted heart runs near 100. (a) is the intrinsic rate of the AV junctional pacemaker, which takes over if the SA node fails. (b) is the normal modulated rate. (d) is above the ceiling of pure vagal withdrawal and requires sympathetic drive.
14.6 Which finding on standing indicates neurogenic rather than volume-related orthostatic hypotension?
a) systolic fall of 25 mm Hg with heart rate rising 20 beats/min b) systolic fall of 25 mm Hg with heart rate rising 2 beats/min c) diastolic rise of 10 mm Hg d) dizziness without any pressure change
Answer
b. The baroreflex's fastest response to reduced carotid sinus stretch is vagal withdrawal and tachycardia. Its absence means the efferent limb cannot execute — autonomic neuropathy, a synucleinopathy, or a drug blocking that limb. (a) is the intact reflex responding appropriately to a volume problem. (c) is a normal response. (d) does not meet the definition of orthostatic hypotension at all and suggests another cause of dizziness.
14.7 RMSSD is regarded as an almost pure index of parasympathetic activity because:
a) the vagus is the only nerve that reaches the SA node b) only the vagus can change one R-R interval relative to the next c) sympathetic fibers do not affect heart rate d) RMSSD is measured only during expiration
Answer
b. M2 receptors release a G-protein beta-gamma subunit that opens a potassium channel directly, a membrane-delimited pathway with no diffusible second messenger, so vagal effects appear and disappear within a single cardiac cycle. β1 signalling must run through G_s, adenylyl cyclase, cyclic AMP, and protein kinase A, taking about five seconds — far too slow to alter one interval relative to the next. Since RMSSD is computed from successive differences, it isolates the only limb fast enough to produce them. (a) is false; sympathetic fibers from T1–T5 reach the SA node too. (c) is false. (d) is a fabrication.
Level 2 · Comprehension
14.8 Explain why the two-neuron design of autonomic pathways makes selective pharmacology possible in a way that the one-neuron somatic design does not.
Model answer
Because the extra synapse introduces an extra receptor, and receptors come in subtypes that are distributed unevenly.
Every somatic motor ending in the body uses acetylcholine on a nicotinic N_M receptor. There is one transmitter, one receptor type, and one possible effect, so any drug that blocks it paralyses everything at once — which is exactly what neuromuscular blocking agents do, and why they can only be used with a ventilator.
The autonomic system uses acetylcholine on nicotinic N_N receptors at the ganglion, but then uses acetylcholine on muscarinic M1–M5 or norepinephrine on adrenergic α1, α2, β1, β2, β3 receptors at the target. Because different organs express different subtypes — β1 concentrated in the heart and renal juxtaglomerular cells, β2 in bronchi and skeletal muscle arterioles, M3 on glands and smooth muscle, α1 on vascular smooth muscle — a drug with subtype preference has organ preference.
Two qualifications complete the answer. First, selectivity is only possible where nature was already selective: blocking the ganglionic receptor, which is identical in both divisions, produces the chaos of hexamethonium and is useless. Second, selectivity is a ratio, not a switch: metoprolol's thirtyfold β1 preference is overwhelmed at high dose, and "cardioselective" means relatively selective at ordinary doses.
14.9 Explain why heart rate can rise within one second at the start of exercise but takes 20 to 30 seconds to reach its full value.
Model answer
Two signalling architectures with two different speeds.
The immediate rise is vagal withdrawal. Acetylcholine at the SA node acts on M2 receptors coupled to G_i, whose beta-gamma subunit binds and opens a G-protein-gated inward-rectifier potassium channel directly — a membrane-delimited pathway with no diffusible second messenger. Removing acetylcholine closes that channel almost immediately, so the pacemaker potential depolarizes faster within a single cardiac cycle. Central command from motor cortex silences the cardioinhibitory neurons of the nucleus ambiguus before any metabolic change has occurred, which is why the rate rises before the muscles have used any oxygen.
The slower rise is sympathetic activation. Norepinephrine at β1 receptors acts through G_s → adenylyl cyclase → cyclic AMP → protein kinase A, a cascade with several diffusion and enzymatic steps, taking roughly five seconds to become significant and 20–30 seconds to plateau. Circulating epinephrine from the adrenal medulla is slower still, requiring secretion and circulation.
