Skeletal Muscle Contraction

Skeletal muscle converts chemical energy (ATP) into mechanical work through a precisely choreographed sequence linking an action potential at the neuromuscular junction to shortening of the sarcomere. This is among the most heavily tested physiology areas in NEET PG, with stems on the cross-bridge cycle, excitation–contraction (E-C) coupling, the role of calcium and troponin, and the length–tension relationship recurring almost every year.

Structural organisation: from muscle to filament

Understanding contraction demands a clear hierarchy of structure, because nearly every "identify the band/zone" question hinges on it.

• Muscle → fascicle → muscle fibre (cell) → myofibril → sarcomere → myofilaments.
• The sarcomere is the functional contractile unit, defined as the region between two Z-lines (Z-discs).
• Thick filaments = myosin II; thin filaments = actin + troponin + tropomyosin.

Sarcomere zones and what happens during contraction

Band / Zone	Contains	Behaviour on contraction
A band	Whole thick filament (+ overlap with thin)	Constant width (length of myosin doesn't change)
I band	Thin filament only (no thick)	Shortens
H zone	Thick filament only (no overlap)	Shortens / disappears
Z line	Anchors thin filaments (α-actinin)	Z-lines move closer
M line	Anchors thick filaments (myomesin, creatine kinase)	Stays central

High-yield: During contraction the A band stays constant, while the I band and H zone shorten. This is the single most common sarcomere question — the actual filaments do not shorten; they slide past each other.

Key structural proteins to remember:

• Titin – largest protein in the body; runs from Z-line to M-line; provides passive elasticity and is responsible for passive tension when muscle is overstretched.
• Nebulin – aligns/measures actin thin filament length ("nebulin = ruler").
• Dystrophin – links the actin cytoskeleton to the sarcolemmal dystroglycan complex; deficient in Duchenne (absent) and Becker (reduced) muscular dystrophy.
• α-actinin – anchors actin at the Z-line.

Sliding filament theory

Proposed by Huxley and Hanson / A.F. Huxley and Niedergerke (1954). Core idea: muscle shortens because thin (actin) filaments slide over stationary thick (myosin) filaments toward the centre of the sarcomere, without any change in the length of either filament.

The molecular engine driving this sliding is the cross-bridge cycle, powered by myosin ATPase.

The cross-bridge cycle (myosin ATPase cycle)

This is a stepwise loop. Memorise the order and especially which step uses ATP vs which uses Pi/ADP release.

Resting (energised) state → ATP hydrolysed, myosin head cocked, carrying ADP + Pi.

1. Cross-bridge formation (attachment): