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Part III · Regulation and Integration  ·  Estimated reading time 120 minutes  ·  Prerequisites: Chapters 11, 12

In This Chapter

13. The Peripheral Nervous System

Sensory and Somatic Motor Divisions

Part III · Regulation and Integration  ·  Estimated reading time 120 minutes  ·  Prerequisites: Chapters 11, 12

Case File 13 — "It Doesn't Hurt, So I Didn't Mention It"

Amara Osei, 45, comes to her six-month cardiology follow-up. She is walking three times a week, her weight is down four pounds, and she reports no chest pain. At the end of the visit, almost as an afterthought, the nurse practitioner asks her to take off her shoes.

Amara says her feet have felt "a bit far away" for months — numb across the toes, sometimes prickling at night, as though she were wearing socks she cannot take off. She had not mentioned it to anyone. Her explanation is the one clinicians hear constantly and should never accept: "It doesn't hurt."

Examination Amara's finding Expected at 45
Distribution of numbness Both feet, symmetric, toes to mid-calf — a stocking —
Vibration, 128 Hz fork, great toe Absent bilaterally Clearly felt, > 10 s
Vibration, 128 Hz fork, medial malleolus 5 s right, 4 s left > 10 s
10 g monofilament, 10 plantar sites Not felt at 4 of 10 sites, both feet 10 of 10
Pinprick and cool metal, dorsum of foot Reduced bilaterally Normal
Joint position sense, great toe and ankle Intact — 10 of 10 movements correct Intact
Ankle jerk (Achilles reflex) Absent bilaterally Present
Knee jerk (patellar reflex) Present and symmetric Present
Light touch, dorsum of both hands Normal Normal
Strength, all four limbs 5 / 5 throughout 5 / 5
Foot inspection Dry, fissured heels; a 3 mm callus under the right first metatarsal head —
HbA1c 7.1% (7.4% on admission) < 5.7%

Nothing here is dramatic. There is no weakness, no pain, no visible wound. And yet this examination has found the second-most dangerous thing in Amara's chart, because a foot that cannot feel a stone in a shoe is a foot that will eventually be injured without noticing.

Three things about this pattern should stop you.

Three questions to hold on to.

  1. The numbness begins at the toes and stops at mid-calf, symmetrically, in a stocking shape. Why do the longest axons in the body fail first — and why does that produce a stocking rather than the banded, one-sided pattern of a dermatome?
  2. Vibration sense at the great toe is gone and both ankle jerks are absent, yet joint position sense at the same ankle is perfectly preserved. All three of those tests are usually described as "large fiber" functions. What does that dissociation tell you about which axons are dying and in what order?
  3. In Chapter 1, Amara's cardiac pain was felt in her chest wall, left arm, and jaw — a strictly dermatomal phenomenon produced by a diseased organ that has no sensory nerves of its own. Her foot numbness is not dermatomal at all. Explain the difference in the underlying anatomy, precisely enough that you could predict the shape of each from first principles.

Learning Objectives

By the end of this chapter you should be able to:

  1. Diagram the organization of the peripheral nervous system into sensory and motor divisions, and the motor division into somatic and autonomic branches, and state the single anatomical feature that separates the two motor branches.
  2. Describe the connective tissue architecture of a peripheral nerve — endoneurium, perineurium, epineurium — and distinguish a sensory ganglion from an autonomic ganglion by structure and by function.
  3. Classify sensory receptors three ways — by stimulus type, by location, and by structural complexity — and place any named receptor correctly in all three schemes.
  4. Match each cutaneous and proprioceptive receptor (Merkel disc, Meissner corpuscle, Pacinian corpuscle, Ruffini ending, hair follicle receptor, free nerve ending, muscle spindle, Golgi tendon organ) to the stimulus it detects, the fiber class that carries it, and the adaptation rate it shows.
  5. Explain receptive field size and predict two-point discrimination thresholds across the body from it, using the idea of innervation density.
  6. Distinguish phasic (rapidly adapting) from tonic (slowly adapting) receptors and state a clinical consequence of each.
  7. Describe the three levels of neural integration in sensation — receptor, circuit, and perceptual — and explain why the crossing point of a spinal tract determines the pattern of loss after a cord lesion.
  8. Define a dermatome, read a dermatome map, use it to localize a lesion, and explain referred pain by segmental convergence.
  9. Name all twelve cranial nerves by number and name, classify each as sensory, motor, or mixed, state its function, and describe how it is tested at the bedside.
  10. Trace the formation of a spinal nerve from dorsal and ventral roots through the dorsal and ventral rami, and name the four plexuses, their major terminal nerves, and the deficit produced by injury to each.
  11. Contrast the neuromuscular junction with an autonomic varicosity, and diagram a reflex arc, distinguishing monosynaptic, polysynaptic, ipsilateral, contralateral, and crossed reflexes.
  12. Rank the peripheral nerve fiber classes (Aα, Aβ, Aδ, B, C) by diameter, myelination, and conduction velocity, and state which sensory modality each carries.
  13. Explain length-dependent axonopathy and predict the distribution of sensory loss it produces, distinguishing it from radiculopathy, plexopathy, and mononeuropathy.
  14. Perform and interpret the bedside sensory examination — vibration, monofilament, pinprick, temperature, joint position, and reflexes — stating which fiber population each tool tests and why the tools disagree in early neuropathy.

13.1 The Organization of the Peripheral Nervous System

The central nervous system is a magnificent organ that cannot, on its own, detect anything or do anything. Sealed inside bone and floating in cerebrospinal fluid (Chapter 12), the brain has no direct contact with the world. Everything it knows arrives on a peripheral nerve. Everything it does leaves on one.

The peripheral nervous system (PNS) is that traffic: all neural tissue outside the brain and spinal cord. Anatomically it is a modest inventory — 12 pairs of cranial nerves and 31 pairs of spinal nerves, plus the ganglia, plexuses, receptors, and motor endings attached to them. Functionally it is enormous, because those 43 pairs of cables carry, in both directions, every sensation you will ever have and every movement you will ever make.

The three-part framework

Every function of the nervous system, from a knee jerk to a doctoral thesis, decomposes into three steps. Learn this framework now; it organizes the rest of the chapter and much of the rest of the book.

  1. Sensory (afferent) input. Receptors transduce a physical or chemical stimulus into a change in membrane potential, which becomes a train of action potentials travelling toward the CNS. Afferent comes from Latin ad- (toward) + ferre (to carry): carried toward the centre.
  2. Integration. The CNS compares that input against its internal state, its memory, and its priorities, and decides. Integration can take 20 milliseconds in a spinal reflex arc or twenty years in a career decision. The machinery is the same: excitatory and inhibitory postsynaptic potentials summing at a trigger zone (Chapter 11).
  3. Motor (efferent) output. Commands travel outward to effectors — the only three things the nervous system can command are skeletal muscle, cardiac and smooth muscle, and glands. Efferent: ex- (out) + ferre. Carried away.

The PNS supplies steps 1 and 3. Chapter 12 covered step 2.

The divisions, and why the split is where it is

The sensory division is subdivided by where the information comes from:

  • Somatic sensory fibers carry information from the skin, skeletal muscles, and joints — touch, pressure, vibration, temperature, pain, and body position.
  • Visceral sensory fibers carry information from the organs and blood vessels — stretch of the bladder wall, distension of the gut, blood pressure at the carotid sinus, the oxygen and carbon dioxide content of arterial blood. Most of this never reaches consciousness, which is a mercy.
  • Special sensory fibers carry vision, hearing, equilibrium, taste, and smell. These get their own chapter (Chapter 15) because their receptors are elaborate organs rather than nerve endings.

The motor division is subdivided by what it commands:

  • The somatic motor division (the "voluntary" nervous system) commands skeletal muscle.
  • The autonomic motor division (the "involuntary" or visceral motor system) commands cardiac muscle, smooth muscle, and glands, and itself splits into sympathetic and parasympathetic divisions.

The words "voluntary" and "involuntary" are traditional and half-wrong, and you should be suspicious of them from the start. You cannot voluntarily change your glomerular filtration rate, true. But you also cannot voluntarily stop your diaphragm from contracting for more than a couple of minutes, and skilled practitioners can deliberately alter heart rate and blood pressure through breathing. The honest distinction is not about willpower. It is about anatomy: somatic motor pathways use one neuron from CNS to effector; autonomic pathways use two, with a synapse in a peripheral ganglion. That structural difference is what actually separates the two systems, and it explains almost everything else about them. This chapter follows the sensory division and the somatic motor division to their ends; Chapter 14 takes the autonomic branch and follows it to its own, very different, destinations.

                THE NERVOUS SYSTEM — WHO DOES WHAT, AND WHERE

   ╔══════════════════ CENTRAL NERVOUS SYSTEM  [Chapter 12] ══════════════════╗
   ║          BRAIN  +  SPINAL CORD      ·      INTEGRATION · DECISION        ║
   ╚═══════▲══════════════════════════════════════════════════╤═══════════════╝
           │  AFFERENT — carried TOWARD                       │  EFFERENT
           │                                                  ▼  carried AWAY
   ┌───────┴─────────────────────┐        ┌──────────────────────────────────┐
   │  SENSORY (AFFERENT) DIVISION│        │   MOTOR (EFFERENT) DIVISION      │
   ├─────────────────────────────┤        ├───────────────┬──────────────────┤
   │ SOMATIC SENSORY             │        │   SOMATIC     │   AUTONOMIC      │
   │   skin · muscle · joints    │        │   MOTOR       │   MOTOR (ANS)    │
   │   touch, pain, temp,        │        ├───────────────┼──────────────────┤
   │   vibration, position       │        │ ONE neuron    │ TWO neurons in   │
   │                             │        │ cell body in  │ series; synapse  │
   │ VISCERAL SENSORY            │        │ CNS, axon all │ in a peripheral  │
   │   organs · vessels · gut    │        │ the way out   │ GANGLION         │
   │   stretch, BP, blood gases  │        │               │                  │
   │   (mostly unconscious)      │        │ thick, heavily│ thin pre (B),    │
   │                             │        │ myelinated    │ UNmyelinated     │
   │ SPECIAL SENSORY [Ch 15]     │        │ (A-alpha)     │ post (C)         │
   │   sight · hearing · balance │        │ FAST 80-120   │ SLOW 3-15 and    │
   │   taste · smell             │        │ m/s           │ 0.5-2 m/s        │
   │                             │        │               │                  │
   │ Cell bodies live OUTSIDE    │        │ ACh onto      │ ACh or NE onto   │
   │ the CNS, in DORSAL ROOT     │        │ NICOTINIC     │ nicotinic, musc- │
   │ GANGLIA and cranial ganglia │        │ receptors     │ arinic, adrenerg.│
   │ — pseudounipolar neurons    │        │               │                  │
   │                             │        │ SKELETAL      │ CARDIAC + SMOOTH │
   │                             │        │ MUSCLE only   │ MUSCLE, GLANDS,  │
   │                             │        │               │ adipose          │
   │                             │        │ ALWAYS        │ EXCITES or       │
   │                             │        │ EXCITATORY    │ INHIBITS         │
   └─────────────────────────────┘        └───────────────┴────────┬─────────┘
                                                                   │
                                                    ┌──────────────┴───────────┐
                                                    ▼                          ▼
                                            SYMPATHETIC                PARASYMPATHETIC
                                            "thoracolumbar"            "craniosacral"
                                            T1 – L2                    CN III, VII, IX, X
                                            mobilize · expend          + S2 – S4
                                                                       conserve · restore

   HARDWARE COUNT:  12 pairs cranial nerves  +  31 pairs spinal nerves
                    ( 8 cervical · 12 thoracic · 5 lumbar · 5 sacral · 1 coccygeal )

Figure 13.1 — The functional organization of the peripheral nervous system.

Described: A branching diagram. At the top sits the central nervous system — brain and spinal cord — labelled as the site of integration and decision. An upward arrow on the left represents afferent traffic carried toward the centre; a downward arrow on the right represents efferent traffic carried away. The afferent side is the sensory division, which has three parts: somatic sensory, carrying touch, pain, temperature, vibration and position from skin, muscle and joints; visceral sensory, carrying stretch, blood pressure and blood gas information from organs and vessels, mostly unconsciously; and special sensory, carrying sight, hearing, balance, taste and smell. Sensory cell bodies lie outside the central nervous system in dorsal root ganglia and cranial ganglia and are pseudounipolar. The efferent side is the motor division, split into somatic motor and autonomic motor. Somatic motor uses one neuron whose cell body sits in the central nervous system and whose thick, heavily myelinated A-alpha axon runs all the way to the target at 80 to 120 metres per second, releasing acetylcholine onto nicotinic receptors, exciting skeletal muscle only, and never inhibiting. Autonomic motor uses two neurons in series with a synapse in a peripheral ganglion; the preganglionic axon is thin and lightly myelinated, conducting at 3 to 15 metres per second, and the postganglionic axon is unmyelinated, conducting at 0.5 to 2 metres per second. It releases acetylcholine or norepinephrine onto nicotinic, muscarinic or adrenergic receptors, acts on cardiac and smooth muscle, glands and adipose tissue, and can either excite or inhibit. The autonomic division branches into the sympathetic or thoracolumbar system arising from T1 to L2, which mobilizes and expends, and the parasympathetic or craniosacral system arising from cranial nerves three, seven, nine and ten plus sacral segments two to four, which conserves and restores. A footer gives the hardware count: twelve pairs of cranial nerves and thirty-one pairs of spinal nerves, comprising eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal pair.

Nerves, ganglia, and the vocabulary of the periphery

A nerve is a cable: a bundle of axons wrapped in connective tissue. Its architecture is worth knowing because it explains both how nerves survive being stretched and how they are repaired. Each axon and its Schwann cells are sheathed in endoneurium, a delicate areolar wrapping. Axons are gathered into fascicles bound by perineurium, a tough, cellular, diffusion-limiting sheath that creates a blood–nerve barrier analogous to the blood–brain barrier. Fascicles are bundled by epineurium, dense irregular connective tissue carrying the blood supply — the vasa nervorum. This is precisely the same three-layer scheme you met in skeletal muscle (endomysium, perimysium, epimysium; Chapter 9), which is not a coincidence: both are long, thin, tension-bearing structures that must slide against their neighbours.

A ganglion (plural ganglia) is a cluster of neuron cell bodies outside the CNS. Two functional kinds exist, and confusing them is a common exam casualty:

  • Sensory ganglia — the dorsal root ganglia of spinal nerves and the equivalent cranial ganglia. These contain pseudounipolar sensory cell bodies and no synapses. Signals pass through without being processed.
  • Autonomic (motor) ganglia — sympathetic chain, prevertebral, and terminal ganglia. These contain multipolar postganglionic cell bodies and are synaptic relays.

Nerves are classified by traffic. Mixed nerves carry both sensory and motor fibers; all 31 spinal nerves are mixed. Sensory (afferent) nerves carry only afferents — cranial nerves I, II, and VIII. Motor (efferent) nerves carry only efferents, at least in the classical scheme — cranial nerves III, IV, VI, XI, and XII, though every one of them in fact carries proprioceptive afferents from the muscles it supplies, which is why the "purely motor" label is a teaching simplification rather than a fact.

Check Your Understanding 13.1

  1. A patient has a lesion that destroys one dorsal root ganglion. Predict the deficit precisely: what is lost, what is preserved, and why?
  2. Why does the two-neuron design of autonomic pathways make selective pharmacology possible in a way that the one-neuron somatic design does not?
Show answers
  1. The dorsal root ganglion contains the cell bodies of the sensory neurons for that spinal segment. Destroying it destroys all sensation — touch, pain, temperature, vibration, and proprioception — from that one dermatome and from the corresponding deep structures, on that side only. Motor function is entirely preserved, because motor cell bodies live in the ventral horn of the spinal cord, not in the ganglion, and their axons leave by the ventral root. The patient will have a strip of numb skin with normal strength. In practice the deficit is often subtler than expected because adjacent dermatomes overlap by roughly 50 percent (§13.3).
  2. Because the two-neuron design introduces an extra synapse, and each synapse uses a different receptor. Every somatic motor junction in the body uses acetylcholine on a nicotinic receptor, so any drug that blocks it paralyses everything at once. The autonomic system uses nicotinic receptors at the ganglion but muscarinic or adrenergic receptors at the target — and those target receptors come in subtypes distributed unevenly across organs. That unevenness is the entire foundation of selective drug design, and it is why a drug can be built that slows the heart without slowing the gut or paralysing the diaphragm — the argument Chapter 14 develops in full.

13.2 Sensory Receptors

A sensory receptor is a structure specialized to convert one kind of energy into the only language the nervous system speaks: a change in membrane voltage. The conversion step is transduction, and the resulting local, graded depolarization is a receptor potential (or, when it occurs in the sensory neuron's own membrane, a generator potential). If the receptor potential reaching the trigger zone exceeds threshold, action potentials fire, and their frequency encodes the stimulus intensity (Chapter 11). Nothing else about the action potential changes — a light touch and a crushing pressure produce identical spikes, differing only in how many per second and in which axons carry them.

This is the law of specific nerve energies, stated by Johannes Müller in 1826 and still one of the most important ideas in sensory physiology: what you perceive depends on which fiber fires, not on what stimulated it. Press on your closed eyelid and you see a glowing spot — because mechanical energy has activated a visual pathway, and that pathway's destination in the cortex means "light" regardless of what excited it. Every sensation you have is a construction based on the address of the arriving axon.

Receptors are classified three independent ways. A given receptor gets a label from each scheme, and you should be able to supply all three for any receptor named.

Classification 1 — by the type of stimulus detected

Receptor class Detects Examples
Mechanoreceptors Physical deformation — touch, pressure, vibration, stretch Merkel discs, Meissner and Pacinian corpuscles, Ruffini endings, muscle spindles, Golgi tendon organs, baroreceptors, cochlear hair cells
Thermoreceptors Temperature change Free nerve endings with TRPV1 (heat, > 43 °C) and TRPM8 (cool, < 28 °C) channels
Photoreceptors Light Rods and cones of the retina (Chapter 15)
Chemoreceptors Chemicals in solution Taste and smell receptors; carotid and aortic bodies (O₂, CO₂, pH); osmoreceptors; glucose sensors
Nociceptors Tissue damage, actual or threatened Free nerve endings responding to extreme mechanical force, extreme temperature, and the chemical signature of injury

Nociceptors deserve special comment, because they are the odd one out. They are defined not by the energy they detect but by its magnitude — they are the receptors that only respond when a stimulus is strong enough to threaten tissue. A single nociceptor may be polymodal, firing to a pinch, to 48 °C, and to the potassium, ATP, bradykinin, and hydrogen ions released from damaged cells. Its message is not "there is heat" but "there is harm."

Classification 2 — by location

Class Location Reports on
Exteroceptors At or near the body surface The external world: touch, pressure, pain, temperature; plus all special senses
Interoceptors (visceroceptors) Inside organs and vessels The internal environment: organ stretch, chemical composition, blood pressure, temperature of the blood
Proprioceptors In skeletal muscles, tendons, joint capsules, ligaments The position and movement of the body itself

Proprioception is the sense nobody names when asked to list the senses and nobody can live without. Close your eyes and touch your nose. The information that let you do that came from muscle spindles reporting the length of every muscle in your arm, Golgi tendon organs reporting the tension in every tendon, and joint receptors reporting angles. Patients who lose large-fiber proprioception — through severe peripheral neuropathy or a rare sensory neuronopathy — can still move, but must watch their limbs to control them, and become unable to stand in the dark. Movement without proprioception is possible; movement without proprioception in the dark is not.

Classification 3 — by structural complexity

Here the division is between free (unencapsulated) nerve endings, which are simply bare axon terminals, and encapsulated nerve endings, in which the terminal is wrapped in a purpose-built connective tissue capsule. The capsule is not padding. It is a mechanical filter, and it is the single best example in the body of the principle that structure determines function.

Thread 1 · Structure Determines Function

A Pacinian corpuscle is a bare axon terminal surrounded by 20 to 60 concentric lamellae of flattened fibroblasts separated by fluid — an onion about 1 mm long, visible to the naked eye. Take the capsule off and the axon terminal responds to sustained pressure. Put it back on and the same terminal responds only to changes in pressure, and specifically to vibration around 250 Hz.

Nothing about the neuron changed. The onion is a viscoelastic high-pass filter: when you press steadily, the fluid between the lamellae flows and redistributes within milliseconds, so the lamellae re-equalize and the core stops being deformed. Only a rapid change outruns the fluid redistribution and reaches the axon. The capsule performs, mechanically, what an electrical engineer would build with a capacitor.

This is the whole argument of the book in a single receptor. If you know what a structure looks like, you can predict what it must do — and if you know what it must do, you can often predict what it looks like before you are shown.