The functional division follows and is worth remembering as a rule: the vagus governs heart rates between about 50 and 100 — the intrinsic rate being the ceiling of pure vagal withdrawal — and the sympathetic system governs everything above 100.
14.10 Vessels have essentially no parasympathetic supply. Explain what this implies for blood pressure regulation, and why it means a spinal cord injury above T6 causes profound hypotension.
Model answer
It implies that vascular tone is set by a single dial. Arterioles receive continuous sympathetic α1 discharge at roughly one impulse per second, holding them at about half-maximal constriction, and regulation consists of turning that discharge up or down. There is no vasodilator nerve for the systemic circulation, so dilation is achieved by withdrawing tone, by local metabolic and endothelial signals, or by circulating epinephrine reaching β2 receptors — never by a parasympathetic command.
That single dial is driven from the rostral ventrolateral medulla, whose axons descend in the spinal cord to reach the preganglionic neurons of the lateral horn at T1–L2. A complete injury above T6 severs that descending pathway for most of the sympathetic outflow, including the entire splanchnic bed, which holds roughly a quarter of the blood volume. Vasomotor tone collapses, arterioles dilate, venous capacitance increases, venous return falls, and pressure drops — this is neurogenic shock, and it occurs with a completely normal heart and completely normal vessels.
Two details complete the picture. The heart rate is slow or normal, not fast, because the cardiac sympathetic outflow at T1–T4 is also lost while the vagus, travelling outside the cord, is intact — hypotension with bradycardia, unique among the shock states. And the same anatomy explains autonomic dysreflexia months later: the isolated cord below the lesion can still generate sympathetic discharge, but the medulla's correcting signal can no longer descend to stop it.
Level 3 · Clinical Application
14.11 A 62-year-old smoker presents with six weeks of aching right shoulder and medial arm pain, weakness of the small muscles of the right hand, and a right pupil 2 mm smaller than the left with a mildly droopy right eyelid. The right pupil reacts normally to light, and the anisocoria is greater in a darkened room. Localize the lesion and explain every finding.
Model answer
A Pancoast (superior sulcus) tumour at the right lung apex, invading the inferior trunk of the brachial plexus (C8–T1) and the sympathetic chain / stellate ganglion.
- Medial arm pain and small hand muscle weakness — the inferior trunk carries C8 and T1, which supply all the intrinsic hand muscles through the ulnar and median nerves, and the medial antebrachial cutaneous sensory territory. This is a Klumpke-pattern lower trunk lesion (§13.5).
- Miosis — loss of sympathetic α1 drive to the iris dilator. The parasympathetic sphincter is intact and unopposed, so the pupil is small but still constricts briskly to light. Anisocoria greater in the dark is the confirmatory sign: the normal pupil dilates and the denervated one cannot, so the difference widens.
- Partial ptosis — loss of α1 tone in the superior tarsal (Müller) muscle. It is only 1–2 mm because the main lid elevator, levator palpebrae superioris, is supplied by CN III and is unaffected.
- The triad is Horner syndrome, and its combination with a C8–T1 plexopathy localizes the lesion to the lung apex — the one place where the T1 root, the sympathetic chain, and lung tissue lie within a centimetre of each other. Ipsilateral facial anhidrosis would complete it.
Two points of reasoning. First, Horner syndrome by itself localizes nothing: the pathway is three neurons long and runs from hypothalamus to orbit, so the accompanying findings do the localizing. Second, the pupil reacting normally to light is what separates this from a CN III palsy, in which the pupil would be large and unreactive. Apraclonidine drops would reverse the anisocoria through denervation hypersensitivity, confirming the diagnosis at the bedside.
14.12 A 34-year-old is brought in after agricultural pesticide exposure. Pupils are pinpoint, he is drooling, vomiting, incontinent, wheezing with copious secretions, sweating profusely, and his heart rate is 44. Muscle fasciculations are visible in the chest and thighs. Explain the mechanism, separate the muscarinic from the nicotinic findings, and state the treatment with its rationale.
