Part II · Support and Movement · Estimated reading time 105 minutes · Prerequisites: Chapters 1–4 — especially §3.3–3.5, membrane transport and membrane potential, and §4.6 and §4.9, muscle tissue and tissue repair
In This Chapter
- Learning Objectives
- 9.1 Three Tissues, One Job
- 9.2 The Architecture of a Skeletal Muscle
- 9.3 The Sarcomere
- 9.4 The Neuromuscular Junction and the Motor Unit
- 9.5 Excitation–Contraction Coupling
- 9.6 The Mechanics of Contraction
- 9.7 Muscle Metabolism
- 9.8 Fiber Types and the Size Principle
- 9.9 Advanced Topic · Smooth and Cardiac Muscle
- Chapter Summary
- Case File 9 · Resolution
- Systems Integration Case File · Entry 9
- Review
- Key Terms
9. The Muscular System I
Muscle Tissue Physiology — How Muscles Contract
Case File 9 — "Two Muscles, One Patient"
It is hospital day 3. Amara Osei is out of the coronary care unit and sitting up in a step-down bed, and the conversation has moved from what is happening to what was lost and what happens next. Two different muscles are now the whole story, and they are behaving in opposite ways.
Muscle one — her heart. A bedside echocardiogram shows that a wedge of her left ventricle, in the territory of the circumflex artery that was stented on day 1, is not moving. The rest of the wall thickens and squeezes with each beat; this segment simply sits there and gets dragged along. The laboratory has also been tracking a protein in her blood.
| Measurement | Value | Reference / meaning |
|---|---|---|
| Cardiac troponin I, 08:45 (1 h) | 0.09 ng/mL | 99th percentile 0.04 ng/mL |
| Cardiac troponin I, 14:30 (7 h) | 2.40 ng/mL | Unequivocal necrosis, still climbing |
| Cardiac troponin I, 20:30 (12 h) | 4.10 ng/mL | Peak; infarct size moderate |
| Cardiac troponin I, day 3 | 0.91 ng/mL | Falling, as expected |
| Creatine kinase (total), day 1 | 310 U/L | 30–200 U/L |
| Echocardiogram, day 3 | Hypokinesis of the lateral wall, with the mid-lateral segments frankly akinetic (~14% of LV mass) | Every segment should thicken and move inward |
| Left ventricular ejection fraction | 48% | 55–70% |
Muscle two — everything else. Before discharge she is started on atorvastatin 80 mg nightly, and the pharmacist gives her the standard counselling: tell us about new muscle aches, especially in the thighs and shoulders, and especially if your urine turns dark. Amara, who is a nurse, asks the question nobody on the team enjoys answering: the drug works on cholesterol, in my liver. Why would it hurt my legs?
Three questions to hold on to.
- Troponin is the number everyone is watching, and every clinician calls it a "cardiac marker." But markers are side effects of a molecule's day job. What is troponin actually doing inside a working muscle cell, and why does its presence in blood mean what it means?
- Toby tore a hamstring in high school and it healed. Amara's ventricle will not. Both are striated muscle, built from the same proteins, running the same contractile cycle. Why can skeletal muscle regenerate and cardiac muscle cannot?
- Why does a drug that inhibits an enzyme of cholesterol synthesis in the liver produce pain in skeletal muscle — and why, very rarely, can it dissolve muscle outright?
Learning Objectives
By the end of this chapter you should be able to:
- Compare skeletal, cardiac, and smooth muscle on structure, control, calcium source, speed, fatigue resistance, and regenerative capacity.
- State the four universal properties of muscle tissue — excitability, contractility, extensibility, elasticity — and give the structural basis of each.
- Trace the structural hierarchy from muscle to fascicle to fiber to myofibril to sarcomere to myofilament, naming the connective tissue sheath at each level.
- Explain how epimysium, perimysium, and endomysium converge into a tendon, and why force must pass through connective tissue rather than through muscle cells alone.
- Describe the sarcolemma, T-tubules, sarcoplasmic reticulum, terminal cisternae, and triads, and explain the diffusion problem T-tubules solve.
- Label the A band, I band, H zone, M line, and Z disc of a sarcomere, and predict exactly which of them change width during contraction and which do not.
- Name the proteins of the thick and thin filaments, including titin, nebulin, tropomyosin, and the three troponin subunits, and assign a function to each.
- Sequence the events at the neuromuscular junction from action potential to end plate potential, and explain the role of acetylcholinesterase and the junctional safety factor.
- Define the motor unit and relate innervation ratio to precision, using the extraocular muscles and the gastrocnemius as the two extremes.
- Sequence excitation–contraction coupling from action potential through the DHP receptor, ryanodine receptor, calcium release, troponin C, and tropomyosin displacement.
- Describe the four steps of the cross-bridge cycle, identify both roles of ATP within it, and use rigor mortis to prove that ATP is required for cross-bridge release.
- Distinguish twitch, wave summation, tetanus, and treppe, and explain graded force production by recruitment and rate coding.
- Distinguish isotonic (concentric and eccentric) from isometric contraction, and explain the length–tension and force–velocity relationships from filament overlap and cross-bridge kinetics.
- Compare the three ATP-regenerating systems by time domain, power output, and capacity, and explain EPOC.
- State the actual causes of muscle fatigue, and explain precisely why "lactic acid build-up" is the wrong answer.
- Compare type I, IIa, and IIx fibers on at least ten properties, apply the size principle to recruitment order, and distinguish the adaptations produced by endurance versus resistance training.
- Explain calmodulin/MLCK regulation and the latch state in smooth muscle, and explain why the absence of tetanus in cardiac muscle is not a limitation but a requirement.
9.1 Three Tissues, One Job
Muscle is the only tissue in the body that can shorten forcefully. Everything else the body does mechanically — moving a limb, moving blood, moving food, moving air, closing a sphincter, raising a hair — is downstream of that one capability, executed by one of three tissues built on the same molecular principle and packaged three very different ways.
Roughly 40% of body mass in a lean adult male and 32% in a lean adult female is skeletal muscle. Another 10% or so is smooth and cardiac muscle. There is no other single tissue that accounts for so much of you.
The four universal properties
Every muscle cell, of every type, has four properties. Two are shared with other excitable tissues; two are unique to muscle.
| Property | Definition | Structural basis |
|---|---|---|
| Excitability (responsiveness) | The ability to receive a stimulus and respond with a change in membrane potential | Voltage-gated and ligand-gated ion channels in the plasma membrane; shared with neurons |
| Contractility | The ability to shorten forcefully when stimulated | Interdigitating actin and myosin filaments plus a myosin ATPase — unique to muscle |
| Extensibility | The ability to be stretched beyond resting length without damage | Titin's spring-like domains, plus the collagen and elastin of the surrounding sheaths |
| Elasticity | The ability to recoil to resting length after being stretched | The same elements, storing and returning strain energy |
The last two are the ones students skip, and they are exactly the two that explain why muscles work in antagonistic pairs. A muscle can pull; it cannot push. It cannot even lengthen itself. Every contraction must eventually be undone by something else — an opposing muscle, gravity, or elastic recoil — and extensibility is what makes that safe. A tissue that could only shorten and could not be safely stretched would tear the first time its antagonist fired.
Thread 1 · Structure Determines Function
Hold the four properties in mind as you read the rest of the chapter, because every one of them will turn out to be a specific protein. Excitability will be the acetylcholine receptor and the voltage-gated sodium channel. Contractility will be the myosin head. Extensibility and elasticity will be titin — a single molecule spanning half a sarcomere, behaving like a molecular spring. This is the thread stated as sharply as it can be stated: a property of a tissue, when you look closely enough, is always a shape of a molecule.
The three types compared
SKELETAL CARDIAC SMOOTH
═══════════════════════════════════════════════════════════════════════════
LOCATION attached to bone heart wall only walls of hollow
(+ some skin, organs, vessels,
sphincters) airways, iris, hair
───────────────────────────────────────────────────────────────────────────
CELL SHAPE long cylinder short branched fusiform (spindle)
10–100 µm wide cylinder 2–10 µm wide
up to 30 cm long 10–20 µm wide 20–200 µm long
50–100 µm long (500 µm in uterus)
───────────────────────────────────────────────────────────────────────────
NUCLEI MANY (hundreds), 1–2, central ONE, central
peripheral
───────────────────────────────────────────────────────────────────────────
STRIATIONS YES YES NO
SARCOMERES YES, aligned YES, aligned none — oblique
lattice on dense
bodies
───────────────────────────────────────────────────────────────────────────
CONTROL VOLUNTARY involuntary involuntary
somatic motor autonomic autonomic + hormones
neuron, 1 per MODULATES an + local chemicals
fiber, obligatory intrinsic rhythm + stretch
───────────────────────────────────────────────────────────────────────────
PACEMAKER none — silent YES (SA node) YES in single-unit
without a nerve autorhythmic (e.g. gut)
───────────────────────────────────────────────────────────────────────────
CELL-TO-CELL NONE. Each fiber GAP JUNCTIONS in GAP JUNCTIONS in
COUPLING is electrically intercalated single-unit types;
private discs → syncytium none in multi-unit
───────────────────────────────────────────────────────────────────────────
Ca2+ SOURCE ~100% from SR ~20–25% extra- mostly EXTRACELLULAR
(no extracellular cellular, which + a small SR
Ca2+ needed) TRIGGERS ~75–80%
from SR (CICR)
───────────────────────────────────────────────────────────────────────────
Ca2+ RECEPTOR TROPONIN C TROPONIN C CALMODULIN → MLCK
(thin filament) (thin filament) (thick filament
regulation)
───────────────────────────────────────────────────────────────────────────
SPEED fast (twitch intermediate VERY SLOW (0.5–3 s)
7–100 ms) (~250–300 ms) but tension/area
equals or exceeds
skeletal
───────────────────────────────────────────────────────────────────────────
TETANUS? YES — and it is NO — long YES, and can hold
the normal mode refractory period tension for HOURS
of use forbids it at ~1/300 the ATP
cost ("latch")
───────────────────────────────────────────────────────────────────────────
REGENERATION LIMITED but REAL essentially NONE GOOD — retains
(satellite cells) (scar instead) true mitosis
═══════════════════════════════════════════════════════════════════════════
Figure 9.1 — The three muscle tissue types compared across the properties that matter.
Described: A three-column comparison of skeletal, cardiac, and smooth muscle. Skeletal muscle attaches to bone, consists of long cylindrical cells 10 to 100 micrometers wide and up to 30 centimeters long with hundreds of peripheral nuclei, is striated with aligned sarcomeres, is under voluntary somatic control with one motor neuron obligatory per fiber, has no pacemaker and no electrical coupling between fibers, obtains essentially all of its calcium from the sarcoplasmic reticulum without needing extracellular calcium, uses troponin C as its calcium receptor, twitches in 7 to 100 milliseconds, uses tetanus as its normal working mode, and regenerates to a limited but real extent using satellite cells. Cardiac muscle is found only in the heart wall, consists of short branched cells 10 to 20 micrometers wide and 50 to 100 micrometers long with one or two central nuclei, is striated, is involuntary with autonomic input that modulates an intrinsic rhythm generated by pacemaker cells in the sinoatrial node, is electrically coupled through gap junctions in intercalated discs so the tissue behaves as a functional syncytium, draws 20 to 25 percent of its activating calcium from outside the cell which then triggers release of the remaining 75 to 80 percent from the sarcoplasmic reticulum, uses troponin C, contracts over roughly 250 to 300 milliseconds, cannot be tetanized because its refractory period is nearly as long as its contraction, and essentially cannot regenerate — it forms scar. Smooth muscle lies in the walls of hollow organs, vessels, airways, the iris, and hair follicles, consists of spindle-shaped cells 2 to 10 micrometers wide and 20 to 200 micrometers long, up to 500 micrometers in the pregnant uterus, with a single central nucleus, has no striations because its filaments form an oblique lattice anchored to dense bodies, is involuntary and responds to autonomic nerves, hormones, local chemicals, and stretch, has pacemaker activity in single-unit types, obtains most of its calcium from outside the cell, uses calmodulin and myosin light chain kinase rather than troponin, contracts very slowly over half a second to three seconds while generating tension per unit area equal to or greater than skeletal muscle, can maintain tension for hours at roughly one three-hundredth of the ATP cost in the latch state, and retains genuine capacity for cell division.
What skeletal muscle is for, beyond movement
Six functions, and only the first is obvious.
- Producing movement of the skeleton, and of the eyes, face, and tongue.
- Maintaining posture and position. Continuous, low-level, largely unconscious adjustment against gravity — you will meet the specific antigravity muscles in Chapter 10.
- Stabilizing joints. Muscle tension across a joint is a far more important stabilizer than ligament in most positions; this is why muscle weakness produces joint injury.
- Generating heat. Muscle contraction is roughly 20–25% efficient; the other 75–80% of the energy becomes heat. Skeletal muscle produces about 85% of body heat, and shivering can raise heat production four- to fivefold. Thermoregulation (Chapter 1) is executed by muscle.
- Acting as a glucose sink and a protein reserve. Skeletal muscle takes up roughly 80% of a glucose load after a meal, which is why muscle insulin resistance is the central lesion of metabolic syndrome (Chapter 24) — and Amara's. In starvation, muscle protein is catabolized to supply amino acids for gluconeogenesis.
- Assisting venous return and lymph flow. The skeletal muscle pump, developed fully in §10.8, moves roughly as much blood upward from the legs as the heart could on its own.
Check Your Understanding 9.1
- A muscle can pull but not push. Name three different mechanisms the body uses to return a shortened muscle to its resting length.
- Cardiac muscle cells are electrically coupled by gap junctions; skeletal muscle fibers are not. State one functional advantage of each arrangement.
- Smooth muscle contracts one hundred times more slowly than skeletal muscle but generates at least as much tension per square centimeter. What does that tell you about where its design compromise was made?
Show answers
- (a) Contraction of the antagonist muscle on the other side of the joint; (b) gravity or an external load; (c) elastic recoil of stretched elastic elements — titin within the sarcomere, and the collagen and elastin of the tendon and sheaths. A fourth, in hollow organs, is the pressure of the contents themselves refilling the organ.
- Gap junctions let cardiac tissue behave as a functional syncytium: one wave of excitation spreads cell to cell so the whole chamber contracts as a coordinated unit, which is the only way to generate pressure in a bag. Electrical privacy lets skeletal muscle be graded: because each fiber is controlled independently by its motor neuron, the nervous system can activate 2% of a muscle or 100% of it, which is what allows the same biceps to hold an egg and to hold a suitcase.
- The compromise was made in speed, not in force. Smooth muscle's myosin ATPase hydrolyzes ATP roughly ten to one hundred times more slowly than skeletal myosin, so cross-bridges cycle slowly — but a slowly cycling cross-bridge still generates full force while attached, and slow cycling means each cross-bridge spends a larger fraction of its time attached. Slow, strong, and cheap is exactly the specification for a tissue that must hold an arteriole half-closed for seventy years.
9.2 The Architecture of a Skeletal Muscle
A skeletal muscle is an organ. It contains muscle tissue, dense connective tissue, nervous tissue, and blood vessels — at least four tissue types, which is the definition from §1.2. Understanding it requires holding six levels of packaging in your head at once, and the packaging is not decorative: it is the force transmission system.
MUSCLE (organ) e.g. biceps brachii
┌──────────────────────────────────────────────────────────────────┐
│ EPIMYSIUM — dense irregular collagen, wraps the whole muscle │
│ ┌────────────────────────────────────────────────────────────┐ │
│ │ FASCICLE — a bundle of 10–100 fibers, visible to the eye │ │
│ │ (this is the "grain" you see in a steak) │ │
│ │ PERIMYSIUM — collagen sheath around each fascicle; │ │
│ │ carries the arteries, veins and nerves INTO the muscle │ │
│ │ ┌──────────────────────────────────────────────────────┐ │ │
│ │ │ MUSCLE FIBER = ONE CELL. 10–100 µm wide. │ │ │
│ │ │ Multinucleate. Up to 30 cm long (sartorius). │ │ │
│ │ │ ENDOMYSIUM — fine reticular fibers around EACH cell; │ │ │
│ │ │ holds the capillaries (3–5 per fiber) and the │ │ │
│ │ │ satellite cells │ │ │
│ │ │ ┌────────────────────────────────────────────────┐ │ │ │
│ │ │ │ MYOFIBRIL — 1–2 µm wide; hundreds to thousands │ │ │ │
│ │ │ │ per fiber; ~80% of the cell's volume │ │ │ │
│ │ │ │ ┌──────────────────────────────────────────┐ │ │ │ │
│ │ │ │ │ SARCOMERE — 2.0–2.2 µm at rest. │ │ │ │ │
│ │ │ │ │ THE contractile unit. ~10,000 in a row │ │ │ │ │
│ │ │ │ │ along one 2-cm myofibril. │ │ │ │ │
│ │ │ │ │ ┌────────────────────────────────────┐ │ │ │ │ │
│ │ │ │ │ │ MYOFILAMENTS │ │ │ │ │ │
│ │ │ │ │ │ THICK = myosin (~15 nm, 1.6 µm) │ │ │ │ │ │
│ │ │ │ │ │ THIN = actin (~8 nm, 1.0 µm) │ │ │ │ │ │
│ │ │ │ │ │ ELASTIC = titin (Z disc → M line) │ │ │ │ │ │
│ │ │ │ │ └────────────────────────────────────┘ │ │ │ │ │
│ │ │ │ └──────────────────────────────────────────┘ │ │ │ │
│ │ │ └────────────────────────────────────────────────┘ │ │ │
│ │ └──────────────────────────────────────────────────────┘ │ │
│ └────────────────────────────────────────────────────────────┘ │
└───────────────────────────┬──────────────────────────────────────┘
│
ALL THREE SHEATHS FUSE AND CONTINUE AS THE TENDON
│
▼
┌─────────────────────────────────────────────────┐
│ TENDON (cord) or APONEUROSIS (flat sheet) │
│ → blends with the PERIOSTEUM of bone │
│ → force finally reaches the skeleton │
└─────────────────────────────────────────────────┘
FORCE PATH: myosin head → actin → Z disc → costamere → sarcolemma
→ endomysium → perimysium → epimysium → tendon → bone
(a break ANYWHERE in this chain is a muscle disease)
Figure 9.2 — The structural hierarchy of a skeletal muscle, and the path force must travel.
Described: Six nested levels of organization. The whole muscle, such as the biceps brachii, is wrapped in epimysium, a sheath of dense irregular collagen. Within it lie fascicles, bundles of ten to one hundred fibers that are visible to the naked eye as the grain of meat; each fascicle is wrapped in perimysium, which also carries the arteries, veins, and nerves into the muscle. Within each fascicle lie muscle fibers, each of which is a single multinucleate cell 10 to 100 micrometers wide and up to 30 centimeters long in the sartorius; each fiber is wrapped in endomysium, a fine sheath of reticular fibers that holds the three to five capillaries serving that fiber and the satellite cells lying against it. Within each fiber lie hundreds to thousands of myofibrils, each one to two micrometers wide, occupying about eighty percent of the cell's volume. Each myofibril is a chain of roughly ten thousand sarcomeres, each 2.0 to 2.2 micrometers long at rest, which are the contractile units. Each sarcomere contains myofilaments: thick filaments of myosin about 15 nanometers across and 1.6 micrometers long, thin filaments of actin about 8 nanometers across and 1.0 micrometer long, and elastic titin filaments running from the Z disc to the M line. All three connective tissue sheaths fuse at the end of the muscle and continue as a tendon, a cord, or an aponeurosis, a flat sheet, which blends with the periosteum of bone. The force path runs from the myosin head to actin, to the Z disc, through the costamere to the sarcolemma, then to endomysium, perimysium, epimysium, tendon, and finally bone; a break anywhere along this chain constitutes a muscle disease.