The cutaneous receptors, one at a time

          THE SENSORY RECEPTORS OF SKIN — a vertical slice, ~2 mm deep

  ═════ skin surface ═══════════════════════════════════════════════════════
   │ E                    ╭─ hair shaft                                     │
   │ P    ○ ○ ○ ○ ○       │                                                 │
   │ I   ─────────────────┼──── FREE NERVE ENDINGS ───────────────────      │
   │ D    stratum basale  │     • pain (nociceptors)  • temperature         │
   │ E                    │     • crude touch · itch                        │
   │ R   ▪▪ MERKEL DISCS ─┼──── at the epidermal-dermal junction            │
   │ M   (tactile discs)  │     ► steady pressure, edges, texture, points   │
   │ I                    │     ► TONIC (slowly adapting, type I)           │
   │ S                    │     ► SMALL receptive field ~ 2-5 mm            │
  ═╪═════════════════════╪═══════════════════════════════════════════════════
   │ D  ╭──╮ MEISSNER     │  ╭─────╮ HAIR FOLLICLE RECEPTOR                 │
   │ E  │≡≡│ CORPUSCLE    │  │ ▓▓▓ │ ► spiral of free endings around the    │
   │ R  ╰──╯ in dermal    │  │ ▓▓▓ │   follicle; detects hair BENDING       │
   │ M       papillae     │  ╰──┬──╯ ► PHASIC — you stop feeling your       │
   │ I  ► light stroking, │     │      clothes within seconds               │
   │ S    flutter 10-50Hz │     │                                           │
   │      slip of objects │  ┌──┴────────────────────────────────────┐      │
   │ ► PHASIC (RA type I) │  │  RUFFINI ENDING (bulbous corpuscle)   │      │
   │ ► field ~3-5 mm      │  │  ► spindle of collagen continuous     │      │
   │ ► DENSE in fingertip │  │    with dermal fibers                 │      │
   │                      │  │  ► SKIN STRETCH + sustained pressure  │      │
   │                      │  │  ► TONIC (SA type II) · field 10-50mm │      │
   │                      │  └───────────────────────────────────────┘      │
  ═╪═════════════════════════════════════════════════════════════════════════
   │ H        ╭───────────────╮  PACINIAN (LAMELLATED) CORPUSCLE            │
   │ Y       ╭┤ (((( • )))) ├╮  ► 20-60 fluid-separated lamellae, ~1 mm     │
   │ P       ╰┤ (((( • )))) ├╯  ► DEEP pressure + VIBRATION, peak 250 Hz    │
   │ O        ╰───────────────╯  ► VERY PHASIC (RA type II): on/off only    │
   │ D                           ► HUGE receptive field, whole finger pad   │
   │ E                           ► also in periosteum, joint capsules,      │
   │ R                             mesentery, pancreas, bladder wall        │
   │ M                                                                       │
  ═════════════════════════════════════════════════════════════════════════

   DEEPER STILL — in muscle and tendon (not skin):
   ┌──────────────────────────────────────┬────────────────────────────────┐
   │ MUSCLE SPINDLE                       │ GOLGI TENDON ORGAN             │
   │ intrafusal fibers, in PARALLEL with  │ in SERIES with muscle, at the  │
   │ the muscle belly                     │ musculotendinous junction      │
   │ ► reports muscle LENGTH and RATE     │ ► reports muscle TENSION       │
   │   of change of length                │ ► fires when muscle CONTRACTS  │
   │ ► fires when muscle is STRETCHED     │ ► drives the tendon reflex     │
   │ ► drives the stretch reflex          │   (protective relaxation)      │
   └──────────────────────────────────────┴────────────────────────────────┘

Figure 13.2 — Cutaneous and deep sensory receptors, shown by depth, with the stimulus each detects and its adaptation rate.

Described: A vertical section through skin about two millimetres deep, with receptors drawn at their true depths. In the epidermis, free nerve endings branch upward between keratinocytes and detect pain, temperature, crude touch and itch; Merkel discs, also called tactile discs, sit at the epidermal–dermal junction, respond tonically and slowly to steady pressure, edges, texture and fine points, and have small receptive fields of two to five millimetres. In the dermis, Meissner corpuscles occupy the dermal papillae just beneath the epidermis, respond phasically to light stroking and flutter between ten and fifty hertz and to objects slipping from the grip, have receptive fields of three to five millimetres, and are densest in the fingertips. Alongside them, hair follicle receptors wrap spirals of free endings around each follicle and detect hair bending, adapting rapidly, which is why you stop feeling your clothes within seconds of dressing. Deeper in the dermis, Ruffini endings are spindle-shaped structures whose internal collagen is continuous with the surrounding dermal fibers; they respond tonically to skin stretch and sustained pressure over large receptive fields of ten to fifty millimetres. In the hypodermis lie Pacinian, or lamellated, corpuscles: bare terminals surrounded by twenty to sixty fluid-separated concentric lamellae, about a millimetre long, responding only to changes in deep pressure and to vibration peaking near two hundred fifty hertz, with very large receptive fields covering a whole fingerpad; they also occur in periosteum, joint capsules, mesentery, pancreas and bladder wall. A separate lower panel shows two deep receptors that are not in skin: the muscle spindle, whose intrafusal fibers lie in parallel with the muscle belly and report muscle length and the rate of change of length, firing when the muscle is stretched and driving the stretch reflex; and the Golgi tendon organ, which lies in series with the muscle at the musculotendinous junction, reports muscle tension, fires when the muscle contracts, and drives the protective tendon reflex.

Free nerve endings are the majority of cutaneous receptors and the least glamorous. Bare, branching terminals of small-diameter A-delta (thinly myelinated, 5–30 m/s) and C (unmyelinated, 0.5–2 m/s) fibers, they weave between epidermal keratinocytes and end without ceremony. They mediate pain, temperature, itch, and crude touch. Their humility is deceptive: they are the receptors that keep you alive, and the two conduction velocities produce the familiar double pain of a stubbed toe — a sharp, well-localized first pain on A-delta fibers, followed roughly a second later by a dull, poorly localized, sickening second pain on C fibers. Time the gap yourself sometime. It is a direct measurement of axonal conduction velocity in your own body.

Two specialized derivatives of free endings deserve names. Tactile (Merkel) discs are free endings that flatten against a Merkel cell, a modified epidermal cell in the stratum basale that is itself mechanosensitive and releases transmitter onto the terminal. They are slowly adapting, exquisitely sensitive to edges and points, and are the receptors you are using when you read Braille or judge the texture of fabric. Hair follicle receptors wrap a spiral of free endings around a hair follicle; bending the hair deforms the endings. They are rapidly adapting, which is why a spider walking on your forearm is unmistakable and the sleeve covering it is invisible.

Encapsulated endings are all mechanoreceptors, and the capsule in each case tunes the response:

  • Meissner corpuscles (tactile corpuscles) — spiralling flattened Schwann cells stacked inside an oval capsule, sitting in the dermal papillae directly beneath the epidermis, only in hairless (glabrous) skin: fingertips, palms, soles, lips, nipples, eyelids. They are rapidly adapting and tuned to flutter, 10–50 Hz. Their real job is grip control: when an object begins to slip from your fingers, the micro-slip generates exactly the frequencies Meissner corpuscles detect, and a reflex tightens your grip within about 70 ms — before you notice.
  • Pacinian (lamellated) corpuscles — the onions described above, deep in the dermis and hypodermis, and also scattered through periosteum, joint capsules, mesentery, pancreas, and the bladder wall. They are the fastest-adapting receptors in the body, firing at the onset and offset of a stimulus and nothing in between, tuned to about 250 Hz. They are why you can feel the texture of a road through a car's steering wheel — vibration transmitted through a rigid object is read as the surface at the far end of it, which is also how a surgeon feels tissue through an instrument and a blind person reads the pavement through a cane.
  • Ruffini endings (bulbous corpuscles) — elongated capsules in the deep dermis and joint capsules whose internal collagen fibers are continuous with the surrounding dermal collagen. Stretch the skin and the collagen pulls on the endings. Slowly adapting, large receptive fields, and important for sensing the direction of skin stretch and therefore for finger position and grip force.

Histology · Why an Encapsulated Receptor Looks the Way It Does

Under the microscope, an H&E section of thick skin from a fingertip gives you two of these receptors reliably.

A Meissner corpuscle appears in a dermal papilla as an ovoid, 30–140 µm tall, oriented with its long axis perpendicular to the skin surface — standing on end, pressed up against the epidermis. Inside, flattened lamellar cells (modified Schwann cells) stack like coins, with the axon spiralling between them. The vertical orientation is functional: it puts the receptor as close to the surface as any structure can get without being in the epidermis, and it is why Meissner corpuscles resolve fine spatial detail.

A Pacinian corpuscle is unmistakable and often the first thing students find: a large oval in the hypodermis, 0.5–2 mm long, sectioned into the classic "sliced onion" of concentric lamellae with a pale, unstained fluid space between each pair and a single central axon core. The fluid spaces are the point. In a well-fixed section you can count the lamellae, and their number correlates with how sharply the corpuscle filters out steady pressure.

Note what you will not see: free nerve endings. Their terminals are unmyelinated, sub-micron, and invisible on routine stains. Demonstrating them requires silver impregnation or immunostaining for PGP9.5, and this histological invisibility has a clinical echo — a skin punch biopsy stained for intraepidermal nerve fiber density is the standard test for small fiber neuropathy, a condition in which patients have burning pain and completely normal nerve conduction studies, because conduction studies only measure the large myelinated fibers that are not the ones dying.

Receptive fields and two-point discrimination

A receptive field is the area of the body surface which, when stimulated, changes the firing of one sensory neuron. Field size varies over roughly two orders of magnitude across the body, and it — together with receptor density — sets the spatial resolution of touch.

Two-point discrimination is the bedside measurement of that resolution: the smallest separation at which two simultaneous points are felt as two rather than one. Use a paper clip bent into a U and you can measure it on yourself in five minutes.

Body region Two-point threshold Receptor density Cortical representation
Fingertip 2–4 mm Very high Enormous
Lips, tongue tip 2–5 mm Very high Enormous
Palm 8–12 mm High Large
Forehead 15 mm Moderate Moderate
Forearm 35–40 mm Low Small
Back, thigh, calf 45–70 mm Very low Very small

Three facts explain this table, and they are the same fact stated three ways. Where receptive fields are small, receptor density is high; where receptor density is high, the strip of primary somatosensory cortex devoted to that region is large. The sensory homunculus of Chapter 12 — the grotesque cortical figure with vast lips and hands and a tiny trunk — is a picture of this table. Cortical area is allocated in proportion to innervation density, not to body surface area, because cortex processes information and information arrives per axon, not per square centimetre.

Predict This

You are testing two-point discrimination on a patient's fingertip and find a threshold of 3 mm. You then test the same fingertip after applying a topical anesthetic strong enough to eliminate about half the small fibers but leave large myelinated fibers intact.

Commit to a prediction: does the two-point threshold get better, get worse, or stay roughly the same?

(Answer: it stays roughly the same. Two-point discrimination is a property of large myelinated A-beta mechanoreceptor afferents — Merkel and Meissner. Small fibers carry pain and temperature and contribute almost nothing to spatial acuity for touch. This dissociation is the basis of the entire bedside sensory examination: you test large-fiber function with a tuning fork, joint position, and light touch, and small-fiber function with a pin and a cold object, because they travel in different fibers, ascend in different spinal tracts, and fail in different diseases.)

Adaptation: phasic and tonic receptors

Adaptation is a decline in receptor potential, and therefore in firing rate, during a constant stimulus. Every receptor adapts to some degree; what differs is how fast and how completely.

Phasic (rapidly adapting) receptors fire a burst at the onset of a stimulus, fall silent during it, and often fire again at its offset. Meissner corpuscles, Pacinian corpuscles, and hair follicle receptors are phasic. Functionally they are change detectors, and they answer the question what just happened?

Tonic (slowly adapting) receptors fire continuously, with only modest decline, for as long as the stimulus lasts. Merkel discs, Ruffini endings, muscle spindles, and — critically — nociceptors are tonic. They answer the question what is the situation right now?

The clinical meaning of each is direct and important:

  • Because touch receptors are largely phasic, you cannot feel your own clothes. This is not a curiosity; it is essential filtering. If every one of your roughly 5 million cutaneous receptors reported continuously, the somatosensory cortex would be swamped by information about a shirt. Phasic receptors implement the rule that only change carries information.
  • Because nociceptors are tonic and in fact frequently sensitize — firing more over time as inflammatory mediators accumulate — pain does not fade. This asymmetry is deliberate: a body that adapted to tissue damage would stop protecting the damaged part. It is also why chronic pain is such a hard problem. There is no built-in off-switch to exploit.
  • Because proprioceptors are tonic, postural control never sleeps. A muscle spindle discharges continuously at rest and modulates around that baseline, which is why proprioceptive information is available instantly rather than only when you move.
  • Because Pacinian corpuscles are extremely phasic, a pressure sore forms without warning. A patient sitting still on an ischial tuberosity gets no ongoing signal from deep pressure receptors after the first few seconds. Healthy people shift position every few minutes anyway, driven by low-level discomfort from tonic nociceptors; a patient with impaired sensation does not, and the tissue dies under a pressure they never felt.

Aging · Two-Point Discrimination from 20 to 90

Cutaneous mechanoreception declines steadily and measurably across adult life, and the decline is not uniform across receptor types.

Meissner corpuscles are the most vulnerable. Their density in fingertip skin falls from about 30 per square millimetre at age 20 to under 10 by age 70 — a two-thirds loss — and the survivors become larger, more irregular, and less well coupled to the epidermis as the dermal papillae flatten. Pacinian corpuscles decline in number too, and the surviving ones develop thickened, less compliant lamellae, which shifts their tuning and blunts vibration sense. Merkel cell density falls more slowly.

The measurable consequences: fingertip two-point threshold roughly doubles between 20 and 80, from about 2–3 mm to 5–7 mm. Vibration detection threshold at 250 Hz rises severalfold, and vibration sense at the great toe is often the first thing to go — which is why a 128 Hz tuning fork at the medial malleolus is a standard part of the neurological exam in older adults, and why an abnormal result there must be interpreted against age-adjusted norms rather than treated as automatically pathological.

The functional cost is grip. Older adults grip objects with substantially greater force than young adults for the same task, because the Meissner-driven slip-detection reflex has lost sensitivity and the nervous system compensates by squeezing harder — an entirely rational strategy that also causes fatigue and makes fine manipulation clumsy. Add a diabetic neuropathy on top of normal aging, as in Amara's mother's generation and increasingly in Amara's own, and the safety margin disappears.

Check Your Understanding 13.2

  1. A Pacinian corpuscle and a Merkel disc are both mechanoreceptors, both exteroceptors, and both respond to pressure. Give the two features that separate them and explain what each difference is for.
  2. Explain why the sensory homunculus has enormous hands and a tiny back, using the word innervation density.
  3. Why is it dangerous that nociceptors do not adapt — and why is it also necessary?
Show answers
  1. Adaptation rate and receptive field size. The Merkel disc is tonic with a receptive field of 2–5 mm; it therefore reports sustained, spatially precise pressure and is built for reading fine surface detail — edges, points, texture. The Pacinian corpuscle is extremely phasic with a receptive field covering an entire fingerpad; it reports transient, high frequency events with no spatial precision at all and is built for detecting vibration, including vibration transmitted through a held object. One reports the shape of the world, the other reports events in it.
  2. Cortical territory is allocated per axon, not per square centimetre. The hand contains roughly 17,000 mechanoreceptive afferents in the glabrous skin alone; the entire skin of the back contains a small fraction of that in the same area. Because innervation density in the hand is one to two orders of magnitude higher, the hand delivers vastly more information per unit area and requires vastly more cortex to process it. The homunculus is a map of incoming information, not of anatomy.
  3. Dangerous because it means pain has no natural termination — there is no receptor-level mechanism that quiets a chronic nociceptive signal, so persistent injury or inflammation produces persistent suffering, and central sensitization can make it worse over time. Necessary because the purpose of pain is to protect damaged tissue, and protection must last as long as the damage does. A nociceptor that adapted like a Meissner corpuscle would stop reporting a broken bone after ten seconds, and you would walk on it. Congenital insensitivity to pain is instructive here: affected children rarely survive to adulthood without severe joint destruction, corneal scarring, and repeated unnoticed fractures.

13.3 Sensory Processing, Perception, and Dermatomes

Sensation is not perception. Sensation is the arrival of information. Perception is the conscious interpretation the brain constructs from it, and the construction adds a great deal that was never in the signal. Sensory processing happens at three levels, and problems can occur at any of them.

Level 1 — the receptor level

At the receptor, three things must happen. The stimulus must be of the right modality for the receptor (a Pacinian corpuscle is deaf to temperature). It must fall within the receptor's receptive field. And it must exceed threshold intensity — the minimum required to produce enough receptor potential to trigger the first action potential. Below threshold there is a graded receptor potential but no signal transmitted, which is why a stimulus can be present and genuinely undetectable rather than merely ignored.

Intensity above threshold is coded two ways at once. Frequency coding: a stronger stimulus produces a larger receptor potential and therefore a higher firing rate in the same axon. Population coding (recruitment): a stronger stimulus spreads far enough to exceed threshold in additional receptors, so more axons fire. Together these give the nervous system a dynamic range of about a million to one using signals that are individually all-or-none.

Level 2 — the circuit level

Afferent axons deliver their signal to ascending pathways in the spinal cord and brainstem, and the pathway a given fiber joins determines a great deal about how the sensation will feel. Chapter 12 developed these tracts; the essential points for the PNS are:

  • The dorsal column–medial lemniscal pathway carries fine discriminative touch, vibration, pressure, and conscious proprioception on large myelinated A-beta fibers. It ascends ipsilaterally in the spinal cord and crosses in the medulla.
  • The spinothalamic (anterolateral) pathway carries pain, temperature, and crude touch on A-delta and C fibers. It synapses in the dorsal horn and crosses within one or two segments in the spinal cord itself, then ascends contralaterally.
  • The spinocerebellar pathways carry unconscious proprioception to the cerebellum for motor coordination, and never reach consciousness at all.

That difference in where the crossing happens is one of the most clinically productive facts in neurology. A hemisection of the spinal cord (Brown-Séquard syndrome) produces loss of vibration and position sense on the same side as the lesion and loss of pain and temperature on the opposite side, starting a segment or two below it. A patient who cannot feel a pin on the left leg but cannot feel a tuning fork on the right has told you, with no imaging at all, that the problem is in the spinal cord and which half of it.

Level 3 — the perceptual level

The primary somatosensory cortex and its association areas extract the perceptual detection (is anything there?), magnitude estimation (how strong?), spatial discrimination (where, and how many points?), feature abstraction (what combination of qualities?), quality discrimination (submodalities within a sense — sweet versus sour, sharp versus dull), and pattern recognition (a familiar shape, a familiar face, the letter e in Braille).

The cortex also fills in, predicts, and ignores, and the results can be startling. Your somatosensory system suppresses self-generated touch, which is why you cannot tickle yourself. It builds a body map so committed to its own model that when a limb is amputated the map persists, producing a phantom limb — a perception with no sensation behind it at all, proving that perception is a construction of the CNS rather than a readout of the periphery.

Perception of the four somatic submodalities

Touch. Constructed from the four cutaneous mechanoreceptor channels running in parallel. Merkel afferents supply spatial form, Meissner afferents supply flutter and slip, Pacinian afferents supply high-frequency vibration, and Ruffini afferents supply skin stretch. The percept "I am holding a warm ceramic mug with a chipped rim" is a synthesis across all four channels plus thermoreceptors.

Temperature. Two populations of free nerve endings with different molecular sensors: cold receptors using TRPM8, most active between about 10 and 35 °C, and warm receptors using TRPV1-family channels, most active between about 30 and 45 °C. Both are phasic — step into a pool and it is briefly shocking, then merely cool — and both are outnumbered roughly three to one by cold receptors, which is why cooling is detected faster than warming. Below 10 °C and above 45 °C, thermoreceptors fall silent and nociceptors take over; this is why extreme cold and extreme heat both feel like pain rather than temperature, and why they can feel confusingly similar. TRPM8 is also opened by menthol and TRPV1 by capsaicin, which is the entire explanation for why mint feels cold and chili feels hot: the molecules bind the sensor directly, and the brain believes the sensor.

Pain. Nociceptive information is exceptional in how heavily it is modulated. The gate control mechanism in the dorsal horn allows large-diameter A-beta touch afferents to excite inhibitory interneurons that presynaptically suppress C-fiber input — which is why rubbing a banged elbow genuinely reduces the pain, and why transcutaneous electrical nerve stimulation works at all. Descending pathways from the periaqueductal gray release endogenous opioids onto the same synapses, which is why pain is worse when you are frightened and can vanish entirely in the first minutes after a serious injury.

Referred pain is the piece of this that matters most for Amara. Visceral afferents from an organ enter the spinal cord at the same segments as somatic afferents from a particular skin region, and converge on the same second-order neurons. The brain, which has spent a lifetime receiving signals on those neurons from skin, interprets the visceral signal as coming from the skin. Cardiac afferents enter at T1–T5, the same segments that serve the medial arm, shoulder, and — through the cervical cord and its overlap with the trigeminal nucleus — the jaw. This is why Amara's myocardial ischaemia in Chapter 1 was felt in her chest wall, left arm, and mandible. The heart cannot be felt; the segments it shares can.

Proprioception. Assembled from muscle spindle length signals, Golgi tendon organ tension signals, joint capsule receptors, and — importantly — an internal copy of the motor command itself (corollary discharge), which lets the brain predict where a limb should be and compare that to where the afferents say it is.