Model answer
Mechanism: organophosphate inhibition of acetylcholinesterase. Acetylcholine is no longer hydrolysed, so it accumulates at every cholinergic synapse simultaneously — autonomic ganglia of both divisions, all parasympathetic effector junctions, sympathetic cholinergic sweat glands, the neuromuscular junction, and the CNS.
Muscarinic findings (M2 and M3): pinpoint pupils (M3, iris sphincter), drooling (M3, salivary glands), vomiting and incontinence (M3, gut and detrusor), bronchorrhoea and wheeze (M3, bronchial glands and smooth muscle), profuse sweating (M3, sympathetic cholinergic), bradycardia at 44 (M2, SA node).
Nicotinic findings: the fasciculations, and the weakness that follows. Persistent depolarization of the end plate first causes spontaneous firing and then inactivates voltage-gated sodium channels, producing depolarizing paralysis — which, because the diaphragm is skeletal muscle, threatens respiratory arrest.
Treatment: (1) Atropine in large, repeated doses, titrated not to heart rate or pupil size but to drying of pulmonary secretions, because it is bronchorrhoea and bronchospasm that kill. Atropine is a competitive muscarinic antagonist and reverses every muscarinic finding. (2) Pralidoxime, which reactivates acetylcholinesterase by displacing the phosphate group — the only agent that treats the nicotinic features — given before the enzyme–inhibitor complex "ages" into an irreversible form. (3) Airway support and a benzodiazepine for seizures.
The critical teaching point: atropine does not touch nicotinic receptors. A patient can be fully atropinized, dry and pink, and still become paralysed and die of respiratory failure. Two receptor families require two drugs.
14.13 A 28-year-old with a complete spinal cord injury at T4, six months post-injury, develops a pounding headache, blotchy flushing and sweating of the face and neck, a blood pressure of 226/120, and a heart rate of 48. Below the nipple line the skin is pale and cool. Explain, name the condition, and give the first three management steps in order.
Model answer
Autonomic dysreflexia. A noxious stimulus below the lesion — most often a blocked catheter or distended bladder, next most often faecal impaction, then a pressure sore or ingrown toenail — is exciting afferents that enter the cord below T4. Because the descending inhibitory pathways from the medulla cannot cross the lesion, the isolated cord segments generate an unregulated mass sympathetic discharge; the splanchnic bed constricts and pressure rises catastrophically.
The baroreceptors and medulla work perfectly and attempt to correct. The sympathetic half of the correction cannot descend past T4, so vasoconstriction below the lesion continues unopposed — hence the pale, cool skin below the nipple line. The vagal half leaves the brainstem in CN X and never enters the cord, so it is delivered normally — hence the bradycardia of 48 in the face of severe hypertension, a combination unique to this condition, and one that cannot correct the pressure because the vagus does not innervate resistance vessels. Above the lesion, where descending control is intact, compensatory vasodilation and sweating produce the flushing, sweating, and pounding headache.
Management, in order: (1) Sit the patient upright and lower the legs — immediate, free, and it uses orthostatic pooling against the mechanism causing the problem. (2) Loosen all constrictive clothing and devices, including abdominal binders and leg-bag straps. (3) Find and remove the stimulus — check and unkink the catheter or catheterize the bladder first, using lidocaine gel, since bladder distension causes the large majority of episodes; then examine for faecal impaction, again with local anaesthetic lubricant, because the examination itself can worsen the discharge. Short-acting antihypertensives (topical nitrate, immediate-release nifedipine) are added only if the pressure remains dangerous after the stimulus is addressed, since removing the stimulus usually resolves the episode within minutes and a long-acting agent then causes profound hypotension. Note also that a "normal" reading of 120/80 may represent severe hypertension in a patient whose baseline is 90/60.
Level 4 · Integration and Synthesis
14.14 Amara is on metoprolol 50 mg daily. Two weeks later she develops a respiratory infection with wheeze, and separately reports one episode of shakiness and confusion at 3 a.m. that resolved after eating. Discuss mechanistically how her beta blocker relates to each event, what would change if she were on propranolol, and what you would monitor.