The sheaths are not wrapping paper
Read the force path at the bottom of the figure again. A myosin head generates about 3 to 5 piconewtons. A quadriceps can generate several thousand newtons. Getting from one to the other requires summing billions of cross-bridges and then transmitting the sum, and the transmission is done by collagen, not by muscle.
Three consequences follow immediately:
- Connective tissue is why a muscle has a shape and a line of pull. The arrangement of fascicles within the epimysium — parallel, pennate, convergent — determines how much force a muscle makes and how far it can move. That is the whole of §10.3.
- Force is transmitted laterally as well as end-on. A muscle fiber does not usually run the full length of its fascicle. Force passes sideways from the sarcolemma into the endomysium through protein assemblies called costameres, which physically rivet the Z discs of the outermost myofibrils to the cell membrane. Dystrophin is the key protein of that rivet.
- The weak link is usually the junction, not the belly. Muscles most often tear at the myotendinous junction, where the material properties change abruptly.
Clinical Connection · Duchenne Muscular Dystrophy — When the Rivet Fails
Dystrophin is a long rod-shaped protein that links the actin cytoskeleton inside the fiber to a complex of membrane proteins that in turn grips laminin in the basal lamina outside. It is a shock absorber and a force coupler. The gene is enormous — 2.4 million base pairs, the largest in the human genome — and lies on the X chromosome, which is why Duchenne muscular dystrophy affects roughly 1 in 3,500 to 5,000 male births.
Without dystrophin, every contraction transmits force through a membrane that is not properly tied to the cytoskeleton. The sarcolemma tears microscopically. Calcium leaks in down its 10,000-fold gradient; calcium-activated proteases (calpains) degrade cellular proteins; mitochondria are overloaded; the fiber dies. Serum creatine kinase, an enzyme that should be inside muscle, runs 10 to 100 times normal from birth, long before weakness appears.
Satellite cells (§9.2, and the Development sidebar below) repair the damage at first, which is why boys are typically normal until age 3–5. Eventually the regenerative reserve is exhausted, and muscle is replaced by fat and fibrous tissue — producing the characteristic pseudohypertrophy of the calves, which look large and are weak. Weakness is proximal and symmetric; the child uses Gower's manoeuvre, walking his hands up his own thighs to stand, because his hip extensors cannot do it. Ambulation is usually lost by the early teens, and the cardiac and respiratory muscles — which contain the same protein — become the limiting problem.
Note what this disease teaches about normal physiology: dystrophin does not generate force, and a boy with Duchenne has entirely normal myosin, actin, and troponin. The contractile machinery is intact; the anchor is not. Force is useless if it cannot be transmitted.
The membrane system: sarcolemma, T-tubules, and sarcoplasmic reticulum
The muscle fiber's plasma membrane is the sarcolemma; its cytoplasm is the sarcoplasm. The sarcoplasm is unusually specialized: it holds large stores of glycogen in granules called glycosomes, the oxygen-binding protein myoglobin in high concentration, and a mitochondrial content that varies enormously with fiber type (§9.8).
Now the problem the fiber has to solve. An action potential arrives at one point on the sarcolemma and sweeps over the surface in a few milliseconds. But the myofibrils that must respond fill the entire cross-section of a cell that may be 100 micrometers across. Calcium released only at the surface would take, by simple diffusion, on the order of seconds to reach the center — a hundred times too slow. If the fiber relied on diffusion from the surface, the outer myofibrils would contract while the core did nothing.
The solution is anatomical: bring the membrane inside.
- T-tubules (transverse tubules) are deep, narrow invaginations of the sarcolemma that run transversely across the fiber and branch among the myofibrils. Their lumen is continuous with the extracellular space; their membrane is continuous with the sarcolemma and carries the same voltage-gated machinery. An action potential therefore propagates into the fiber at full amplitude, reaching every myofibril within about 2 milliseconds. In mammalian skeletal muscle, T-tubules penetrate at the A–I band junctions, so there are two per sarcomere.
- The sarcoplasmic reticulum (SR) is smooth endoplasmic reticulum specialized for one purpose: storing and releasing calcium. It forms a sleeve around each myofibril, and at each T-tubule it swells into paired terminal cisternae.
- A triad is one T-tubule flanked by two terminal cisternae — the three-element structure where excitation is converted into calcium release. Cardiac muscle has dyads (one T-tubule, one cisterna); smooth muscle has neither.
The calcium numbers are worth memorizing because everything in §9.5 follows from them.
| Compartment | Free Ca²⁺ concentration | Note |
|---|---|---|
| Resting sarcoplasm | ~50 nanomolar (0.00005 mM) | Kept there by SERCA at ATP cost |
| Activated sarcoplasm | ~1–10 micromolar | A 20- to 200-fold rise, in ~2 ms |
| Inside the SR (free) | ~1 millimolar | A ~20,000-fold gradient across the SR |
| Inside the SR (total) | ~10–20 millimolar | Most of it bound to calsequestrin |
| Extracellular fluid | ~1.2 millimolar (ionized) | Not required for skeletal contraction |
Calsequestrin is the trick that makes the storage possible. Each molecule binds approximately 40 calcium ions with low affinity, so the SR can hold a very large total load of calcium while keeping the free concentration low enough that SERCA can still pump against it and low enough that the store does not simply leak out. Low affinity matters: the calcium must come off instantly when the ryanodine receptor opens.
Histology · Telling the Three Muscles Apart on a Slide
Under the microscope, three questions in sequence will identify any muscle section correctly.
- Are there striations? Alternating dark and light cross-bands mean aligned sarcomeres, which means skeletal or cardiac. No striations means smooth.
- If striated — where are the nuclei, and do the cells branch? Skeletal fibers are long, unbranched, parallel-sided cylinders with many flattened nuclei pressed against the sarcolemma, at the very edge of the cell. Cardiac cells are shorter, obviously branched, and have one or two large, pale, central nuclei with a clear perinuclear halo of glycogen.
- If striated and branched — look for intercalated discs. These appear as dark transverse lines crossing the fiber at intervals, often with a stepped or staircase profile. They are unique to cardiac muscle and are the single most reliable sign.
Smooth muscle in longitudinal section looks like overlapping spindles with a single central, cigar-shaped nucleus; in cross-section it looks like a mosaic of circles of different diameters, some containing a nucleus and some not — because a section through the tapering end of a fusiform cell is smaller than one through its middle, and misses the nucleus. Students consistently misread that variation as artifact. It is geometry.
One further skeletal-muscle detail: in a well-fixed longitudinal section you can resolve the banding itself — a wide dark A band and a lighter I band with a fine dark line, the Z disc, bisecting it. The striations of a whole fiber align because the myofibrils inside are held in register by intermediate filaments of desmin tying adjacent Z discs together. Without desmin the myofibrils drift out of phase and the cell loses its stripes.
Development · Why a Muscle Cell Has Six Hundred Nuclei
Skeletal muscle fibers are the largest cells in the body, and they are large because they were built by fusion. In the embryo, mesodermal precursors called myoblasts proliferate, align end to end, and then fuse their membranes to form multinucleate myotubes. Each myotube then synthesizes myofibrils, which push the nuclei out to the periphery — which is exactly why an adult skeletal muscle fiber has its nuclei flattened against the sarcolemma, and cardiac and smooth muscle do not.
The arrangement solves a logistical problem. A single nucleus can support only so much cytoplasm — a volume called its myonuclear domain, roughly 10,000 to 30,000 cubic micrometers. A fiber 30 centimeters long needs hundreds of them, distributed along its length, so that no region of the cell is far from a source of messenger RNA.
Not all myoblasts fuse. A population is set aside beneath the basal lamina as satellite cells — mononucleate, quiescent, marked by the transcription factor Pax7, making up 2–8% of the nuclei associated with adult muscle. They are the reason skeletal muscle can repair itself at all, and they are the reason Case File question 2 has the answer it does. Cardiac muscle never sets aside an equivalent reserve, and this is a decision made before birth.
Two practical consequences. First, muscle hypertrophy in an adult is mostly the addition of myofibrils to existing fibers, but sustained growth requires satellite cells to fuse in and donate nuclei — you cannot expand a fiber indefinitely without expanding its transcriptional capacity. Second, those donated nuclei appear to persist for a long time after training stops (myonuclear permanence), which is a plausible cellular basis for "muscle memory": retraining a previously trained muscle is faster than training a naive one.
Check Your Understanding 9.2
- A drug destroys T-tubules without affecting any other structure. The fiber is stimulated at its neuromuscular junction. Predict, precisely, what happens.
- Calsequestrin binds calcium with low affinity. Why would a high-affinity calcium-binding protein be a worse design for the SR lumen?
- Serum creatine kinase is elevated in Duchenne muscular dystrophy from birth, years before any weakness. What does that tell you about the sequence of events in the disease?
Show answers
- The action potential still propagates normally over the surface of the fiber, because the sarcolemma is intact. But it never reaches the interior, so the DHP receptors at the triads are never activated and the SR does not release calcium — except perhaps in a thin rim just beneath the surface. The fiber would produce a weak, slow, incomplete contraction confined to its periphery. This is the clearest possible demonstration that T-tubules exist to solve a diffusion-distance problem.
- Because the calcium has to come off again, essentially instantaneously, when the ryanodine receptor opens. A high-affinity buffer would hold onto it, slowing and reducing release. Low affinity plus very high capacity (about 40 ions per molecule) gives you a large store that empties fast — the same design principle as a shallow, wide reservoir with a big outlet valve.
- It tells you that membrane damage and cell death come first, and weakness comes later. Creatine kinase in the serum means sarcolemmas are leaking from the beginning. The boy is not weak yet because satellite cells are keeping up with the destruction. Clinical weakness marks the point at which the regenerative reserve is exhausted, not the point at which the damage begins — which is why treatments aimed only at the late weakness are aimed too late.
9.3 The Sarcomere
The sarcomere is the smallest unit of a muscle that can do the job of a muscle. Everything above it in the hierarchy is packaging and plumbing; everything below it is chemistry.
A sarcomere is defined as the region from one Z disc to the next, and at resting length it is 2.0–2.2 micrometers long. A myofibril 2 centimeters long therefore contains about 10,000 sarcomeres in series. That number explains something important: if each sarcomere shortens by only 0.5 micrometers — about 25% — the myofibril shortens by 5 millimetres. Small displacements, multiplied ten thousand times, become useful movement.
══════════════ THE SARCOMERE AT REST (2.2 µm) ═══════════════════════════
Z disc M line Z disc
│ │ │
║◄──────── I band ──────►║ │ ║
║ (thin only) ║ │ ║
║ ◄──────────────── A band (1.6 µm) ──────────────► ║
║ ║ (full length of the THICK filament) ║ ║
║ ║ ◄──── H zone ────► ║ ║
║ ║ (thick only, │ ║ ║
║ ║ no overlap) │ ║ ║
║ ║ │ ║ ║
║────┼───────────────────────┼───────────────────────┼─────║
═══█ │ ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬│▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ │ █═══
█────┼───────────────────────┼───────────────────────┼─────█
═══█ ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬│▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ █═══
█────┼───────────────────────┼───────────────────────┼─────█
═══█ │ ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬│▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ │ █═══
║ ║ │ ║ ║
═══ = THIN filament (actin + tropomyosin + troponin), 1.0 µm
▬▬▬ = THICK filament (myosin), 1.6 µm, heads at both ends,
BARE ZONE (no heads) at the centre, at the M line
█ = Z disc (α-actinin) — anchors thin filaments of TWO sarcomeres
~~~~ = TITIN runs Z disc → M line inside each half-sarcomere
(not drawn; it is the spring that keeps the thick filament
centred and supplies passive tension)
══════════════ THE SAME SARCOMERE CONTRACTED (1.7 µm) ═══════════════════
║◄─ I band ─►║ ║
║ NARROWER ║ ║
║ ◄────────── A band — UNCHANGED (1.6 µm) ────► ║
║ ║ H zone GONE (filaments now overlap) ║
║───┼─────────────────┼──────────────────┼────────║
═══█ ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬│▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ █═══
█───┼─────────────────┼──────────────────┼────────█
═══█▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬│▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ █═══
║ ║ │ ║ ║
WHAT CHANGED WHAT DID NOT CHANGE
───────────────── ────────────────────────────────────────────
I band → narrower A band width (= thick filament length)
H zone → narrower Thick filament length
or absent Thin filament length
Z–Z → shorter Number of filaments
▲ THE FILAMENTS DO NOT SHORTEN. THEY SLIDE.
Figure 9.3 — The sarcomere at rest and contracted: what changes and what does not.
Described: Two drawings of the same sarcomere, first at a resting length of 2.2 micrometers and then contracted to 1.7 micrometers. In each, the sarcomere runs from one Z disc to the next. Z discs, built of alpha-actinin, anchor the thin filaments of two adjacent sarcomeres. Thin filaments, 1.0 micrometer long and made of actin with tropomyosin and troponin, project inward from each Z disc. Thick filaments of myosin, 1.6 micrometers long, occupy the centre; they carry projecting heads along their length except for a bare zone at their exact centre, where the M line cross-links them. The A band is defined as the full length of the thick filaments and is therefore 1.6 micrometers wide. The I band is the region containing thin filaments only, and it spans the Z disc, so half of it belongs to each neighbouring sarcomere. The H zone is the central part of the A band containing thick filaments only, with no thin filament overlap. Titin, not drawn, runs from the Z disc to the M line inside each half sarcomere and acts as a spring that centres the thick filament and supplies passive tension. In the contracted drawing, the thin filaments have slid inward toward the M line. The I band is narrower, the H zone is narrower or has disappeared entirely, and the two Z discs are closer together. The A band width is exactly unchanged, as are the lengths of the thick and thin filaments and their number. The filaments do not shorten; they slide past one another.
Why the bands look the way they do
The striations that name the tissue are a pure optical consequence of filament overlap, and once you see that, the whole pattern is derivable rather than memorizable.
- The A band is dark because it is dense: it is the region occupied by thick filaments. It is anisotropic under polarized light — it rotates the plane of polarization — which is what the "A" stands for. Because it is defined as the length of the thick filament, and the thick filament never changes length, the A band width is invariant. This is the single most useful fact in the whole diagram, and the fastest way to catch a wrong answer.
- The I band is light because thin filaments alone scatter less light. It is isotropic. It is bisected by the Z disc, so any given I band is shared between two sarcomeres.
- The H zone (from helle, German for bright) is the middle of the A band where thick filaments sit alone. It exists only when the sarcomere is long enough that the thin filaments do not reach the centre. Shorten the sarcomere and it closes.
- The M line is a set of cross-links (chiefly the protein myomesin) holding the thick filaments in a hexagonal lattice at their midpoints. Without it the thick filaments would be pulled out of register by the first contraction, since each half is being tugged in the opposite direction.
- The Z disc is a lattice of α-actinin into which the plus ends of thin filaments from both directions are anchored. Z is for Zwischenscheibe, "between disc."
Notice the geometry that makes the whole thing work: the myosin heads on the two halves of a thick filament point in opposite directions, away from the M line, because the myosin tails in each half point toward it. So when heads on the left half pull, they pull actin rightward toward the centre, and heads on the right half pull actin leftward toward the centre. Both Z discs come in. A thick filament with all its heads pointing the same way would drag the whole sarcomere sideways and accomplish nothing.
The proteins, by job
| Protein | Location | Job |
|---|---|---|
| Myosin II | Thick filament | Motor. Two heavy chains form a coiled-coil tail and two globular heads; four light chains sit at the head–neck junction. Each head has an actin-binding site and an ATP-binding site with ATPase activity. About 300 molecules per thick filament |
| Actin (F-actin) | Thin filament | Track. A double helix of globular G-actin monomers, each carrying one myosin-binding site |
| Tropomyosin | Thin filament | Gate. A rod-shaped dimer lying end to end in the actin groove, each molecule covering seven actin monomers and, at rest, their myosin-binding sites |
| Troponin T (TnT) | Thin filament | Binds the troponin complex to tropomyosin — the "T" is for tropomyosin |
| Troponin I (TnI) | Thin filament | Inhibitory. Binds actin and holds tropomyosin in the blocking position when calcium is low. This is the protein in Amara's blood |
| Troponin C (TnC) | Thin filament | The calcium switch. Binds Ca²⁺; when it does, it grips TnI and pulls it off actin |
| Titin | Z disc → M line | The largest protein known (~3–3.7 million daltons, ~27,000–34,000 amino acids). A molecular spring: supplies passive tension, prevents overstretch, and keeps the thick filament centred |
| Nebulin | Along thin filament | A molecular ruler that sets thin filament length |
| α-Actinin | Z disc | Anchors thin filaments |
| Myomesin | M line | Cross-links thick filaments |
| Desmin | Around Z discs | Intermediate filament tying neighbouring myofibrils in register |
| Dystrophin | Sarcolemma | Links the cytoskeleton to the extracellular matrix; transmits force laterally |
Two of these deserve a second look. Titin is why a relaxed muscle resists being stretched: its immunoglobulin-like domains and its PEVK segment unfold progressively under load, like opening a series of tiny folded ribbons, generating a restoring force that rises steeply as the sarcomere is pulled past about 2.4 micrometers. That is elasticity and extensibility — two of the four universal properties — reduced to one molecule.
And troponin I is the direct answer to Case File question 1, held in reserve until §9.5.
Predict This
An electron micrograph shows a sarcomere in which the H zone has vanished entirely and the I band is very narrow, but the A band measures exactly 1.6 micrometers, the same as in a relaxed sarcomere from the same muscle.
Before reading on, commit to answers: is this muscle contracted or relaxed? And what would you predict happens to the tension this sarcomere can generate compared with one at 2.2 micrometers?
(Answers: contracted — the disappearance of the H zone and narrowing of the I band are the signature, and the unchanged A band confirms the filaments slid rather than shortened. Tension will be lower than at 2.2 micrometers, because thin filaments from opposite ends have begun to overlap each other and interfere with cross-bridge formation. This is the ascending limb of the length–tension curve, §9.6, and it is the reason a muscle already shortened is a weak muscle.)
Check Your Understanding 9.3
- A student states that "the A band narrows during contraction because the myosin filaments shorten." Give the two independent pieces of evidence in Figure 9.3 that refute this.
- Tropomyosin covers seven actin monomers. What does that number predict about how many myosin-binding sites are exposed by a single movement of one tropomyosin molecule, and why is that cooperative behaviour useful?
- In a muscle stretched far beyond resting length, which band would you expect to be widest, and what would happen to active tension?
Show answers
- First, the A band is defined as the length of the thick filament, and it is measured to be unchanged in contracted sarcomeres; if myosin shortened, the A band would shrink by definition. Second, the I band and H zone narrow while the A band does not — a pattern that is only possible if thin filaments are moving into the A band region. Sliding explains every observation; shortening explains none of them.
- Moving one tropomyosin molecule exposes seven binding sites at once, so calcium binding at one troponin complex switches on a seven-actin stretch of the filament. This is cooperativity: it makes activation steep and switch-like rather than gradual, so a small rise in calcium produces a large jump in the number of available sites. Cooperativity is further amplified because the first strongly bound myosin heads themselves push tropomyosin further into the groove, recruiting neighbouring sites — the filament effectively turns itself on.
- The I band would be widest, and the H zone would also be wide, because the thin filaments have been pulled out toward the Z discs and away from the centre. Active tension would be reduced, because fewer myosin heads lie opposite actin; extended far enough (about 3.6 micrometers, where overlap reaches zero) active tension falls to nothing. Passive tension from titin, meanwhile, rises steeply. This is the descending limb of the length–tension curve.