Clinical Connection · Shingles Draws a Dermatome on the Skin

Varicella-zoster virus causes chickenpox, then retreats into the dorsal root ganglia and cranial sensory ganglia, where it persists silently for decades inside sensory cell bodies. When cell-mediated immunity wanes — with age, illness, or immunosuppression — the virus reactivates, replicates, and travels anterograde down the sensory axons to the skin.

The result is the most literal demonstration in medicine that dermatomes are real. The rash appears as a band of grouped vesicles that stops exactly at the midline and covers precisely one dermatome, usually one of T3–L2 or the ophthalmic division of the trigeminal nerve. It does not cross to the other side, because a dorsal root ganglion serves one side only. It does not spread to the dermatome above or below, because those are served by different ganglia. Patients often describe burning pain two to three days before any rash appears — the virus is in the nerve before it reaches the skin.

Two clinical consequences follow directly from the anatomy. Herpes zoster ophthalmicus, reactivation in the ophthalmic (V1) division, threatens the cornea, and involvement of the tip of the nose (Hutchinson sign) predicts eye involvement because the nasociliary branch supplies both. And postherpetic neuralgia — pain persisting months after the rash heals — occurs because the reactivation destroyed neurons in the ganglion; the pain is now generated by damaged central circuitry, not by anything happening in the skin, which is why it responds to drugs that act on neuronal excitability rather than to anti-inflammatories.

Dermatomes

A dermatome is the area of skin supplied by the sensory fibers of a single spinal nerve — that is, by a single dorsal root and its ganglion. Thirty of the thirty-one spinal nerves have one. C1 does not, because it carries essentially no cutaneous afferents.

             DERMATOMES — the skin territory of each spinal nerve
                  (anterior view, left; posterior noted right)

                        ┌─────────────┐
              V1 ───────┤   forehead  │      TRIGEMINAL, not spinal:
              V2 ───────┤   cheek     │      the face is supplied by CN V,
              V3 ───────┤   jaw       │      NOT by cervical dermatomes.
                        └──────┬──────┘      The boundary is the jawline.
                     C2 ┌──────┴──────┐ C2   ← back of scalp
                     C3 │   neck      │ C3
              ┌─────C4──┤ shoulder cap├──C4─────┐  ← "cape" distribution
              │  C5     │             │    C5   │
         C5 ──┤  lateral│  T2  ═══════│         ├── C5   LANDMARKS
              │  arm    │  T3         │         │   ─────────────────
         C6 ──┤ ┌───────┤  T4 ── NIPPLE LINE ───┼── T4   nipple = T4
    thumb ────┤ │ C6    │  T5         │         │   xiphoid = T6
              │ │       │  T6 ── XIPHOID ───────┼── T6   umbilicus = T10
         C7 ──┤ │ C7    │  T7         │         │   inguinal = L1
   mid 3 ─────┤ │       │  T8         │         │
    fingers   │ │       │  T9         │   C6    │   thumb        = C6
              │ │ C8    │  T10 ─ UMBILICUS ─────┼── middle finger= C7
         C8 ──┤ │       │  T11        │   C7    │   little finger= C8
   little ────┤ │       │  T12        │         │   nipple       = T4
   finger     │ └───┬───┤  L1 ── INGUINAL ──────┼── umbilicus    = T10
              │  T1 │   │             │   C8    │   knee, anterior = L3
         T1 ──┤ medial  │             │         │   great toe    = L5
              │  arm    │             │         │   little toe   = S1
              └─────────┴──┬───┬──────┴─────────┘   perineum     = S4-S5
                           │L2 │L2
                           │   │              POSTERIOR (dorsal) surface:
                        ┌──┴───┴──┐           back of thigh  = S2
                        │ L3   L3 │  ← knee   buttock        = S3
                        │         │           anus, "saddle" = S3-S5
                        │ L4   L4 │
                        │ ┌─────┐ │           ADJACENT DERMATOMES OVERLAP
                        │ │L5 L5│ │           by roughly 50%. Cutting ONE
                        │ │     │ │           dorsal root produces almost
                        │ │S1 S1│ │           NO detectable numbness. THREE
                        └─┴──┬──┴─┘           adjacent roots must be lost
                          L5 │ S1             to produce a clear sensory
                       great │ little         level.
                         toe   toe

Figure 13.3 — Dermatome map with the clinically memorized landmarks.

Described: A schematic human figure divided into bands of skin, each labelled with the spinal nerve that supplies it. The face is excluded and marked as the territory of the trigeminal nerve in three divisions — V1 for the forehead, V2 for the cheek, V3 for the jaw — with the jawline as the boundary between trigeminal and cervical supply. C2 and C3 cover the back of the scalp and neck; C4 covers the shoulder cap in a cape-like distribution. The upper limb is supplied from C5 laterally through C6, C7, and C8 to T1 medially, following the limb around: C5 the lateral arm, C6 the thumb, C7 the middle three fingers, C8 the little finger, and T1 the medial arm. The trunk carries stacked horizontal bands from T2 down to T12 and L1. Five landmarks are marked for memorization: the nipple line is T4, the xiphoid process T6, the umbilicus T10, the inguinal region L1, and the perineum S4 to S5. In the lower limb, L2 covers the upper anterior thigh, L3 reaches the knee, L4 the medial leg, L5 the great toe, and S1 the little toe; on the posterior surface, S2 covers the back of the thigh, S3 the buttock, and S3 to S5 the saddle region around the anus. A note states that adjacent dermatomes overlap by roughly fifty percent, so cutting a single dorsal root produces almost no detectable numbness and three adjacent roots must be lost before a clear sensory level appears.

The clinical uses of dermatomes are worth listing explicitly, because this is one of the few maps in anatomy that students are genuinely expected to carry in their heads.

  • Localizing a radiculopathy. A herniated disc compressing one nerve root produces pain and altered sensation in that root's dermatome. Numbness of the thumb and lateral forearm points to C6; numbness of the middle finger to C7; numbness of the great toe and lateral leg to L5.
  • Determining a spinal cord injury level. The neurological level is defined as the lowest segment with normal sensory and motor function on both sides. Because the landmarks are fixed, an examiner with a pin and a wisp of cotton can establish a level in two minutes.
  • Checking a regional anesthetic. After a spinal or epidural block, the anesthetist tests with cold and asks how high the level has risen. A block to T4 (nipple) has reached the level of the cardiac sympathetic outflow and predicts bradycardia and hypotension — a direct application of the sympathetic anatomy in Chapter 14.
  • Recognizing shingles, as above.
  • Interpreting referred pain. Diaphragmatic irritation refers to the C4 dermatome over the shoulder tip, because the phrenic nerve arises from C3–C5. A patient with blood under the diaphragm after a ruptured spleen complains of shoulder pain (Kehr sign) and nothing in the shoulder is wrong.

The 50 percent overlap between adjacent dermatomes deserves emphasis, because it is the reason dermatome maps in different textbooks disagree with each other. Each patch of skin is supplied by fibers from two or three adjacent roots. The maps show the centre of each territory, which is reproducible; the borders are not. Practically: a single root lesion produces a region of reduced sensation with a small zone of true numbness at its core, not a crisp rectangle of anesthesia.

Development · Why Dermatomes Spiral Down the Limb

On the trunk the dermatome map is exactly what a segmented animal should have: stacked horizontal bands, T2 through T12, each one a neat ring. On the limbs it becomes a spiral, and students reasonably ask why anatomy suddenly stopped being tidy. The answer is that the limbs were built later, out of the same segments, and then moved.

In the fifth week the limb buds appear as paddles projecting laterally from segments C5–T1 and L2–S3. Each bud carries in with it the myotome and dermatome of every segment it grew from, and the nerves of those segments grow into it and stay attached to the tissue they were originally wired to. Nothing is ever rewired afterwards. Whatever the tissue does next, the nerve follows it.

What the tissue does next is rotate. The upper limb bud rotates roughly 90 degrees laterally, so that the original ventral (flexor) surface comes to face anteriorly and the thumb ends up on the lateral side. The lower limb bud rotates roughly 90 degrees medially, so the original ventral surface comes to face posteriorly and the great toe ends up on the medial side. The consequence is visible on any dermatome map. In the arm, the sequence C5–C6–C7–C8–T1 runs down the lateral side, around the hand, and back up the medial side; in the leg, the lumbar segments sweep down the front while the sacral segments end up on the back of the thigh and the sole. The two limbs spiral in opposite directions because they rotated in opposite directions.

Three practical consequences follow. First, the knee is L3 in front but S2 behind, which is why "pain in the knee" is a poor localizing complaint until you ask which surface. Second, the great toe is L5 and the little toe is S1 — adjacent toes on adjacent roots, which is why an L5–S1 disc herniation can be localized to the correct root by testing two square centimetres of skin. Third, the axial line — the boundary where non-adjacent dermatomes are brought face to face by the spiral — is the one place on the body where sensation changes abruptly rather than gradually, because there is no overlap between roots that were never neighbours.

The same logic explains the plexuses of §13.5. A limb assembled from five segments and then rotated needs its segmental nerves re-sorted into bundles that follow the adult compartments rather than the embryonic segments. A plexus is the wiring harness that performs that re-sort.

Check Your Understanding 13.3

  1. A patient has a knife wound to the right side of the spinal cord at T10. Predict the sensory findings on each side of the body below the lesion.
  2. A patient reports numbness of the medial forearm and little finger. Is this more likely a C8 nerve root problem or an ulnar nerve problem — and what one extra test would distinguish them?
  3. Why does a patient with a ruptured spleen complain of left shoulder pain?
Show answers
  1. Right side: loss of vibration, fine touch, and conscious proprioception below T10, because the dorsal columns ascend ipsilaterally and have not yet crossed. Left side: loss of pain and temperature beginning one or two segments below T10 (around T11–T12), because the spinothalamic fibers cross within a segment or two of entry and are therefore already carrying left-sided information on the right side of the cord. There will also be right-sided weakness, since the corticospinal tract has crossed in the medulla. The dissociated, crossed sensory picture is Brown-Séquard syndrome.
  2. Both produce medial forearm and little-finger symptoms, so the distinguishing test must find territory that belongs to one and not the other. The medial antebrachial cutaneous nerve, which supplies the medial forearm skin, arises from the medial cord of the brachial plexus proximal to the ulnar nerve — so ulnar neuropathy at the elbow or wrist spares the forearm and affects only the hand. C8 root pathology affects both. Practically: numbness limited to the hand suggests ulnar neuropathy; numbness extending up the medial forearm suggests a C8 or lower-trunk lesion. Testing the C8-innervated but non-ulnar muscles (for example, the flexor pollicis longus, median-innervated but C8) adds further separation.
  3. The diaphragm is innervated by the phrenic nerve from C3, C4, and C5. Blood in the left upper abdomen irritates the underside of the left hemidiaphragm; the resulting afferent traffic enters the cord at C3–C5, where the brain has learned that such signals come from the C4 dermatome over the shoulder tip. This is referred pain by segmental convergence, and it is called Kehr sign. It is also a reminder that pain location is a statement about spinal segments, not about anatomy.

13.4 The Cranial Nerves

Twelve pairs of nerves attach directly to the brain rather than to the spinal cord. They are numbered I to XII in rostral-to-caudal order of attachment, and with two exceptions (I and II) they emerge from the brainstem. They are not simply "spinal nerves of the head" — they lack dorsal and ventral roots, several carry parasympathetic fibers, and four of them carry special sensory information that has no spinal equivalent at all.

Learning them is unavoidable and repays the effort immediately, because cranial nerve testing is the fastest, cheapest neurological localization tool in existence. A competent examiner can test all twelve in four minutes with a penlight, a cotton wisp, a tuning fork, and a tongue depressor, and can thereby localize a lesion to a specific few millimetres of brainstem.

        THE TWELVE CRANIAL NERVES — inferior (basal) view of the brain
              ANTERIOR (front of head) at top · POSTERIOR at bottom

                        ╭───────────────────────╮
                        │   OLFACTORY BULBS     │
      I  OLFACTORY  ────┤  ○                 ○  │  [S] smell
                        │   FRONTAL LOBES       │
                        ╰───────╮       ╭───────╯
      II OPTIC       ────────────╲─────╱───────    [S] vision
                          OPTIC   ╲───╱  CHIASM
                                   │ │
                        ╭──────────┴─┴──────────╮
      III OCULOMOTOR ───┤ ◄─ MIDBRAIN ─►        │  [M+P] most eye muscles,
                        │                       │        pupil constriction,
      IV TROCHLEAR  ────┤ (emerges POSTERIORLY  │        lens accommodation,
                        │  — the only one)      │  [M]   superior oblique
                        ╰──────────┬────────────╯
                        ╭──────────┴────────────╮
      V TRIGEMINAL  ────┤ ══════ PONS ══════    │  [B] FACE SENSATION +
         V1 ophthalmic  │   (largest CN)        │      muscles of MASTICATION
         V2 maxillary   │                       │
         V3 mandibular  │                       │
      VI ABDUCENS   ────┤ at pons-medulla       │  [M] lateral rectus
      VII FACIAL    ────┤ junction              │  [B+P] FACIAL EXPRESSION,
      VIII VESTIBULO- ──┤                       │        taste ant. 2/3,
          COCHLEAR      │                       │        tears + saliva
                        ╰──────────┬────────────╯  [S] hearing + balance
                        ╭──────────┴────────────╮
      IX GLOSSO-    ────┤                       │  [B+P] taste post. 1/3,
         PHARYNGEAL     │      MEDULLA          │        carotid baro/chemo,
      X  VAGUS      ────┤                       │        parotid gland
                        │                       │  [B+P] THORAX + ABDOMEN,
      XI ACCESSORY  ────┤  (roots from upper    │        larynx, swallowing
                        │   cervical cord,      │  [M] sternocleidomastoid
      XII HYPOGLOSSAL ──┤   ascend through      │      + trapezius
                        │   foramen magnum)     │  [M] tongue muscles
                        ╰───────────────────────╯
                             ▼ to spinal cord

   CODE:  [S] purely SENSORY   [M] purely MOTOR   [B] BOTH (mixed)
          [P] also carries PARASYMPATHETIC fibers  — only III, VII, IX, X do

   ┌──────────────────────────────────────────────────────────────────────┐
   │ SENSORY / MOTOR / BOTH, in order:                                    │
   │   I    II   III  IV   V    VI   VII  VIII IX   X    XI   XII         │
   │   S    S    M    M    B    M    B    S    B    B    M    M           │
   │                                                                       │
   │ PARASYMPATHETIC OUTFLOW — the "1-9-7-3" rule:                        │
   │   CN III → ciliary ganglion → pupil + lens                           │
   │   CN VII → pterygopalatine + submandibular ganglia → tears, saliva   │
   │   CN IX  → otic ganglion → parotid gland                             │
   │   CN X   → terminal ganglia IN the organ walls → heart, lungs, gut   │
   │   CN X ALONE carries ~75% of all parasympathetic fibers in the body  │
   └──────────────────────────────────────────────────────────────────────┘

Figure 13.4 — The twelve cranial nerves at their brainstem attachments, coded by function.

Described: An inferior view of the brain with the twelve cranial nerves shown at their points of attachment, running from anterior at the top to posterior at the bottom. Cranial nerve one, the olfactory nerve, attaches at the olfactory bulbs beneath the frontal lobes and is purely sensory for smell. Cranial nerve two, the optic nerve, converges at the optic chiasm and is purely sensory for vision. From the midbrain emerge cranial nerve three, the oculomotor, which is motor plus parasympathetic and supplies most eye muscles, pupillary constriction and lens accommodation, and cranial nerve four, the trochlear, purely motor to the superior oblique and the only cranial nerve to emerge from the posterior surface of the brainstem. From the pons emerge cranial nerve five, the trigeminal, the largest cranial nerve, mixed, carrying sensation from the face in three divisions — ophthalmic, maxillary and mandibular — and motor supply to the muscles of mastication. At the pons–medulla junction emerge cranial nerve six, the abducens, motor to the lateral rectus; cranial nerve seven, the facial, mixed with parasympathetic fibers, supplying facial expression, taste from the anterior two-thirds of the tongue, and tear and saliva production; and cranial nerve eight, the vestibulocochlear, purely sensory for hearing and balance. From the medulla emerge cranial nerve nine, the glossopharyngeal, mixed with parasympathetic fibers, carrying taste from the posterior third of the tongue, carotid baroreceptor and chemoreceptor input, and supply to the parotid gland; cranial nerve ten, the vagus, mixed with parasympathetic fibers, supplying thorax and abdomen, larynx and swallowing; cranial nerve eleven, the accessory, motor to sternocleidomastoid and trapezius, whose roots arise from the upper cervical cord and ascend through the foramen magnum; and cranial nerve twelve, the hypoglossal, motor to the tongue muscles. A key gives the sensory, motor or both classification in order as sensory, sensory, motor, motor, both, motor, both, sensory, both, both, motor, motor. A second box states that parasympathetic fibers travel only in cranial nerves three, seven, nine and ten — the one-nine-seven-three rule — relaying in the ciliary ganglion for the pupil and lens, the pterygopalatine and submandibular ganglia for tears and saliva, the otic ganglion for the parotid gland, and terminal ganglia within the organ walls for the heart, lungs and gut, with the vagus alone carrying about seventy-five percent of all parasympathetic fibers in the body.

The full table

# Name Type Function Bedside test
I Olfactory Sensory Smell Occlude one nostril; identify a non-irritating odor (coffee, vanilla). Do not use ammonia — it stimulates CN V, not I.
II Optic Sensory Vision Visual acuity chart each eye separately; confrontation visual fields; fundoscopy; afferent limb of pupillary light reflex
III Oculomotor Motor + parasympathetic Superior, inferior, medial rectus and inferior oblique; levator palpebrae superioris (lid elevation); parasympathetic: pupillary sphincter and ciliary muscle Follow a finger in an H pattern; look for ptosis; pupil size and light reflex (efferent limb); accommodation
IV Trochlear Motor Superior oblique — depresses and intorts the adducted eye Ask the patient to look down and in toward the nose; vertical diplopia on stairs
V Trigeminal Both Sensation from face, scalp anterior to the vertex, cornea, nasal and oral mucosa, anterior two-thirds of tongue (touch, not taste), teeth, dura; motor to masseter, temporalis, pterygoids, tensor tympani Light touch and pin in all three divisions; corneal reflex (afferent limb); clench teeth and palpate masseter; jaw opens toward a weak side
VI Abducens Motor Lateral rectus — abducts the eye Look laterally; failure produces horizontal diplopia worse on gaze to that side
VII Facial Both + parasympathetic Motor to all muscles of facial expression, stapedius, posterior digastric; sensory taste from anterior two-thirds of tongue; parasympathetic to lacrimal, submandibular, sublingual glands Raise eyebrows, close eyes tightly against resistance, smile, puff cheeks; taste with sugar or salt on the anterior tongue; ask about dry eye and hyperacusis
VIII Vestibulocochlear Sensory Cochlear division: hearing. Vestibular division: linear and angular acceleration, head position Finger rub or whisper each ear; Rinne and Weber with a 512 Hz fork; head impulse test, Dix-Hallpike, nystagmus (Chapter 15)
IX Glossopharyngeal Both + parasympathetic Taste and general sensation from posterior third of tongue and pharynx; carotid sinus baroreceptors and carotid body chemoreceptors; motor to stylopharyngeus; parasympathetic to parotid gland Gag reflex (afferent limb); palatal sensation; hard to test in isolation
X Vagus Both + parasympathetic Motor to pharynx, soft palate, larynx (phonation and swallowing); sensory from pharynx, larynx, aortic arch baroreceptors, and thoracic/abdominal viscera; parasympathetic to heart, lungs, and gut to the splenic flexure Say "ahh" — the uvula deviates away from a weak side; listen for hoarseness; gag reflex (efferent limb); swallow assessment
XI Accessory Motor Sternocleidomastoid and upper trapezius Shrug shoulders against resistance; turn head against resistance (tests the opposite sternocleidomastoid)
XII Hypoglossal Motor All intrinsic and most extrinsic tongue muscles Protrude the tongue — it deviates toward a weak side; look for atrophy and fasciculation

Mnemonics abound. For the names in order: On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops. For the sensory/motor/both sequence: Some Say Marry Money, But My Brother Says Big Brains Matter More. Use them, then discard them — the goal is to know the nerves, not the poem.

The five that matter most clinically

CN III (oculomotor). This nerve carries two very different populations of axons, and their spatial arrangement inside the nerve produces one of the most useful signs in emergency medicine. The parasympathetic fibers destined for the pupil run in a superficial, peripheral ring around the outside of the nerve; the somatic motor fibers to the eye muscles run in the core. Superficial fibers get their blood supply from surrounding pial vessels; core fibers depend on the vasa nervorum penetrating the centre.

The consequence: compression of the nerve from outside — by an expanding aneurysm of the posterior communicating artery, or by the uncus of the temporal lobe herniating through the tentorium as intracranial pressure rises — squeezes the superficial fibers first and produces a dilated, unreactive pupil ("blown pupil") that may precede any eye-movement problem. Ischaemia of the nerve — as in diabetic or hypertensive microvascular disease, both relevant to Amara — kills the core first and produces ptosis and an eye deviated "down and out" while sparing the pupil. Pupil-involving third nerve palsy is a neurosurgical emergency until proven otherwise; pupil-sparing third nerve palsy in a diabetic patient usually resolves in three months. The whole distinction rests on where the axons sit inside the cable.