Model answer
The wheeze. Bronchial smooth muscle relaxation depends on β2 receptors reached chiefly by circulating epinephrine, since airways have little direct sympathetic innervation. Metoprolol is about thirtyfold selective for β1, so at 50 mg it largely spares that reserve, and the wheeze is most likely infective. On propranolol, which blocks β1 and β2 equally, the epinephrine-mediated bronchodilator reserve would be removed, leaving vagal M3 bronchoconstriction unopposed — a genuine risk of severe bronchospasm, and the reason non-selective agents are avoided in reactive airway disease. Two caveats: selectivity is a ratio, not a switch, and is substantially lost above about 200 mg daily; and if she needs a β2-agonist rescue inhaler, metoprolol will blunt its effect somewhat even at ordinary doses.
The 3 a.m. episode. Likely nocturnal hypoglycaemia, and beta blockade interacts with it three ways. (1) Warning symptoms are masked: tremor and palpitations are β2- and β1-mediated adrenergic responses and are blunted. Sweating is not blunted, because it is sympathetic cholinergic acting on M3 — which is why diaphoresis becomes the only remaining warning sign, and why her being found sweating matters. (2) Recovery is impaired: hepatic glycogenolysis and gluconeogenesis are driven partly by β2, so a non-selective agent slows the counter-regulatory glucose rise; metoprolol interferes less. (3) α2 receptors on pancreatic beta cells inhibit insulin release while β2 receptors stimulate it, so beta blockade shifts insulin dynamics slightly.
What to monitor: peak flow or symptoms during the respiratory illness; capillary glucose, particularly overnight; supine and standing blood pressure and heart rate, since her blunted baroreflex (SDNN 42 ms, 30:15 ratio 1.02) makes her prone to orthostatic drops; and her HRV, which should improve on beta blockade as vagal influence is unmasked. If she developed true asthma, a more β1-selective agent such as bisoprolol, or a different class, would be reasonable — noting that in ischaemic heart disease the survival benefit of beta blockade is large enough that non-use is rarely the right answer.
14.15 Compare, mechanistically, why a transplanted (denervated) heart, a heart under full beta blockade, and the heart of a trained endurance athlete each have an unusual resting rate — and predict how each responds to standing up quickly.
Model answer
Transplanted heart. All autonomic connections were cut. The SA node runs at its intrinsic rate of roughly 95–110, which is why transplant recipients characteristically have a resting rate near 100. There is no vagal restraint to withdraw and no direct sympathetic drive. On standing: no immediate rate response at all, because both fast mechanisms are absent. The rate rises only after 30–60 seconds, when circulating catecholamines from the adrenal medulla reach β1 receptors — a hormonal route that requires no innervation. These patients are prone to orthostatic symptoms and are taught to stand slowly and to warm up and cool down gradually. They also feel no anginal pain, since the afferents were cut too.
Full beta blockade. Vagal input is intact and unopposed; sympathetic β1 input is pharmacologically removed. The rate is low — 50s to low 60s — and HRV is often higher than before, because the vagal signal is unmasked. On standing: vagal withdrawal still works and produces a small, prompt rise, but the sympathetic component that normally completes the response is blocked, so the tachycardic response is blunted and the risk of orthostatic hypotension rises. This is Amara — with the added problem that her vagal limb is itself damaged, so both limbs are impaired for different reasons.
Trained endurance athlete. Both limbs are intact, but the balance is shifted and the mechanics have changed. A resting rate in the 40s results from markedly elevated vagal tone — demonstrable because atropine raises an athlete's rate far more than an untrained person's — and from structural remodelling: a larger, more compliant left ventricle with a greater end-diastolic volume delivers a larger stroke volume, so the same cardiac output needs fewer beats. Nia's resting rate of 44 with a VO₂max of 58.4 mL/kg/min is this picture. On standing: an excellent response — rapid vagal withdrawal, brisk sympathetic recruitment, and a large venous return reserve.
The synthesis: three hearts with abnormal resting rates for three entirely different reasons — absent modulation, blocked modulation, and shifted modulation — and the way to tell them apart is not the resting number but the response to a challenge. This is why clinical physiology consists so largely of provoking a system and watching what it does, and it is why a single vital sign, recorded once, is nearly uninterpretable.