9.4 The Neuromuscular Junction and the Motor Unit
A skeletal muscle fiber is electrically private and functionally deaf. It does nothing until a motor neuron tells it to, and every fiber has exactly one neuromuscular junction, near its midpoint, for its entire life. There is no back-up. Cut the nerve and the muscle is paralysed and will atrophy — a fact that will matter for Toby's quadriceps in Chapter 10.
MOTOR NEURON AXON (myelinated, 12–20 µm, 70–120 m/s)
│
│ loses myelin, branches into
▼
┌─────────── AXON TERMINAL (synaptic knob) ───────────┐
│ ~300,000 synaptic vesicles │
│ each holding ~5,000–10,000 ACh molecules │
│ mitochondria · active zones · voltage-gated Ca2+ │
└──┬──────┬──────┬──────┬──────┬──────┬──────┬─────────┘
│ │ │ │ │ │ │
▼ ▼ ▼ ▼ ▼ ▼ ▼ ← exocytosis
░░░░░░░░░░░░░░░░ SYNAPTIC CLEFT (~50 nm) ░░░░░░░░░░░░░░
░ ACh diffuses across in ~0.1 ms ░
░ ACETYLCHOLINESTERASE sits in the basal lamina HERE ░
░ (hydrolyses ACh in ~1 ms; ~5,000 molecules/s each) ░
░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
╔══╗▲▲▲╔══╗▲▲▲╔══╗▲▲▲╔══╗▲▲▲╔══╗▲▲▲╔══╗ MOTOR END PLATE
║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ (junctional folds)
║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║
║██║ ║██║ ║██║ ║██║ ║██║ ║██║ ██ = voltage-gated
╚══╝ ╚══╝ ╚══╝ ╚══╝ ╚══╝ ╚══╝ Na+ channels
(fold DEPTHS)
▲▲▲ = nicotinic ACh receptors, ~10,000–20,000 per µm²
on the CRESTS of the folds. Ligand-gated cation
channel; needs TWO ACh to open; Na+ in > K+ out
RESULT ─────────────────────────────────────────────────────
END PLATE POTENTIAL ~ +30 to +40 mV, GRADED, LOCAL
Threshold needed ~ +15 mV
SAFETY FACTOR ~ 3–5× → transmission NEVER fails
in health (1 nerve AP =
1 muscle AP, always)
─────────────────────────────────────────────────────────────
LESION SITES botulinum toxin → blocks ACh RELEASE (flaccid)
curare → blocks the RECEPTOR (flaccid)
myasthenia → destroys RECEPTORS (fatigable)
organophosphate→ blocks the ESTERASE (spastic)
tetanus toxin → acts in the CNS, not here
Figure 9.4 — The neuromuscular junction, its safety factor, and where drugs and diseases break it.
Described: A vertical sequence from motor neuron to muscle membrane. A myelinated motor axon, 12 to 20 micrometers across and conducting at 70 to 120 metres per second, loses its myelin and branches into axon terminals. Each terminal contains about 300,000 synaptic vesicles holding 5,000 to 10,000 acetylcholine molecules each, along with mitochondria, active zones, and voltage-gated calcium channels. Vesicles release acetylcholine by exocytosis into a synaptic cleft about 50 nanometers wide, which the transmitter crosses in roughly one tenth of a millisecond. Acetylcholinesterase is anchored in the basal lamina within the cleft and hydrolyses acetylcholine within about a millisecond, each enzyme molecule handling some 5,000 molecules per second. The muscle membrane beneath is the motor end plate, thrown into deep junctional folds. Nicotinic acetylcholine receptors, packed at 10,000 to 20,000 per square micrometer, sit on the crests of the folds; each is a ligand-gated cation channel requiring two acetylcholine molecules to open, and it admits more sodium inward than it lets potassium outward. Voltage-gated sodium channels are concentrated in the depths of the folds. The result is an end plate potential of roughly 30 to 40 millivolts, which is graded and local, against a threshold requirement of about 15 millivolts — a safety factor of three to five times, so that in health one nerve impulse always produces exactly one muscle impulse. Lesion sites are listed: botulinum toxin blocks acetylcholine release causing flaccid paralysis; curare blocks the receptor, also flaccid; myasthenia gravis destroys receptors, producing fatigable weakness; organophosphates block the esterase, producing sustained depolarization and spastic paralysis; and tetanus toxin acts in the central nervous system rather than at this junction.
The sequence, step by step
- An action potential travelling at 70–120 m/s reaches the axon terminal.
- Depolarization opens voltage-gated Ca²⁺ channels in the terminal membrane. Calcium enters from the extracellular fluid, where it is roughly 10,000 times more concentrated than in the cytosol.
- Calcium binds synaptotagmin, the calcium sensor of the release machinery, triggering SNARE-mediated fusion of synaptic vesicles with the membrane at the active zones. Roughly 50–300 vesicles — quanta — release together.
- Acetylcholine diffuses across the 50 nm cleft in about 0.1 ms.
- Two ACh molecules bind each nicotinic acetylcholine receptor, opening its intrinsic cation channel. Sodium rushes in far faster than potassium leaves, because sodium is much further from its equilibrium potential.
- The resulting local, graded depolarization is the end plate potential (EPP) — typically 30–40 mV, against a threshold requirement of roughly 15 mV.
- The EPP spreads to the fold depths, where voltage-gated Na⁺ channels are concentrated, and triggers a genuine, propagating, all-or-none muscle action potential that sweeps along the sarcolemma and down the T-tubules at about 3–5 m/s.
- Acetylcholinesterase, anchored in the basal lamina of the cleft, hydrolyses ACh into acetate and choline within about a millisecond. Choline is recovered by a transporter and re-used. Without this step the receptor would be re-stimulated continuously and the fiber would be unable to repolarize.
The number to hold on to is the safety factor. Central nervous system synapses are probabilistic: one presynaptic spike usually does not fire the postsynaptic cell, and computation happens in that uncertainty. The neuromuscular junction is the opposite. It is deliberately over-engineered so that transmission is obligatory: one nerve impulse produces one muscle impulse, essentially every time, for a lifetime. All the computation has already been done upstream, in the spinal cord. The junction's only job is not to be the weak link.
Which means that when the junction does become the weak link, the clinical picture is unmistakable.
Clinical Connection · Myasthenia Gravis — Eating the Safety Factor
Myasthenia gravis is an autoimmune disease in which antibodies attack the nicotinic ACh receptor (in about 85% of cases) or the clustering protein MuSK. Receptors are cross-linked and internalized, complement damages the junctional folds, and the postsynaptic surface flattens. Receptor numbers fall to roughly a third of normal.
Now do the arithmetic. Normal EPP is 30–40 mV against a 15 mV threshold. Cut the receptors to a third and the EPP falls to perhaps 12–15 mV — right at threshold. Transmission still works, but the margin is gone.
That predicts the disease exactly. Fatigability is the hallmark: with repeated stimulation, the number of vesicles released per impulse normally declines slightly over the first few impulses (a normal phenomenon, invisible in health because of the safety factor). In myasthenia, that ordinary decline drops the EPP below threshold, and fibers begin to drop out one by one. Strength therefore decays with use and recovers with rest — the patient is strongest in the morning and weakest at night, the eyelid droops as the day goes on, and speech becomes progressively nasal during a long sentence.
The most heavily used and most delicately controlled muscles fail first, which is why ptosis and diplopia are the presenting symptoms in more than half of patients: extraocular muscles fire at very high rates and have small motor units with the least margin to spare.
Treatment follows from mechanism. Acetylcholinesterase inhibitors (pyridostigmine) leave ACh in the cleft longer, so the remaining receptors are hit more often — restoring the EPP without restoring the receptors. Immunosuppression and thymectomy attack the antibody production itself. And the reason a myasthenic crisis is a respiratory emergency is that the diaphragm is subject to the same failing safety factor as the eyelid.
Clinical Connection · Botulism and Tetanus — Opposite Lesions, Opposite Pictures
These two toxins are produced by related Clostridium bacteria, are among the most potent poisons known, and are both zinc proteases that cleave SNARE proteins to block vesicle fusion. They produce opposite diseases, and the difference is entirely a matter of which synapse they reach.
Botulinum toxin is taken up at the neuromuscular junction and cleaves SNAP-25 in the motor nerve terminal. Acetylcholine cannot be released. The result is flaccid paralysis: descending, symmetric, beginning with the cranial nerves — blurred vision, ptosis, dysphagia, dysarthria — and progressing to the limbs and diaphragm. The patient is fully conscious and sensation is normal, because only the motor terminal is affected. Recovery requires the axon to sprout entirely new terminals, which takes weeks to months. Therapeutic doses exploit exactly this: a few units injected into an overactive muscle produce local, reversible weakness.
Tetanus toxin binds at the neuromuscular junction too, but instead of acting there it is carried by retrograde axonal transport up the motor neuron into the spinal cord, where it crosses into the terminals of inhibitory interneurons and cleaves synaptobrevin. Glycine and GABA can no longer be released. Motor neurons lose their inhibition and fire continuously. The result is spastic paralysis: sustained, painful, involuntary co-contraction of agonists and antagonists together — trismus ("lockjaw"), risus sardonicus, opisthotonos, and the possibility of fractures from the force of the muscle contraction itself.
One toxin removes the signal to contract; the other removes the signal to stop. Set side by side, they demonstrate that normal movement depends as much on inhibition as on excitation — a principle that returns as reciprocal inhibition in §10.1.
The motor unit
A motor unit is one motor neuron plus all the muscle fibers it innervates. It is the smallest functional unit the nervous system can command: fire the neuron and every fiber in that unit contracts, all-or-none, together. Grading force is therefore a matter of choosing how many units to activate and how fast to fire them.
The innervation ratio — fibers per motor neuron — varies over two orders of magnitude, and it varies exactly as the precision requirement does.
| Muscle | Fibers per motor unit (approx.) | What that buys |
|---|---|---|
| Extraocular muscles | 3–10 | The finest control in the body; eye position must be accurate to a few arc-minutes for binocular vision |
| Laryngeal muscles | 2–5 | Pitch and articulation |
| Intrinsic hand muscles | 100 | Threading a needle, playing an instrument |
| Biceps brachii | ~750 | Useful force with moderate control |
| Tibialis anterior | ~600 | Controlled foot placement |
| Gastrocnemius | 1,000–2,000 | Maximum force per motor neuron; the ankle does not need fine grading |
| Quadriceps (vastus) | ~1,000+ | Large force, coarse control |
The smallest possible increment of force in a muscle is one motor unit. In the extraocular muscles that increment is a handful of fibers; in the gastrocnemius it is over a thousand. Precision costs neurons, and the body spends them where they are worth spending.
One more structural point that is easy to miss and matters clinically: the fibers of a single motor unit are not clustered together. They are scattered throughout the muscle, intermingled with fibers of other units, so that a single unit's fibers might make up 1–5% of the cells in any given region. Contraction is therefore smooth and evenly distributed rather than lumpy. When a nerve is damaged and surviving axons sprout to adopt orphaned fibers, this scattering is lost — the surviving unit takes over a contiguous patch of tissue, producing fiber-type grouping on biopsy. That histological change is the fingerprint of denervation and reinnervation, and it accumulates quietly with age.
Imaging · EMG, Muscle MRI, and Amara's Echocardiogram
Three ways of looking at muscle, each answering a different question.
Electromyography (EMG) records the electrical activity of muscle fibers, either with surface electrodes or with a fine needle in the muscle belly. It measures motor unit action potentials, and it distinguishes the two great categories of muscle weakness. In a neurogenic lesion — a damaged nerve — surviving motor units have adopted extra fibers, so their potentials are abnormally large and long, and there are fewer of them; at rest, the denervated fibers fire spontaneously, producing fibrillations. In a myopathic lesion — damaged muscle, as in muscular dystrophy or statin myopathy — each motor unit has lost fibers, so its potentials are abnormally small and brief, and many units must be recruited early to produce any force. Nerve or muscle: a needle answers it in twenty minutes.
Muscle MRI shows the same distinction anatomically. Oedema on fluid-sensitive (STIR) sequences marks acutely inflamed or damaged muscle; fatty replacement on T1 marks chronic loss. The pattern of which muscles are involved is often diagnostic in itself.
Echocardiography is ultrasound of the heart — real-time, portable, harmless (§1.8), and the study that produced Amara's key finding. The sonographer divides the left ventricle into 16 or 17 standard segments and grades each one: normal (thickens and moves inward), hypokinetic (reduced), akinetic (does not move), or dyskinetic (bulges outward during systole). The report on Amara reads hypokinesis of the lateral wall, with akinesis of the mid-lateral segments — the territory of the circumflex artery that was stented on day 1.
That word is doing a great deal of work. Wall motion depends on regional blood supply, so a regional wall motion abnormality maps the territory of one coronary artery onto the muscle it feeds. It appears within seconds of coronary occlusion — before ECG changes, and long before troponin rises — because contraction is the first thing to fail when a myocyte runs out of ATP. And its persistence at three days, after flow has been restored, distinguishes muscle that is stunned but alive from muscle that is dead. The 14% of Amara's left ventricle that is akinetic is the physical answer to Case File question 2.
Check Your Understanding 9.4
- Why does an anticholinesterase drug improve strength in myasthenia gravis but cause paralysis in organophosphate poisoning? Both block the same enzyme.
- A patient has weakness that is worst after activity and best after rest, and no sensory loss. Where in the motor pathway is the lesion, and what single physiological parameter is abnormal?
- Predict the minimum force increment available to the nervous system in the extraocular muscles versus the gastrocnemius, and explain what functional difference this produces.
Show answers
- It is a matter of degree, and of the safety factor. In myasthenia the EPP has fallen to the edge of threshold; a modest prolongation of ACh action restores it to a working value. In organophosphate poisoning the enzyme is blocked essentially completely and irreversibly, so ACh accumulates massively; the end plate remains depolarized, voltage-gated sodium channels at the fold depths become inactivated, and the fiber can no longer generate an action potential at all. This is depolarizing block — the membrane is stuck above threshold rather than below it. Too little transmitter and too much transmitter both cause paralysis, by opposite mechanisms.
- At the neuromuscular junction, and the abnormal parameter is the safety factor — the margin by which the end plate potential exceeds threshold. Absent sensory loss excludes a peripheral nerve or root lesion; fatigability with recovery on rest is specific for a junction whose margin has been eaten away.
- Roughly 3–10 fibers in an extraocular muscle versus 1,000–2,000 in the gastrocnemius. The eye can therefore be aimed in extremely fine increments, which is required because a half-degree error produces double vision. The ankle cannot be graded so finely, and does not need to be: the consequence of a small error in plantarflexion force is nothing at all. Fine control is metabolically and anatomically expensive — it requires many motor neurons for the same mass of muscle — and the body buys it only where it pays.
9.5 Excitation–Contraction Coupling
Excitation–contraction coupling is the set of events that converts an electrical signal on the sarcolemma into a mechanical event in the myofibrils. It is the hinge of the whole chapter, and it is worth learning as an ordered list you can recite, because almost every muscle disease and every muscle drug acts at one identifiable step.
1 ACTION POTENTIAL sweeps along the SARCOLEMMA (~3–5 m/s)
│
▼
2 … and DOWN THE T-TUBULE, reaching the fiber's core in ~2 ms
│
▼
3 DHP RECEPTOR (Cav1.1) in the T-tubule membrane senses the voltage
change. In SKELETAL muscle it is a VOLTAGE SENSOR that moves
MECHANICALLY — arranged in tetrads, physically touching the RyR.
NO extracellular Ca2+ is required.
┌─────────────────────────────────────────────────────────────┐
│ CARDIAC DIFFERENCE: the DHP receptor (Cav1.2) opens and │
│ admits Ca2+, and THAT Ca2+ opens RyR2. │
│ = CALCIUM-INDUCED CALCIUM RELEASE. Extracellular Ca2+ │
│ is ABSOLUTELY REQUIRED. (Why Ca-channel blockers weaken │
│ the heart but not the biceps.) │
└─────────────────────────────────────────────────────────────┘
│
▼
4 RYANODINE RECEPTOR (RyR1) — the SR calcium release channel —
OPENS. Ca2+ floods out of the terminal cisternae:
sarcoplasmic Ca2+ 50 nM ────► 1–10 µM in ~2 ms
(a 20–200 fold rise, down a
20,000-fold gradient)
│
▼
5 Ca2+ BINDS TROPONIN C (2 regulatory sites on the N-lobe)
│
▼
6 TnC grips TnI and PULLS IT OFF ACTIN.
TnT then rotates TROPOMYOSIN ~25° deeper into the actin groove.
MYOSIN-BINDING SITES ON ACTIN ARE NOW EXPOSED.
│
▼
7 CROSS-BRIDGE CYCLING BEGINS (Figure 9.6) → the filaments slide
│
▼
8 RELAXATION: ACh destroyed → AP stops → RyR closes →
SERCA pumps Ca2+ back into the SR (2 Ca2+ per ATP) →
sarcoplasmic Ca2+ falls to 50 nM → Ca2+ leaves TnC →
tropomyosin re-covers the binding sites → cross-bridges cannot
re-form → tension falls.
▲ THE MUSCLE DOES NOT LENGTHEN ITSELF. Something else must
stretch it: an antagonist, gravity, titin, or organ filling.
Figure 9.5 — Excitation–contraction coupling as a numbered sequence, with the cardiac difference marked at step 3.
Described: An eight-step vertical flow. Step one: an action potential sweeps along the sarcolemma at three to five metres per second. Step two: it continues down the T-tubule, reaching the core of the fiber within about two milliseconds. Step three: the dihydropyridine receptor, a voltage-gated calcium channel called Cav1.1, senses the voltage change; in skeletal muscle it acts as a purely mechanical voltage sensor, arranged in groups of four that physically touch the ryanodine receptor, so no extracellular calcium is required. A boxed note records the cardiac difference: there the dihydropyridine receptor Cav1.2 actually opens and admits calcium, and that calcium opens the cardiac ryanodine receptor RyR2 — calcium-induced calcium release — so extracellular calcium is absolutely required, which is why calcium channel blockers weaken the heart but not the biceps. Step four: the ryanodine receptor RyR1, the calcium release channel of the sarcoplasmic reticulum, opens, and calcium floods from the terminal cisternae, raising sarcoplasmic calcium from 50 nanomolar to between 1 and 10 micromolar in about two milliseconds, a twenty- to two-hundred-fold rise down a twenty-thousand fold gradient. Step five: calcium binds troponin C at two regulatory sites on its amino-terminal lobe. Step six: troponin C grips troponin I and pulls it off actin, and troponin T then rotates tropomyosin about twenty-five degrees deeper into the actin groove, exposing the myosin-binding sites. Step seven: cross-bridge cycling begins and the filaments slide. Step eight, relaxation: acetylcholine is destroyed, the action potential stops, the ryanodine receptor closes, SERCA pumps calcium back into the sarcoplasmic reticulum at a cost of one ATP per two calcium ions, sarcoplasmic calcium falls back to 50 nanomolar, calcium leaves troponin C, tropomyosin re-covers the binding sites, cross-bridges cannot re-form, and tension falls. A final note emphasises that the muscle does not lengthen itself: an antagonist muscle, gravity, titin's recoil, or the refilling of a hollow organ must stretch it.
The cross-bridge cycle
Everything above exists to expose a binding site on actin. What follows is the engine itself: a four-step cycle, repeated by each of roughly 300 myosin heads per thick filament, several times per second, asynchronously, so that at any instant some heads are pulling while others are detached and the filament never slips backward.