CN V (trigeminal). The largest cranial nerve, and the reason the face is not part of any dermatome. Its three divisions leave the skull through three different foramina — V1 the superior orbital fissure, V2 the foramen rotundum, V3 the foramen ovale — which lets a lesion be localized by which divisions are involved. It supplies the afferent limb of the corneal reflex (touch the cornea, both eyes blink; the efferent limb is CN VII), and the afferent limb of the jaw jerk. Trigeminal neuralgia produces paroxysms of lancinating, electric-shock pain in one division, classically V2 or V3, triggered by trivial stimuli such as a breeze or chewing. It is usually caused by an artery pulsating against the nerve root at its entry zone, demyelinating it, so that touch fibers cross-excite pain fibers — a purely mechanical fault producing a purely sensory catastrophe.

CN VII (facial). Discussed at length in the sidebar below, because the distinction between a facial nerve palsy and a stroke is one of the few pieces of neuroanatomy that a member of the public can usefully know.

CN VIII (vestibulocochlear). Two nerves in one sheath. The cochlear division carries hearing; the vestibular division carries balance. A slow-growing schwannoma at the cerebellopontine angle compresses them in order — progressive unilateral sensorineural hearing loss and tinnitus first, because the cochlear fibers are more vulnerable, then imbalance, then, as the tumour grows, CN V (facial numbness, lost corneal reflex) and finally CN VII. The order of symptoms maps the local anatomy. Chapter 15 develops this nerve fully.

CN X (vagus). The most consequential nerve in this chapter. Vagus means "wandering," and it does: from the medulla through the jugular foramen, down the neck in the carotid sheath, into the thorax, around the aortic arch on the left and the right subclavian artery on the right, along the esophagus, through the diaphragm, and across the abdominal viscera as far as the splenic flexure of the colon. It carries roughly 75 percent of all parasympathetic fibers in the body — and, less famously but just as importantly, 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, and inflammatory signals from the gut, and this vast afferent stream is the reason vagal reflexes are so fast and so powerful. When Amara's beta-blocker takes effect, the vagus is the system it is allowing to win.

Clinical Connection · Bell Palsy Versus Stroke — Why the Forehead Decides It

A patient presents with a drooping face on one side. This is either a lower motor neuron lesion of the facial nerve itself (Bell palsy — usually idiopathic, probably viral reactivation in the geniculate ganglion) or an upper motor neuron lesion in the contralateral motor cortex or corticobulbar tract (a stroke). One is an outpatient problem treated with steroids and eye protection; the other is a time-critical emergency. The distinguishing feature takes two seconds to test: ask the patient to raise their eyebrows.

Here is why it works. The facial nucleus in the pons has an upper part, supplying the frontalis and orbicularis oculi muscles of the forehead and eye, and a lower part, supplying the muscles of the cheek and mouth. The upper facial nucleus receives corticobulbar input from both cerebral hemispheres. The lower facial nucleus receives input from the contralateral hemisphere only.

  • Stroke (upper motor neuron). One hemisphere is damaged. The lower face on the opposite side loses its only input and is weak. The upper face on that side still receives input from the intact hemisphere, so the forehead still wrinkles and the eye still closes. The patient has a drooping mouth and a normal forehead.
  • Bell palsy (lower motor neuron). The facial nerve itself is damaged after the nucleus, so every fiber it carries fails. The forehead does not wrinkle, the eye will not close, the mouth droops, and the nasolabial fold flattens — the whole half-face.

Two extra findings support Bell palsy and confirm the level of the lesion. If the branch to stapedius is involved, ordinary sounds become uncomfortably loud (hyperacusis), because the muscle that damps ossicular movement is paralysed. If the branch to the lacrimal gland is involved, the eye is dry — which, combined with an eyelid that will not close, is why lubricating drops and taping the eye at night are the single most important part of treatment. The face may recover; a scarred cornea does not.

The bilateral innervation of the upper face is not an accident. Bilateral cortical control is the pattern for muscles that normally act together and symmetrically — the forehead, the pharynx, the upper trapezius, the diaphragm — and unilateral control is the pattern for muscles used independently on the two sides, like the hand and the lower face. Anatomy predicting function once again.

Check Your Understanding 13.4

  1. A patient's uvula deviates to the right when they say "ahh," and their tongue deviates to the left when protruded. Which nerves are affected, and on which side is each lesion?
  2. A 58-year-old with poorly controlled diabetes develops sudden ptosis and an eye that rests "down and out." The pupil is 3 mm and reacts briskly. Should this patient go straight to angiography? Justify anatomically.
Show answers
  1. Uvula deviating to the right indicates a left vagus (CN X) lesion: the intact right levator veli palatini pulls the palate up on the right, dragging the uvula toward the strong side, so the uvula points away from the lesion. Tongue deviating to the left indicates a left hypoglossal (CN XII) lesion: the intact right genioglossus protrudes its half of the tongue while the weak left side does not, so the tongue points toward the lesion. Both lesions are on the left. The opposite directions are not arbitrary — they follow from whether the muscle pushes the structure away from itself (genioglossus) or pulls it toward itself (levator veli palatini). Remembering the mechanics is more reliable than remembering the rule.
  2. No — this is a classic pupil-sparing third nerve palsy and is very likely microvascular. The parasympathetic pupillary fibers ride superficially in CN III and are supplied by pial vessels; an ischaemic insult to the vasa nervorum infarcts the core of the nerve, producing ptosis and impaired ocular motility while the superficial pupil fibers survive on their separate supply. Aneurysmal compression, by contrast, squeezes from outside in and takes the pupil first. A completely pupil-sparing palsy in a diabetic or hypertensive patient over 50 is usually observed and typically resolves within about three months. Any pupil involvement, any progression, any additional cranial nerve, or any pain out of proportion changes the answer immediately — the cost of missing a posterior communicating artery aneurysm is catastrophic, so the threshold for imaging is deliberately low.

13.5 Spinal Nerves and the Plexuses

Thirty-one pairs of spinal nerves leave the vertebral canal: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. Note the arithmetic problem in the neck — there are seven cervical vertebrae and eight cervical nerves. The first seven cervical nerves exit above their correspondingly numbered vertebra; C8 exits between C7 and T1; and from T1 down, every nerve exits below its numbered vertebra. This is a bookkeeping detail with real consequences: a disc herniation at C5–C6 compresses the C6 root, whereas a herniation at L4–L5 compresses the L5 root. Same anatomy, different rule, because the switch happens at C8.

Formation: roots, rami, and what belongs to which

Every spinal nerve is formed the same way, and the sequence is worth committing to memory because each named part has a distinct fiber content.

  1. Rootlets emerge from the spinal cord in two rows. The dorsal (posterior) rootlets carry sensory axons in; the ventral (anterior) rootlets carry motor axons out.
  2. Rootlets merge into a dorsal root — which bears the dorsal root ganglion containing the pseudounipolar sensory cell bodies — and a ventral root.
  3. The two roots unite inside the intervertebral foramen to form the spinal nerve, which is therefore mixed and is only about 1–2 cm long. Everything before the union is a root; everything after it is a ramus.
  4. Immediately outside the foramen the spinal nerve divides into a dorsal ramus and a ventral ramus. Both are mixed. - The dorsal ramus is small and supplies the deep muscles and skin of the back in a strict segmental strip. It never joins a plexus. - The ventral ramus is large and supplies everything else — the limbs and the entire anterolateral trunk. In the thorax the ventral rami remain segmental as the intercostal nerves; everywhere else they interweave into plexuses.
  5. Two additional small branches leave near the origin: the meningeal branch, which turns back into the vertebral canal to supply the meninges, vertebrae, and — crucially for back pain — the outer annulus of the intervertebral disc and the facet joints; and the rami communicantes, the connections to the sympathetic chain (§13.7), present at T1–L2.

The distinction between root and ramus is not pedantry. A root lesion produces a dermatomal and myotomal pattern; a peripheral nerve lesion produces the pattern of that nerve, which is a mixture of several roots. Being able to tell them apart is most of peripheral neurology.

Bell-Magendie law — dorsal roots are sensory, ventral roots are motor — was established in the 1820s and remains one of the cleanest structure-function statements in anatomy. It has one famous complication: a small number of unmyelinated afferent C fibers do run in ventral roots, which is why cutting a dorsal root does not always abolish pain, and why dorsal rhizotomy for intractable pain often disappointed the surgeons who tried it.

The four plexuses

A plexus (Latin for "braid") is a network in which the ventral rami of several spinal nerves exchange fibers before re-sorting into named peripheral nerves. Plexuses form wherever the ventral rami serve a limb, and nowhere else. Thoracic ventral rami, serving a segmental body wall, do not braid.

Why braid at all? Two reasons, both defensive. First, redundancy: because each terminal nerve draws fibers from several roots, damage to one root weakens many muscles slightly rather than paralysing a few completely. Second, developmental logistics: a limb bud grows out from several segments at once and its muscles migrate and fuse, so a single adult muscle may be assembled from myotomes of three different segments. The plexus is the wiring harness that reconciles a segmental spinal cord with a non-segmental limb.

Plexus Roots Major nerves Territory
Cervical C1–C4 (some C5) Lesser occipital, great auricular, transverse cervical, supraclavicular; phrenic (C3, C4, C5); ansa cervicalis Skin and muscles of neck; the diaphragm
Brachial C5–T1 Musculocutaneous, axillary, radial, median, ulnar; plus long thoracic, thoracodorsal, suprascapular The entire upper limb
Lumbar L1–L4 Femoral, obturator; iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous Anterior and medial thigh, anterior leg via saphenous
Sacral L4–S4 Sciatic (= tibial + common fibular), superior and inferior gluteal, pudendal, posterior femoral cutaneous Buttock, posterior thigh, and everything below the knee; perineum

"C3, 4, 5 keep the diaphragm alive" is the most valuable four-word mnemonic in clinical medicine. It explains why a complete spinal cord injury at C3 requires permanent mechanical ventilation and one at C6 usually does not; why phrenic nerve injury during cardiac surgery produces an elevated hemidiaphragm on chest X-ray; and why diaphragmatic irritation refers pain to the shoulder tip (§13.3).

      THE BRACHIAL PLEXUS — Roots · Trunks · Divisions · Cords · Branches
        mnemonic: "Randy Travis Drinks Cold Beer"    (R-T-D-C-B)

 ROOTS          TRUNKS          DIVISIONS        CORDS           BRANCHES
 (ventral                      (ant = flexors                   (terminal)
  rami)                         post = extensors)

 C5 ──┐
      ├──► SUPERIOR ──┬─ ant ──┐
 C6 ──┘    TRUNK      └─ post ─┼──┐
                                │  ├─► POSTERIOR ─┬──► AXILLARY  (C5,C6)
 C7 ─────► MIDDLE ────┬─ post ──┘  │    CORD      │    deltoid, teres minor
           TRUNK      └─ ant ───┐  │              │    skin over deltoid
                                │  │              │
 C8 ──┐                         │  │              └──► RADIAL (C5-T1)
      ├──► INFERIOR ──┬─ post ──┘  │                   ALL extensors of arm
 T1 ──┘    TRUNK      │            │                   + forearm; triceps
                      │            │                   + posterior skin
                      │            │
                      │            └─► LATERAL ─┬──► MUSCULOCUTANEOUS
                      │                CORD     │      (C5,C6,C7) biceps,
                      │  (ant. divs of          │      brachialis, coraco-
                      │   sup + mid trunks)     │      brachialis; skin of
                      │                         │      lateral forearm
                      │                         └──┐
                      │                            ├─► MEDIAN (C6-T1)
                      └─► MEDIAL ──┬───────────────┘   most forearm flexors,
                          CORD     │                   thenar muscles, LOAF
                    (ant. div of   │                   skin of lateral palm
                     inf trunk)    │
                                   └──► ULNAR (C8,T1)
                                        FCU, medial FDP, ALL interossei,
                                        adductor pollicis, hypothenar,
                                        medial 1.5 digits

 ┌── INJURY PATTERNS ──────────────────────────────────────────────────────┐
 │ UPPER trunk (C5-C6) "Erb palsy"  · birth traction, motorcycle fall      │
 │    arm adducted, internally rotated, elbow extended, forearm pronated   │
 │    = "waiter's tip" position. Deltoid + biceps out.                     │
 │ LOWER trunk (C8-T1) "Klumpke palsy" · hyperabduction, Pancoast tumour   │
 │    intrinsic hand muscles out = claw hand; may add Horner syndrome      │
 │    because T1 carries the sympathetic supply to the head.               │
 │ AXILLARY  · surgical neck humerus fracture / anterior shoulder          │
 │    dislocation → cannot abduct 15-90°, numb "regimental badge" patch    │
 │ RADIAL    · midshaft humerus fracture / "Saturday night palsy"          │
 │    → WRIST DROP, cannot extend wrist or fingers; numb dorsal web space  │
 │ MEDIAN    · carpal tunnel / supracondylar fracture                      │
 │    → thenar wasting, weak thumb opposition, "hand of benediction"       │
 │ ULNAR     · medial epicondyle fracture / cubital tunnel                 │
 │    → CLAW HAND (4th+5th digits), Froment sign, wasted first web space   │
 │ LONG THORACIC (C5,6,7) · axillary surgery → WINGED SCAPULA              │
 └─────────────────────────────────────────────────────────────────────────┘

Figure 13.5 — The brachial plexus, from roots to terminal branches, with the deficit produced by injury at each level.

Described: A left-to-right flow diagram of the brachial plexus in five stages — roots, trunks, divisions, cords, and terminal branches. The ventral rami of C5 and C6 join to form the superior trunk; C7 continues alone as the middle trunk; C8 and T1 join to form the inferior trunk. Each trunk splits into an anterior division destined for flexor muscles and a posterior division destined for extensors. All three posterior divisions unite to form the posterior cord, which gives the axillary nerve, supplying deltoid and teres minor and the skin over the deltoid, and the radial nerve, supplying every extensor of the arm and forearm including triceps and the posterior skin. The anterior divisions of the superior and middle trunks form the lateral cord, which gives the musculocutaneous nerve to biceps, brachialis and coracobrachialis and the skin of the lateral forearm, and contributes to the median nerve. The anterior division of the inferior trunk forms the medial cord, which contributes the other half of the median nerve and gives the ulnar nerve. The median nerve supplies most forearm flexors, the thenar muscles and the lateral palm; the ulnar nerve supplies flexor carpi ulnaris, the medial half of flexor digitorum profundus, all interossei, adductor pollicis, the hypothenar muscles, and the medial one and a half digits. A boxed panel lists injury patterns: upper trunk injury from birth traction or a motorcycle fall causes Erb palsy with the arm adducted, internally rotated, elbow extended and forearm pronated in the waiter's tip position; lower trunk injury from hyperabduction or a Pancoast tumour causes Klumpke palsy with clawing of the hand and sometimes Horner syndrome because T1 carries sympathetic fibers to the head; axillary nerve injury from a surgical neck fracture or anterior shoulder dislocation prevents abduction between fifteen and ninety degrees and numbs the regimental badge patch; radial nerve injury from a midshaft humeral fracture causes wrist drop; median nerve injury causes thenar wasting and weak thumb opposition; ulnar nerve injury causes claw hand affecting the fourth and fifth digits with a positive Froment sign; and long thoracic nerve injury causes a winged scapula.

The lower limb plexuses in brief

The lumbar plexus (L1–L4) forms within the psoas major muscle and its two great branches are the femoral and the obturator.

  • The femoral nerve (L2–L4) passes under the inguinal ligament lateral to the femoral artery and supplies the quadriceps, sartorius, and iliacus, plus sensation to the anterior thigh and, through its long saphenous branch, the medial leg and foot. Femoral nerve injury abolishes knee extension and the patellar reflex — the patient's knee buckles going downstairs. Because the nerve is the most lateral structure in the femoral triangle, the mnemonic NAVEL (Nerve, Artery, Vein, Empty space, Lymphatics, lateral to medial) tells you where to find it and, just as importantly, where not to put a needle aimed at the femoral vein.
  • The obturator nerve (L2–L4) passes through the obturator foramen to the adductor group. Injury abolishes thigh adduction and numbs the medial thigh.

The sacral plexus (L4–S4) lies on the posterior pelvic wall and produces the sciatic nerve, the largest nerve in the body — about the width of a thumb where it leaves the greater sciatic foramen. The sciatic is really two nerves in a common sheath from the start: the tibial division (posterior thigh, all of the posterior compartment of the leg, and the sole) and the common fibular (peroneal) division (short head of biceps femoris, then the anterior and lateral leg compartments and the dorsum of the foot).

  • Sciatic injury (posterior hip dislocation, badly placed gluteal injection) produces weakness of hamstrings and everything below the knee, with sensory loss over most of the leg and foot.
  • Common fibular injury is far more common because the nerve wraps around the neck of the fibula with almost no soft tissue over it — a plaster cast, a tight knee brace, crossing the legs habitually, or prolonged squatting is enough. The result is foot drop: the patient cannot dorsiflex, and compensates with a high-stepping gait. Sensory loss covers the dorsum of the foot and the lateral leg.
  • The pudendal nerve (S2–S4) supplies the perineum, the external urethral and anal sphincters, and the genitalia, and carries the afferents for the sexual and micturition reflexes described in Chapter 14. "S2, 3, 4 keep the urine off the floor."

Clinical Connection · Carpal Tunnel Versus Ulnar Neuropathy — Two Tunnels, Two Hands

Both are entrapment neuropathies, both cause a numb hand, and telling them apart at the bedside is pure applied anatomy.

Carpal tunnel syndrome is compression of the median nerve beneath the flexor retinaculum at the wrist, in a tunnel whose floor and walls are bone and whose roof is an unyielding ligament. Anything that increases the contents — tenosynovitis, fluid retention in pregnancy, hypothyroid myxoedema, amyloid, rheumatoid synovitis — raises pressure in a compartment that cannot expand.

  • Sensory: numbness and tingling of the thumb, index, middle, and radial half of the ring finger — the palmar surfaces. The thenar eminence itself is spared, because the palmar cutaneous branch leaves the median nerve proximal to the retinaculum and passes over it, not through it. Palm-sparing numbness is nearly diagnostic.
  • Motor: weakness and eventual wasting of the thenar muscles — abductor pollicis brevis, opponens pollicis, and the superficial head of flexor pollicis brevis (the LOAF muscles, with the two lateral lumbricals). Loss of thumb abduction and opposition is the key.
  • Classic history: waking at 3 a.m. shaking the hand. Sleep flexion of the wrist raises tunnel pressure; shaking restores flow.

Ulnar neuropathy most often occurs at the cubital tunnel behind the medial epicondyle, where the nerve is superficial enough to be the "funny bone."

  • Sensory: numbness of the little finger and the ulnar half of the ring finger, including the dorsal surface — because the dorsal cutaneous branch arises 5–8 cm proximal to the wrist. If the dorsal hand is numb, the lesion is at or above the elbow; if it is spared, the lesion is at the wrist (Guyon canal).
  • Motor: all the interossei, adductor pollicis, hypothenar muscles, and the medial two lumbricals. Hence the claw hand — the ring and little fingers hyperextend at the metacarpophalangeal joints and flex at the interphalangeal joints, because the lumbricals and interossei that normally flex the MCPs and extend the IPs are gone. Hence also Froment sign: asked to pinch a sheet of paper, the patient substitutes flexor pollicis longus (median) for the paralysed adductor pollicis, and the thumb interphalangeal joint visibly flexes.
  • The ulnar paradox: a lesion at the elbow produces less clawing than a lesion at the wrist, because a high lesion also paralyses the medial half of flexor digitorum profundus and so weakens the very flexion that creates the claw. The more proximal, more severe injury looks milder. Remembering this prevents a real diagnostic error.

Two tunnels, two nerves, two completely different hands — and the pattern of sensory sparing localizes the lesion to within a few centimetres before any test is ordered.

Imaging · Nerve Conduction Studies, EMG, and Ultrasound-Guided Blocks

Nerve conduction studies (NCS). A surface electrode stimulates a nerve at two points along its course while another records the response. Two numbers come out. Conduction velocity (normal 50–65 m/s in large upper-limb motor fibers) is dominated by myelination, so a slowed velocity, especially with prolonged distal latency, points to a demyelinating process — carpal tunnel syndrome, Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy. Amplitude of the compound muscle or sensory action potential reflects the number of surviving axons, so a reduced amplitude with preserved velocity points to an axonal process — diabetic polyneuropathy, toxic and metabolic neuropathies. Velocity is insulation; amplitude is wiring. In carpal tunnel syndrome the specific finding is prolonged median distal sensory latency across the wrist with normal values elsewhere in the same limb: the fault is localized to a 3 cm segment.

Electromyography (EMG). A fine needle records electrical activity from within a muscle. A normal muscle at rest is electrically silent. Fibrillation potentials and positive sharp waves at rest indicate muscle fibers that have lost their nerve supply — evidence of denervation, appearing 2–3 weeks after injury. On voluntary effort, chronically reinnervated muscle shows large, long-duration, polyphasic motor units, because surviving axons have sprouted to adopt orphaned fibers, enlarging each motor unit. EMG therefore dates an injury and distinguishes nerve disease from primary muscle disease.