Concept Map to Complete
Copy this onto blank paper and fill every bracket from memory before checking the chapter.
AUTONOMIC NERVOUS SYSTEM
( [ __ ] neurons in series )
│
┌──────────────────┴──────────────────┐
SYMPATHETIC PARASYMPATHETIC
"[ ______________ ]" "[ ______________ ]"
outflow: [ __ ] to [ __ ] outflow: CN [_,_,_,_]
lateral horn + S[_] - S[_]
│ │
PREgang fiber = [ short / long ] PREgang = [ short / long ]
ganglion = [ ____________ ] ganglion = [ ____________ ]
+ [ ____________ ] on/in the ORGAN
divergence 1 : [ ______ ] divergence 1 : [ ____ ]
⇒ response is [ ___________ ] ⇒ response is [ _________ ]
│ │
▼ ▼
POSTgang transmitter = [ ______ ] transmitter = [ ______ ]
(EXCEPT sweat glands = [ ______ ] )
│ │
┌──────────┴──────────┐ ┌───────────┴──────────┐
α1 → [ ______________ ] M2 → [ _______________ ]
α2 → [ ______________ ] M3 → [ _______________ ]
β1 → [ ______________ ] + [ ______ ] release + endothelium →
β2 → [ ______________ ] [ __________ ]
β3 → [ ______________ ]
G protein: α1 = [ __ ] · α2 = [ __ ] · β = [ __ ] · M2 = [ __ ] · M3 = [ __ ]
═══ THE HEART ═════════════════════════════════════════════════════════
intrinsic SA node rate = [ ____ ] /min resting rate = intrinsic
vagal tone subtracts about [ __ ] beats minus [ ___________ ]
M2 acts in [ ____________ ] because it gates [ __________ ] DIRECTLY
β1 acts in [ ____________ ] because it must build [ __________ ]
⇒ beat-to-beat HRV is therefore [ ______________ ]
RMSSD measures [ ______ ] · SDNN measures [ ______________ ]
═══ THE BARORECEPTOR REFLEX ═══════════════════════════════════════════
receptor: [ ______________ ] (CN [__]) · [ ____________ ] (CN [__])
they sense [ ______________ ], not pressure
centre: [ ______________________ ] of the medulla
efferent: vagus [ ↑/↓ ] sympathetic [ ↑/↓ ]
effectors: SA node · arterioles · [ ______ ] · [ __________ ] cells
time to correct: [ ______ ] heartbeats · resets over [ ______ ] days
Lab / Self-Exploration
- Measure your own respiratory sinus arrhythmia. Take your radial pulse and breathe slowly and deeply at about six breaths per minute — five seconds in, five seconds out. Feel the pulse quicken on inspiration and slow on expiration. Count the fastest and slowest 10-second rates and multiply by six to estimate the swing. Under age 50 it is usually well over 15 beats per minute; over 70 it is often undetectable. This is vagal tone, palpable at the wrist, and it is the same measurement as Amara's "3 beats per minute."
- Watch the baroreflex work. Lie down for five minutes, take your pulse for 30 seconds, then stand quickly and take it immediately and again at one and three minutes. The rate should rise 10–20 beats within seconds and settle. If you have a home blood pressure cuff, add supine and standing pressures and calculate the drop. Then repeat after standing still for two minutes first, and note how much smaller the response is when there is nothing to correct.
- Demonstrate the ciliospinal reflex and pupillary responses. In a dim room with a partner, shine a penlight into one eye: both pupils constrict — direct and consensual, CN II in and CN III out. Then have them look at a distant object and switch to a finger 15 cm away: the pupils constrict and the eyes converge. Finally, pinch the skin on the side of the neck: the pupil on that side dilates 1–2 mm — the ciliospinal reflex, a pure sympathetic response travelling from the cord at T1 up the chain to the superior cervical ganglion. Three cranial nerves and one sympathetic pathway in thirty seconds.