┌──────────── STEP 1 ────────────┐
│ CROSS-BRIDGE FORMATION │
│ Energised head (ADP + Pi │
│ bound, "cocked" at ~90°) │
│ binds the exposed actin site. │
│ Weak binding → STRONG binding;│
│ Pi is released. │
└───────────────┬────────────────┘
│
┌──────────── STEP 4 ────────┐ ▼ ┌─────────── STEP 2 ─────────┐
│ RE-COCKING (energising) │ │ THE POWER STROKE │
│ Myosin ATPase HYDROLYSES │ │ Head pivots 90° → 45°, │
│ ATP → ADP + Pi, both │◄──────┤ dragging the thin filament│
│ staying bound. Energy is │ │ ~10 nm toward the M line. │
│ stored in the COCKED head.│ │ Force ~3–5 pN per head. │
│ ★ ATP ROLE 2: RE-COCKING │ │ ADP is released. │
└───────────────┬────────────┘ └─────────────┬──────────────┘
│ │
│ ┌────────── STEP 3 ─────────┘
└──────┤ CROSS-BRIDGE DETACHMENT
│ A NEW ATP binds the head.
│ Binding (not hydrolysis) collapses
│ myosin's affinity for actin.
│ The head lets go.
│ ★ ATP ROLE 1: DETACHMENT
└───────────────────────────
THE CYCLE REPEATS while Ca2+ remains bound to troponin C.
Each cycle slides the filament ~10 nm. Thousands of cycles per
second across a sarcomere; ~5 cycles/s per head under moderate load.
═══ ATP IS USED FOUR TIMES IN ONE CONTRACTION ═══════════════════
1. DETACHMENT of the head from actin (step 3) ← the one
everyone
forgets
2. RE-COCKING of the head (step 4)
3. SERCA pumping Ca2+ back into the SR (relaxation)
4. Na+/K+ ATPase restoring ion gradients (after every AP)
═══ RIGOR MORTIS — THE EXPERIMENT NATURE RUNS ═══════════════════
Death → no ATP → (a) SERCA stops, sarcolemma leaks, so
sarcoplasmic Ca2+ RISES and cross-bridges FORM;
(b) no ATP is available for STEP 3, so they CANNOT LET GO.
Result: rigid, locked muscle. Onset 3–4 h, maximal ~12 h,
resolving 24–72 h as autolysis digests the proteins.
▲ PROOF that ATP is required for RELEASE, not only for pulling.
Figure 9.6 — The four-step cross-bridge cycle, with both roles of ATP marked, and rigor mortis as the proof.
Described: A four-station loop. Step one, cross-bridge formation: an energised myosin head carrying bound ADP and inorganic phosphate, cocked at roughly ninety degrees, binds the exposed site on actin; binding progresses from weak to strong and inorganic phosphate is released. Step two, the power stroke: the head pivots from ninety degrees to forty-five degrees, dragging the thin filament about ten nanometers toward the M line and generating three to five piconewtons of force, after which ADP is released. Step three, cross-bridge detachment: a new molecule of ATP binds the head, and the act of binding — not its hydrolysis — collapses myosin's affinity for actin so the head lets go; this is marked as ATP role one. Step four, re-cocking: the myosin ATPase hydrolyses that ATP into ADP and inorganic phosphate, both of which remain bound, and the released energy is stored in the re-cocked conformation of the head; this is marked as ATP role two. The cycle then repeats for as long as calcium remains bound to troponin C, each cycle sliding the filament about ten nanometers, with each head cycling roughly five times per second under moderate load. A summary panel lists the four uses of ATP in a single contraction: detachment of the head from actin, re-cocking of the head, SERCA pumping calcium back into the sarcoplasmic reticulum during relaxation, and the sodium-potassium ATPase restoring ion gradients after every action potential. A final panel explains rigor mortis: after death there is no ATP, so SERCA stops and the sarcolemma leaks, sarcoplasmic calcium rises and cross-bridges form, but no ATP is available for step three so they cannot let go, producing rigid locked muscle beginning three to four hours after death, maximal at about twelve hours, and resolving over twenty-four to seventy-two hours as autolysis digests the proteins. This is the proof that ATP is required for cross-bridge release, not only for generating force.
Two features of that cycle repay a second reading.
ATP binding, not ATP hydrolysis, causes detachment. This is counterintuitive and it is the single most commonly misremembered fact in the chapter. The head does not "spend energy to let go"; it lets go because a nucleotide docking in its cleft changes its shape and destroys its grip. Hydrolysis happens afterwards, and pays for the re-cocking.
Rigor mortis is the control experiment. If ATP were needed only to generate force, a corpse would be limp — no ATP, no pulling, no tension. Instead the muscle locks solid, because the cross-bridges that formed cannot be released. That is as clean a demonstration as physiology offers, and it also explains why rigor eventually passes: nothing releases the bridges, but autolytic enzymes eventually destroy the proteins holding them.
Clinical Connection · Malignant Hyperthermia — A Ryanodine Receptor That Will Not Close
Malignant hyperthermia is an inherited (usually autosomal dominant) defect of the RyR1 ryanodine receptor, or occasionally of the DHP receptor. It affects roughly 1 in 2,000 people by genotype, and produces no symptoms whatsoever until the person is exposed to a triggering agent: a volatile anaesthetic (halothane, sevoflurane, isoflurane) or the depolarizing muscle relaxant succinylcholine.
The mutant receptor, on exposure, opens and fails to close. Calcium pours from the SR continuously. Now run the consequences forward using only §9.5:
- Cytosolic calcium stays high, so cross-bridges cycle without stopping. Muscle becomes rigid — classically, masseter spasm as the first sign, which is disastrous during intubation.
- Every cross-bridge cycle consumes ATP, and SERCA simultaneously runs at maximum trying to pump the calcium back, consuming still more. ATP demand becomes enormous.
- Roughly 75–80% of that energy appears as heat. Core temperature rises as fast as 1 °C every 5 minutes and can exceed 43 °C.
- Oxygen is consumed at three to five times normal and CO₂ production rises correspondingly — the earliest reliable sign is a rising end-tidal CO₂ that will not respond to increased ventilation.
- ATP depletion causes membrane failure: potassium, phosphate, myoglobin, and creatine kinase pour out of the fibers. Hyperkalaemia threatens the heart; myoglobinuria threatens the kidney (§9.7's rhabdomyolysis pathway).
The antidote, dantrolene, does exactly one thing: it inhibits RyR1 and stops the calcium leak. Before dantrolene, mortality was around 80%; with prompt recognition and treatment it is under 5%. There is no better illustration in medicine of why knowing the molecular step matters — the entire treatment is one channel blocker aimed at one protein at one step of Figure 9.5.
Check Your Understanding 9.5
- Skeletal muscle contracts normally in a calcium-free bath for many minutes; cardiac muscle stops within a few beats. Explain, naming the two receptors involved.
- A toxin locks the myosin head in the state immediately after the power stroke. What is the mechanical result, and which normal event has been prevented?
- Why does a muscle need ATP to relax as well as to contract? Name both ATP-consuming steps of relaxation.
Show answers
- In skeletal muscle the DHP receptor (Cav1.1) acts as a pure voltage sensor that is mechanically coupled to RyR1; no calcium needs to cross the membrane, and the SR store is large enough for many contractions. In cardiac muscle the DHP receptor (Cav1.2) actually conducts calcium into the cell, and that trigger calcium is what opens RyR2 — calcium-induced calcium release. Remove extracellular calcium and the trigger disappears, so the SR is never told to release, and contraction fails within a few beats.
- The head is attached to actin with ADP released and no new ATP bound — the rigor state. The muscle becomes rigid and cannot be stretched, because the prevented event is step 3, ATP binding and detachment. This is exactly the state of rigor mortis, and it demonstrates again that detachment is an active, nucleotide-dependent event.
- Because relaxation requires the calcium to be removed against its gradient, and because the membrane must be reset. SERCA pumps two calcium ions back into the SR per ATP hydrolysed, restoring a 20,000-fold gradient. The Na⁺/K⁺ ATPase simultaneously restores the sodium and potassium gradients spent on each action potential. A muscle deprived of ATP cannot relax — which is the whole of rigor mortis, and much of malignant hyperthermia.
9.6 The Mechanics of Contraction
We now move from one sarcomere to a whole muscle producing measurable force. Three questions organize this: how does force grow over time, how does it depend on length, and how does it depend on speed?
Twitch, summation, tetanus, treppe
A twitch is the mechanical response of a motor unit to a single action potential. It has three phases:
- Latent period (~2 ms): the interval between the action potential and any measurable tension. Nothing appears to happen — but excitation–contraction coupling is running. This is Figure 9.5, steps 1 through 6, made visible as a delay.
- Contraction phase (10–100 ms): cross-bridges cycle, tension rises to a peak.
- Relaxation phase (10–100+ ms, and usually longer than contraction): SERCA reclaims calcium. Relaxation is slower than contraction because pumping calcium uphill is slower than letting it fall downhill.
Twitch duration varies with fiber type in exactly the way you would predict: extraocular muscle ~7.5 ms, gastrocnemius ~40 ms, soleus ~100 ms.
A single twitch is essentially useless for movement — it is over before it accomplishes anything. Real contractions are built by two mechanisms working together:
1 · Wave summation (rate coding). If a second stimulus arrives before relaxation is complete, its tension adds on top of the residual tension. Two reasons: cytosolic calcium has not yet been fully re-sequestered, so more cross-bridges are already available; and the series elastic elements — tendon, titin, and the connective tissue sheaths — have already been pulled taut, so the second contraction's shortening goes straight into force rather than into taking up slack. Increase the stimulus frequency and tension climbs through unfused (incomplete) tetanus, a bumpy plateau, to fused (complete) tetanus, a smooth maximum, at roughly 50 Hz in slow muscle and 80–100 Hz in fast muscle. Fused tetanic tension is three to five times peak twitch tension.
2 · Multiple motor unit summation (recruitment). The nervous system adds motor units. This is the coarse control; rate coding is the fine control. In small muscles, recruitment is essentially complete by about 50% of maximum force and everything above that is rate coding; in large muscles recruitment continues to nearly 100%.
Treppe (German for "staircase") is a distinct phenomenon: when a rested muscle is stimulated repeatedly at a constant, maximal frequency, the first several contractions grow progressively stronger — by 10–30% — before plateauing. Three causes: cytosolic calcium rises slightly with each stimulus because SERCA cannot quite keep up; enzymes work faster as the muscle warms; and the myosin regulatory light chains become phosphorylated, increasing the sensitivity of the cross-bridges to calcium. Treppe is the physiological basis of a warm-up, and it is worth the two minutes it takes.
Note what tetanus implies about the normal state of your muscles. Voluntary contractions in daily life are almost always partially or fully fused tetani of asynchronously firing motor units — the units take turns, so the whole-muscle output is smooth even though individual units are twitching. This is why you can hold a coffee cup without vibrating.
Isotonic and isometric contraction
Contractions are classified by what happens to length.
| Type | Muscle length | Relationship of tension to load | Everyday example |
|---|---|---|---|
| Isotonic — concentric | Shortens | Tension exceeds load | Lifting a bag; the upward phase of a curl |
| Isotonic — eccentric | Lengthens while contracting | Load exceeds tension | Lowering a bag under control; walking downstairs |
| Isometric | Unchanged | Tension equals load | Holding a bag still; postural muscles standing |
"Isotonic" means equal tension and is an idealization — as a joint moves, its leverage changes, so tension changes throughout the range (§10.2). The clinically useful distinction is simply shortening, lengthening, or neither.
Eccentric contraction deserves special attention because it is counterintuitive and because it is where most injury and most adaptation happen. A muscle contracting eccentrically produces 1.2 to 1.8 times more force than the same muscle can produce concentrically or isometrically, while consuming less ATP and less oxygen. That combination — more force, less cost — sounds like a free lunch. It is not.
The mechanism is that cross-bridges are being pulled apart while attached. Strain-dependent detachment forces heads off actin mechanically rather than waiting for ATP binding, and they re-attach immediately at the next available site; in addition, titin stiffens when calcium is bound, taking up load passively. So force is high because it is partly borne rather than generated. And forcible detachment does mechanical damage: Z discs are streamed and broken, membranes are torn, and calcium leaks in.
Exercise & Sport · Eccentric Work, DOMS, and Why Damage Is the Point
Walk downhill for an hour and you will be sore for two days. Walk uphill for an hour and you will not. Same muscles, same duration, similar energy expenditure — and only the downhill, eccentric work produces delayed-onset muscle soreness (DOMS).
The time course is diagnostic. DOMS begins 8–12 hours after exercise, peaks at 24–72 hours, and resolves over 5–7 days. Blood lactate, by contrast, returns to baseline within about an hour. Lactate therefore cannot be the cause of DOMS, and the fact that this explanation persists in gyms is a good illustration of how a timeline can settle a mechanism argument outright.
What actually happens:
- Mechanical disruption. Forcible cross-bridge detachment under stretch damages sarcomeres — and it damages them unevenly, because the weakest sarcomeres in a myofibril lengthen most and therefore take the most strain (the "popping sarcomere" hypothesis). Z discs stream; titin and desmin are disrupted; the sarcolemma tears.
- Calcium influx through the damaged membrane activates calpains, calcium-dependent proteases that degrade structural proteins, extending the damage over hours.
- Inflammation. Neutrophils arrive within hours, macrophages over 1–3 days. They clear debris and release prostaglandins, bradykinin, and cytokines that sensitize group III and IV nociceptive afferents in the muscle's connective tissue. That sensitization is the soreness — the pain comes from inflammatory chemistry acting on nerve endings, not from the tearing itself, which is why the pain is delayed and why it is worst on stretch and palpation.
- Regeneration. Satellite cells activate, proliferate, and fuse; damaged proteins are replaced by more, and stronger, copies.
Step 4 is why eccentric work is prescribed rather than avoided. The repeated bout effect is dramatic: a second identical eccentric session two weeks later produces roughly half the soreness, half the strength loss, and far less creatine kinase release. Adaptations include added sarcomeres in series (which shifts the length–tension curve so the muscle operates on a safer part of it), stronger cytoskeletal proteins, and better motor unit distribution of load.
Practical implications: eccentric loading is the best-supported rehabilitation for tendinopathy; eccentric hamstring work (the Nordic curl) reduces hamstring strain injury substantially in sprinting sports; and the first session of any new eccentric programme should be deliberately small, because the muscle has no protection at all until it has been damaged once. Note also what DOMS is not: it is not a measure of workout quality, and its absence does not mean the session was wasted.
The length–tension relationship
Here the sarcomere geometry of §9.3 pays off completely. The active force a muscle can generate depends on its length, and the dependence is predictable from filament overlap alone.
TENSION
(% max)
120 ┤ ┌ TOTAL = active + passive
│ ╱
100 ┤ ╭─────────╮ PLATEAU ╱
│ ╱ ╲ optimal ╱
80 ┤ ╱ ╲ 2.0–2.2 ╱ ← PASSIVE tension (titin)
│ ╱ ╲ µm ╱ rises steeply past 2.4 µm
60 ┤ ╱ ╲ ╱
│ ╱ ASCENDING ╲ ╱ DESCENDING LIMB
40 ┤ ╱ LIMB ╲╱ (fewer cross-bridges available)
│ ╱ ╱ ╲
20 ┤╱ ╱ ╲
├─────────────────────────────────╲──────────────────►
1.2 1.6 2.0 2.2 2.6 3.0 3.6 SARCOMERE
LENGTH (µm)
── A ── 1.3 µm ────────── ── B ── 2.2 µm ── ── C ── 3.2 µm ──
═══════█═══════════ ═══█═══════ ═══█═════
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ ▬▬▬▬▬▬▬▬ ▬▬▬▬▬▬
═══════█═══════════ ═══█═══════ ═══█═════
thin filaments from EVERY head lies thin filaments
OPPOSITE ends OVERLAP opposite actin; have been pulled
each other and interfere; NO interference; OUT; many heads
thick filaments collide MAXIMUM number face empty space
with the Z discs of cross-bridges
→ TENSION LOW → TENSION MAX → TENSION LOW
Active tension is ZERO at ~1.27 µm and again at ~3.6 µm
(= thick 1.6 µm + thin 1.0 µm + thin 1.0 µm — the length at which
overlap disappears entirely). The curve is arithmetic, not biology.
IN THE INTACT BODY, joints restrict sarcomere length to roughly
70–130% of optimal, so you normally work near the plateau.
Figure 9.7 — The length–tension relationship, with filament overlap at three sarcomere lengths.
Described: A graph of tension as a percentage of maximum against sarcomere length in micrometers, from 1.2 to 3.6. Active tension rises along an ascending limb from zero at about 1.27 micrometers, reaches a plateau of one hundred percent between 2.0 and 2.2 micrometers, and then falls along a descending limb to zero at about 3.6 micrometers. A separate passive tension curve, contributed by titin, is near zero at short lengths and rises steeply beyond about 2.4 micrometers; total tension is the sum of the two. Beneath the graph, three sarcomere diagrams explain the shape. At A, 1.3 micrometers, the thin filaments projecting from opposite Z discs overlap each other and interfere, and the thick filaments have collided with the Z discs, so tension is low. At B, 2.2 micrometers, every myosin head lies opposite an actin binding site with no interference from opposing thin filaments, giving the maximum possible number of cross-bridges and maximum tension. At C, 3.2 micrometers, the thin filaments have been pulled out of the thick filament array so that many heads face empty space, and tension is low. Active tension reaches zero at about 3.6 micrometers, which is simply the sum of one thick filament length of 1.6 micrometers and two thin filament lengths of 1.0 micrometer each — the curve is arithmetic rather than biology. A closing note records that in the intact body, joint ranges restrict sarcomere length to roughly seventy to one hundred thirty percent of optimal, so muscles normally operate near the plateau.
The practical version of this curve is something you already know with your body. You cannot grip strongly with the wrist fully flexed, because the finger flexors are already shortened and sitting on the ascending limb; extend the wrist and grip strength roughly doubles. You cannot generate much force at the very end of a bicep curl. And a muscle that has been immobilized in a shortened position for weeks — Toby's quadriceps in a brace — physically removes sarcomeres in series, re-optimizing its length–tension curve for the shortened position, which is one reason rehabilitation must restore range of motion before it restores strength.
The force–velocity relationship
Force and speed trade off, hyperbolically.
- At zero load, a muscle shortens at its maximum velocity (V_max), which depends almost entirely on myosin ATPase rate — hence fiber type.
- As load rises, shortening velocity falls. The reason is kinetic: at higher loads, each cross-bridge must remain attached longer to move the filament, so fewer complete cycles occur per second, and a greater proportion of heads are attached at any instant simply bearing load.
- At the load equal to maximum isometric force (P₀), velocity is zero — the muscle holds.
- Beyond P₀ the muscle is lengthened while contracting: eccentric action, where force rises steeply to 1.2–1.8 × P₀.
Power is force multiplied by velocity, and it is therefore zero at both extremes: an isometric hold produces no power (no movement), and an unloaded twitch produces no power (no force). Maximum power occurs at roughly 30% of maximum force and 30% of maximum velocity — which is precisely why athletes training for power train with moderate loads moved fast, rather than with maximal loads moved slowly or light loads moved carelessly.
Check Your Understanding 9.6
- Why is the relaxation phase of a twitch usually longer than the contraction phase?
- You cannot lift a heavy suitcase quickly. Name the relationship that explains this and give its mechanistic cause at the cross-bridge level.
- A patient's finger flexors are strong when the wrist is extended and weak when the wrist is flexed, with no nerve or muscle disease. Explain.
Show answers
- Because contraction is driven by calcium moving down its gradient through an open channel — fast and passive — while relaxation requires SERCA to pump calcium up a 20,000-fold gradient at a cost of one ATP per two ions. Downhill is always faster than uphill. This asymmetry is also what makes wave summation possible: residual calcium is still present when the next stimulus arrives.