Ultrasound-guided regional block. High-frequency ultrasound (10–15 MHz) resolves a peripheral nerve as a honeycomb: hypoechoic (dark) fascicles set in hyperechoic (bright) connective tissue. Because the operator watches the needle tip in real time, local anesthetic can be deposited in a ring around the nerve — an interscalene block for shoulder surgery targeting the superior trunk, a supraclavicular block at the trunk/division level for the whole arm, a femoral or adductor canal block for knee surgery. Ultrasound guidance has roughly halved the volume of anesthetic needed and largely eliminated the old technique of hunting for paraesthesias with a needle. It has also made §13.5 something an anesthetist looks at on a screen every day rather than something recalled from a textbook.

Check Your Understanding 13.5

  1. A patient falls asleep with their arm over the back of a chair and wakes with wrist drop. Name the nerve, the mechanism, and predict two sensory findings.
  2. Why does a C5–C6 disc herniation compress C6, while an L4–L5 herniation compresses L5?
  3. Explain, from plexus anatomy, why a single nerve root injury rarely paralyses a muscle completely.
Show answers
  1. The radial nerve, compressed in the spiral groove of the humerus — "Saturday night palsy." The nerve is pressed between the hard chair edge and the bone, and prolonged compression causes focal demyelination (neurapraxia), which usually recovers in weeks. Sensory findings: numbness over the dorsal first web space (the superficial branch of the radial nerve, its most reliable autonomous zone) and over the posterior forearm (posterior antebrachial cutaneous nerve). Note that triceps is usually spared, because its branches leave the radial nerve proximal to the spiral groove — another example of the level of a lesion being readable from which branches are working.
  2. Because the numbering rule changes at C8. In the cervical spine, each nerve exits above its numbered vertebra, so the nerve traversing the C5–C6 disc space is C6. From T1 down, each nerve exits below its numbered vertebra, so at L4–L5 the exiting nerve is L4 — but a posterolateral disc herniation, which is the common kind, compresses the traversing root that is on its way to the level below, which is L5. The exiting root is only caught by a far lateral herniation.
  3. Because a plexus redistributes fibers. Almost every limb muscle receives motor axons from two or three spinal segments — biceps from C5 and C6, deltoid from C5 and C6, tibialis anterior from L4 and L5. Losing one root removes only that root's share of the motor units, producing weakness rather than paralysis. Complete paralysis of a muscle therefore points to a peripheral nerve lesion (which carries all of that muscle's supply) rather than a root lesion, and this contrast is one of the most useful discriminators in the clinic.

13.6 Motor Endings, the Somatic Motor System, and Reflexes

Motor endings

A motor ending is where an efferent axon meets its effector. There are two structural types, and the difference between them is the difference between the two motor divisions.

The neuromuscular junction (NMJ) of skeletal muscle, developed in Chapter 9, is the most elaborate synapse in the body: a myelinated A-alpha axon loses its myelin, branches into terminal boutons that sit in a synaptic trough on the muscle fiber, and releases acetylcholine into a 30–50 nm cleft whose postsynaptic membrane is thrown into junctional folds packed with about 10,000 nicotinic receptors per square micrometre. Acetylcholinesterase in the basal lamina destroys the transmitter within about a millisecond. The design is built for one purpose: guaranteed one-to-one transmission. A single presynaptic action potential always produces a muscle fiber action potential, with a safety factor of three to four. The somatic motor system does not negotiate.

Varicosities are the autonomic alternative, and they are almost the opposite of an NMJ. A thin, unmyelinated postganglionic axon runs across a sheet of smooth muscle or a gland, swelling every few micrometres into a bead — a varicosity — packed with vesicles. There is no specialized postsynaptic membrane, no fold, no trough, and the gap can be 20 nm or 2,000 nm. Transmitter is sprayed into the interstitial space and diffuses to whatever receptors it finds. This is volume transmission: slow, diffuse, and imprecise, which is exactly right for an effector that is electrically coupled by gap junctions and needs to contract as a sheet. One axon can influence hundreds of muscle cells. Nothing about it is one-to-one.

The second motor division, in one paragraph

The varicosity is your first sight of the autonomic motor division, and the difference in the ending is a symptom of a deeper difference in the wiring. A somatic motor pathway uses one neuron: its cell body sits in the ventral horn or a cranial motor nucleus, its thick myelinated A-alpha axon runs uninterrupted to the muscle at 80–120 m/s, it releases acetylcholine onto nicotinic receptors, and the effect is always excitation — there is no inhibitory neuromuscular junction in human skeletal muscle, so somatic inhibition happens entirely inside the CNS by silencing the motor neuron. An autonomic motor pathway uses two neurons in series with a synapse in a peripheral ganglion, ends in varicosities rather than end plates, releases acetylcholine or norepinephrine onto any of eight receptor subtypes, and can excite or inhibit the same organ depending on which receptor the target cell expresses. One further contrast matters for this chapter's Case File: cut the somatic nerve to a muscle and the muscle is paralysed and wastes; cut the autonomic nerve to the heart and the heart beats faster, because it has lost a restraint rather than a command. Chapter 14 builds the full comparison axis by axis and shows why that single structural difference is the foundation of most of pharmacology.

Reflexes

A reflex is a rapid, involuntary, stereotyped response to a stimulus. It is the smallest complete unit of nervous system function, and every reflex has the same five components:

receptor → afferent (sensory) neuron → integration centre → efferent (motor) neuron → effector

Reflexes are classified along four axes: by development (innate versus learned), by integration site (spinal versus cranial), by effector (somatic versus autonomic), and by complexity (monosynaptic versus polysynaptic).

The stretch reflex is the only monosynaptic reflex in the human body, and its speed is the point. Tap the patellar tendon; the quadriceps is stretched by about a millimetre; muscle spindles report the stretch on large Ia afferents that synapse directly on the alpha motor neurons of the same muscle; the muscle contracts about 20–30 ms later. One synapse, no interneuron, no time wasted. Simultaneously, Ia collaterals excite inhibitory interneurons that suppress the motor neurons of the antagonist — reciprocal inhibition — so the hamstrings relax as the quadriceps fires.

The stretch reflex is not for doctors with hammers. It is the servo loop that maintains posture. Stand still and your body sways; the sway stretches a muscle; the spindle detects it; the muscle contracts and corrects it, continuously, below awareness. Gamma motor neurons keep the system sensitive: they contract the ends of the intrafusal fibers in parallel with the whole-muscle contraction driven by alpha motor neurons (alpha-gamma coactivation), so the spindle stays taut and stays informative even as the muscle shortens. Without gamma drive a shortening muscle would go slack around its spindle and the sensor would fall silent exactly when it was needed.

The tendon (Golgi) reflex is the counterweight. Golgi tendon organs lie in series with the muscle at the musculotendinous junction and report tension rather than length. High tension excites Ib afferents which, through inhibitory interneurons, inhibit the contracting muscle and excite its antagonist — the reverse of the stretch reflex. Classically this was described as a protective circuit-breaker preventing tendon avulsion; modern work shows it is better understood as a continuous tension-regulating servo that smooths force output and allows delicate grip. It is what lets you hold an egg.

The flexor (withdrawal) reflex is polysynaptic and ipsilateral: nociceptor input drives, through chains of interneurons, flexion at every joint of the limb, pulling it away from the stimulus. Because it involves many muscles and many segments, it takes longer — 50–100 ms — and it cannot be voluntarily suppressed, though its force can be modulated.

The crossed-extensor reflex is what makes withdrawal survivable. Step on a tack and the flexor reflex lifts that leg — which would drop you on the floor, except that collaterals cross the cord and drive extension of the opposite leg to take the weight. It is a contralateral, polysynaptic reflex, and it is the clearest demonstration in the body that a spinal cord is not a cable but a computer.

Thread 2 · Homeostasis Is the Master Concept

It is easy to read a chapter on sensation as a chapter about experience — about what things feel like. It is not. Almost everything in the sensory division exists to defend a set point, and conscious perception is an occasional by-product.

Consider what the afferent traffic of one quiet minute is actually doing. Muscle spindles report the length of every postural muscle continuously, and the stretch reflex corrects deviations before you can perceive them; the variable being defended is joint angle, and the loop closes in the spinal cord without the brain being told. Golgi tendon organs report tension and keep force output smooth; the defended variable is muscle tension. Thermoreceptors in the skin report the temperature of the shell so that the hypothalamus can defend a core temperature you never feel. Baroreceptors in the carotid sinus report arterial wall stretch and defend blood pressure, a variable you have no conscious access to at all. Nociceptors defend tissue integrity, and they do it by being the one receptor class that refuses to adapt.

Every one of those is the afferent limb of a negative feedback loop of exactly the form Chapter 1 laid out: sensor, control centre, effector. This chapter supplies the sensors. Chapter 12 supplied the control centres. Chapter 14 supplies the efferent limb for the visceral loops, and Chapter 15 supplies the sensors that grew into whole organs.

The clinical corollary is the one Amara is living. When a sensor fails, the loop it belonged to does not merely lose accuracy — it loses the ability to know that it is wrong. A foot with no nociceptors is not a foot that hurts less. It is a foot with the alarm disconnected while the building continues to burn.

Exercise & Sport · Proprioception, Balance Training, and Why It Prevents Injury

When Toby Reyes ruptured his anterior cruciate ligament in Chapter 8, he lost more than a ligament. The ACL is densely populated with Ruffini endings, Pacinian corpuscles, and Golgi-like tension receptors, and their afferents feed a reflex arc to the hamstrings that fires within about 70 ms of anterior tibial translation. Rupturing the ligament removes the sensor. Athletes after ACL reconstruction show measurable proprioceptive deficits — increased joint position sense error, delayed hamstring onset latency, greater postural sway — that persist long after the graft is mechanically strong. A great many second injuries happen in a knee that is strong but blind.

This is why balance training is not a warm-up. Programs built on single-leg stance, unstable surfaces, perturbation training, and landing mechanics work by three measurable mechanisms: (1) increasing gamma motor neuron drive, which raises spindle sensitivity so smaller perturbations are detected; (2) shortening reflex latency in the protective muscle (hamstring co-contraction onset falls by 15–30 ms with training); and (3) re-weighting the sensory channels so the nervous system leans more on vestibular and remaining articular input where ligamentous input has been lost. Neuromuscular training programs of this design reduce non-contact ACL injury rates in field-sport athletes by roughly half, and they achieve it without changing strength at all.

A practical demonstration you can run in ten seconds: stand on one leg with your eyes open and count your corrections. Now close your eyes. Sway typically increases two- to threefold — the Romberg phenomenon. Vision was silently contributing about half your balance information, and removing it forces the proprioceptive and vestibular channels to carry the load alone. In a patient with large-fiber sensory loss, closing the eyes does not increase sway; it causes a fall.

Check Your Understanding 13.6

  1. The neuromuscular junction has a "safety factor" of three to four — each nerve impulse releases three to four times the acetylcholine needed to reach threshold in the muscle fiber. Why would evolution build in that much redundancy, and what does it predict about the earliest symptom of a disease that removes half the receptors?
  2. A patient has absent ankle jerks on both sides. Name three different places the lesion could be, using the five components of a reflex arc.
Show answers
  1. Because the somatic motor system is a command system, not a modulatory one, and a command that sometimes fails is worse than useless. A motor unit that dropped one twitch in twenty would make every movement jerky and every posture unreliable, so the junction is engineered for guaranteed one-to-one transmission with a large margin. The prediction is precise and clinically important: because of that margin, a disease can destroy a substantial fraction of the receptors before any weakness appears — and when weakness does appear it appears first as fatigability, since transmitter release runs down slightly during repeated firing and the shrunken margin is exhausted only after several contractions. This is exactly the presentation of myasthenia gravis: normal strength on the first effort, drooping eyelids and a failing voice by the twentieth. A safety factor converts a graded loss into a threshold phenomenon, and it is why the first symptom of many junctional diseases is not "weak" but "weak after a while."
  2. Using the arc: (a) receptor — muscle spindles of gastrocnemius/soleus, damaged in a large-fiber sensory neuropathy such as Amara's early diabetic neuropathy; (b) afferent neuron — the S1 sensory fibers, compressed by an L5–S1 disc herniation; (c) integration centre — the S1 segment of the cord itself; (d) efferent neuron — the S1 motor axons in the tibial nerve, or the tibial nerve itself; (e) effector — the calf muscle, in a myopathy or after Achilles rupture. A single absent reflex is a localizing sign only when combined with the sensory and motor findings that tell you which box is broken.

13.7 Advanced Topic · Peripheral Neuropathy and the Anatomy of the Sensory Examination

Everything in the preceding six sections was built to be used at a bedside. This section puts it to work on one problem — a patient whose feet have gone quiet — because that problem forces you to think in terms of fiber class, axon length, and the shape of a territory, which are the three ideas that convert anatomy into diagnosis.

Fiber classes: the axon is the variable

Peripheral axons are not interchangeable cables. They vary over a thirtyfold range of conduction velocity and a twentyfold range of diameter, and the two are tightly coupled: in myelinated fibers, conduction velocity in metres per second is roughly six times the diameter in micrometres. That single proportionality explains most of what a nerve conduction study measures and most of what a bedside sensory examination detects.

        PERIPHERAL NERVE FIBER CLASSES — diameter, speed, and cargo
                (sensory fibers also carry the Roman-numeral names)

  CLASS   Ø(µm)  MYELIN   SPEED      WHAT IT CARRIES              FAILS
                          (m/s)                                   WHEN?
 ══════════════════════════════════════════════════════════════════════════
  A-alpha 13-20  THICK    80-120   MOTOR to extrafusal muscle    LATE
   (Ia,Ib)                         Ia  = muscle SPINDLE (length)   ▲
                                   Ib  = GOLGI TENDON (tension)    │
                                   ► the AFFERENT LIMB of every    │
                                     tendon reflex you elicit      │
 ──────────────────────────────────────────────────────────────────┼───────
  A-beta   6-12  THICK    35-75    FINE TOUCH · VIBRATION ·        │
   (II)                            PRESSURE · joint position       │  EARLY
                                   ► Merkel · Meissner ·           │  in
                                     Pacinian · Ruffini            │  LENGTH-
                                   ► the DORSAL COLUMN modalities  │  DEPENDENT
 ──────────────────────────────────────────────────────────────────┤  DISEASE
  A-gamma  4-8   thick    15-40    MOTOR to intrafusal fibers      │
                                   (keeps the spindle taut)        │
 ──────────────────────────────────────────────────────────────────┤
  A-delta  1-5   THIN      5-30    FIRST PAIN (sharp, localized)   │
   (III)                           COLD · crude touch              │
                                   ► SPINOTHALAMIC modalities      │
 ──────────────────────────────────────────────────────────────────┤
  B        1-3   thin      3-15    AUTONOMIC PREganglionic         │
 ──────────────────────────────────────────────────────────────────┤
  C       0.2-1.5 NONE   0.5-2.0   SECOND PAIN (dull, diffuse)     │
   (IV)                            WARMTH · ITCH · visceral pain   ▼
                                   AUTONOMIC POSTganglionic      EARLIEST
                                   ► the "small fiber" population  (often
                                                                   silent)
 ══════════════════════════════════════════════════════════════════════════

  RULE OF THUMB (myelinated fibers):  velocity (m/s) ≈ 6 × diameter (µm)

  ┌───────────────────────────────────────────────────────────────────────┐
  │ WHAT THIS PREDICTS AT THE BEDSIDE                                     │
  │   128 Hz tuning fork  ─► A-beta (Pacinian)      LARGE fiber          │
  │   10 g monofilament   ─► A-beta (Merkel/Meiss.) LARGE fiber          │
  │   joint position      ─► A-beta + Ia            LARGE fiber          │
  │   tendon reflex       ─► Ia in, A-alpha out     LARGE fiber, BOTH    │
  │   pinprick            ─► A-delta                SMALL fiber          │
  │   cold metal / warmth ─► A-delta / C            SMALL fiber          │
  │   nerve conduction    ─► LARGE fibers ONLY  ◄── small fiber disease  │
  │     study                                       has a NORMAL study   │
  └───────────────────────────────────────────────────────────────────────┘

Figure 13.6 — Peripheral nerve fiber classes, and which bedside test interrogates each.

Described: A table of peripheral nerve fiber classes ordered from largest and fastest to smallest and slowest. A-alpha fibers are thirteen to twenty micrometres across, thickly myelinated, conduct at eighty to one hundred twenty metres per second, and carry motor output to extrafusal muscle fibers together with the group Ia spindle afferents reporting muscle length and the group Ib Golgi tendon afferents reporting tension; they form the afferent limb of every tendon reflex. A-beta fibers are six to twelve micrometres, thickly myelinated, conduct at thirty-five to seventy-five metres per second, and carry fine touch, vibration, pressure and joint position from Merkel discs, Meissner corpuscles, Pacinian corpuscles and Ruffini endings — the dorsal column modalities. A-gamma fibers, four to eight micrometres, supply the intrafusal fibers of the spindle. A-delta fibers are one to five micrometres, thinly myelinated, conduct at five to thirty metres per second, and carry sharp first pain, cold and crude touch. B fibers are one to three micrometres and carry autonomic preganglionic traffic. C fibers are unmyelinated, 0.2 to 1.5 micrometres, conduct at half to two metres per second, and carry dull second pain, warmth, itch, visceral sensation and all autonomic postganglionic traffic. An arrow down the right-hand margin shows that in length-dependent disease the small unmyelinated fibers fail earliest and often silently, the large myelinated fibers fail next, and motor A-alpha fibers fail last. A rule of thumb states that conduction velocity in metres per second is approximately six times the fiber diameter in micrometres. A final panel maps each bedside test to the fiber class it interrogates: the 128 hertz tuning fork, the ten gram monofilament, joint position sense and the tendon reflex all test large fibers, while pinprick, cold and warmth test small fibers; nerve conduction studies record from large fibers only, which is why pure small fiber neuropathy produces a completely normal study.

Two consequences of that figure are worth stating explicitly, because they are the source of most confusion about neuropathy.

A nerve conduction study is blind to more than half the nerve. Surface electrodes record a summed potential, and that summed potential is dominated by the fastest, largest, most heavily myelinated axons. Unmyelinated C fibers contribute nothing measurable. A patient with severe burning feet from a pure small fiber neuropathy therefore has an entirely normal nerve conduction study, and is told, wrongly, that nothing is wrong. The tests that do detect it are quantitative sensory testing, autonomic function testing, and a 3 mm skin punch biopsy stained for intraepidermal nerve fiber density (§13.2).

"Large fiber" is not one thing. Vibration, light touch, joint position, and the reflex afferent are all carried by large myelinated axons, but they are carried by different large myelinated axons, with different diameters, different receptors, and different degrees of redundancy. They therefore fail at different times, and the order in which they fail is a diagnostic signal rather than a nuisance.

Length-dependent axonopathy: why the longest axons die first

An axon is a metabolically absurd structure. A dorsal root ganglion neuron supplying the great toe of a 5'5" adult has a peripheral process roughly 90 cm long, a cell body about 60 µm across, and no protein synthesis machinery anywhere except in that cell body. Everything the distal terminal needs — mitochondria, membrane, channels, structural proteins, transmitter enzymes — must be manufactured at the soma and carried the entire distance by axonal transport, at 200–400 mm per day for fast anterograde transport and as little as 1 mm per day for the slow component that carries cytoskeleton. A structural protein made today reaches that toe terminal in about two years.

Now consider the arithmetic of maintenance. The volume of an axon scales with its length, so the longest axons contain the most membrane to maintain, the most cytoskeleton to renew, and the most mitochondria to keep supplied — all from one cell body of fixed capacity. They also have the longest transport route, so any impairment of transport starves the distal end first, and the most surface area exposed to whatever is wrong in the interstitial fluid.

Length-dependent axonopathy is what happens when a diffuse metabolic or toxic insult meets that arithmetic. In diabetes, chronic hyperglycaemia damages nerve by at least four routes acting together: flux through the polyol pathway, which consumes NADPH and accumulates sorbitol; advanced glycation end products cross-linking structural and transport proteins; oxidative stress overwhelming reduced antioxidant capacity; and microvascular disease of the vasa nervorum, which narrows the nerve's own blood supply. None of those insults is targeted. They act everywhere on every axon at once — and precisely because the insult is uniform, the longest axons cross the threshold of failure first.

The clinical signature follows without any further assumption. The terminal dies back first, so the disease is a dying-back process that begins at the tips and advances proximally. The toes go before the ankle, the ankle before the calf. Because the two feet are the same length, the process is symmetric. And when the advancing front reaches roughly the mid-calf, the fingers — whose axons are now the next longest in the body — begin to be affected, producing the characteristic stocking-and-glove distribution. The glove is not a coincidence and it is not a second disease. It is the same contour line, drawn at a distance from the spinal cord that happens to include the hand.

Reflexes obey the same rule. The ankle jerk has the longest reflex arc in the body: an Ia afferent from the soleus spindle to the S1 segment and an S1 motor axon back down the tibial nerve. The knee jerk arc, at L3–L4, is 30–40 cm shorter. Length-dependent disease abolishes the ankle jerk long before it touches the knee jerk, which is why a symmetrically absent ankle jerk with an intact knee jerk is one of the most reliable early signs of a distal polyneuropathy — and why finding it in a 45-year-old with an HbA1c of 7.1% is not a normal variant.