- Feel the sympathetic system change your skin. Hold one hand in cold tap water for 60 seconds while a partner watches both hands. The immersed hand blanches — α1-mediated cutaneous vasoconstriction — and, more interestingly, so does the other one, because the response is a reflex, not a local effect. Then look for piloerection on your forearm: same receptor, same nerve, different effector.
- Time a Valsalva at home, safely. Sitting, take a normal breath and bear down gently for 10 seconds (do not do this if you have known heart disease, retinal disease, or glaucoma) while a partner takes your pulse. The rate typically rises during the strain and falls below baseline for a few beats on release — the phase II sympathetic response and the phase IV vagal overshoot of §14.7, in miniature and without instruments.
- Read a drug label as a receptor map. Find any over-the-counter decongestant, antihistamine, or motion sickness tablet in a cupboard. Identify the receptor its active ingredient acts on, then predict its listed side effects before reading them — a decongestant (α1 agonist) should raise blood pressure and cause insomnia; an older antihistamine or hyoscine (muscarinic antagonist) should cause dry mouth, blurred near vision, constipation, and urinary hesitancy. Then check the leaflet. The match is close enough to be slightly unsettling, and it is the fastest way to prove to yourself that Figure 14.4 is not an abstraction.
Key Terms
acetylcholine (ACh) · Transmitter of all autonomic preganglionic fibers, all parasympathetic postganglionic fibers, sympathetic fibers to sweat glands, and the somatic neuromuscular junction; destroyed in the cleft by acetylcholinesterase within a millisecond.
acetylcholinesterase · Enzyme that hydrolyses acetylcholine in the synaptic cleft; irreversibly inhibited by organophosphates and reversibly by neostigmine.
adrenal medulla · A modified sympathetic ganglion of neural crest origin whose chromaffin cells receive preganglionic fibers directly and secrete about 80 percent epinephrine and 20 percent norepinephrine into the blood.
adrenergic · Describing a fiber that releases norepinephrine, or a receptor that binds norepinephrine and epinephrine (α1, α2, β1, β2, β3).
alpha-1 receptor · G_q-coupled adrenergic receptor; constricts vascular smooth muscle, dilates the pupil, contracts the bladder internal sphincter and superior tarsal muscle. The principal vasoconstrictor receptor.
alpha-2 receptor · G_i-coupled adrenergic receptor sited mainly presynaptically and in the brainstem; inhibits norepinephrine release and central sympathetic outflow. Target of clonidine.
atropine · Competitive antagonist at all muscarinic receptors; raises heart rate, dilates the pupil, dries secretions, and abolishes sweating. Has no action at nicotinic receptors.
autonomic dysreflexia · Uncontrolled sympathetic discharge below a spinal cord lesion at or above T6, producing severe hypertension with reflex bradycardia, flushing and sweating above the lesion, and pale cold skin below it.
autonomic nervous system (ANS) · The two-neuron visceral motor division controlling cardiac muscle, smooth muscle, glands, and adipose tissue; comprises sympathetic and parasympathetic divisions.
autonomic tone · Continuous baseline firing in both divisions, allowing regulation by modulating an existing signal up or down rather than switching a system on.
baroreceptor reflex · Negative feedback loop from carotid sinus and aortic arch stretch receptors through the nucleus of the solitary tract to vagal and sympathetic efferents; corrects blood pressure within one to two heartbeats and resets over one to two days.
beta-1 receptor · G_s-coupled adrenergic receptor on cardiac SA node, AV node and myocardium and on renal juxtaglomerular cells; raises rate, conduction, contractility, and renin release.
beta-2 receptor · G_s-coupled adrenergic receptor on bronchial smooth muscle, skeletal muscle and coronary arterioles, liver, and uterus; reached chiefly by circulating epinephrine rather than by nerves.
beta-3 receptor · G_s-coupled adrenergic receptor driving lipolysis in adipose tissue and relaxing the bladder detrusor.
chromaffin cell · Neural-crest-derived cell of the adrenal medulla; a postganglionic sympathetic neuron without an axon, secreting catecholamines into the blood.
cholinergic · Describing a fiber that releases acetylcholine, or a receptor that binds it (nicotinic or muscarinic).
denervation hypersensitivity · Up-regulation of receptors on an effector that has lost its nerve supply, causing exaggerated responses to circulating agonists; the basis of the apraclonidine test in Horner syndrome.
dual innervation · Supply of an organ by both autonomic divisions, holding it at an intermediate operating point from which it can be moved rapidly in either direction.
gray ramus communicans · Unmyelinated postganglionic sympathetic axons rejoining a spinal nerve; present at all 31 spinal levels.
heart rate variability (HRV) · Beat-to-beat variation in the R-R interval within normal sinus rhythm; SDNN measures total variability, RMSSD isolates vagal activity. Low HRV indicates reduced regulatory reserve and predicts mortality.