- The force–velocity relationship. At a high load, each cross-bridge must stay attached longer to translocate the filament against resistance, so the cycling rate falls and fewer power strokes occur per second. Attached-but-not-yet-detached heads also resist further sliding. The consequence is a hyperbolic fall in shortening velocity as load rises, reaching zero velocity at maximum isometric force.
- Length–tension. The long finger flexors cross the wrist as well as the fingers. Flexing the wrist shortens them further, pushing their sarcomeres onto the ascending limb where thin filaments interfere with each other, so fewer effective cross-bridges can form. Extending the wrist restores them toward optimal length. This is active insufficiency, and it is the reason grip dynamometers specify wrist position.
9.7 Muscle Metabolism
A resting muscle fiber holds about 5 millimoles of ATP per kilogram of wet tissue. At maximal contraction it consumes ATP at roughly 3–4 mmol/kg per second. Divide one by the other and you get the central fact of muscle metabolism: the stored ATP lasts about two seconds.
Everything else is regeneration. Three systems do it, and they differ in exactly the way engineers would expect: the fastest has the smallest tank, and the largest tank is the slowest to open.
Predict This
A 100-metre sprint takes about 10 seconds; a 400-metre run takes about 50. Both are run at essentially maximal effort, and the 400 is only four times longer.
Before reading on, commit to an answer: why is the 400 metres almost universally described as the most painful event in athletics, while the 100 is not — and which ATP system does that difference implicate?
(Answer: the 100 m is powered mainly by stored ATP and creatine phosphate, which run out at about 8–10 seconds and leave behind almost nothing. The 400 m forces the athlete deep into anaerobic glycolysis for 40 seconds, producing a large accumulation of inorganic phosphate, hydrogen ions, and lactate — blood lactate after a 400 m can reach 20–25 mmol/L against a resting 1. The pain is a metabolic state, not a distance.)
POWER
(mmol ATP ┌── 1. IMMEDIATE / PHOSPHAGEN ── creatine phosphate
/kg/s) │ CP + ADP ──(creatine kinase)──► ATP + creatine
3.6 ┤███████ │ Store: ~20–25 mmol/kg. ONE enzyme, ONE step,
│███████ │ no oxygen, no membranes crossed.
3.0 ┤███████ │ PEAK POWER · TINY CAPACITY
│███████╲
2.4 ┤███████ ╲ ┌── 2. GLYCOLYTIC (anaerobic) ── glucose/glycogen
│███████ ╲ │ → 2 pyruvate → 2 LACTATE. Net 2 ATP/glucose
1.8 ┤███████ ╲│ (3 from stored glycogen). 10+ enzyme steps.
│████████████████████ blood lactate can reach
1.2 ┤████████████████████╲ 15–25 mmol/L (rest ~1)
│█████████████████████╲
0.6 ┤███████████████████████████████████████████████████████████
│ 3. OXIDATIVE: glycolysis → acetyl-CoA → citric acid cycle
│ → electron transport. ~30–32 ATP/glucose; ~106/palmitate
└──┬────┬─────┬──────┬───────┬────────┬────────┬────────────►
2 s 10 s 30 s 1 min 2 min 10 min 1 h TIME
═══════════════════════════════════════════════════════════════════
SYSTEM DOMINATES LIMIT IS REAL EVENT
─────────────────────────────────────────────────────────────────
Stored ATP 0–2 s quantity a single jump
Phosphagen 2–10 s CP store 100 m sprint, 1RM lift
Glycolytic 10 s–2 min H+ / Pi / rate 400–800 m; a hard rally
Oxidative >2 min O2 DELIVERY 5 km to a marathon
═══════════════════════════════════════════════════════════════════
▲ ALL THREE RUN AT ALL TIMES. The question is never "which system"
but "what PROPORTION". At rest you are ~100% oxidative and mostly
burning FAT; the crossover to carbohydrate occurs near 60–65% VO2max.
Figure 9.8 — The three ATP-regenerating systems plotted against time, with power output and real-world time domains.
Described: A graph of ATP-regenerating power, in millimoles of ATP per kilogram per second, against time from two seconds to one hour on a compressed scale. The immediate or phosphagen system, in which creatine kinase transfers a phosphate from creatine phosphate to ADP, has the highest power at about 3.6 millimoles per kilogram per second but the smallest store, roughly 20 to 25 millimoles per kilogram; it is a single enzymatic step requiring no oxygen and crossing no membranes, and it dominates for the first two to ten seconds. The glycolytic system, converting glucose or glycogen through ten or more enzymatic steps to pyruvate and then lactate, yields a net two ATP per glucose or three per glucosyl unit taken from glycogen, produces about 1.6 to 2.4 millimoles per kilogram per second, and dominates from roughly ten seconds to two minutes, during which blood lactate can rise from a resting value near one millimole per litre to fifteen to twenty-five. The oxidative system, running glycolysis into acetyl-CoA, the citric acid cycle, and the electron transport chain, yields about thirty to thirty-two ATP per glucose and about one hundred and six per palmitate, produces only about 0.4 to 0.7 millimoles per kilogram per second, and dominates beyond two minutes with essentially unlimited capacity. A table pairs each system with what limits it and with a real event: stored ATP for zero to two seconds limited by sheer quantity, as in a single jump; the phosphagen system for two to ten seconds limited by the creatine phosphate store, as in a 100 metre sprint or a single maximal lift; the glycolytic system for ten seconds to two minutes limited by hydrogen ion and phosphate accumulation, as in a 400 or 800 metre run; and the oxidative system beyond two minutes limited by oxygen delivery, as in events from five kilometres to a marathon. A closing note emphasises that all three systems run at all times and the question is always one of proportion; at rest a person is essentially entirely oxidative and burning mostly fat, with the crossover toward carbohydrate occurring near sixty to sixty-five percent of maximal oxygen uptake.
Two supporting reactions deserve names. Creatine kinase (CK) catalyses the phosphagen reaction, and it is abundant in muscle — which is exactly why CK appears in the blood when muscle is damaged, and why Amara's peak total CK of 310 U/L was recorded alongside her troponin. Adenylate kinase (myokinase) converts two ADP into one ATP plus one AMP, squeezing a last increment out of the adenine nucleotide pool; the AMP it generates is a powerful activator of glycolysis and of AMP-activated protein kinase, so it also functions as a low-energy alarm.
Oxygen debt and EPOC
Stop a hard sprint and your oxygen consumption does not return to baseline for many minutes. The classic name is oxygen debt; the modern one is excess post-exercise oxygen consumption (EPOC), and the change of name reflects a change of understanding: it is not a single debt being repaid but a collection of restorations and residual costs.
- Resynthesis of creatine phosphate — the largest single component of the fast phase, complete in 2–5 minutes.
- Re-oxygenation of myoglobin in muscle and haemoglobin in venous blood.
- Clearance of lactate, most of which is oxidized as fuel by the heart, brain, and other muscle fibers, with a minority converted back to glucose in the liver (the Cori cycle).
- Elevated core temperature, which raises metabolic rate directly (a 1 °C rise raises metabolic rate roughly 10–13%).
- Residual catecholamines, and the continuing extra work of an elevated heart rate and respiratory rate.
- Restoration of ion gradients by the Na⁺/K⁺ ATPase after millions of action potentials.
EPOC has a fast component of minutes and a slow component that can persist for hours; the total amounts to roughly 6–15% of the oxygen cost of the exercise itself, and it scales with intensity far more than with duration.
Muscle fatigue — and the lactic acid problem
Fatigue is an exercise-induced decline in the ability to produce force. It is not the same as running out of ATP: ATP concentration rarely falls below about 60% of resting even in exhausting exercise, because the cell shuts down before it depletes its own currency. Fatigue is better understood as a set of protective failures that appear well before catastrophe.
Thread 2 · Homeostasis Is the Master Concept
Fatigue looks like a failure, and it is easier to understand as a defended limit. A muscle that could genuinely run its ATP to zero would enter rigor and destroy itself — exactly what happens in malignant hyperthermia, where the protective limits are bypassed. Instead, the accumulating products of ATP hydrolysis themselves inhibit the machinery that is hydrolysing it, and the central nervous system reduces drive before the periphery is damaged. Fatigue is negative feedback with the muscle's structural integrity as the regulated variable.
Now the misconception, stated carefully, because it is the most persistent error in this subject.
"Lactic acid builds up and burns the muscle" is wrong in at least four separate ways.
- Lactate is not lactic acid. At intracellular pH, lactic acid is more than 99% dissociated. The molecule that exists in muscle is the lactate anion, and it is not an acid.
- Producing lactate consumes a proton rather than releasing one. The lactate dehydrogenase reaction is pyruvate + NADH + H⁺ → lactate + NAD⁺. It removes a hydrogen ion from the cytosol. The acidosis of intense exercise comes from ATP hydrolysis outrunning mitochondrial ATP resynthesis: each ATP hydrolysed releases a proton, and when the mitochondria cannot consume protons as fast as the myofibrils release them, pH falls. Lactate production is correlated with acidosis because both follow from high glycolytic flux — but correlation is not the causal arrow.
- Lactate is a fuel, not a waste product. It is continuously produced at rest, exported by monocarboxylate transporters, and oxidized by the heart, by slow oxidative muscle fibers, and by the brain. Trained athletes are better at clearing lactate, not worse at making it, and the lactate shuttle is now understood as a major route for moving carbohydrate energy between tissues.
- The experimental evidence points elsewhere. Patients with McArdle disease cannot break down muscle glycogen and therefore produce essentially no lactate at all — and they fatigue faster, not slower. Skinned-fiber experiments at physiological temperature (rather than the 10–15 °C of the classic 1970s experiments) show that acidosis reduces force far less than was believed, and there is good evidence that acidosis actually protects excitability by counteracting the depolarizing effect of potassium accumulation.
So what does cause fatigue? Different mechanisms at different intensities:
| Cause | Mechanism | Where it dominates |
|---|---|---|
| Inorganic phosphate (Pi) accumulation | Pi from ATP and CP breakdown reduces force per cross-bridge and lowers myofibrillar Ca²⁺ sensitivity; it also enters the SR and precipitates with calcium as calcium phosphate, reducing releasable Ca²⁺ | The single most important cause of high-intensity fatigue |
| Reduced SR calcium release | RyR1 becomes leaky and less responsive; SR calcium content falls | Sustained maximal work |
| Extracellular K⁺ accumulation | Repeated action potentials export K⁺; in the narrow T-tubule lumen it can reach 10–15 mmol/L, depolarizing the membrane and inactivating Na⁺ channels, so the AP fails to invade the fiber | High-frequency stimulation |
| Reactive oxygen and nitrogen species | Oxidize contractile and SR proteins, reducing Ca²⁺ sensitivity | Prolonged submaximal work |
| Glycogen depletion | Both whole-fiber depletion and, importantly, depletion of the small intramyofibrillar glycogen pool sitting next to the triads, which impairs SR calcium release even when total glycogen remains | 90 minutes to 3 hours — "hitting the wall" |
| Central fatigue | Reduced voluntary drive from the motor cortex, driven partly by group III and IV muscle afferents reporting the metabolic state, and modulated by serotonin and dopamine | Every duration; larger than most people assume |
Notice how much of that list is signalling rather than exhaustion. Fatigue is largely the body deciding to stop.
Clinical Connection · Statin Myopathy and Rhabdomyolysis
Statins inhibit HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway. That pathway's best-known product is cholesterol, but it is not the only one. The same pathway produces ubiquinone (coenzyme Q10) for the electron transport chain, dolichol for glycoprotein synthesis, heme A, and the isoprenoids farnesyl pyrophosphate and geranylgeranyl pyrophosphate, which are attached to small GTPases (Ras, Rho, Rab) to anchor them in membranes. Inhibit the enzyme and you reduce all of them.
Several plausible mechanisms follow, and the evidence supports more than one:
- Reduced coenzyme Q10 impairs mitochondrial electron transport in a tissue with very high and very variable oxidative demand.
- Reduced protein prenylation disturbs membrane trafficking and calcium handling; there is evidence for increased SR calcium leak through RyR1 and reduced SERCA activity — which would raise resting cytosolic calcium and activate proteases.
- Activation of the ubiquitin–proteasome system through atrogin-1 and MuRF-1, the same atrophy programme used in disuse and starvation.
- Pharmacokinetics. Skeletal muscle is 40% of body mass, and lipophilic statins distribute into it. Variants in the SLCO1B1 gene, which encodes the liver uptake transporter OATP1B1, reduce hepatic extraction and raise plasma statin concentration several-fold, and are a well-established risk factor.
The clinical spectrum is a ladder, and the numbers matter because they determine what to do:
| Presentation | Creatine kinase | Approximate frequency |
|---|---|---|
| Myalgia — aching, usually proximal, symmetric, worse with exertion | Normal | 5–10% report it; blinded rechallenge shows only ~1–2% is truly drug-attributable — the nocebo effect here is large and well documented |
| Myopathy — weakness with muscle symptoms | > 10 × upper limit | ~1 in 1,000 to 1 in 10,000 |
| Rhabdomyolysis — muscle breakdown with systemic effects | > 40 × upper limit, often > 10,000 U/L | ~1–3 per 100,000 patient-years |
| Statin-associated autoimmune myopathy — anti-HMGCR antibodies; progresses after stopping the drug | Very high | ~2 per 100,000 |
Rhabdomyolysis is where §9.2's structure and §9.7's chemistry meet. Sarcolemmal integrity fails; the cell's contents enter the circulation. Potassium and phosphate rise, and hyperkalaemia is the immediate threat to the heart. Myoglobin — 17.8 kDa, small enough to be freely filtered — reaches the kidney, where it causes renal vasoconstriction, forms obstructing casts in the tubules, and is directly toxic to tubular cells; the result is acute kidney injury. The urine turns tea-coloured and tests positive for blood on dipstick while showing no red cells on microscopy, because the dipstick detects heme, not cells. For a patient like Amara, already destined for chronic kidney disease (Chapter 26), this is not an abstract risk.
The counselling Amara received is therefore precisely targeted: proximal, symmetric muscle ache plus dark urine is the pattern that matters. And the correct response to muscle aches on a statin three days after a myocardial infarction is to investigate, not to stop — because the absolute benefit of high-intensity statin therapy in the year after an infarct is large, and most aches are not the drug.
Check Your Understanding 9.7
- A patient with McArdle disease cannot produce lactate and fatigues faster than normal. What does this observation eliminate as a cause of fatigue, and what does it suggest instead?
- A 100 m sprinter and a marathon runner both finish exhausted. Name the dominant fatigue mechanism in each.
- Why does the dipstick read "blood" in rhabdomyolysis when there are no red cells in the urine?
Show answers
- It eliminates lactate accumulation as a necessary cause: you cannot blame a molecule that is absent in a patient who fatigues more readily. It suggests instead that the limiting factor is the supply of glycolytic ATP — these patients cannot mobilize muscle glycogen, so they cannot support high-intensity work at all, and must rely on blood glucose and fat. Characteristically they experience a "second wind" after 6–10 minutes as fatty acid delivery and blood glucose uptake increase.
- The sprinter is limited chiefly by inorganic phosphate accumulation — plus creatine phosphate depletion, potassium accumulation in the T-tubules, and impaired SR calcium release. The marathon runner is limited chiefly by glycogen depletion, including the intramyofibrillar pool, together with central fatigue, hyperthermia, and dehydration. Same word, entirely different physiology.
- The dipstick detects the heme group, which is present in both haemoglobin and myoglobin. Filtered myoglobin therefore produces a positive reaction. Microscopy finds no red cells because none were filtered. The mismatch between a positive dipstick and a negative microscopic examination is itself the diagnostic clue.
9.8 Fiber Types and the Size Principle
Not all skeletal muscle fibers are alike. Three types exist in adult humans, defined primarily by which myosin heavy chain isoform they express, and the isoform determines the ATPase rate, which determines shortening velocity, which sets everything else.
═══════════════════════════════════════════════════════════════════════
TYPE I TYPE IIa TYPE IIx
slow fast oxidative- fast
oxidative glycolytic glycolytic
═══════════════════════════════════════════════════════════════════════
Myosin heavy MHC-I MHC-IIa MHC-IIx
chain / ATPase SLOW FAST FASTEST
Time to peak 80–100 ms ~50 ms ~25–30 ms
Fiber diameter small intermediate LARGE
Mitochondria VERY MANY many few
Capillaries/fiber 4–6 3–4 1–2
Myoglobin HIGH (red) high (red-pink) LOW (white)
Glycogen store low high VERY HIGH
Oxidative enzymes HIGH high low
Glycolytic enzymes low high VERY HIGH
SR / SERCA density low high VERY HIGH
Fatigue resistance VERY HIGH moderate VERY LOW
Power output low high VERY HIGH
Motor neuron SMALL cell large LARGEST
low threshold highest threshold
Typical use posture, walking to sprinting,
standing, running, jumping, maximal
marathon repeated efforts lifts
Example muscle soleus, deep quadriceps mix extraocular,
erector spinae triceps surface
═══════════════════════════════════════════════════════════════════════
═══ THE SIZE PRINCIPLE (Henneman) ════════════════════════════════════
Motor units are recruited in a FIXED ORDER, smallest motor neuron
first, because a small cell has a HIGH input resistance (R = ρ/area)
and therefore reaches threshold with LESS synaptic current.
FORCE
100% ┤ ╔═══════════════╗
│ ╔═╣ TYPE IIx ║
60% ┤ ╔════════════╝ ║ recruited ║
│ ╔════════╣ TYPE IIa ║ LAST, only ║
20% ┤ ╔════════╣ TYPE I ║ ║ at high force║
│══╝ TYPE I ║ ║ ║ or high RATE ║
0 └────────────────────────────────────────────────────►
low ◄──── DESCENDING NEURAL DRIVE ────► maximal
CONSEQUENCE 1 Light loads NEVER recruit type II fibers, however
many repetitions you do — until fatigue of the
type I units forces recruitment upward.
CONSEQUENCE 2 To train type II fibers you must produce HIGH FORCE
(heavy load) or HIGH RATE OF FORCE DEVELOPMENT
(ballistic movement) or train to genuine failure.
CONSEQUENCE 3 Electrical stimulation VIOLATES the order — it
recruits by axon diameter and proximity, reaching
large fast axons FIRST. This is why NMES can train
type II fibers in a patient who cannot lift.
═══════════════════════════════════════════════════════════════════════
Figure 9.9 — The three human fiber types compared, and the size principle governing recruitment order.
Described: A comparison table of the three adult human skeletal muscle fiber types across fifteen properties, followed by a diagram of the size principle. Type I fibers are slow oxidative: they express the slow myosin heavy chain isoform with the slowest ATPase, take 80 to 100 milliseconds to reach peak tension, are small in diameter, contain very many mitochondria and four to six capillaries per fiber, are rich in myoglobin and therefore red, store little glycogen, have high oxidative and low glycolytic enzyme activity, have sparse sarcoplasmic reticulum, are very fatigue resistant, produce low power, are driven by small low-threshold motor neurons, and are used for posture, standing, and endurance; the soleus and the deep erector spinae are examples. Type IIa fibers are fast oxidative-glycolytic, intermediate on nearly every measure, reaching peak tension in about 50 milliseconds, with many mitochondria, three to four capillaries, high glycogen, high activity of both enzyme systems, moderate fatigue resistance and high power, driven by large motor neurons, and used from walking through running and repeated efforts. Type IIx fibers are fast glycolytic: fastest ATPase, peak tension in 25 to 30 milliseconds, largest diameter, few mitochondria and only one to two capillaries, low myoglobin and therefore pale, very high glycogen and glycolytic enzyme activity, very dense sarcoplasmic reticulum and SERCA, very low fatigue resistance, very high power, driven by the largest and highest-threshold motor neurons, and used for sprinting, jumping, and maximal lifts. The size principle diagram shows force rising from zero to one hundred percent as descending neural drive increases, with type I units recruited first at low force, type IIa units added in the middle range, and type IIx units recruited last, only at high force or high rate of force development. The stated reason is that a small motor neuron has a high input resistance, so a given synaptic current depolarizes it more and it reaches threshold first. Three consequences are listed: light loads never recruit type II fibers however many repetitions are performed unless fatigue of the type I units forces recruitment upward; training type II fibers requires high force, high rate of force development, or training to genuine failure; and electrical stimulation violates the recruitment order because it excites by axon diameter and proximity, reaching large fast axons first, which is why neuromuscular electrical stimulation can train type II fibers in a patient who cannot lift.