Predict This

Amara's vibration sense at the great toe is absent, her monofilament sensation is reduced at 4 of 10 plantar sites, and her ankle jerks are gone — all large-fiber findings. Her pinprick and temperature sensation on the dorsum of the foot are also reduced, which is a small-fiber finding.

Commit to a prediction before reading on: if she has a nerve conduction study next week, will it be normal, mildly abnormal, or grossly abnormal — and which measurement will be abnormal first, conduction velocity or response amplitude?

(Answer: mildly abnormal, and the abnormality will be in amplitude — specifically, reduced or absent sural sensory nerve action potentials in both legs, with conduction velocity normal or only slightly slowed. Amplitude reports the number of surviving axons; velocity reports the quality of myelination. Diabetic polyneuropathy is primarily an axonal process, so axons are lost while the myelin on the survivors remains largely intact, and the study shows small responses conducted at nearly normal speed. The opposite pattern — markedly slowed velocity with preserved amplitude — points to a demyelinating disease such as Guillain-Barré syndrome or chronic inflammatory demyelinating polyneuropathy. Note also what the study will not show: her small-fiber loss is invisible to it, so a "mildly abnormal" study will substantially understate her disease.)

Reading the shape of a sensory territory

The single most useful question in peripheral neurology is not what is lost but what shape is the loss. Four shapes account for nearly everything, and each is a direct statement about which anatomical unit is diseased.

      FOUR SHAPES OF SENSORY LOSS — and what each one localizes to

  ╔═══════════════════════╗   ╔═══════════════════════╗
  ║ 1 · LENGTH-DEPENDENT  ║   ║ 2 · RADICULOPATHY     ║
  ║     POLYNEUROPATHY    ║   ║     (one ROOT)        ║
  ║                       ║   ║                       ║
  ║      \   /            ║   ║        \  /           ║
  ║      |   |            ║   ║        |  |           ║
  ║      |   |            ║   ║      ==+  |    ← BAND ║
  ║     [▓] [▓] ← glove   ║   ║      //|  |      one  ║
  ║      |   |   (later)  ║   ║     //  |  |     side ║
  ║      |   |            ║   ║        |  |           ║
  ║     ▓▓▓ ▓▓▓  ← stocking   ║        |▓▓|    ← follows║
  ║     ▓▓▓ ▓▓▓    BOTH   ║   ║        |▓▓|      a     ║
  ║      ▓   ▓     sides  ║   ║        |▓ |      DERMA-║
  ║                       ║   ║         ▓          TOME║
  ║ UNIT = the AXON       ║   ║ UNIT = the ROOT + DRG  ║
  ║ boundary = DISTANCE   ║   ║ boundary = SEGMENT     ║
  ║ from the cell body    ║   ║ ± myotomal weakness    ║
  ║ symmetric · gradual   ║   ║ ± lost reflex · PAIN   ║
  ║ distal reflexes go    ║   ║ ONE side, ONE band     ║
  ╚═══════════════════════╝   ╚═══════════════════════╝

  ╔═══════════════════════╗   ╔═══════════════════════╗
  ║ 3 · MONONEUROPATHY    ║   ║ 4 · SPINAL CORD LEVEL ║
  ║     (one NERVE)       ║   ║     (a TRACT)         ║
  ║                       ║   ║                       ║
  ║        \  /           ║   ║        \  /           ║
  ║        |  |           ║   ║        |  |           ║
  ║        |  |           ║   ║   ═════|══|═════ ← a  ║
  ║       [▓▓]|  ← sharp  ║   ║        |▓▓|    HORIZ- ║
  ║        |  |    nerve  ║   ║        |▓▓|    ONTAL  ║
  ║        |  |    map    ║   ║        |▓▓|    LEVEL  ║
  ║        |  |    ± motor║   ║        |▓▓|           ║
  ║        |  |    in the ║   ║        |▓▓|  ± crossed║
  ║        |  |    SAME   ║   ║        |▓▓|    dissoc-║
  ║        |  |    nerve  ║   ║        ▓  ▓    iation ║
  ║ UNIT = the NERVE      ║   ║ UNIT = the TRACT      ║
  ║ boundary = ANATOMY of ║   ║ boundary = a LEVEL,   ║
  ║ that one cable        ║   ║ everything below it   ║
  ╚═══════════════════════╝   ╚═══════════════════════╝

   ▓ = territory of reduced or absent sensation

   THE DIAGNOSTIC QUESTION IS ALWAYS:  what anatomical unit has a border
   in that shape?   distance → axon · band → root · patch → nerve ·
   horizontal line → cord

Figure 13.7 — Four shapes of sensory loss, and the anatomical unit each one identifies.

Described: Four schematic human figures, each shaded to show a different pattern of sensory loss. Panel one is length-dependent polyneuropathy: both feet and lower legs are shaded to mid-calf in a stocking, with both hands shaded later in a glove; the diseased unit is the axon, the boundary is set by distance from the cell body rather than by any anatomical border, the pattern is symmetric and gradual, and the most distal reflexes are lost first. Panel two is radiculopathy: a single oblique band on one side of the body running from the spine into the limb, following a dermatome; the diseased unit is the nerve root and its dorsal root ganglion, the boundary is segmental, and the loss is often accompanied by radiating pain, a lost reflex, and weakness in the corresponding myotome. Panel three is mononeuropathy: a single sharply bounded patch on one limb corresponding exactly to the cutaneous map of one peripheral nerve, usually with weakness of the muscles that same nerve supplies; the diseased unit is one cable and the boundary is that cable's anatomy. Panel four is a spinal cord lesion: shading below a horizontal line that runs straight across the trunk, with everything below the level affected and often a crossed dissociation in which vibration is lost on one side and pain and temperature on the other; the diseased unit is a tract and the boundary is a segmental level. A closing line states the diagnostic question: which anatomical unit has a border of that shape — distance implicates the axon, a band the root, a patch a nerve, and a horizontal line the cord.

A fifth pattern is worth knowing because it breaks the rules. Mononeuritis multiplex is the sequential, asymmetric failure of named individual nerves — a foot drop on Tuesday, a wrist drop three weeks later — and it looks like a polyneuropathy only once enough nerves have failed to blur together. Its shape says the disease is attacking nerves through their blood supply, one vasa nervorum at a time, which points to vasculitis, and it is a medical emergency in a way that a symmetric stocking neuropathy is not.

And a sixth, the sensory ganglionopathy (or neuronopathy), attacks the dorsal root ganglion cell bodies themselves rather than their axons. Because the cell body supplies both the peripheral and the central process, loss is not length-dependent: it is patchy, often asymmetric, involves the face and trunk as readily as the feet, and destroys proprioception profoundly enough that the patient cannot stand with the eyes closed. Kill the axon and you get a stocking. Kill the cell body and you get chaos.

The bedside examination, tool by tool

Each instrument in the sensory examination is a probe aimed at a specific fiber population. Used without that understanding, the examination is a ritual; used with it, it is a fiber-class assay performed in ninety seconds with objects that fit in a pocket.

Tool Fiber class What it actually tests Notes on technique
128 Hz tuning fork A-beta (Pacinian, Meissner) Vibration — the earliest large-fiber modality to fail Strike, apply to the bony prominence of the great toe or medial malleolus, and time to extinction. Compare with your own perception at the same instant. Under 10 s at the malleolus is abnormal under age 60
10 g Semmes-Weinstein monofilament A-beta (Merkel, Meissner) Protective pressure sensation — the threshold below which a foot cannot detect injury Press until the filament buckles, hold 1–2 s, 10 sites per foot, avoiding callus. Failure at ≥4 sites predicts ulceration
Cotton wisp A-beta Light touch Least sensitive of the large-fiber tests; often normal when vibration is already gone
Neurological pin (single use) A-delta Sharp/dull discrimination — first pain Ask "sharp or dull?", not "does it hurt?"
Cold metal (tuning fork base) A-delta Cold detection The simplest small-fiber screen available
Passive joint movement A-beta + Ia, plus joint receptors Conscious proprioception Hold the digit at the sides, move 1–2°, eyes closed. Preserved late because several receptor populations report it
Tendon hammer Ia in, A-alpha out The entire large-fiber reflex arc, afferent and efferent The ankle jerk is the longest arc and the first to go
Romberg test Large-fiber proprioception Whether vision is compensating for lost proprioception Sway with eyes open, fall with eyes closed, is a positive test — a proprioceptive sign, not a cerebellar one

Three principles govern the interpretation.

Symmetry is information. A symmetric distal deficit is a statement about axon length. An asymmetric deficit is a statement about a named structure — a root, a nerve, a vessel.

A gradient is information. Length-dependent loss has a gradient: sensation is worst at the toes, better at the ankle, normal at the knee, with no sharp edge anywhere. A root or nerve lesion has an edge. Run the pin or the fork proximally up the limb and ask the patient to say when it changes; the presence or absence of a discrete transition is often more diagnostic than the mapping itself.

Dissociation is information. Loss of pinprick and temperature with preserved vibration and position sense is small-fiber disease (or a spinothalamic tract lesion). Loss of vibration and position with preserved pinprick is large-fiber disease (or a dorsal column lesion). Loss of both in a symmetric distal gradient, as in Amara, is a mixed axonal polyneuropathy — which is what diabetes produces, because hyperglycaemia is not selective for a fiber class.

Thread 3 · The Body Is Integrated

Amara's numb feet are a nervous system finding that is not, in any useful sense, a nervous system problem. Follow the arrows.

Endocrine → nervous. Four years of undiagnosed insulin resistance, then an HbA1c of 7.4% falling to 7.1%, delivered chronic hyperglycaemia to every peripheral nerve she has (Chapter 16, Chapter 24). Sorbitol accumulation, glycation of transport proteins, and oxidative stress attacked the axons with the largest maintenance burden.

Cardiovascular → nervous. The same metabolic state that narrowed her left circumflex artery narrowed the vasa nervorum, the microscopic arterioles inside the epineurium (§13.1). A nerve is an organ with its own circulation, and ischaemic nerve behaves exactly like ischaemic myocardium: it stops working before it dies, and it dies from the end furthest from the supply (Chapter 19).

Nervous → integumentary. A foot that cannot feel pressure does not shift position, and skin under sustained pressure ulcerates (Chapter 5). Her 3 mm plantar callus is the visible early stage of that process: repetitive load that would normally be corrected unconsciously, uncorrected because it is unfelt. Small-fiber loss also removes the sympathetic cholinergic supply to the sweat glands of the foot, which is why the skin is dry and fissured — an autonomic finding (Chapter 14) sitting on the same foot as the sensory one.

Nervous → skeletal → nervous. Loss of the ankle jerk and of distal proprioception degrades postural control (§13.6). Sway increases, gait becomes broad-based, and falls become more likely — in Amara now, and in her mother Adwoa, whose femoral neck T-score of −2.9 (Chapter 6) turns a fall into a fracture.

Four systems, one examination finding, and every arrow points in a stated direction. This is what the Systems Integration Case File is training you to do.

Check Your Understanding 13.7

  1. A 60-year-old has burning, unpleasant sensations in both feet for two years. Vibration, monofilament, joint position, and ankle jerks are all completely normal. Nerve conduction studies are normal. What is the most likely diagnosis, and which two investigations would support it?
  2. Two patients each have numbness of the little finger. Patient A also has numbness extending up the medial forearm and mild weakness of finger abduction; patient B has numbness confined to the little finger and the ulnar half of the ring finger, including the dorsal surface, with wasting of the first dorsal interosseous. Using Figure 13.7, name the shape of each and localize both.
  3. Explain, in terms of axon length, why Amara's knee jerks are normal while her ankle jerks are absent — and predict which reflex would be lost next if the process advanced.
Show answers
  1. Pure small fiber neuropathy. Every test listed interrogates large myelinated fibers — vibration and monofilament through A-beta, joint position through A-beta and Ia, the ankle jerk through Ia and A-alpha, and the nerve conduction study through the large myelinated population exclusively. All of them can be normal while the A-delta and C fiber population is destroyed, because those axons contribute nothing to a surface-recorded compound potential. The two supporting investigations are quantitative sensory testing (formal thermal detection thresholds, which isolate A-delta and C function) and a 3 mm skin punch biopsy stained by immunohistochemistry for PGP9.5 to count intraepidermal nerve fiber density, which is reduced. Autonomic function testing is a defensible third answer, since postganglionic autonomic fibers are C fibers and are frequently involved in the same process. The common causes are diabetes and impaired glucose tolerance — which means this patient needs an HbA1c even though nothing in the examination pointed at the pancreas.
  2. Patient A has a root pattern (shape 2): a C8 radiculopathy. Sensory loss extends into the medial forearm, which is C8/T1 dermatome but is not ulnar nerve territory — the medial antebrachial cutaneous nerve leaves the medial cord proximal to the ulnar nerve. Extension beyond a single nerve's map means the lesion is proximal to where the nerves were sorted, i.e. at the root or lower trunk. Patient B has a nerve pattern (shape 3): an ulnar neuropathy. The territory matches the ulnar cutaneous map exactly and stops at its border, and the inclusion of the dorsal surface of the hand places the lesion at or above the elbow, since the dorsal cutaneous branch leaves 5–8 cm proximal to the wrist (§13.5). Wasting of the first dorsal interosseous confirms motor involvement in the same nerve. The general rule: territory larger than one nerve implicates a root or plexus; territory equal to one nerve implicates that nerve.
  3. The ankle jerk arc runs from the soleus muscle spindle up the tibial and sciatic nerves to the S1 segment and back — the longest reflex arc in the body, roughly 1.5 m of axon in each direction. The knee jerk arc, from the quadriceps spindle to L3–L4, is 30–40 cm shorter in each limb. In a length-dependent axonopathy the insult is uniform along every axon, so the axon that accumulates the most damage per neuron is the longest one, and it crosses the threshold of conduction failure first. The Ia afferents of the ankle arc therefore fail while those of the knee arc are still working. If the process advanced, the next reflexes to be lost would be those with the next-longest arcs: the knee jerks in the legs, and in the upper limb the brachioradialis and triceps reflexes before the biceps reflex, since the radial nerve branches to those muscles are longer. Complete areflexia is a late finding, and by the time it appears there is usually distal weakness as well, because motor A-alpha axons — the largest and longest of all — have finally been reached.

Chapter Summary

§13.1 The peripheral nervous system is everything outside the brain and spinal cord: 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the ganglia, plexuses, receptors, and motor endings attached to them. It supplies the sensory input and motor output of the three-step framework — input, integration, output — whose middle step belongs to the CNS. The sensory division subdivides by where information comes from (somatic, visceral, special); the motor division subdivides by what it commands, and that split is anatomical rather than a matter of willpower: somatic pathways use one neuron from CNS to skeletal muscle, autonomic pathways use two with a synapse in a peripheral ganglion. A nerve is built of endoneurium around each axon, perineurium around each fascicle, and epineurium carrying the vasa nervorum — the same three-layer scheme as skeletal muscle, for the same mechanical reason. Ganglia are either sensory (pseudounipolar cell bodies, no synapses) or autonomic (multipolar, synaptic relays).

§13.2 Receptors transduce energy into a graded receptor potential whose amplitude sets action potential frequency; by the law of specific nerve energies, what you perceive depends on which fiber fires, not on what stimulated it. Receptors classify by stimulus (mechano-, thermo-, photo-, chemo-, nociceptor), by location (extero-, intero-, proprioceptor), and by structure (free versus encapsulated). The capsule is a mechanical filter, not padding: Merkel discs are tonic and spatially precise, Meissner corpuscles phasic and tuned to flutter, Pacinian corpuscles extremely phasic and tuned near 250 Hz, Ruffini endings tonic and tuned to skin stretch. Receptive field size sets two-point discrimination and predicts cortical representation, because cortex is allocated per axon rather than per square centimetre. Phasic receptors report change, which is why you cannot feel your clothes; tonic receptors report state, which is why pain does not fade and posture never lapses.

§13.3 Sensory processing occurs at receptor, circuit, and perceptual levels. Large-fiber information — fine touch, vibration, conscious proprioception — ascends ipsilaterally in the dorsal columns and crosses in the medulla; small-fiber information — pain and temperature — crosses within one or two segments of entry and ascends contralaterally in the spinothalamic tract. That difference in crossing point is why a cord hemisection produces dissociated, crossed sensory loss. A dermatome is the skin field of one dorsal root; adjacent dermatomes overlap by about 50 percent, so three roots must be lost for a clear sensory level, and the memorized landmarks (T4 nipple, T6 xiphoid, T10 umbilicus, L1 inguinal) localize lesions in seconds. The limbs' dermatomes spiral because the limb buds rotated after their nerves were assigned. Referred pain arises from convergence of visceral and somatic afferents on the same second-order neurons, which is why Amara's ischaemic heart hurt in her jaw and left arm.

§13.4 Twelve cranial nerves: I and II are outgrowths of the brain, the rest emerge from the brainstem in rostrocaudal order, and the sensory–motor–both sequence is S S M M B M B S B B M M. Four (III, VII, IX, X) carry parasympathetic fibers; the vagus carries about 75 percent of all parasympathetic fibers and is itself roughly 80 percent afferent. Two details do most of the clinical work: CN III's pupillary fibers ride superficially, so compression takes the pupil while ischaemia spares it; and the upper facial nucleus receives bilateral corticobulbar input, which is why a wrinkling forehead separates a stroke from Bell palsy.

§13.5 A spinal nerve forms from a sensory dorsal root bearing the dorsal root ganglion and a motor ventral root (Bell-Magendie law), then divides immediately into a segmental dorsal ramus and a large ventral ramus. Ventral rami braid into four plexuses — cervical (phrenic, C3–C5), brachial (C5–T1), lumbar (femoral, obturator), sacral (sciatic, pudendal) — which provide redundancy and reconcile a segmental cord with a rotated, non-segmental limb. Root lesions produce dermatomal and myotomal patterns with partial weakness; peripheral nerve lesions produce that nerve's map with potentially complete weakness, and the sparing of proximal cutaneous branches localizes the level to within centimetres.

§13.6 Somatic motor endings are neuromuscular junctions built for guaranteed one-to-one transmission, with junctional folds, ~10,000 nicotinic receptors per square micrometre, and a safety factor of three to four; autonomic endings are varicosities that spray transmitter into interstitial space for volume transmission. Reflexes have five components. The stretch reflex is the only monosynaptic human reflex and works as a postural servo kept sensitive by alpha-gamma coactivation; the tendon reflex regulates force through Golgi tendon organs in series with the muscle; the flexor withdrawal and crossed-extensor reflexes show the cord computing across segments and across the midline.

§13.7 Fiber class determines everything about how a modality behaves and when it fails. Conduction velocity is roughly six times diameter in myelinated fibers, so A-alpha and A-beta fibers are fast and carry motor output, vibration, touch, and the reflex afferent, while A-delta and C fibers are slow and carry pain, temperature, itch, and autonomic traffic. Length-dependent axonopathy follows from the metabolic arithmetic of maintaining the longest axons from one cell body: a uniform metabolic insult crosses the threshold of failure first in the longest fibers, so loss begins at the toes, is symmetric, advances proximally, and adds a glove when the front reaches mid-calf. The shape of a sensory territory names the diseased unit — distance implicates the axon, a band the root, a patch a nerve, a horizontal line the cord — and each bedside tool interrogates a specific fiber class, which is why a normal nerve conduction study does not exclude neuropathy.

The Three Threads in Chapter 13

Structure → Function. Strip the lamellae from a Pacinian corpuscle and the same axon terminal stops reporting vibration and starts reporting pressure — the tuning was in the capsule, not the nerve. A dorsal root ganglion has round cells because it contains no synapses; an autonomic ganglion has angular cells because it does. CN III's parasympathetic fibers ride on the outside of the cable, and that single geometric fact separates a neurosurgical emergency from an outpatient diabetic palsy. Dermatomes spiral because limb buds rotated. And an axon's length — the least glamorous parameter in the whole system — determines which sensations a person loses first. In every case the anatomy came first and the physiology is its consequence.

Homeostasis. The sensory division is the afferent limb of nearly every homeostatic loop in the body, and most of what it reports never reaches consciousness: spindle length, tendon tension, skin temperature, arterial wall stretch, tissue damage. Regulation requires measurement, and this chapter is the measurement. The clinical corollary runs the other way: when a sensor fails, the loop does not simply lose precision — it loses the ability to detect its own error, which is why a foot that cannot feel is a foot that will be injured.

Integration. Amara's numb feet are simultaneously an endocrine finding (four years of hyperglycaemia), a vascular finding (disease of the vasa nervorum is the same process that narrowed her left circumflex artery), an integumentary finding (a dry, callused, ulcer-prone foot), and a musculoskeletal finding (absent ankle jerks and degraded balance in a family where a fall means a fractured femoral neck). One tuning fork, applied to one toe, is a statement about five systems at once.

Case File 13 · Resolution

Question 1 — Why do the longest axons fail first, and why does that produce a stocking rather than a dermatomal pattern?

Because a peripheral axon is maintained entirely from its cell body, and the cost of that maintenance scales with length.