Horner syndrome · Partial ptosis, miosis, and anhidrosis on one side from interruption of the three-neuron sympathetic pathway to the head.
intramural / terminal ganglion · Parasympathetic ganglion lying in or on the wall of the target organ, giving very short postganglionic fibers and organ-specific effects.
length-dependent autonomic neuropathy · Distal loss of unmyelinated autonomic C fibers, affecting the longest nerve — the vagus — first; the mechanism of reduced HRV in diabetes.
muscarinic receptor · G-protein-coupled acetylcholine receptor, M1–M5; M2 slows the heart through a directly gated potassium channel, M3 contracts smooth muscle and stimulates glands.
nicotinic receptor · Ligand-gated cation channel binding acetylcholine; always excitatory; N_N at all autonomic ganglia and the adrenal medulla, N_M at the neuromuscular junction.
norepinephrine (NE) · Transmitter of most sympathetic postganglionic fibers; terminated chiefly by reuptake through the norepinephrine transporter rather than by enzymatic destruction, so its effects last seconds.
orthostatic hypotension · Fall of at least 20 mm Hg systolic or 10 mm Hg diastolic within three minutes of standing; neurogenic when unaccompanied by an appropriate rise in heart rate.
parasympathetic division · Craniosacral outflow (CN III, VII, IX, X and S2–S4) with long preganglionic and short postganglionic fibers ending in terminal ganglia; discrete, organ-specific effects.
prevertebral (collateral) ganglion · Sympathetic ganglion on the anterior aorta — celiac, superior mesenteric, aorticorenal, inferior mesenteric — receiving splanchnic nerves and supplying abdominal and pelvic viscera.
Raynaud phenomenon · Episodic digital vasospasm on cold or stress, mediated by cold-induced translocation of alpha-2C receptors to the smooth muscle membrane; triphasic white–blue–red colour change.
RMSSD · Root mean square of successive differences between adjacent R-R intervals; a near-pure index of cardiac vagal activity because only the vagus acts fast enough to change one interval relative to the next.
SDNN · Standard deviation of all normal-to-normal R-R intervals over 24 hours; total heart rate variability from respiratory, baroreflex, thermoregulatory, hormonal, and circadian sources.
splanchnic nerves · Preganglionic sympathetic fibers passing through the chain without synapsing to reach prevertebral ganglia; greater (T5–T9), lesser (T10–T11), least (T12), and lumbar (L1–L2).
sympathetic chain (trunk) · Paired string of about 22–23 paravertebral ganglia running the length of the vertebral column, allowing a T1–L2 outflow to reach the whole body.
sympathetic division · Thoracolumbar outflow (T1–L2) with short preganglionic and long postganglionic fibers and 1:10–20 divergence; diffuse, body-wide effects.
vagal tone · Continuous parasympathetic discharge holding the SA node about 30 beats below its intrinsic rate of roughly 100 per minute.
vagus nerve (CN X) · Carries about 75 percent of all parasympathetic fibers, is about 80 percent afferent, supplies thoracic and abdominal viscera to the splenic flexure, and is the longest autonomic nerve in the body.
varicosity · A swelling along an autonomic postganglionic axon that releases transmitter diffusely into interstitial space rather than into a specialized cleft; the basis of volume transmission.
white ramus communicans · Myelinated preganglionic sympathetic axons entering the chain; present only at spinal levels T1–L2.
Next: Chapter 15 · The Special Senses — the receptors that grew into whole organs, two of which Amara's mother is losing, and the retina in which Amara's own microvascular disease will first become visible.