Two clarifications. First, humans possess a IIb gene but essentially do not express it in limb muscle; what older textbooks call human type IIb is now correctly called IIx. Second, fibers are not neatly categorical — hybrid fibers co-expressing two isoforms are common, and the continuum I → IIa → IIx is genuinely continuous.
Exercise & Sport · What Actually Changes With Training
The first six weeks of resistance training are mostly neural. Strength can rise 20–40% with little measurable change in muscle cross-sectional area. What changes is the nervous system: more motor units recruited, higher firing rates (rate coding), better synchronization between units, reduced co-activation of antagonists, and reduced inhibition from Golgi tendon organs. This is why beginners get dramatically stronger without getting visibly bigger, and why early-phase strength gains transfer poorly between exercises — you are learning a movement, not just building tissue.
Hypertrophy follows. Mechanical tension is the primary stimulus; it is sensed by mechanotransduction pathways converging on mTORC1, which raises myofibrillar protein synthesis for 24–48 hours after a session. New myofibrils are added in parallel, increasing cross-sectional area by 20–60% over months of training. Sustained growth also requires satellite cells to fuse in and donate nuclei, because each nucleus can only support so much cytoplasm. Type II fibers hypertrophy more than type I — typically about twice as much for the same programme. Fiber hyperplasia (new fibers) is minimal in humans.
Endurance training changes the engine, not the frame. Mitochondrial volume rises 40–50% through PGC-1α-driven biogenesis; capillary density rises 20–30%; myoglobin increases; enzymes of fat oxidation and of the citric acid cycle increase; the muscle burns proportionally more fat at any given absolute workload, sparing glycogen. Fiber cross-sectional area changes little or falls slightly.
Fiber type is partly plastic and partly not. The type I versus type II distinction is largely genetically determined and changes little with training. The IIx ↔ IIa transition, however, is highly plastic: almost any training of any kind shifts IIx toward IIa. A curious consequence is that detraining after a period of hard training produces an overshoot of IIx above baseline — which is part of why a taper before competition works.
Detraining is fast and asymmetric. Mitochondrial adaptations begin to reverse within 1–2 weeks of complete inactivity; hypertrophy is lost more slowly, over weeks to months; and neural adaptations persist longest. Chapter 10 follows this in Toby's atrophying quadriceps and in Amara's rehabilitation prescription.
Aging · Sarcopenia
Sarcopenia is the age-related loss of muscle mass and function. Mass falls roughly 3–8% per decade after age 30, accelerating after 60; strength falls faster than mass, at about 1.5–3% per year after 60; and power — force times velocity — falls fastest of all. That ordering is the clinically important part, because power, not strength, is what recovers a stumble.
The cellular story has three strands, and each one is a concept from this chapter running backwards.
- Preferential loss of type II fibers. Type II fiber cross-sectional area falls by 30–40% between ages 20 and 80; type I area is comparatively preserved. Since type II fibers are the fast, powerful ones, this alone explains why power declines faster than mass.
- Motor unit remodelling. Motor neurons are lost — roughly 25–50% of them between 20 and 80, disproportionately the large ones. Surviving axons sprout to adopt orphaned fibers, so the remaining motor units become larger and coarser, and the adopted fibers convert to the type of their new neuron. Two consequences follow: total muscle mass is partially preserved at the cost of precision, and the histological picture becomes one of fiber type grouping (§9.4). Fine control degrades before strength does.
- Reduced anabolic responsiveness. Older muscle mounts a smaller protein-synthetic response to the same dose of protein or the same bout of resistance exercise — anabolic resistance — so more of both stimuli is needed for the same result.
None of this is fixed. Progressive resistance training produces measurable hypertrophy and large strength gains in people in their eighties and nineties, and power-oriented training (moving moderate loads quickly) improves function more than heavy slow training does. The consequences of ignoring it are followed in Chapter 10 through Amara's mother Adwoa: sarcopenia, falls, and grip strength as a mortality predictor.
Check Your Understanding 9.8
- A patient can produce only 20% of maximum voluntary force because of pain inhibition. Which fiber types are being trained by his exercises, and what does that predict about his recovery of power?
- Why does neuromuscular electrical stimulation recruit fast fibers at low force levels when voluntary contraction does not?
- A sprinter tapers for two weeks before a championship and finds she feels faster. Give a fiber-type explanation.
Show answers
- Only type I and possibly the lowest-threshold type IIa units are being recruited, by the size principle. Type IIx units are never reached. He will regain endurance and low-force control long before he regains power, because the fibers responsible for power are not being loaded at all. This is precisely the problem in post-injury rehabilitation, and it is why programmes eventually must include high-force or high-velocity work.
- Because electrical stimulation bypasses the motor neuron's synaptic input entirely and excites axons directly. Excitability of an axon depends on its diameter — large axons have lower thresholds to external current — and on its proximity to the electrode. Large axons supply fast motor units, so stimulation recruits them early or randomly rather than in the physiological order. This is a genuine clinical advantage in a patient who cannot voluntarily generate high force.
- During intense training, IIx fibers shift toward IIa. Reducing training volume for one to three weeks allows the IIx proportion to rebound, typically overshooting the original baseline. Combined with full glycogen and creatine phosphate stores and complete repair of eccentric damage, the muscle is genuinely faster than it was at the peak of training. The sensation is real, and it is fiber-type plasticity.
9.9 Advanced Topic · Smooth and Cardiac Muscle
The two involuntary muscles use the same sliding filaments and the same myosin ATPase, and then diverge in every element of packaging, regulation, and control. Each divergence is a solution to a different problem.
Smooth muscle: slow, cheap, and built to hold
Smooth muscle has no sarcomeres, and therefore no striations. Its thick and thin filaments run in oblique lattices anchored to dense bodies in the cytoplasm — which contain α-actinin and are functionally Z discs without alignment — and to dense bands on the inner face of the plasma membrane. Intermediate filaments of desmin and vimentin tie the dense bodies into a three-dimensional network. When the filaments slide, the whole cell shortens and twists, contracting to as little as 20–30% of its resting length; a skeletal fiber manages only about 65%. That is the payoff of abandoning the sarcomere: enormous excursion, which is what a bladder or a uterus or a stomach requires.
There are no T-tubules; shallow membrane pockets called caveolae partly substitute. The SR is small — 2–6% of cell volume — so most activating calcium enters from outside the cell through voltage-gated and ligand-gated channels.
And there is no troponin at all. Regulation in smooth muscle is thick-filament linked:
- Cytosolic Ca²⁺ rises and binds calmodulin (four calcium ions per molecule).
- The Ca²⁺–calmodulin complex activates myosin light chain kinase (MLCK).
- MLCK phosphorylates serine-19 of the 20-kDa regulatory myosin light chain on the myosin head.
- Phosphorylation activates the myosin ATPase, and cross-bridge cycling begins.
- Relaxation requires myosin light chain phosphatase (MLCP) to remove that phosphate.
Because the switch is a covalent modification rather than a conformational change, it is slow — contraction takes 0.5 to 3 seconds — and it is tunable. The RhoA/Rho-kinase pathway inhibits MLCP, so a given calcium concentration produces more force: calcium sensitization, which is how many vasoconstrictors work, and the target of several classes of drug.
The most remarkable property is the latch state. Once tension is established, cross-bridges can be dephosphorylated while still attached, whereupon they cycle extremely slowly and detach extremely slowly. Tension is maintained at roughly one three-hundredth of the ATP cost of maintaining the same tension in skeletal muscle. Every arteriole in your body is held partly constricted by this mechanism, every hour of your life, essentially for free. A skeletal muscle attempting the same job would exhaust itself in minutes.
Two further properties follow from the loose organization. Stress-relaxation (plasticity): stretch smooth muscle and tension rises transiently, then decays, so a hollow organ can fill over a large volume range without a rise in pressure — the bladder's key trick. And smooth muscle retains genuine mitotic capacity: the pregnant uterus grows by both hypertrophy and hyperplasia, and vascular smooth muscle proliferation is central to atherosclerosis and to restenosis after stenting, which is directly relevant to Amara's coronary artery.
Finally, smooth muscle comes in two organizational flavours. Single-unit (visceral) smooth muscle — gut, uterus, ureter, small vessels — is coupled by gap junctions and behaves as a syncytium, often with pacemaker cells (the interstitial cells of Cajal in the gut) setting a rhythm the nerves merely modulate. Multi-unit smooth muscle — the iris, arrector pili, large airways, large arteries — has few or no gap junctions, each cell separately innervated, allowing graded, precise control much as a skeletal motor unit does. There is no discrete neuromuscular junction in either: autonomic axons have swellings called varicosities that release transmitter into a diffuse junction over many cells.
Cardiac muscle: the tissue that cannot rest, and cannot be tetanized
Cardiac muscle is striated and uses troponin, so at the level of the sarcomere it works exactly like skeletal muscle. Four differences matter.
1 · The intercalated disc. Cardiac cells are short and branched, and they join end-to-end at specialized junctions containing three elements: desmosomes, which resist the mechanical pull of contraction; fascia adherens, which anchor actin filaments so that sarcomeres are continuous across cell boundaries; and gap junctions built of connexin 43, which couple the cells electrically. The result is a functional syncytium — a wave of excitation spreads cell to cell without a synapse, and the whole chamber contracts as a unit. The heart has no motor units and no recruitment; it is always all-or-none at the organ level.
2 · Calcium-induced calcium release. As noted at step 3 of Figure 9.5, cardiac excitation–contraction coupling requires extracellular calcium. Roughly 20–25% of the activating calcium enters through L-type channels in T-tubules at the Z discs, and that trigger opens RyR2 to release the remaining 75–80%. Calcium is removed by SERCA2a — regulated by phospholamban, which inhibits the pump until it is phosphorylated by protein kinase A — and by the Na⁺/Ca²⁺ exchanger, which trades three sodium ions in for one calcium ion out. This gives the heart something skeletal muscle does not have: the amount of calcium released per beat is variable, and therefore contractile force is continuously adjustable by hormones and nerves. Sympathetic stimulation, acting through β₁ receptors, cAMP, and PKA, phosphorylates the L-type channel (more trigger calcium), phospholamban (faster reuptake, so faster relaxation and a fuller store), and troponin I (faster calcium release from TnC). One signal, three targets, all in the same direction.
3 · Autorhythmicity. About 1% of cardiac cells are pacemakers rather than contractile cells. They have no stable resting potential: after each action potential, a slow inward "funny" current (I_f) plus calcium currents drift the membrane back up to threshold. The sinoatrial node does this fastest, at an intrinsic ~100 per minute, restrained to about 70 by resting vagal tone. Cardiac muscle therefore does not require innervation to contract — a denervated transplanted heart beats — and the autonomic nervous system modulates a rhythm rather than commanding a contraction. This is the exact opposite of skeletal muscle.
4 · No tetanus — and this is the point. The cardiac action potential has a long plateau phase, held up for 200–300 ms by inward calcium current balancing outward potassium current. Because the absolute refractory period lasts nearly as long as the plateau, the cell cannot be re-excited until it has almost finished contracting. Wave summation is therefore impossible, and cardiac muscle cannot be tetanized.
Thread 3 · The Body Is Integrated
Read that last paragraph as an engineering requirement rather than a limitation.
A pump works by alternately filling and emptying. A tetanized skeletal muscle produces its maximum useful output — sustained force. A tetanized heart would produce its minimum useful output: it would squeeze once, stay squeezed, and stop circulating blood. Cardiac output would fall to zero within seconds and the tissue causing the problem would be among the first to die for lack of coronary flow.
So the long refractory period is not a design compromise. It is a safety interlock that makes sustained contraction physically impossible, hard-wired into the ion channels of every cardiac cell. And it explains why the heart must grade its force by different means entirely: by length (the Frank–Starling relationship — the length–tension curve of §9.6, applied to a chamber that fills) and by contractility (how much calcium is released per beat). Chapter 18 is largely the elaboration of those two sentences.
The interlock can fail. When a region of ventricle depolarizes chaotically and asynchronously — ventricular fibrillation — the chamber never fills and never ejects, and death follows in minutes. The commonest substrate for that chaos is a border zone of scar next to surviving muscle, which is precisely what Amara now has.
Regeneration. Cardiomyocytes leave the cell cycle in the first weeks after birth. Many become binucleate or polyploid. The reason appears to be mechanical as much as genetic: mitosis requires the sarcomeric apparatus to be disassembled, and a cell that must contract sixty times a minute for eighty years cannot afford to take itself apart. Carbon-dating studies of human myocardium put annual turnover at roughly 1% at age 25, falling to 0.45% at age 75 — under 50% of a person's cardiomyocytes are replaced across an entire lifetime, and none of that capacity can be marshalled to repair an infarct. There is no satellite cell equivalent.
Repair therefore proceeds exactly as it does for any tissue that cannot regenerate (§4.9, "Tissue Repair — and Why Scar Is Not Muscle"): inflammation for the first 3 days, granulation tissue from days 3 to 14, and a mature collagen scar by 4–8 weeks. The scar is strong, but it is non-contractile, non-conducting, and stiffer than the muscle it replaced. Those three adjectives are the whole of Amara's prognosis: reduced ejection fraction, an arrhythmia substrate, and a ventricle that resists filling — the beginning of the heart failure with preserved ejection fraction that Chapter 18 develops.
Check Your Understanding 9.9
- Verapamil, an L-type calcium channel blocker, reduces the force of cardiac contraction but has essentially no effect on skeletal muscle strength. Explain using Figure 9.5.
- Why can smooth muscle hold tension for hours at a fraction of the ATP cost of skeletal muscle?
- Suppose a mutation shortened the cardiac action potential plateau to 20 ms. Predict the consequence.
Show answers
- In cardiac muscle the L-type channel (Cav1.2) is the conduit for trigger calcium, and without that trigger, RyR2 does not open. Block it and less calcium enters, less is released, and force falls. In skeletal muscle the homologous channel (Cav1.1) works as a mechanical voltage sensor physically coupled to RyR1; whether it conducts calcium is irrelevant to contraction. One protein family, two mechanisms, and a drug that distinguishes them.
- Because of the latch state. Cross-bridges are dephosphorylated while still attached and then cycle and detach extremely slowly, so tension is maintained by bridges that are not consuming ATP at anything like the normal rate. Add slow myosin ATPase kinetics generally, and the cost of holding tension falls to roughly one three-hundredth of the skeletal equivalent.
- The absolute refractory period would shorten with it, so cardiac muscle could be re-excited during contraction and would become capable of wave summation and tetanus. A tetanized ventricle cannot fill; cardiac output would collapse. The mutation would also create a substrate for re-entrant arrhythmia, because a short refractory period allows a wave of excitation to re-enter tissue it has already passed through — which is the mechanism of the short QT syndromes.
Chapter Summary
§9.1 Three muscle tissues share four universal properties — excitability, contractility, extensibility, elasticity — and differ in almost everything else. Skeletal muscle is voluntary, striated, multinucleate, electrically private, and independent of extracellular calcium; cardiac is involuntary, striated, branched, gap-junction coupled, autorhythmic, and absolutely dependent on extracellular calcium; smooth is involuntary, unstriated, slow, extraordinarily economical, and the only one that regenerates well. Skeletal muscle also produces 85% of body heat and absorbs 80% of a glucose load. The tissue-level introduction to all three is §4.6.
§9.2 A muscle is an organ built in six nested levels, each wrapped in connective tissue: epimysium around the muscle, perimysium around fascicles, endomysium around fibers, all converging into the tendon. Force travels from myosin head to bone through that collagen, via costameres and dystrophin — which is why Duchenne muscular dystrophy is a disease of force transmission in a fiber with a normal contractile apparatus. T-tubules solve the diffusion-distance problem by carrying the action potential into the fiber's core; the sarcoplasmic reticulum stores calcium at a 20,000-fold gradient using calsequestrin; a T-tubule plus two terminal cisternae is a triad.
§9.3 The sarcomere runs Z disc to Z disc, 2.0–2.2 µm at rest. The A band is the thick filament's length and never changes; the I band and H zone narrow during contraction because the filaments slide rather than shorten. Myosin is the motor, actin the track, tropomyosin the gate, troponin the calcium-operated latch on the gate, and titin the spring.
§9.4 The neuromuscular junction is deliberately over-engineered: an end plate potential of 30–40 mV against a 15 mV threshold gives a safety factor of 3–5, so one nerve impulse produces one muscle impulse without fail. Myasthenia gravis eats that margin; botulinum toxin blocks release; tetanus toxin blocks the inhibition of motor neurons in the cord. A motor unit is one neuron and its fibers; innervation ratio buys precision, from 3–10 fibers in the eye to over a thousand in the calf.
§9.5 Excitation–contraction coupling: action potential → T-tubule → DHP receptor → ryanodine receptor → calcium from 50 nM to 1–10 µM → troponin C → troponin I released from actin → tropomyosin rotates → cross-bridges cycle. The cycle has four steps, and ATP is used twice within it: binding causes detachment, hydrolysis re-cocks the head. Rigor mortis proves the first of those, because a muscle with no ATP locks rather than going limp. Relaxation costs ATP too, at SERCA.
§9.6 Twitch, wave summation, tetanus (3–5× twitch tension), and treppe describe force over time; recruitment and rate coding are how the nervous system grades it. Contractions are concentric, eccentric, or isometric, and eccentric is the strongest, cheapest, and most damaging. The length–tension curve is filament-overlap arithmetic; the force–velocity curve is cross-bridge kinetics; maximum power sits at about 30% of maximum force.
§9.7 ATP lasts two seconds and is regenerated by three systems with an inverse relationship between power and capacity: phosphagen (2–10 s), glycolytic (10 s–2 min), oxidative (beyond). EPOC is the collection of restorations afterwards. Fatigue is caused chiefly by inorganic phosphate, impaired SR calcium release, potassium accumulation, reactive oxygen species, glycogen depletion, and central drive — not by lactate, which is a fuel whose production consumes a proton rather than releasing one.
§9.8 Type I, IIa, and IIx fibers differ across every measurable axis because they differ in myosin ATPase rate. The size principle recruits small motor neurons first, so type II fibers are reached only at high force or high rate of force development. Early strength gains are neural; hypertrophy follows through mTORC1 and satellite cell fusion; endurance training builds mitochondria and capillaries; the IIx–IIa boundary is highly plastic and the I–II boundary is not.
§9.9 Smooth muscle replaces troponin with calmodulin and MLCK, abandons the sarcomere for enormous excursion, and holds tension in the latch state at 1/300 the cost. Cardiac muscle couples its cells electrically, requires extracellular calcium, generates its own rhythm, and cannot be tetanized because its refractory period spans its contraction — a hard-wired safety interlock without which a pump could not fill. Cardiac muscle also cannot regenerate, and replaces dead muscle with scar that is non-contractile, non-conducting, and stiff.
The Three Threads in Chapter 9
Structure → Function. Every property of muscle in this chapter turned out to be a molecule. Extensibility is titin. The gate is tropomyosin. The switch is troponin C. The rivet is dystrophin. The safety interlock in the heart is a calcium channel holding a plateau. When you can name the molecule, you can predict the disease — and when you meet a disease, you can now ask which molecule.