The sensory neuron supplying Amara's great toe has its cell body in the L5 or S1 dorsal root ganglion, roughly 60 µm across, with a peripheral process about 90 cm long. That cell body contains all of the neuron's protein synthesis machinery; the terminal contains none. Every mitochondrion, every ion channel, every structural protein used at the toe must be built at the soma and carried the full distance by axonal transport — fast anterograde transport moves at 200–400 mm per day, while the slow component carrying cytoskeletal proteins moves at 1 mm per day, so a structural protein made today arrives at that terminal in about two years. The longest axons carry the largest volume of membrane and cytoskeleton per neuron, have the longest supply line, and present the greatest surface area to whatever is wrong in the interstitial fluid.

Chronic hyperglycaemia attacks nerve through four mechanisms at once — polyol pathway flux with sorbitol accumulation and NADPH depletion, advanced glycation end products cross-linking transport and structural proteins, oxidative stress, and microvascular disease of the vasa nervorum that reduces the nerve's own blood supply. None of these is selective. They act uniformly on every axon in the body. And precisely because the insult is uniform, the axons that fail first are the ones with the least margin — the longest ones. The terminal dies back first and the process advances proximally: this is length-dependent axonopathy, or "dying back."

Now the shape. A dermatome is defined by a root: it is the territory of the axons that happen to travel through one dorsal root and its ganglion, and its border is a segmental border on one side of the body. Length-dependent axonopathy has no interest in roots. It attacks axons according to how far the terminal sits from its cell body — which is a distance, and the set of points at a fixed distance from the spinal cord is a contour line around the body, not a segmental band. Both feet are the same length, so the process is symmetric. The contour crosses L4, L5, S1 and S2 territory simultaneously, while sparing the proximal parts of those very same roots — the L5 dermatome is numb at the great toe and normal over the lateral thigh, which no root lesion could ever produce. When the advancing front reaches mid-calf, the next-longest axons in the body are those to the fingertips, and a glove appears on top of the stocking. It is not a second disease. It is the same contour line drawn at a radius that now includes the hand.

That is why the shape is diagnostic. Distance is the boundary, so the unit under attack is the axon.

Question 2 — Vibration and the ankle jerk are gone but ankle proprioception is preserved. What does the dissociation say?

It says two things: that the disease has reached the large myelinated population, and that "large fiber" is not one thing.

Start with what is lost. Vibration is carried by A-beta afferents from Pacinian and Meissner corpuscles — among the thickest, fastest sensory axons in the body — and the great toe is the most distal point at which any of them can be tested. Two vulnerabilities compound there: these are large axons, which have the greatest maintenance burden per unit length, and they are being tested at the far end of the longest peripheral process available. The ankle jerk fails for a parallel reason. Its arc is the longest reflex arc in the body: an Ia afferent from the soleus spindle up the tibial and sciatic nerves to S1, and an A-alpha motor axon all the way back. Both limbs of that arc are large-diameter and both are maximally long. Losing the reflex requires only that one of them fail.

Now what is preserved, and why that is not a contradiction. Conscious joint position sense at the ankle is not the output of a single receptor class. It is assembled from muscle spindle secondary (group II) afferents from several muscles acting across the joint, spindle primaries, Golgi tendon organs, Ruffini endings in the joint capsule and skin, cutaneous stretch information from around the joint, and an internal corollary discharge copy of the motor command (§13.3). It is a redundant, multi-channel percept, and the joint being tested — the ankle — sits proximal to the toes, so the afferents serving it are shorter than those serving the toe pulp. A graded, length-dependent loss therefore removes the single-channel, most-distal, largest-fiber measurements first (vibration at the toe; the S1 reflex arc) and leaves the redundant, slightly more proximal, multi-channel percept intact for years longer. Position sense does eventually go — and when it does, the patient becomes unable to stand with the eyes closed and the Romberg test turns positive — but its preservation now tells you the disease is early rather than that it has spared large fibers.

Two further readings of the same dissociation are worth having. First, her reduced pinprick and temperature sensation on the dorsum of the foot shows that small fibers are involved too: this is a mixed axonal polyneuropathy, not a pure large-fiber one, which is exactly what a non-selective metabolic insult should produce. Second, the pattern rules things out. A pure dorsal column lesion — vitamin B₁₂ deficiency, tabes dorsalis — takes vibration and position sense together, because both ascend in the same tract, and it does not abolish the ankle jerk unless the roots are also involved. Amara's split between vibration and position is a peripheral signature, not a central one.

Question 3 — Her cardiac pain was dermatomal; her foot numbness is not. Explain the difference anatomically.

Two different maps, drawn by two different rules.

The dermatomal map is a map of roots. The heart has no capacity to generate localized sensation of its own — it has no cutaneous receptors, no spatial map in cortex, and only a sparse population of visceral afferents. Those cardiac afferents are A-delta and C fibers that travel with the sympathetic nerves, pass through the sympathetic chain without synapsing, and enter the spinal cord through the dorsal roots of T1–T5. There they synapse on second-order neurons in the dorsal horn — the very same second-order neurons that receive somatic afferents from the chest wall, the medial arm, and, through the cervical cord's continuity with the spinal trigeminal nucleus, the jaw. The brain has spent forty-five years learning that traffic on those neurons means "skin of the chest, arm, and jaw," because that is what has caused it every previous time. When ischaemia sends an unfamiliar signal up the same wire, the brain reads the address, not the source. This is referred pain by segmental convergence, and it is why the location of visceral pain is a statement about spinal segments rather than about anatomy. The boundary of the percept is therefore a segmental boundary: a band, on one side or across the front, matching the dermatomes of T1–T5.

The neuropathic map is a map of axon lengths. Her foot numbness does not involve segments at all. The dorsal root ganglia are intact — she has no radiating pain, no band, no myotomal weakness, and the proximal parts of the same roots are normal. What is failing is the distal end of individual axons, and the failing set is defined by how far each terminal lies from its cell body. The border is therefore a contour of distance, symmetric because the two legs are the same length, gradual because axon length varies continuously with position on the foot and leg, and indifferent to which root any given axon belongs to.

Put the two side by side and the general principle appears: the shape of a sensory disturbance tells you which anatomical unit is diseased. A band on one side means a root. A patch with sharp edges means a named nerve. A horizontal level means a tract in the cord. And a symmetric, graded distal loss with no edge anywhere means the axon itself — the one "unit" in the nervous system whose boundary is not anatomical but metric.

There is a final, unwelcome connection between the two answers. The cardiac visceral afferents that gave Amara her warning pain in Chapter 1 are small A-delta and C fibers, and they enter the cord through the same dorsal root ganglia as the small fibers now dying in her feet. The disease producing her stocking numbness is attacking the same population that produced the symptom that saved her life. Chapter 14 takes that observation to its conclusion.

Systems Integration Case File · Entry 13

Entry 13 — The peripheral nervous system enters the file

New findings this chapter, at the six-month follow-up:

Finding Value
Distribution of numbness Symmetric stocking, toes to mid-calf, both feet
Vibration, 128 Hz, great toes Absent bilaterally
Vibration, medial malleoli 5 s right, 4 s left (expected > 10 s)
10 g monofilament Not felt at 4 of 10 plantar sites, both feet
Pinprick and cold, dorsum of foot Reduced bilaterally
Joint position sense, great toe and ankle Intact
Ankle jerks / knee jerks Absent / present and symmetric
Upper limb sensation and strength Normal
Foot skin Dry, fissured heels; 3 mm plantar callus, right first metatarsal head
HbA1c 7.1% (7.4% on admission)
Referred pain at presentation (Chapter 1) Chest wall, left arm, jaw — dermatomes T1–T5

Your entry:

1 · ADD (2–3 sentences). State what the peripheral nervous system contributes to Amara's picture. Use at least three of the numbers above, name the fiber classes involved, and say what shape her deficit has and what that shape localizes to.

2 · CONNECT (2–3 sentences). Link the PNS to at least two systems already in your file, stating the direction of causation each time. Consider: nervous (Chapters 11, 12), cardiovascular (Chapter 1's presentation and her coronary disease), integumentary (Chapter 5), skeletal (Chapter 6, Adwoa), muscular and rehabilitation (Chapter 10), and the endocrine system arriving in Chapter 16.

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 an early, symmetric, length-dependent, mixed axonal polyneuropathy: absent vibration at both great toes, monofilament failure at 4 of 10 plantar sites, and absent ankle jerks with intact knee jerks establish large-fiber (A-beta and Ia/A-alpha) involvement, while reduced pinprick and cold on the dorsum of the foot establish small-fiber (A-delta and C) involvement. The deficit is shaped as a stocking, which means its boundary is a contour of distance from the cell body rather than a segmental or nerve border — so the diseased unit is the axon, not the root, not a named nerve, and not the cord. Preserved joint position sense at the ankle and normal hand sensation date the process as early, because position sense is redundantly reported and the upper limb axons are shorter.

2 · CONNECT. Endocrine → nervous: four years of hyperglycaemia (HbA1c 7.4% at admission, 7.1% now) causes polyol flux, protein glycation, and oxidative stress in peripheral axons, and the longest axons fail first — the metabolic disease is the cause and the neuropathy is the consequence, not the reverse. Cardiovascular → nervous: the same process that narrowed her left circumflex artery narrows the vasa nervorum inside the epineurium, so ischaemia of the nerve's own blood supply causes further axonal loss — one vascular disease producing two apparently unrelated organ failures. Nervous → integumentary: loss of protective sensation causes uncorrected repetitive pressure, which is what the 3 mm plantar callus is; and loss of sympathetic cholinergic C fibers to the sweat glands causes the dry, fissured skin, so both the sensory and the autonomic limbs of the same neuropathy are visible on the same foot (Chapter 5). Nervous → skeletal: absent ankle jerks and degraded distal proprioception cause increased postural sway and fall risk, which in a family with Adwoa's femoral neck T-score of −2.9 (Chapter 6) converts into fracture risk. Nervous → cardiovascular (reverse arrow): the referred chest, arm, and jaw pain of Chapter 1 was produced by convergence of cardiac visceral afferents onto T1–T5 somatic second-order neurons — the ischaemic heart caused a sensation in the skin.

3 · PREDICT. I expect the autonomic consequences of the same fiber loss to be measurable in Chapter 14 — a raised resting heart rate and a reduced heart rate variability — because postganglionic autonomic fibers are unmyelinated C fibers of exactly the population that is failing in her feet, and the vagus is the longest autonomic nerve in the body. A second defensible prediction: because the small A-delta and C afferents that carry cardiac pain are the same population, I expect at least one future ischaemic episode to be painless, and I expect the retinal examination in Chapter 15 to show microvascular changes, since the same four mechanisms damage retinal capillaries and vasa nervorum alike. A third: a foot ulcer under that callus unless offloading and footwear are addressed, since 4 of 10 monofilament sites is below the threshold of protective sensation.

Review

Level 1 · Recall

13.1 Which receptor is slowly adapting and has a small receptive field, making it the principal receptor for reading fine surface texture?

a) Pacinian corpuscle    b) Meissner corpuscle    c) Merkel disc    d) Ruffini ending

Answer

c — Merkel disc. Slowly adapting (tonic) with a 2–5 mm receptive field. (a) The Pacinian corpuscle is the most rapidly adapting receptor in the body, with a receptive field covering a whole fingerpad — the opposite on both counts. (b) Meissner is rapidly adapting with a small field: good for flutter and slip detection, not for sustained form. (d) Ruffini is slowly adapting but with a large field of 10–50 mm, and it reports skin stretch rather than surface detail.

13.2 Vibration sense tested with a 128 Hz tuning fork is carried to the spinal cord by:

a) unmyelinated C fibers    b) thinly myelinated A-delta fibers    c) large myelinated A-beta fibers    d) B fibers

Answer

c — large myelinated A-beta fibers, chiefly from Pacinian and Meissner corpuscles, conducting at 35–75 m/s and ascending in the dorsal columns. (a) C fibers carry dull second pain, warmth, itch, and postganglionic autonomic traffic. (b) A-delta fibers carry sharp first pain and cold. (d) B fibers are autonomic preganglionic. The practical consequence is that a tuning fork is a large-fiber test and tells you nothing about small-fiber function, which is why a patient can have burning feet and a perfectly normal vibration threshold.

13.3 A patient cannot wrinkle their forehead, cannot close the right eye, and has a drooping right corner of the mouth. The lesion is:

a) left motor cortex    b) right facial nerve    c) right trigeminal nerve    d) left internal capsule

Answer

b — right facial nerve (a lower motor neuron lesion). Forehead involvement is the decisive finding: the upper facial nucleus receives bilateral corticobulbar input, so an upper motor neuron lesion (a and d) spares the forehead. (c) The trigeminal nerve supplies facial sensation and the muscles of mastication, not facial expression.

13.4 "C3, 4, 5 keep the diaphragm alive" refers to the:

a) vagus nerve    b) phrenic nerve    c) long thoracic nerve    d) accessory nerve

Answer

b — the phrenic nerve, from the cervical plexus. It is the sole motor supply to the diaphragm, which is why a complete cord injury above C3 requires permanent ventilation and one at C6 usually does not. (a) The vagus supplies thoracic and abdominal viscera, not the diaphragm's skeletal muscle. (c) The long thoracic nerve (C5–C7) supplies serratus anterior; injury causes a winged scapula. (d) The accessory nerve supplies sternocleidomastoid and trapezius.

13.5 The stretch (myotatic) reflex is unique among human reflexes because it is:

a) contralateral    b) monosynaptic    c) autonomic    d) learned rather than innate

Answer

b — monosynaptic. The Ia afferent from the muscle spindle synapses directly on the alpha motor neuron of the same muscle, with no interneuron, giving a latency of about 20–30 ms. (a) It is ipsilateral; the crossed-extensor reflex is the contralateral one. (c) It is somatic. (d) It is innate.

13.6 Surgical section of a single dorsal root usually produces:

a) complete anesthesia of one dermatome    b) little or no detectable numbness    c) paralysis of the corresponding myotome    d) loss of pain but not touch

Answer

b — little or no detectable numbness. Adjacent dermatomes overlap by roughly 50 percent, so the skin of any one segment is also supplied by the roots above and below; two or three adjacent roots must be lost before a clear sensory level appears. (a) is the common misconception the overlap disproves. (c) is wrong because motor axons leave in the ventral root — Bell-Magendie law. (d) describes a spinothalamic tract lesion, not a root lesion; a dorsal root carries all modalities from its territory.

13.7 A symmetric, gradual, distal sensory loss shaped like a stocking, with no sharp border anywhere, localizes the disease process to:

a) a single nerve root    b) the dorsal columns of the spinal cord    c) the peripheral axon, failing according to length    d) one named peripheral nerve

Answer

c — the peripheral axon. The boundary of the deficit is a distance from the cell body rather than any anatomical border, which is the signature of length-dependent axonopathy. (a) A root lesion produces a band on one side with a segmental edge, often with radiating pain and myotomal weakness. (b) A dorsal column lesion produces a horizontal level below which vibration and position sense are lost bilaterally, typically with a spastic rather than an areflexic picture. (d) A mononeuropathy produces a sharply bounded patch matching one cable's cutaneous map, usually with weakness in that nerve's muscles.

Level 2 · Comprehension

13.8 Explain why a receptor's capsule rather than its axon determines what stimulus it responds to, using the Pacinian corpuscle as your example.

Model answer

The axon terminal in a Pacinian corpuscle contains mechanically gated ion channels that open when the membrane is deformed. Those channels have no intrinsic preference for vibration over sustained pressure — strip the capsule away and the naked terminal responds to steady pressure with a sustained receptor potential.

The capsule creates the selectivity mechanically. Its 20–60 lamellae are separated by fluid. Apply steady pressure and the fluid flows between lamellae within a few milliseconds, redistributing the load so the lamellae re-equalize and the core is no longer deformed — the receptor potential collapses and firing stops. Only a stimulus changing faster than the fluid can redistribute reaches the core, which makes the capsule a viscoelastic high-pass filter with peak sensitivity near 250 Hz.

The general lesson is that sensory tuning is often an accessory-structure property, not a membrane property. The same principle appears in the ear, where the basilar membrane's stiffness gradient rather than any property of the hair cells creates the tonotopic map (Chapter 15).

13.9 Distinguish a nerve root lesion from a peripheral nerve lesion on clinical grounds, and explain from plexus anatomy why the distinction works.

Model answer

A root lesion produces a dermatomal sensory pattern and a myotomal motor pattern — mild weakness spread across several muscles that happen to share that segment, often with a reduced reflex, and pain radiating in a band. A peripheral nerve lesion produces the sensory territory and muscle list of that specific nerve, with weakness that can be complete.

The anatomy behind it is the plexus. Each terminal nerve is assembled from fibers of two to four roots, and each root distributes fibers into several terminal nerves. So a single root lesion removes only that root's share of the motor units of any given muscle — biceps loses its C5 contribution but keeps C6 — producing weakness rather than paralysis. A peripheral nerve carries all of a muscle's supply, so cutting it denervates the muscle completely.

Sensory sparing patterns refine it further, because cutaneous branches leave at known points: a spared thenar eminence in carpal tunnel syndrome (the palmar cutaneous branch leaves proximal to the retinaculum), a spared dorsal hand in a wrist-level ulnar lesion (the dorsal cutaneous branch leaves 5–8 cm proximal to the wrist), spared triceps in a spiral-groove radial palsy. The lesion's level is read from which branches still work.

13.10 A patient has two years of burning, unpleasant sensations in both feet. Vibration, monofilament, joint position sense, and ankle jerks are normal, and the nerve conduction study is entirely normal. Explain how the study can be normal in a patient with genuine nerve disease.

Model answer

A nerve conduction study records a summed potential from a population of axons through surface electrodes, and the summed potential is dominated by the fastest and largest fibers in that population. Unmyelinated C fibers, conducting at 0.5–2 m/s, and thinly myelinated A-delta fibers contribute essentially nothing measurable: their responses are tiny, dispersed over tens of milliseconds, and buried in the trace. The study is, in effect, an assay of large myelinated axons only.

Every bedside test listed in the question interrogates the same large-fiber population — vibration and monofilament through A-beta afferents, joint position through A-beta and group Ia, and the ankle jerk through Ia in and A-alpha out. So a pure small fiber neuropathy can destroy the A-delta and C population entirely while leaving every one of those measurements normal. The patient's symptoms are real and the objective tests are silent, which is a common and damaging source of misdiagnosis.

The tests that do detect it are quantitative sensory testing of thermal detection thresholds, autonomic function testing (postganglionic autonomic fibers are C fibers, so sudomotor testing is often abnormal), and a 3 mm skin punch biopsy stained for PGP9.5 to count intraepidermal nerve fiber density. The general principle worth extracting: every test has a fiber-class sensitivity, and a normal test excludes only what that test can see.

Level 3 · Clinical Application

13.11 A 68-year-old vegetarian with a history of gastric surgery reports numb, clumsy hands and an unsteady gait, worse in the dark. On examination, vibration and joint position sense are lost to the knees and wrists, pinprick is preserved, the Romberg test is strongly positive, knee reflexes are brisk, and both plantar responses are extensor. Explain the findings and localize the lesion.

Model answer

This is subacute combined degeneration of the spinal cord from vitamin B₁₂ deficiency, and the localization is central, not peripheral, despite the distal distribution.

  • Vibration and joint position lost together places the lesion in the dorsal columns, where both modalities ascend in the same tract. Note the contrast with a length-dependent polyneuropathy such as Amara's, which dissociates them — vibration goes years before position sense, because position sense is reported redundantly by several receptor populations.
  • Pinprick preserved confirms the spinothalamic tract is spared. A peripheral neuropathy attacking axons non-selectively would not spare it so cleanly.
  • Romberg strongly positive is a proprioceptive sign: with the eyes open, vision substitutes for the missing dorsal column input; close the eyes and the substitution is removed and the patient falls. It is not a cerebellar test — a cerebellar patient is unsteady with the eyes open.
  • Brisk reflexes and extensor plantars are the decisive finding. These are upper motor neuron signs, from involvement of the lateral corticospinal tracts — the "combined" in subacute combined degeneration. A peripheral neuropathy causes absent reflexes, never brisk ones. Any patient with a distal sensory deficit and increased reflexes has a cord lesion until proven otherwise.
  • Hands as well as feet, early is a further clue against a length-dependent process, in which the hands are involved only after the legs are affected to mid-calf.

The mechanism is impaired methylmalonyl-CoA mutase and methionine synthase activity, producing defective myelin maintenance in the large myelinated tracts. Gastric surgery removes the parietal cells that make intrinsic factor, and a vegetarian diet removes the dietary source (Chapter 23, Chapter 24). Treatment is parenteral B₁₂; the sensory deficits often improve, but established corticospinal signs may not.

13.12 A 44-year-old develops a right foot drop after a period of prolonged squatting while tiling a floor. Examination shows weak dorsiflexion and eversion, normal inversion, normal plantar flexion, numbness over the dorsum of the foot and lateral leg, and a normal ankle jerk. A colleague suggests an L5 radiculopathy. Argue for the correct diagnosis using three specific findings.

Model answer

This is a common fibular (peroneal) neuropathy at the fibular neck, not an L5 radiculopathy. Three findings separate them, and each is pure applied anatomy.

  1. Inversion is normal. Foot inversion is performed chiefly by tibialis posterior, which is L5-innervated but travels in the tibial nerve. An L5 root lesion weakens every L5 muscle regardless of which peripheral nerve carries it, so it weakens inversion. A common fibular lesion cannot, because tibialis posterior is not in that nerve. This single test is the most useful discriminator at the bedside.
  2. Hip abduction is normal (implied by an otherwise normal examination). Gluteus medius is L5-innervated through the superior gluteal nerve; L5 radiculopathy commonly weakens it, giving a positive Trendelenburg sign. A fibular neuropathy never does.
  3. The mechanism and the site fit. The common fibular nerve winds around the neck of the fibula with almost no soft tissue over it, and prolonged squatting, leg crossing, a tight cast, or a knee brace compresses it there. Percussion over the fibular neck often reproduces paraesthesiae (Tinel sign), and the sensory loss stops at the borders of the superficial and deep fibular cutaneous territories rather than following the L5 dermatome down to the great toe and up the lateral thigh.

A supporting negative: there is no back pain and no radiating pain in a band, which most symptomatic L5 radiculopathies have. The normal ankle jerk is consistent with both — the ankle jerk is S1, not L5 — and so is worth mentioning only to note that it does not discriminate. Nerve conduction studies would confirm focal slowing or conduction block across the fibular head, which is the classic demyelinating (neurapraxic) picture and predicts recovery over weeks to a few months with padding and avoidance of the posture.

13.13 A 39-year-old pregnant woman wakes at 3 a.m. most nights with tingling in her right hand and shakes it until it settles. Examination shows reduced sensation over the palmar surfaces of the thumb, index, middle, and radial half of the ring finger, normal sensation over the thenar eminence itself, and mild weakness of thumb abduction. Explain each finding anatomically and state why the spared patch of skin is nearly diagnostic.

Model answer

Carpal tunnel syndrome — compression of the median nerve beneath the flexor retinaculum.

  • Why pregnancy: the carpal tunnel is a rigid compartment with a bony floor and walls and an unyielding ligamentous roof. Fluid retention increases the volume of its contents in a space that cannot expand, so pressure rises. The same logic explains the association with hypothyroid myxoedema, rheumatoid synovitis, and amyloid.
  • Why 3 a.m.: the wrist is held flexed during sleep, and wrist flexion raises carpal tunnel pressure substantially. Ischaemia of the nerve within the tunnel produces the paraesthesiae; shaking the hand restores perfusion, which is why the manoeuvre works and why patients describe it so consistently.
  • The sensory territory is the median nerve's palmar distribution: thumb, index, middle, and radial half of the ring finger.
  • The spared thenar eminence is the key. The palmar cutaneous branch of the median nerve arises 5–8 cm proximal to the wrist and passes superficial to the flexor retinaculum, not through the tunnel. A lesion inside the tunnel therefore cannot affect it, and the skin over the thenar eminence stays normal. Numbness that includes the thenar eminence points to a lesion proximal to the wrist — a proximal median neuropathy, a medial cord lesion, or a C6–C7 radiculopathy — not to the carpal tunnel.
  • Weak thumb abduction reflects involvement of the recurrent motor branch to abductor pollicis brevis and opponens pollicis (the LOAF muscles). Abduction and opposition are the movements to test, because they are the ones no other nerve can substitute for.

The general principle is the one that runs through §13.5 and §13.7: the level of a nerve lesion is read from which branches still work. A spared cutaneous branch is as informative as an affected one.

Level 4 · Integration and Synthesis

13.14 Build the causal chain from Amara's twenty years of rotating night shift (Chapter 12) to the 3 mm callus under her right first metatarsal head, naming at least four organ systems and stating the direction of causation at every step. Then identify two points at which the chain becomes a positive feedback loop, and one point at which an intervention would break it.

Model answer

The chain.

  1. Circadian → nervous → endocrine. Twenty years of rotating night shift chronically misaligns the suprachiasmatic nucleus from the light–dark cycle, shortens and fragments sleep, raises evening cortisol, and reduces insulin sensitivity by 20–30 percent after even a few nights (Chapters 12, 16).
  2. Endocrine → metabolic. Insulin resistance raises fasting glucose and HbA1c — hers was 7.4% on admission and is 7.1% now — and promotes visceral adiposity and dyslipidaemia (Chapter 24).
  3. Metabolic → nervous (peripheral). Chronic hyperglycaemia damages peripheral axons by polyol flux, advanced glycation end products, oxidative stress, and microvascular disease of the vasa nervorum. Because the insult is uniform, the longest axons fail first (§13.7): absent vibration at the great toes, monofilament failure at 4 of 10 sites, absent ankle jerks.
  4. Metabolic → cardiovascular → nervous. The same metabolic state that produced her 90% proximal left circumflex stenosis narrows the arterioles inside the epineurium, so ischaemia of the nerve's own blood supply accelerates axonal loss. One vascular disease, two organs.
  5. Nervous → integumentary (sensory limb). Loss of protective sensation means repetitive pressure under the first metatarsal head is not detected and not corrected. Normally, low-level discomfort from tonic nociceptors makes you shift weight unconsciously many times an hour (§13.2); with the nociceptors gone, the load repeats identically thousands of times a day. Skin responds to repetitive shear with hyperkeratosis — the callus is the visible record of uncorrected load.
  6. Nervous → integumentary (autonomic limb). Loss of sympathetic cholinergic C fibers to the sweat glands of the foot causes anhidrosis, so the skin is dry and fissured — which is why the heels are cracked (Chapter 5, and Chapter 14).
  7. Integumentary → the next chapter of her life. A callus concentrates pressure on the tissue beneath it, raising local pressure by up to 30 percent; the tissue under it breaks down first, producing a subkeratotic haematoma and then an ulcer through skin that cannot report the injury.

Positive feedback loop 1 — the pressure–callus loop. Uncorrected pressure produces callus; callus raises the local pressure it is responding to; higher pressure produces more callus and faster tissue breakdown underneath. The loop has no sensory brake because the brake is precisely what has been lost.

Positive feedback loop 2 — the glucose–nerve loop. Hyperglycaemia damages autonomic and sensory nerves; autonomic damage causes gastroparesis and erratic carbohydrate absorption and blunts the counter-regulatory catecholamine response to hypoglycaemia; both worsen glycaemic control; worse control accelerates nerve damage. A third defensible loop: neuropathy degrades balance and gait, reducing activity, which worsens insulin sensitivity, which accelerates the neuropathy.

Where to break it. The highest-yield single intervention is at step 5: offload the foot — professional debridement of the callus, pressure-redistributing footwear or an insole, and daily visual inspection substituting vision for the missing sensory channel. It is the cheapest intervention in the chain and it interrupts the loop with the shortest cycle time. Glycaemic control (step 2) is the intervention with the largest long-term effect but the slowest — it slows progression of the neuropathy and does not restore lost axons. Note that the neurological finding is what makes the podiatric intervention urgent, which is the point of examining the feet of a cardiology patient at all.

13.15 Three patients each present with numb feet. Patient A has a symmetric stocking deficit with vibration lost before position sense, absent ankle jerks, and an HbA1c of 8.9%. Patient B has vibration and position sense lost together to the knees with brisk reflexes and extensor plantars. Patient C has patchy, asymmetric loss of all modalities involving the face, trunk, and both arms as severely as the feet, with profound sensory ataxia and areflexia. For each, name the anatomical unit that is diseased, predict the Romberg result, and explain how the shape of the deficit gave you the answer.

Model answer

Patient A — the axon. Length-dependent diabetic polyneuropathy. The diseased unit is the distal peripheral axon, and the boundary of the deficit is a contour of distance from the cell body, which is why it is symmetric, graded, and edgeless. Vibration precedes position sense because vibration is a single-channel, most-distal, largest-fiber measurement while position sense is reported redundantly by spindles, joint capsule receptors, cutaneous stretch, and corollary discharge. The ankle jerk goes first because it has the longest reflex arc in the body. Romberg: negative early, becoming positive later — when the loss finally reaches the redundant proprioceptive channels. Amara is at the early stage of exactly this.

Patient B — the tract. Subacute combined degeneration of the cord from B₁₂ deficiency. The diseased unit is the dorsal column (with the lateral corticospinal tracts), and the boundary is a level, not a distance. Vibration and position sense are lost together because they travel in the same tract; pinprick is spared because the spinothalamic tract is separate. The decisive finding is brisk reflexes with extensor plantars — upper motor neuron signs, which no peripheral neuropathy can produce. Romberg: strongly positive, because the dorsal column input is gone wholesale and vision is the only remaining substitute.

Patient C — the cell body. A sensory ganglionopathy (neuronopathy) attacking dorsal root ganglion neurons themselves — paraneoplastic (classically small-cell lung cancer with anti-Hu antibodies), Sjögren syndrome, cisplatin toxicity, or pyridoxine excess. Because the cell body supplies both the peripheral and the central process, its death removes the whole neuron, and the loss is not length-dependent at all: it is patchy, asymmetric, and non-length-dependent, involving face and trunk — territory no distal axonopathy reaches until very late. All modalities go because ganglion cells of every size are attacked. Romberg: dramatically positive, often with pseudoathetosis of the outstretched hands, because proprioception is destroyed globally rather than gradually.

The synthesis. All three patients say "my feet are numb," and all three deficits look distal at a glance. What separates them is the shape: a distance contour indicts the axon, a horizontal level indicts the tract, and a patchy non-length-dependent distribution indicts the cell body. The accompanying signs then confirm it — areflexia points peripheral, brisk reflexes point central, and proximal or facial involvement points to the ganglion. This is why the question at the bedside is never "is there sensory loss?" but always "what is the shape of it, and what anatomical unit has a border that shape?"

Concept Map to Complete

Copy this onto blank paper and fill every bracket from memory before checking the chapter.

                        PERIPHERAL NERVOUS SYSTEM
                                    │
              ┌─────────────────────┴────────────────────┐
        [ __________ ] DIVISION                    [ __________ ] DIVISION
         (carried toward CNS)                       (carried away from CNS)
              │                                            │
      ┌───────┼───────┐                        ┌───────────┴───────────┐
   somatic  visceral  special              [ ________ ]          [ ________ ]
   sensory  sensory   senses               ONE neuron            TWO neurons
                                           effector =            effector =
   RECEPTORS classified by:                [ __________ ]        [ ____________ ]
   · STIMULUS: [ ____ ]receptor            transmitter =         ► Chapter 14
               [ ____ ]receptor            [ ______ ]
               [ ____ ]receptor            receptor =
               [ ____ ]receptor            [ ______ ]
               [ ____ ]receptor            effect always
   · LOCATION: [ ____ ] / [ ____ ]         [ __________ ]
               / [ ____ ]                  ending = [ ___________ ]
   · STRUCTURE: free  vs  [ ________ ]     safety factor = [ ___ ]

   ADAPTATION                              REFLEX ARC — five parts
   ┌──────────────┬──────────────┐         [ _______ ] → [ ________ ] →
   [ ______ ]     [ ______ ]               [ ___________ ] → [ ________ ] →
   rapidly adapt  slowly adapt             [ ________ ]
   examples:      examples:                monosynaptic example = [ ______ ]
   [ _____ ]      [ _____ ]                contralateral example = [ ______ ]
   [ _____ ]      [ _____ ]
   MEANING:       MEANING:                 FIBER CLASSES — fastest to slowest
   [ __________ ] [ ___________ ]          A-alpha  → [ ______________ ]
                                           A-beta   → [ ______________ ]
   TERRITORY SHAPE  ⇒  DISEASED UNIT       A-delta  → [ ______________ ]
   stocking, symmetric  ⇒ [ ______ ]       C        → [ ______________ ]
   band, one side       ⇒ [ ______ ]       velocity ≈ [ __ ] × diameter
   sharp patch          ⇒ [ ______ ]
   horizontal level     ⇒ [ ______ ]       ASCENDING PATHWAYS
                                           dorsal column carries [ _________ ]
   TWO-POINT THRESHOLD                       and crosses in the [ ________ ]
   fingertip = [ ____ ] mm                 spinothalamic carries [ ________ ]
   back      = [ ____ ] mm                   and crosses in the [ ________ ]
   set by [ _________________ ]            ⇒ hemisection gives [ ___________ ]

Lab / Self-Exploration

  1. Map your own two-point discrimination. Bend a paper clip into a U and adjust the gap. With a partner's eyes closed, touch one or two points simultaneously on the fingertip, palm, forearm, and back of the neck, and find the smallest gap felt as two. Plot threshold against body region and compare with the table in §13.2. Randomly intersperse one-point trials to control for guessing.
  2. Demonstrate phasic adaptation. Have a partner close their eyes while you rest a coin on the back of their forearm. Ask them to say when they stop feeling it — typically 10–20 seconds. Now slide the coin one centimetre. The sensation returns instantly. You have just shown that the receptor reports change, not state.
  3. Time your own double pain. Pinch the web of your thumb sharply and attend carefully. A sharp, well-localized first pain arrives on A-delta fibers; a duller, more diffuse second pain follows roughly a second later on C fibers. Repeat on the toe, where the axons are longer, and note that the gap is wider. You have measured conduction velocity in your own body.
  4. Elicit the stretch reflex and demonstrate reinforcement. Cross one leg over the other and tap the patellar tendon with the edge of your hand. Then repeat while clenching your teeth and pulling your interlocked hands apart (the Jendrassik manoeuvre). The reflex is larger, because the effort raises gamma motor neuron drive and increases spindle sensitivity.
  5. Run a Romberg test on yourself, safely. Stand with feet together beside a wall or a partner. Count your postural corrections for 30 seconds with the eyes open, then repeat with the eyes closed. Sway typically increases two- to threefold: vision was silently supplying about half your balance information. Then repeat standing on a folded towel, which degrades cutaneous and ankle proprioceptive input, and note that the eyes-closed condition becomes far harder. You have just reproduced, in ninety seconds, the reason a patient with large-fiber sensory loss falls in the dark.
  6. Test vibration and light touch against each other. Borrow a tuning fork (128 Hz if possible; a 512 Hz fork from a music shop works for demonstration). Strike it and place the base on the bony prominence of your great toe, then your medial malleolus, then your knee, and time how long you feel it at each. Then repeat with a wisp of cotton for light touch. In a healthy young adult both are easy everywhere; the point of the exercise is to learn the technique and the normal duration, so that an abnormal result is recognizable when you meet one.
  7. Map a dermatome on a partner. Using a cool metal spoon, run a line from the clavicle straight down the front of the trunk and ask your partner to report the temperature as "the same" or "different" every 3 cm. Nothing should change — which is the point: overlap makes boundaries invisible in a healthy person. Now identify the landmarks by palpation instead — nipple (T4), xiphoid (T6), umbilicus (T10) — and commit them to memory, because these three are the ones you will actually use.

Key Terms

adaptation · Decline in receptor potential and firing rate during a constant stimulus; rapid in phasic receptors, slow or absent in tonic receptors.

A-beta fiber · Large myelinated sensory axon, 6–12 µm, conducting at 35–75 m/s; carries fine touch, pressure, vibration, and conscious proprioception.

A-delta fiber · Thinly myelinated sensory axon, 1–5 µm, conducting at 5–30 m/s; carries sharp first pain and cold.

axonal transport · Movement of proteins, organelles, and membrane between the cell body and the axon terminal; fast anterograde transport runs at 200–400 mm/day, the slow cytoskeletal component at about 1 mm/day.

Bell-Magendie law · Dorsal roots are sensory and ventral roots are motor.

C fiber · Unmyelinated axon, 0.2–1.5 µm, conducting at 0.5–2 m/s; carries dull second pain, warmth, itch, visceral sensation, and all postganglionic autonomic traffic.

corollary discharge · An internal copy of a motor command sent to sensory areas, allowing the brain to predict the sensory consequences of its own movements.

cranial nerves · The twelve pairs of nerves attaching directly to the brain; four (III, VII, IX, X) carry parasympathetic fibers.

crossed-extensor reflex · Polysynaptic contralateral reflex that extends the opposite limb to bear weight while the stimulated limb withdraws.

dermatome · The strip of skin supplied by the sensory fibers of a single spinal nerve; adjacent dermatomes overlap by roughly 50 percent.

dorsal column–medial lemniscal pathway · Ascending pathway for fine touch, vibration, and conscious proprioception; travels ipsilaterally in the cord and crosses in the medulla.

dorsal root ganglion · Cluster of pseudounipolar sensory cell bodies on the dorsal root; contains no synapses.

encapsulated nerve ending · A sensory terminal wrapped in a connective tissue capsule that acts as a mechanical filter determining the stimulus to which it responds.

epineurium, perineurium, endoneurium · The three connective tissue sheaths of a peripheral nerve, wrapping the whole nerve, each fascicle, and each axon respectively.

exteroceptor / interoceptor / proprioceptor · Receptors classified by location: at the body surface, within viscera, and within muscles, tendons, and joints.

free nerve ending · An unencapsulated sensory terminal; mediates pain, temperature, itch, and crude touch on A-delta and C fibers.

ganglion · A cluster of neuron cell bodies outside the CNS; sensory ganglia contain no synapses, autonomic ganglia are synaptic relays.

generator potential / receptor potential · The local, graded depolarization produced by transduction; if it reaches threshold at the trigger zone, action potentials fire at a frequency proportional to its amplitude.

Golgi tendon organ · Proprioceptor in series with muscle at the musculotendinous junction; reports muscle tension and drives the tendon reflex.

law of specific nerve energies · What is perceived depends on which fiber fires and where it projects, not on what stimulated it.

length-dependent axonopathy · Distal dying-back degeneration in which a uniform metabolic or toxic insult causes the longest axons to fail first, producing a symmetric stocking-and-glove deficit.

Meissner corpuscle · Rapidly adapting encapsulated mechanoreceptor in dermal papillae of glabrous skin; detects flutter at 10–50 Hz and object slip.

Merkel disc · Slowly adapting receptor at the epidermal–dermal junction; detects sustained pressure, edges, and fine texture with a small receptive field.

monofilament (10 g Semmes-Weinstein) · Calibrated nylon filament used to test protective pressure sensation through A-beta afferents; failure at four or more of ten plantar sites predicts foot ulceration.

mononeuritis multiplex · Sequential, asymmetric failure of individual named nerves, indicating disease of the vasa nervorum rather than a length-dependent process.

muscle spindle · Proprioceptor lying in parallel with muscle fibers; reports muscle length and rate of change of length, and drives the stretch reflex.

neuromuscular junction · The somatic motor ending; junctional folds packed with nicotinic receptors give guaranteed one-to-one transmission with a safety factor of three to four.

nociceptor · Receptor responding to actual or threatened tissue damage; tonic and often sensitizing, so pain does not fade.

Pacinian (lamellated) corpuscle · Deep, extremely rapidly adapting mechanoreceptor whose fluid-separated lamellae make it a mechanical high-pass filter tuned near 250 Hz.

phasic (rapidly adapting) receptor · Reports change; falls silent during a sustained stimulus.

plexus · A network of ventral rami exchanging fibers before re-sorting into peripheral nerves; cervical, brachial, lumbar, and sacral.

radiculopathy · Disease of a nerve root, producing a dermatomal sensory pattern, myotomal weakness, radiating pain, and a lost reflex on one side.

receptive field · The area of the body surface whose stimulation alters the firing of one sensory neuron; small fields give fine two-point discrimination.

referred pain · Visceral pain perceived in a somatic region sharing the same spinal segments, because visceral and somatic afferents converge on the same second-order neurons.

reflex arc · Receptor, afferent neuron, integration centre, efferent neuron, effector; the minimal functional unit of the nervous system.

Romberg test · Comparison of postural sway with the eyes open and closed; increased sway or falling with the eyes closed indicates loss of large-fiber proprioception.

Ruffini ending · Slowly adapting encapsulated receptor in deep dermis and joint capsules; detects skin stretch and the direction of stretch.

sensory ganglionopathy (neuronopathy) · Disease of dorsal root ganglion cell bodies producing patchy, non-length-dependent sensory loss including face and trunk, with profound sensory ataxia.

small fiber neuropathy · Selective loss of A-delta and C fibers, causing burning pain and thermal loss with entirely normal nerve conduction studies; diagnosed by thermal thresholds and intraepidermal nerve fiber density.

spinal nerve · The short mixed nerve formed by union of a dorsal and a ventral root, dividing immediately into dorsal and ventral rami; 31 pairs.

spinothalamic (anterolateral) pathway · Ascending pathway for pain, temperature, and crude touch; crosses within one or two segments of entry and ascends contralaterally.

stretch (myotatic) reflex · The only monosynaptic human reflex; a postural servo maintained by alpha-gamma coactivation.

tonic (slowly adapting) receptor · Reports state; continues firing throughout a sustained stimulus.

transduction · Conversion of a physical or chemical stimulus into a change in receptor membrane potential.

two-point discrimination · The smallest separation at which two simultaneous touches are felt as two; 2–4 mm at the fingertip, 45–70 mm on the back.

varicosity · A swelling along an autonomic postganglionic axon that releases transmitter diffusely into the interstitial space rather than into a specialized cleft.

vibration sense · A-beta-mediated large-fiber modality tested with a 128 Hz tuning fork; the earliest large-fiber function to fail in a length-dependent neuropathy.

Next: Chapter 14 · The Autonomic Nervous System — the other half of the motor division, where two neurons in series and eight receptor subtypes let the nervous system speed a heart, empty a bladder, and be talked out of all of it by a drug — and where the same small fibers failing in Amara's feet turn out to be failing at her sinoatrial node.