Muscle Physiology
Usually weeks 5-6 of a medical physiology course, once the neuromuscular junction is builtMuscle is where the membrane and synapse units finally move something. The course builds skeletal muscle first — sarcomere structure, the calcium handoff from T-tubule to sarcoplasmic reticulum, and the ATP-driven cross-bridge cycle — then asks how force is graded and why smooth and cardiac muscle do it differently. The exam lives in three places: the sequence of excitation-contraction coupling (and the drug, toxin, or mutation at each step), the two classic graphs (length-tension and force-velocity), and the regulatory contrast between troponin-controlled striated muscle and myosin-controlled smooth muscle. The cardiovascular unit will reuse every one of them.
This guide is the frame. The exams are written from your lectures— drop this unit's slides on the dashboard to get flashcards and board-style questions from your own course, and quiz them all semester (a course whose exams re-test earlier units rewards nothing more than early, repeated self-testing). The physiology playbook has the weekly loop.
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AI-authored study notes, not faculty-reviewed and not medical advice. Verify every mechanism against your course materials and faculty; programs differ in emphasis.
High-Yield Pearl
Name who the calcium sensor is — troponin C in striated muscle, calmodulin in smooth muscle — and where the calcium comes from — the SR alone in skeletal, entry-triggered release in the heart — and nearly every muscle drug, toxin, and disease question sorts itself.
What the exam asks
Mechanisms, curves, and what happens when each one fails.
- The sarcomere and sliding filaments. A skeletal muscle fiber is packed with myofibrils, each a chain of sarcomeres running from one Z disc to the next. Thin filaments (actin, with tropomyosin lying in its groove and the troponin complex — C, I, T — bound along it) anchor at the Z discs; thick filaments (myosin) sit in the center, tethered at the M line and held in register by the giant elastic protein titin. The A band is the length of the thick filaments; the I band is thin filament alone; the H zone is thick filament alone. In contraction the filaments SLIDE past each other: the A band keeps its length while the I band and H zone shorten and the Z discs draw together.
- Excitation-contraction coupling in skeletal muscle. The end-plate potential fires a muscle action potential that spreads along the sarcolemma and down the T-tubules. There, dihydropyridine receptors (L-type Ca2+ channels acting as voltage SENSORS) are mechanically linked to ryanodine receptors (RyR1) in the adjacent sarcoplasmic reticulum — the triad. Depolarization pulls RyR1 open and Ca2+ floods the cytosol from the SR. Ca2+ binds troponin C, the troponin complex shifts tropomyosin off actin's myosin-binding sites, and cross-bridges form. Relaxation requires the SERCA pump to spend ATP pulling Ca2+ back into the SR, where calsequestrin stores it. Skeletal muscle does not need extracellular Ca2+ to contract.
- The cross-bridge cycle and what ATP actually does. (1) ATP binds the myosin head and releases it from actin; (2) ATP hydrolysis cocks the head into its high-energy position; (3) the head binds an exposed site on actin; (4) release of Pi triggers the power stroke, pulling the thin filament toward the M line; (5) ADP leaves and the head stays bound in the rigor state until a new ATP arrives. ATP therefore powers the stroke AND is required to detach — which is why an ATP-depleted muscle locks rigid (rigor mortis) rather than going limp.
- Length-tension — the preload graph. Passive tension (from stretching titin and connective tissue) is near zero at short lengths and rises steeply as the muscle is stretched. Active tension — what cross-bridges add when the muscle is stimulated — peaks at the optimal sarcomere length (roughly 2.0–2.2 µm), where thick and thin filaments overlap so that every myosin head can reach actin. Shorter than that, thin filaments collide and overlap each other and thick filaments butt into the Z discs; longer, thin filaments are pulled out of reach. Total tension is the sum. Preload is the length a muscle is set at before it contracts — in the heart, that is the Frank-Starling mechanism.