Homeostasis. Two loops ran through the chapter. Calcium is held at 50 nanomolar in resting sarcoplasm against a 20,000-fold gradient, at continuous ATP cost, and every muscle emergency in this chapter — malignant hyperthermia, rigor, dystrophy, ischaemia — is that gradient failing. And fatigue itself is negative feedback: the products of ATP hydrolysis inhibit the machinery hydrolysing it, and the nervous system reduces drive, so the muscle stops before it destroys itself.
Integration. Amara's troponin result is a cell biology fact (§9.3), an imaging fact (§9.4), a tissue fact (Chapter 4), and a prognosis (§9.9) simultaneously. A regulatory protein of the thin filament is in her blood; therefore cardiac sarcolemmas failed; therefore cells died; therefore — because cardiac muscle has no satellite cells — fibroblasts will lay down collagen; therefore 14% of her left ventricle will never contract again; therefore her ejection fraction is 48%; therefore her exercise prescription in Chapter 10 looks the way it does.
Case File 9 · Resolution
Question 1 — What is troponin actually doing inside a working muscle cell, and why does its presence in blood mean what it means?
Troponin is not a contractile protein. It generates no force and it is not part of the motor. It is the calcium-operated switch of the thin filament — a three-subunit complex spaced every 38.5 nanometres along the actin helix, one complex for every tropomyosin molecule and therefore for every seven actin monomers (§9.3).
At rest, cytosolic calcium is about 50 nanomolar. Troponin I binds actin and holds tropomyosin parked over the myosin-binding sites; troponin T is the physical link to tropomyosin; troponin C sits waiting with empty calcium sites. No myosin head can attach. The muscle is not "relaxed" in a passive sense — it is being actively held off, and troponin I is the protein doing the holding.
When calcium arrives, it binds the regulatory sites on troponin C. TnC changes shape, exposes a hydrophobic pocket, and grips the switch peptide of troponin I — pulling TnI off actin. Freed, TnT rotates tropomyosin about 25 degrees deeper into the actin groove, and seven myosin-binding sites appear at once. Cross-bridges form; the filaments slide. Remove the calcium and the whole sequence reverses within milliseconds.
So the answer to what is troponin doing is: it converts a chemical signal into mechanical permission. Calcium does not cause contraction; calcium removes an inhibition. This is a crucial distinction, and it is the reason that the number of proteins involved in switching a muscle on is larger than the number involved in making it pull.
Now the blood test. Cardiac muscle expresses its own isoforms of troponin I and troponin T, encoded by different genes from the skeletal isoforms, with distinct amino acid sequences — cardiac troponin I carries a unique 31-residue extension at its N-terminus that no skeletal isoform has. Antibodies can therefore be raised that bind cardiac troponin I and nothing else, which is why the assay is cardiac-specific in a way that creatine kinase is not. CK is found in all muscle, and her peak CK of 310 U/L would be equally consistent with a bruised thigh. Her cardiac troponin I of 2.40 ng/mL against a 99th-percentile reference of 0.04 ng/mL, rising to a peak of 4.10, is not ambiguous at all.
Troponin is a structural protein, bolted to the thin filament. About 5–8% of it floats free in the cytosol, and that fraction escapes early when a membrane fails — which is why troponin begins to rise 3–4 hours after the injury. The remaining 92–95% is released slowly over days as the myofibrils of dead cells are proteolysed, which is why the level peaks at 12–24 hours and stays elevated for 7–10 days, and why Amara's peak of 4.10 ng/mL came at twelve hours while her day-3 value of 0.91 ng/mL is falling exactly as it should. The magnitude of the peak correlates with the mass of muscle lost.
There is one more inference, and it is the one that matters clinically. A protein that is normally bolted to the inside of a sarcomere cannot reach the bloodstream unless the sarcolemma has failed. Troponin in blood is therefore not a report of ischaemia; it is a report of cell death. That is the entire logic of the test, and it is a statement about §9.2's membrane, not about §9.3's switch.
Question 2 — Why can skeletal muscle regenerate after a tear and cardiac muscle cannot?
Because of a decision made in the embryo (§9.2, Development sidebar).
When skeletal muscle is built, not every myoblast fuses into the myotube. A population is set aside, wedged between the sarcolemma and the basal lamina of each fiber, mononucleate and quiescent, marked by the transcription factor Pax7. These are satellite cells, and they make up 2–8% of the nuclei associated with adult skeletal muscle.
When Toby tore his hamstring, the injury did three things at once: it damaged fibers, it released growth factors from the damaged matrix, and it left the basal lamina largely intact. Satellite cells activated within hours, proliferated over days, and then followed the myogenic programme — MyoD, then myogenin — either fusing into the surviving ends of damaged fibers or fusing with one another to form new myotubes inside the empty basal lamina tubes, which act as a scaffold telling the new muscle where to grow and where to reconnect. Within weeks the tissue was muscle again, with a small amount of fibrosis. The regeneration is not perfect and it is not unlimited — Duchenne dystrophy is what exhausting it looks like — but it is real.
Cardiac muscle set aside nothing. Cardiomyocytes exit the cell cycle within weeks of birth; many become binucleate or polyploid, which is what a cell does when it grows without dividing. The obstacle appears to be as much mechanical as genetic: mitosis requires disassembling the sarcomeric apparatus, and a cell contracting sixty times a minute for eighty years cannot take itself apart. Isotope-dating of human myocardium puts turnover at about 1% per year at age 25, falling to 0.45% at 75 — enough to replace under half the heart across a lifetime, and nothing like enough to rebuild a region killed in a single afternoon.
So Amara's ventricle repairs by the only route available to a tissue without a stem cell reserve, and it is the route Chapter 4 described for any permanent tissue: inflammatory cells clear the dead muscle over the first three days, fibroblasts migrate in and lay down granulation tissue over days 3 to 14, and collagen matures into scar over 4 to 8 weeks.
The scar is strong — stronger in tensile terms than the muscle it replaced. It is also, in three words, non-contractile, non-conducting, and stiff. The echocardiogram's word akinetic is the first of those made visible: that 14% of her left ventricle will not thicken or move inward again, which is why her ejection fraction is 48% rather than 60%. The second, non-conducting, creates a border zone where surviving muscle is interleaved with insulating collagen — the classic substrate for re-entrant ventricular arrhythmia. The third, stiffness, means the ventricle resists filling, which raises left atrial pressure, which is the beginning of the heart failure with preserved ejection fraction that Chapter 18 develops in full.
One consolation, and it is a genuine one. The remaining myocytes hypertrophy, and the heart has substantial functional reserve. Rehabilitation works on the muscle that survived, not on the muscle that died — which is exactly the premise of Chapter 10.
Question 3 — Why does a drug that inhibits cholesterol synthesis in the liver cause pain in skeletal muscle?
Because the enzyme the drug inhibits does not only make cholesterol.
HMG-CoA reductase catalyses the rate-limiting step of the mevalonate pathway, and cholesterol is only one of that pathway's products. It also yields ubiquinone (coenzyme Q10), the mobile electron carrier of the respiratory chain; dolichol; heme A; and the isoprenoids farnesyl and geranylgeranyl pyrophosphate, which are covalently attached to small GTPases such as Ras, Rho, and Rab to anchor them in membranes. Inhibit the enzyme and every branch downstream is reduced, not just the cholesterol branch.
Skeletal muscle is unusually exposed to that, for three reasons drawn from this chapter. It is 40% of body mass, so a lipophilic statin distributes into a great deal of it. It is a tissue whose whole business is ATP turnover, so a reduction in respiratory chain efficiency matters more there than in most tissues. And its function depends on maintaining a 20,000-fold calcium gradient across the SR membrane (§9.2) — a gradient defended by proteins whose localization and regulation depend on prenylation and on membrane composition. There is experimental evidence for increased RyR1 calcium leak and reduced SERCA function under statin exposure, and for activation of the atrogin-1/MuRF-1 ubiquitin–proteasome atrophy programme. Add pharmacokinetics — SLCO1B1 variants that reduce hepatic uptake and raise plasma concentrations several-fold — and you have a mechanism, a dose dependence, and a genetic risk factor.
The clinical spectrum runs from myalgia with a normal CK, through myopathy with CK above ten times normal, to rhabdomyolysis, in which sarcolemmal integrity fails outright and the fiber's contents enter the circulation. That last one is the reason for the counselling about dark urine: filtered myoglobin obstructs and poisons the renal tubules, and potassium released from dying muscle threatens the heart directly. In a woman who will develop stage 3 chronic kidney disease (Chapter 26), this matters more than it would in most patients.
Two pieces of honesty belong in the answer. First, the nocebo effect here is unusually large: in blinded randomized trials and in n-of-1 rechallenge studies, most muscle symptoms reported on a statin also occur on placebo, and only about 1–2% of symptoms are truly attributable to the drug. Second, and more importantly for Amara: three days after a myocardial infarction, the absolute benefit of high-intensity statin therapy is among the largest in cardiology. The right response to muscle aches is to measure CK, characterize the pattern, and rechallenge — not reflexively to stop.
Note what has just happened across these three questions. A single molecule in her blood (troponin) explained the thin filament; a single absence in her heart (satellite cells) explained her prognosis; and a single enzyme in her liver explained a symptom in her thighs. None of those connections is visible if you study muscle one system at a time.
Systems Integration Case File · Entry 9
Entry 9 — Two muscles, and the difference between them
Amara's file now contains a proven cardiac muscle injury with a measured size, a measured functional cost, and a permanent structural consequence. Add the muscular system to your model.
Your entry:
1 · ADD. In two or three sentences, state what the muscular system contributes to Amara's picture. Use her actual numbers: troponin I peak 4.10 ng/mL, akinetic segment ~14% of the left ventricle, ejection fraction 48%. Name the specific cellular reason the akinesis is permanent.
2 · CONNECT. Link muscle to at least two systems already in your file, stating the direction of causation each time. Then identify one feedback loop in which muscle appears on both sides.
3 · PREDICT. Amara will begin cardiac rehabilitation in Chapter 10. Predict one specific restriction the exercise physiologist will place on her programme, and justify it from this chapter's physiology.
Model responses — read only after writing your own
1 · ADD. Cardiac muscle is the tissue that was lost. A cardiac troponin I peaking at 4.10 ng/mL against a 99th-percentile reference of 0.04 is a structural thin-filament protein appearing in plasma, which is only possible if cardiac sarcolemmas failed and cells died (§9.3, §9.5). The echocardiogram localizes that loss to roughly 14% of the left ventricle, which is akinetic, and the functional cost is an ejection fraction of 48% against a normal 55–70%. The akinesis is permanent because cardiac muscle sets aside no satellite cell reserve and its myocytes leave the cell cycle shortly after birth (§9.9); the dead muscle is replaced by fibroblast-derived collagen scar, which is non-contractile, non-conducting, and stiff.
2 · CONNECT. Cardiovascular → muscular: the narrowed coronary artery reduced oxygen delivery to cardiac muscle, ATP fell, cross-bridges could not cycle or detach, and the cells died — flow caused the muscle loss, not the reverse. Muscular → cardiovascular: the loss of 14% of contractile mass reduced stroke volume and ejection fraction, which lowers the pressure the vascular system receives — the muscle loss now causes the circulatory deficit. Skeletal muscular → endocrine/metabolic: skeletal muscle takes up roughly 80% of a glucose load, and her insulin-resistant muscle is a principal cause of the elevated glucose that drove the coronary disease in the first place (Chapter 2, and forward to Chapter 24). Muscular → integumentary and nervous: sympathetic activation to defend cardiac output constricts skin arterioles — smooth muscle acting as an effector for a nervous decision about a muscular failure, which is exactly Chapter 1's cool, pale skin.
The feedback loop: damaged cardiac muscle → lower stroke volume → lower arterial pressure → baroreceptor unloading → sympathetic activation → increased heart rate and contractility → increased myocardial oxygen demand → greater ischaemia in muscle already supplied by a narrowed artery → further loss of contractile muscle. Cardiac muscle is both the failing element and the effector of the compensation, which is why the loop is dangerous. This is the positive feedback runaway of §1.5 with a cellular mechanism now attached to every arrow.
3 · PREDICT. A reasonable prediction: she will be told not to hold her breath or perform heavy isometric lifting. Justification from this chapter: sustained isometric contraction above roughly 20% of maximum voluntary force occludes the muscle's own blood supply, so no local vasodilation offsets the pressor response; systemic pressure therefore rises steeply, raising the tension the surviving left ventricle must generate — and that tension is a principal determinant of myocardial oxygen demand in a heart whose supply is already limited. Rhythmic contraction, by contrast, alternates with relaxation, permits functional hyperaemia, and is therefore a flow load rather than a pressure load. Chapter 10 develops exactly this, together with the skeletal muscle pump.
Review
Level 1 · Recall
8.1 During contraction, which of the following does not change?
a) I band width b) H zone width c) A band width d) sarcomere length
Answer
c — the A band. The A band is defined as the length of the thick filament, and thick filaments neither shorten nor lengthen; they slide past thin filaments. The I band narrows, the H zone narrows or disappears, and the sarcomere shortens. Any answer implying that filaments shorten is wrong.
8.2 ATP binding to the myosin head causes:
a) the power stroke b) detachment of the head from actin c) calcium release from the SR d) tropomyosin to move
Answer
b — detachment. ATP binding collapses myosin's affinity for actin; ATP hydrolysis afterwards re-cocks the head. The power stroke is driven by energy already stored in the cocked head. Rigor mortis is the proof: with no ATP available to bind, cross-bridges cannot release, and muscle locks rather than going limp.
8.3 The protein that binds calcium to initiate skeletal muscle contraction is:
a) calmodulin b) calsequestrin c) troponin C d) tropomyosin
Answer
c — troponin C. Calmodulin is the calcium sensor in smooth muscle, working through MLCK. Calsequestrin is the low-affinity calcium buffer inside the SR. Tropomyosin is the gate that troponin operates, not the sensor.
8.4 A triad consists of:
a) three myofibrils b) one T-tubule and two terminal cisternae c) actin, myosin, and titin d) two T-tubules and one cisterna
Answer
b. A triad is the structure where excitation is converted into calcium release: DHP receptors in the T-tubule membrane physically coupled to ryanodine receptors in the flanking terminal cisternae. Cardiac muscle has dyads — one T-tubule and one cisterna, at the Z disc.
8.5 Which fiber type has the largest diameter, the highest glycolytic enzyme activity, and the highest recruitment threshold?
a) type I b) type IIa c) type IIx d) all are equal
Answer
c — type IIx. It is driven by the largest motor neuron, which by the size principle has the lowest input resistance and therefore the highest current requirement to reach threshold — so it is recruited last. Type I is the smallest, most oxidative, and most fatigue resistant.
8.6 Which statement about lactate is correct?
a) it causes delayed-onset muscle soreness b) its production releases a proton c) it is oxidized as a fuel by the heart and by other muscle fibers d) it is produced only when oxygen is absent
Answer
c. Lactate is shuttled by monocarboxylate transporters and oxidized by the heart, brain, and oxidative muscle fibers. It is produced continuously even at rest and in full oxygen. Its production consumes a proton (pyruvate + NADH + H⁺ → lactate + NAD⁺). DOMS peaks at 24–72 hours, long after lactate has cleared in about an hour, and is caused by eccentric mechanical damage and the inflammation that follows.
8.7 Cardiac muscle cannot be tetanized because:
a) it lacks troponin b) its refractory period lasts nearly as long as its contraction c) it has no sarcoplasmic reticulum d) its motor units are too large
Answer
b. The long plateau of the cardiac action potential, sustained by inward calcium current, keeps the cell refractory until contraction is nearly over, so a second stimulus cannot summate. This is a safety interlock, not a limitation: a tetanized heart could not fill, and cardiac output would fall to zero. Cardiac muscle has troponin and a sarcoplasmic reticulum, and has no motor units at all.
8.8 Malignant hyperthermia results from a defect in:
a) acetylcholinesterase b) dystrophin c) the ryanodine receptor d) SERCA
Answer
c — the ryanodine receptor (RyR1). On exposure to volatile anaesthetics or succinylcholine it opens and fails to close, so calcium pours from the SR, cross-bridges cycle uncontrollably, and ATP consumption by both cross-bridges and SERCA generates enormous heat. Dantrolene works by inhibiting RyR1 — a treatment aimed at exactly one step of Figure 9.5.
Level 2 · Comprehension
8.9 Explain why relaxation, not contraction, is the step that fails in rigor mortis, and what that reveals about the role of ATP.
Model answer
After death, ATP production ceases. Two things follow. SERCA stops pumping and the sarcolemma and SR begin to leak, so cytosolic calcium rises — meaning troponin C is occupied, tropomyosin moves aside, and cross-bridges form. But cross-bridge detachment requires a new ATP molecule to bind the myosin head, and there is none. The bridges therefore lock in the attached state and the muscle becomes rigid.
If ATP were required only to generate force, a body with no ATP would be flaccid. It is not. So ATP has at least two distinct roles: it is hydrolysed to re-cock the head, and — separately, and by binding rather than hydrolysis — it is required to release the head. Rigor resolves after 24 to 72 hours not because the bridges detach but because autolytic enzymes destroy the proteins holding them.
8.10 A muscle held in a shortened position generates less force than the same muscle at resting length. Explain from filament geometry, and give a clinical or everyday example.
Model answer
At sarcomere lengths below about 2.0 micrometers, thin filaments projecting from opposite Z discs begin to overlap each other in the centre of the sarcomere, interfering with cross-bridge formation on the far half of each thick filament. Below about 1.65 micrometers the thick filaments themselves collide with the Z discs and are compressed, and force falls steeply, reaching zero near 1.27 micrometers. Fewer effective cross-bridges means less force.
Everyday example: grip strength roughly halves when the wrist is flexed, because the long finger flexors cross the wrist and are already shortened — which is why dynamometers specify wrist position. Clinical example: a quadriceps immobilized in a shortened position for weeks removes sarcomeres in series, re-optimizing its curve for the shortened position, so restoring range of motion must precede restoring strength.
8.11 The neuromuscular junction has a safety factor of 3–5. Explain what that means, why it is desirable there but not at central synapses, and what happens when it is lost.
Model answer
The end plate potential is 30–40 mV where only about 15 mV is needed to reach threshold, so the signal is three to five times larger than required. Consequently one nerve impulse produces one muscle impulse essentially without fail.
This is desirable at the neuromuscular junction because no computation happens there. The decision to contract was made in the spinal cord and motor cortex; the junction's only job is to be a reliable relay. Central synapses are the opposite: a single input usually does not fire the postsynaptic neuron, and integration of many inputs is precisely the computation being performed. Reliability there would destroy the function.
Lose the margin — as in myasthenia gravis, where receptor numbers fall to about a third — and the normal small decline in transmitter release with repeated firing begins to drop the EPP below threshold. Fibers drop out one by one, so strength decays with use and recovers with rest. The muscles with the least margin, the extraocular muscles, fail first.
8.12 Two athletes have the same maximum oxygen uptake. One is a 400-metre runner and one is a marathon runner. Predict how their muscle differs and how they will differ in fatigue mechanism.
Model answer
The 400-metre runner will have a higher proportion of type IIa and IIx fibers, larger fiber diameters, higher glycogen stores, higher glycolytic enzyme activity (phosphofructokinase, LDH), greater SR and SERCA density, and greater buffering capacity. The marathon runner will have more type I fibers, greater mitochondrial volume and capillary density, more myoglobin, higher fat-oxidation enzyme activity, and a higher lactate threshold as a percentage of maximum oxygen uptake.
Fatigue in the 400-metre runner is dominated by inorganic phosphate accumulation, creatine phosphate depletion, potassium accumulation in the T-tubules, and impaired SR calcium release — all within about 50 seconds. Fatigue in the marathon runner is dominated by glycogen depletion (including the intramyofibrillar pool adjacent to the triads), central fatigue, hyperthermia, and dehydration, over two to four hours. Identical VO₂max, entirely different limiting physiology.
Level 3 · Clinical Application
8.13 A 62-year-old man on simvastatin 80 mg begins a new gym programme and three weeks later develops severe thigh and shoulder aching and passes dark urine. CK is 46,000 U/L; potassium is 6.1 mEq/L; creatinine has risen from 0.9 to 2.4 mg/dL. Explain the entire sequence mechanistically, and identify the most immediately life-threatening abnormality.
Model answer
Statin exposure plus unaccustomed eccentric exercise is an additive insult. The statin reduces mevalonate pathway output — coenzyme Q10 for the electron transport chain, and isoprenoids for prenylation of the small GTPases that govern membrane trafficking and calcium handling — and there is evidence for increased RyR1 calcium leak and reduced SERCA function. New eccentric work simultaneously produces mechanical disruption of sarcomeres and sarcolemma (§9.6). Together they exceed the fiber's capacity to maintain the 20,000-fold SR calcium gradient; cytosolic calcium rises, calpains activate, and fibers necrose.
Sarcolemmal failure releases the cell's contents: creatine kinase (46,000 U/L, over 200 times normal), myoglobin, potassium, and phosphate. Myoglobin at 17.8 kDa is freely filtered; in the tubule it causes vasoconstriction, forms obstructing casts, and is directly cytotoxic, producing acute kidney injury — hence the creatinine rise from 0.9 to 2.4 mg/dL. The dark urine is myoglobin, not blood; a dipstick will read positive for heme while microscopy shows no red cells.
The most immediately life-threatening abnormality is the potassium of 6.1 mEq/L. Potassium has the narrowest homeostatic window of any regulated variable in the body (§1.5), and hyperkalaemia depolarizes cardiac cells, inactivates their sodium channels, and can cause asystole or ventricular fibrillation within minutes — long before the kidney injury would kill anyone. Failing kidneys cannot excrete the potassium, so the two problems reinforce each other.
8.14 A 3-year-old boy has difficulty climbing stairs, walks his hands up his thighs to stand, and has calves that look unusually large. CK is 12,000 U/L. Explain why the calves are large and weak at once, why the CK was already abnormal in infancy, and why the weakness appeared only now.
Model answer
This is Duchenne muscular dystrophy, an X-linked absence of dystrophin, the protein linking the internal actin cytoskeleton through a membrane complex to laminin in the basal lamina. Without it, force generated by intact contractile machinery is transmitted through a membrane not properly tethered to the cytoskeleton, and the sarcolemma tears microscopically with every contraction. Calcium enters, calpains activate, fibers die.
CK was elevated from infancy because membrane leakage begins immediately — the disease is present and active long before it is visible. Escape of an intracellular enzyme is direct evidence of membrane failure, exactly as troponin is for cardiac muscle.
Weakness appeared only now because satellite cells were keeping pace with the destruction. Clinical weakness marks the point where the regenerative reserve is exhausted, not where the damage starts — which is why therapies aimed only at late weakness are aimed too late.
The calves are large and weak because the muscle has been replaced by fat and fibrous tissue — pseudohypertrophy. Volume is preserved while contractile tissue is not, which is a useful reminder that muscle size and muscle function are different measurements. Gower's manoeuvre — walking the hands up the thighs — is a specific sign of proximal hip extensor weakness, and proximal-predominant weakness is characteristic.
8.15 A patient develops sudden masseter rigidity during induction of anaesthesia with sevoflurane and succinylcholine. End-tidal CO₂ rises from 35 to 68 mm Hg despite increased ventilation; temperature is 38.9 °C and climbing. Explain each finding, name the drug required, and state its mechanism.
Model answer
This is malignant hyperthermia from a mutant RyR1 that opens on exposure to volatile anaesthetic or succinylcholine and fails to close.
- Masseter rigidity: cytosolic calcium remains high, so troponin C stays occupied, tropomyosin stays displaced, and cross-bridges cycle continuously. The muscle cannot relax because relaxation requires calcium removal, not merely the absence of stimulation.
- Rising end-tidal CO₂ despite increased ventilation: the earliest and most sensitive sign. Uncontrolled cross-bridge cycling plus maximal SERCA activity consumes ATP enormously; oxidative phosphorylation runs at three to five times normal, so CO₂ production outstrips even increased ventilation.
- Rising temperature: contraction is only 20–25% efficient, so most of that ATP turnover appears as heat. Core temperature can rise 1 °C every five minutes.
The drug is dantrolene, and its mechanism is to inhibit RyR1 directly, closing the leak. Supportive care addresses the downstream consequences — cooling, hyperkalaemia from dying muscle, and myoglobinuric renal injury. Mortality fell from roughly 80% to under 5% with dantrolene, which is what happens when a treatment is aimed at the exact molecular step of the lesion.
Level 4 · Integration and Synthesis
8.16 Amara's echocardiogram shows akinesis of the mid-lateral segments. Construct the full causal chain from her coronary artery narrowing to her ejection fraction of 48%, naming the specific step of excitation–contraction coupling that fails first when a myocyte becomes ischaemic, and explain why contraction fails before the ECG changes and long before troponin rises.
Model answer
The chain. Coronary narrowing (plaque plus thrombus) → reduced blood flow to the lateral wall myocardium → reduced oxygen delivery → oxidative phosphorylation fails → ATP falls within seconds, because a cardiac myocyte holds only a few seconds' worth and has almost no creatine phosphate buffer relative to its demand.
What fails first. Two ATP-dependent steps fail almost immediately, and both are in §9.5. SERCA stops clearing calcium efficiently, so relaxation is impaired first — diastolic dysfunction precedes systolic dysfunction, and this is measurable. Then cross-bridge cycling itself fails, because both the re-cocking step and the detachment step require ATP. Contraction stops. The cell is not yet dead: this is stunned or hibernating myocardium, and it can recover if flow is restored quickly.
Why contraction fails before the ECG changes. Contraction is the most ATP-expensive thing a myocyte does, so it is the first function sacrificed. The ECG reports membrane ion currents, which are cheaper and continue for longer; ST-segment changes require enough cells to be sufficiently ischaemic to alter the resting membrane potential across a region. This is exactly why bedside echocardiography can identify a regional wall motion abnormality in a patient whose ECG is still normal.
Why troponin rises last. Troponin is bolted to the thin filament and cannot leave a cell with an intact membrane. Its appearance requires irreversible sarcolemmal failure — cell death, not cell distress — and then requires the protein to diffuse into the interstitium and be cleared into the blood. The first detectable rise is 3–4 hours later.
Ejection fraction. Roughly 14% of the ventricular mass no longer shortens, and because cardiac muscle cannot regenerate (§9.9), it never will. Stroke volume falls; ejection fraction falls from a normal 55–70% to 48%. The remaining muscle hypertrophies and works harder, which is both the compensation and the beginning of the next problem.
8.17 Design a drug that would selectively weaken smooth muscle in the airways without affecting skeletal or cardiac muscle. Name at least two molecular targets that would achieve this, and explain why each is specific.
Model answer
The specificity comes from the fact that smooth muscle regulation is thick-filament linked and uses a protein set the striated muscles do not have.
Target 1 — myosin light chain kinase, or the pathway that activates it. Skeletal and cardiac muscle regulate contraction through troponin C on the thin filament; smooth muscle regulates it through Ca²⁺–calmodulin activation of MLCK, which phosphorylates the regulatory myosin light chain. Inhibiting MLCK, or reducing cytosolic calcium in a smooth-muscle-selective way, weakens smooth muscle only. In practice this is what β₂-adrenergic agonists do: they raise cAMP, activate protein kinase A, reduce MLCK activity and lower cytosolic calcium, and the β₂ receptor subtype is expressed on airway smooth muscle far more than on cardiac muscle (which is predominantly β₁) — a second layer of specificity built on receptor distribution.
Target 2 — myosin light chain phosphatase, via the RhoA/Rho-kinase pathway. Enhancing MLCP activity, or blocking Rho-kinase's inhibition of it, dephosphorylates the light chain and relaxes smooth muscle. Striated muscle does not regulate contraction this way at all, so the target simply is not present in the relevant role.
Target 3 — the calcium source. Smooth muscle depends heavily on extracellular calcium entering through voltage-gated and receptor-operated channels, and has only a small SR. Skeletal muscle needs no extracellular calcium whatsoever. A channel blocker selective for the smooth-muscle isoform would therefore spare skeletal muscle entirely — though not cardiac muscle, which also depends on extracellular calcium, which is exactly why systemic calcium channel blockers have cardiac effects and why the vascular-selective dihydropyridines were developed.
A good answer notes the general principle: pharmacological selectivity is usually achieved by targeting a step that exists in only one tissue, or a receptor subtype distributed unevenly — and the more different two tissues' regulatory machinery is, the easier selectivity becomes, even when their contractile machinery is nearly identical.
8.18 Argue for or against: "Fatigue is a failure of the muscle." Use at least three lines of evidence from this chapter, and state what a better formulation would be.
Model answer
A strong answer argues against, with a qualification.
Evidence that fatigue is regulated rather than exhausted:
- ATP is never depleted. Muscle ATP rarely falls below about 60% of resting even in exhausting exercise. If fatigue were simple exhaustion, ATP would approach zero. It does not, because the cell reduces its own output first. Malignant hyperthermia shows what genuine ATP depletion looks like, and it is catastrophic and rare.
- The inhibitors are the products. Inorganic phosphate, the direct product of ATP and creatine phosphate hydrolysis, reduces force per cross-bridge, reduces myofibrillar calcium sensitivity, and precipitates calcium inside the SR to reduce release. That is a feedback inhibition loop, structurally identical to any other negative feedback in this book.
- Central fatigue is substantial. Voluntary drive falls before peripheral capacity is exhausted; superimposed electrical stimulation during a maximal voluntary contraction can still elicit extra force. Group III and IV muscle afferents report the metabolic state to the central nervous system, which reduces drive.
- The lactate story is backwards. The molecule blamed for fatigue is a fuel; its production consumes a proton; and patients who cannot produce it at all fatigue faster.
The qualification: peripheral limits are real. Glycogen depletion in a marathon is a genuine substrate limitation, and potassium accumulation genuinely impairs action potential propagation. Fatigue is not purely a decision.
A better formulation: fatigue is a set of protective negative feedback mechanisms, peripheral and central, that reduce force output before structural damage occurs. The regulated variable is not force; it is the integrity of the cell. Framed that way, fatigue belongs with fever and with the baroreflex rather than with mechanical breakdown — and it explains why training improves it through both metabolic adaptation and altered tolerance of the same signals.
Concept Map to Complete
Copy onto blank paper and fill in every bracket from memory, then check.
ACTION POTENTIAL at the [ ___________ ] junction
│
EPP of [ __ ] mV vs threshold [ __ ] mV
│ safety factor [ __ ]
▼
AP along SARCOLEMMA → down the [ _________ ]
│
[ ______ ] RECEPTOR (voltage sensor)
│ mechanically opens
[ ______ ] RECEPTOR (SR release channel)
│
Ca2+ : [ ___ ] nM ──────► [ ___ ] µM
│
binds [ __________ ]
│
which pulls [ _______ ] off actin
│
so [ ___________ ] rotates and exposes
[ ______ ] binding sites per molecule
│
┌───────────────────────────┴────────────────────────────┐
│ CROSS-BRIDGE CYCLE (4 steps) │
│ 1 [ __________ ] 2 [ ___________ ] │
│ 3 [ __________ ] ← ATP role: [ ________ ] │
│ 4 [ __________ ] ← ATP role: [ ________ ] │
└───────────────────────────┬────────────────────────────┘
│
RELAXATION: [ _______ ] pumps Ca2+ back, [ _ ] Ca2+ per ATP
│
NO ATP → cross-bridges [ _________ ] → [ ______ ____ ]
Lab / Self-Exploration
- Demonstrate the length–tension curve on yourself. Squeeze a rolled towel as hard as you can with your wrist fully flexed, then again with the wrist extended about 30 degrees. The difference is large and immediate. Explain it in terms of sarcomere overlap in the long finger flexors.
- Find your own tetanus threshold. Hold your arm out straight, palm up, with a light book on it. Watch for the fine tremor after 30–60 seconds. That visible tremor is motor units firing out of phase as recruitment is forced upward by fatigue — asynchronous firing becoming synchronous.
- Feel the difference between concentric, eccentric, and isometric. Do a slow push-up: lower over 5 seconds (eccentric), hold halfway for 5 seconds (isometric), push up over 2 seconds (concentric). Rank them by difficulty during the movement. Then predict which one will make you sore in 48 hours, and check.
- Time your own phosphagen system. Sprint as hard as you can and note the second at which you feel a clear, unmistakable drop in power output. For most untrained people this falls between 6 and 12 seconds — the creatine phosphate store running out. Do not do this without warming up, and not at all if you have been told to avoid maximal effort.
- Test the size principle. Curl a very light object slowly and note which muscles you feel. Then curl the heaviest object you can safely manage. The second recruits fibers the first never touches, no matter how many repetitions of the light one you perform.
- Observe treppe. Measure your maximum grip (a bathroom-scale squeeze works) cold, then after ten submaximal squeezes, then after a further ten. The rise over the first several efforts is treppe, and it is the physiological reason warming up is not optional.
Key Terms
A band · The region of a sarcomere occupied by thick filaments; 1.6 µm wide and invariant during contraction.
acetylcholinesterase · Enzyme anchored in the synaptic cleft's basal lamina that hydrolyses acetylcholine within about a millisecond, terminating end plate depolarization.
actin · Protein of the thin filament; a double helix of G-actin monomers each carrying a myosin-binding site.
calmodulin · The calcium-binding protein of smooth muscle that, when loaded with Ca²⁺, activates myosin light chain kinase.
calsequestrin · Low-affinity, high-capacity calcium-binding protein inside the sarcoplasmic reticulum; allows a large store with a low free concentration.
contractility · The ability to shorten forcefully; the property unique to muscle tissue.
costamere · Protein assembly riveting peripheral Z discs to the sarcolemma, transmitting force laterally into the endomysium.
creatine phosphate · The immediate phosphate reserve of muscle (~20–25 mmol/kg), transferred to ADP by creatine kinase to regenerate ATP for 8–10 seconds of maximal effort.
cross-bridge cycle · The four-step sequence — attachment, power stroke, ATP-binding detachment, ATP-hydrolysis re-cocking — that slides thin filaments past thick.
DHP receptor (dihydropyridine receptor) · The T-tubule voltage sensor; a mechanical trigger for RyR1 in skeletal muscle, a calcium conduit for RyR2 in cardiac muscle.
dystrophin · Protein linking the actin cytoskeleton to the extracellular matrix through the sarcolemma; absent in Duchenne muscular dystrophy.
eccentric contraction · Contraction during which the muscle lengthens because the load exceeds the tension; produces the most force, the least metabolic cost, and the most damage.
endomysium · Fine reticular connective tissue surrounding each muscle fiber; holds capillaries and satellite cells.
end plate potential (EPP) · The graded local depolarization of the motor end plate produced by acetylcholine; normally 30–40 mV against a 15 mV threshold.
epimysium · Dense irregular connective tissue surrounding an entire muscle.
excitation–contraction coupling · The sequence converting a sarcolemmal action potential into calcium release and cross-bridge cycling.
fascicle · A bundle of muscle fibers wrapped in perimysium; the visible grain of a muscle.
H zone · The central region of the A band containing thick filaments only; narrows or disappears during contraction.
I band · The region containing thin filaments only, bisected by the Z disc and shared between adjacent sarcomeres; narrows during contraction.
intercalated disc · The junction between cardiac cells, containing desmosomes, fascia adherens, and gap junctions; makes the myocardium a functional syncytium.
isometric contraction · Contraction in which tension develops but muscle length does not change.
latch state · The smooth muscle condition in which dephosphorylated cross-bridges remain attached and cycle very slowly, holding tension at roughly 1/300 the ATP cost.
length–tension relationship · The dependence of active force on sarcomere length, explained entirely by filament overlap; optimal at 2.0–2.2 µm.
motor unit · One motor neuron and every muscle fiber it innervates; the smallest unit the nervous system can command.
myofibril · A rod-like bundle of myofilaments, 1–2 µm across, running the length of a fiber; a chain of sarcomeres.
myoglobin · Oxygen-binding protein of muscle; high in type I fibers, and nephrotoxic when released in rhabdomyolysis.
myosin light chain kinase (MLCK) · Smooth muscle enzyme that, activated by Ca²⁺–calmodulin, phosphorylates the regulatory myosin light chain to permit contraction.
neuromuscular junction · The synapse between a somatic motor neuron and a muscle fiber; one per fiber, with a safety factor of 3–5.
perimysium · Connective tissue surrounding each fascicle; carries the muscle's vessels and nerves.
power stroke · The pivot of the myosin head from about 90° to 45° that drags the thin filament roughly 10 nm toward the M line.
rhabdomyolysis · Breakdown of skeletal muscle releasing myoglobin, potassium, phosphate, and creatine kinase into the circulation; threatens the heart through hyperkalaemia and the kidney through myoglobinuria.
rigor mortis · Post-mortem muscle rigidity caused by cross-bridges that formed as calcium leaked but cannot detach without ATP; proof that ATP is required for release.
ryanodine receptor (RyR) · The calcium release channel of the sarcoplasmic reticulum; RyR1 in skeletal muscle, RyR2 in cardiac; defective in malignant hyperthermia.
sarcolemma · The plasma membrane of a muscle fiber.
sarcomere · The contractile unit, running Z disc to Z disc, 2.0–2.2 µm at rest.
sarcoplasmic reticulum (SR) · Smooth endoplasmic reticulum specialized for calcium storage and release; swells into terminal cisternae at each T-tubule.
satellite cell · Quiescent Pax7-positive myogenic stem cell beneath the basal lamina; responsible for skeletal muscle regeneration and for donating nuclei during hypertrophy. Cardiac muscle has no equivalent.
SERCA · The SR calcium ATPase; pumps two Ca²⁺ into the SR per ATP hydrolysed, producing relaxation.
size principle · Motor units are recruited in order of increasing motor neuron size, so type I fibers are always recruited before type II.
sliding filament model · Contraction occurs because thin filaments slide over thick filaments; neither filament changes length.
tetanus (physiological) · Smooth, maximal contraction produced by stimulus frequencies high enough that twitches fuse; 3–5 times peak twitch tension. Impossible in cardiac muscle.
titin · The largest known protein, spanning Z disc to M line; supplies passive tension and centres the thick filament.
treppe · The progressive increase in twitch force over the first several maximal stimuli in a rested muscle; the physiological basis of warming up.
triad · One T-tubule flanked by two terminal cisternae; the site of excitation–contraction coupling.
tropomyosin · Rod-shaped protein lying in the actin groove, covering seven myosin-binding sites at rest.
troponin · Three-subunit regulatory complex of the thin filament: TnT binds tropomyosin, TnI inhibits by binding actin, TnC binds calcium. Cardiac isoforms of TnI and TnT are the basis of the cardiac-specific blood test.
T-tubule · Invagination of the sarcolemma carrying the action potential into the fiber's interior, solving the calcium diffusion-distance problem.
Z disc · The α-actinin lattice anchoring thin filaments and defining the boundaries of a sarcomere.
Next: Chapter 10 · The Muscular System II — where these mechanisms become six hundred named muscles, lever systems, and the exercise prescription that will decide how much of Amara's function comes back.