- Force-velocity — the afterload graph. The heavier the load (afterload), the slower a muscle shortens. Velocity is maximal with no load at all and falls to zero at the load the muscle can just hold — an ISOMETRIC contraction (tension without length change). A contraction that shortens is concentric; one that lengthens while generating force, as when lowering a weight, is ECCENTRIC and produces the most force and the most muscle damage (delayed-onset soreness).
- Grading force — recruitment and summation. A motor unit is one motor neuron and all the fibers it innervates: small units (a few fibers, eye muscles) for precision, large units (hundreds to thousands, the quadriceps) for power. Force rises two ways. RECRUITMENT follows the size principle — small, low-threshold, fatigue-resistant units fire first, large ones last. RATE CODING exploits the fact that the muscle action potential (a few milliseconds) is far shorter than the twitch it triggers (tens of milliseconds): a second stimulus arriving before relaxation adds force (summation), and a fast train fuses into a smooth maximal contraction (tetanus) as Ca2+ release outpaces reuptake.
- Fiber types and energy supply. Slow-twitch oxidative (type I) fibers are red — rich in myoglobin, mitochondria, and capillaries — contract slowly, resist fatigue, and dominate postural muscles like the soleus. Fast-twitch glycolytic (type II) fibers are pale, contract fast and forcefully, and tire quickly — sprinting and lifting. Energy comes first from stored ATP and phosphocreatine (seconds), then anaerobic glycolysis (lactate, tens of seconds to a couple of minutes), then oxidative phosphorylation for anything sustained. Endurance training shifts fibers toward oxidative capacity; resistance training produces hypertrophy.
- Muscle sensors and the spinal reflexes. Muscle spindles are stretch receptors (intrafusal fibers) lying IN PARALLEL with the working fibers; their Ia afferents fire when the muscle lengthens and synapse directly on alpha motor neurons to the same muscle — the monosynaptic stretch reflex (the knee jerk) — while an interneuron inhibits the antagonist (reciprocal inhibition). Gamma motor neurons contract the spindle's ends to keep it taut and sensitive during shortening. Golgi tendon organs sit IN SERIES in the tendon, sense TENSION through Ib afferents, and inhibit the same muscle (autogenic inhibition) to protect it from overload.
- Smooth muscle — regulated at the myosin, not the troponin. Smooth muscle has actin and myosin but no troponin and no sarcomeres (dense bodies anchor the thin filaments, so cells shorten in every direction). Ca2+ enters through L-type channels and is released from the SR by IP3; it binds CALMODULIN, and Ca2+-calmodulin activates myosin light-chain kinase (MLCK), which phosphorylates the myosin light chain so cross-bridges can cycle. Myosin light-chain phosphatase (MLCP) removes the phosphate and relaxes the cell. cAMP inhibits MLCK (β2 agonists relax bronchi); cGMP from nitric oxide activates MLCP (nitrates and NO dilate vessels). The latch state lets smooth muscle hold tone for hours on little ATP. Single-unit (visceral) smooth muscle is gap-junction coupled and often has pacemaker activity — gut, uterus, small vessels; multiunit smooth muscle is fired fiber by fiber — iris, piloerector, large airways.
- Cardiac muscle — a hybrid with its own rules. Cardiac muscle is striated and troponin-regulated like skeletal muscle, but its cells are coupled by gap junctions in intercalated discs, so the heart contracts as a unit and cannot recruit motor units. Its EC coupling is calcium-induced calcium release: Ca2+ entering through L-type channels during the plateau opens RyR2 on the SR — so cardiac force DOES depend on extracellular Ca2+ and is blunted by calcium channel blockers. Its long refractory period outlasts most of the contraction, so the heart cannot be tetanized. Force is graded by the amount of Ca2+ delivered (sympathetic stimulation, digoxin) and by preload (Frank-Starling).
Worth drilling · 38 concepts
Each with the one fact that identifies it. Your course's slides are the authority; use this to check your list against.
- 1Sarcomere — contractile unit from one Z disc to the next
- 2Z disc — anchors the thin filaments and marks each sarcomere's borders
- 3M line — center of the sarcomere where the thick filaments are tethered
- 4A band — the length of the thick filaments; does NOT shorten during contraction
- 5H zone — the thick-filament-only center; shortens (with the I band) during contraction
- 6Titin — giant elastic protein; the source of passive tension and the thick filaments' spring
- 7Troponin C — binds Ca2+ and shifts tropomyosin off actin's myosin sites
- 8Tropomyosin — covers the myosin-binding sites on actin at rest
- 9T-tubule — sarcolemma invagination that carries the action potential deep into the fiber
- 10Dihydropyridine receptor — T-tubule voltage sensor mechanically linked to RyR1 in skeletal muscle
- 11Ryanodine receptor — SR Ca2+ release channel; mutated in malignant hyperthermia
- 12SERCA pump — ATP-driven Ca2+ reuptake into the SR; required for relaxation
- 13Calsequestrin — Ca2+-binding storage protein inside the SR
- 14Power stroke — Pi release lets the myosin head pull the thin filament toward the M line
- 15Rigor state — myosin locked to actin because no ATP is available to detach it
- 16Optimal length — maximal thick-thin overlap and maximal active tension
- 17Passive tension — elastic recoil of titin and connective tissue in a stretched muscle
- 18Afterload — the load a contracting muscle must move; more afterload, slower shortening
- 19Isometric contraction — tension develops with no change in muscle length
- 20Eccentric contraction — muscle lengthens while producing force; the most damaging kind
- 21Motor unit — one motor neuron plus every fiber it innervates
- 22Size principle — small, low-threshold motor units are recruited first
- 23Summation — a second stimulus before relaxation adds force to the first twitch
- 24Tetanus — fused, sustained maximal contraction from high-frequency stimulation
- 25Slow-twitch oxidative fibers — type I; red, myoglobin-rich, fatigue-resistant; posture
- 26Fast-twitch glycolytic fibers — type II; pale, powerful, fatigue quickly; sprinting
- 27Phosphocreatine — regenerates ATP for the first seconds of intense work
- 28Muscle spindle — intrafusal stretch receptor in parallel with the fibers; Ia afferents
- 29Golgi tendon organ — tension receptor in series with the tendon; Ib afferents
- 30Stretch reflex — monosynaptic Ia loop onto alpha motor neurons; the knee jerk
- 31Gamma motor neurons — keep the spindle taut and sensitive during contraction
- 32Calmodulin — smooth muscle's Ca2+ sensor in place of troponin
- 33Myosin light-chain kinase — Ca2+-calmodulin enzyme that switches smooth muscle myosin on
- 34Myosin light-chain phosphatase — relaxes smooth muscle; activated through cGMP
- 35Latch state — smooth muscle holds tension for long periods on little ATP
- 36Single-unit smooth muscle — gap-junction coupled sheets that contract together (gut, uterus)
- 37Calcium-induced calcium release — cardiac trigger: L-type Ca2+ entry opens RyR2
- 38Intercalated disc — gap junctions and desmosomes joining cardiac cells into one working unit
Practice it
Active recall over the drill list — flip and claim, match the pairs, produce the answers cold, fill in the diagram, or read the curves like the exam does.
Every concept on the drill list, one card at a time — the name up front, the fact that identifies it on the flip. Claim each card honestly and the deck learns what to lead with next time. Progress lives in this browser only.
What everyone misses
The distinctions that lose points on this material, year after year.
- The A band never shortens. Contraction is sliding, not shrinking, so the thick filaments — and therefore the A band — keep their length. The I band and H zone narrow and the Z discs move closer. Questions that ask which band stays constant are testing whether you picture filaments sliding or filaments compressing.
- ATP is needed to LET GO. Students credit ATP only with the power stroke and conclude that no ATP means no contraction and a limp muscle. In the cycle, binding a fresh ATP is what releases myosin from actin; without it the heads stay attached — rigor. SERCA's ATP use is also why relaxation, not only contraction, fails in an energy-starved muscle.
- Skeletal muscle doesn't need extracellular calcium; the heart does. In skeletal muscle the DHP receptor pulls RyR1 open mechanically, so contraction proceeds even in Ca2+-free fluid and dihydropyridine calcium channel blockers don't cause skeletal weakness. Cardiac muscle depends on the Ca2+ that enters during the plateau to trigger release (CICR), so the same drugs reduce contractility. The contrast is a favorite two-part question.
- Smooth muscle has no troponin. A reflex answer is 'Ca2+ binds troponin C' for every muscle type. In smooth muscle Ca2+ binds calmodulin, and control lives on the THICK filament through myosin light-chain phosphorylation. That is why cAMP and cGMP pathways — which act on MLCK and MLCP — relax smooth muscle, and why the vasodilator and bronchodilator drugs work.
- Past optimal length, ACTIVE tension falls even as total tension rises. On the length-tension graph, the total curve keeps climbing at long lengths because passive tension is taking over. Students read the rising total and conclude the muscle contracts harder when overstretched. The active component — the cross-bridge part — is falling because filament overlap is shrinking.
- Spindles report length, tendon organs report tension. The spindle sits in parallel and fires with stretch; the Golgi tendon organ sits in series and fires with tension, including tension from the muscle's own contraction. A vignette of sudden relaxation under a heavy load is the tendon organ's inhibition, not a spindle reflex.
Clinical correlations
Where this unit shows up again — in clinic, on rotations, and on the boards.
- Malignant hyperthermia. Minutes into anesthesia with a volatile agent or succinylcholine: masseter rigidity, rising end-tidal CO2, tachycardia, then hyperthermia and hyperkalemia. A mutant RyR1 leaks Ca2+ uncontrollably, so muscle contracts and burns ATP nonstop. Dantrolene blocks RyR1 and is the antidote; stopping the trigger agent and cooling are immediate steps.
- Duchenne muscular dystrophy. An X-linked absence of dystrophin — the protein linking the cytoskeleton to the membrane — lets fibers tear with each contraction. A young boy climbs up his own legs to stand (Gowers sign), has enlarged calves (fat and fibrous replacement), and a markedly elevated CK. Becker dystrophy is the milder, partially functional form.
- Rhabdomyolysis. Crush injury, extreme exertion, statins, or prolonged immobilization rupture muscle cells, spilling CK, myoglobin, potassium, and phosphate. Dark urine that is dipstick-positive for blood with no red cells on microscopy is myoglobin; the dangers are acute kidney injury and hyperkalemic arrhythmia. Aggressive IV fluids are the core treatment.
- Nitrates, PDE5 inhibitors, and β2 agonists. Nitroglycerin releases NO → cGMP → myosin light-chain phosphatase activity → vascular smooth muscle relaxes. Sildenafil slows cGMP breakdown, so combining it with nitrates causes profound hypotension — a contraindication that is pure smooth-muscle physiology. Albuterol raises cAMP in bronchial smooth muscle, inhibiting MLCK and opening the airway.
- Reading the reflex exam. Hyperreflexia with spasticity and an upgoing toe points to an upper motor neuron lesion (lost descending inhibition of the stretch reflex); hyporeflexia with atrophy and fasciculations points to a lower motor neuron lesion. A reflex with a slow relaxation phase suggests hypothyroidism. Each finding is the spindle loop reporting on its supervisors.
- Calcium channel blockers and the heart vs skeletal muscle. Verapamil and diltiazem reduce cardiac contractility and slow nodal conduction because cardiac EC coupling depends on L-type Ca2+ entry; they cause no skeletal weakness because skeletal coupling is mechanical. Worsening heart failure with reduced ejection fraction is the classic warning.
- Disuse atrophy and sarcopenia. Immobilization and aging shrink fibers and lose motor units, preferentially fast-twitch ones, raising fall risk. Resistance exercise is the only intervention that reliably reverses it — which is why early mobilization is standard in hospital care.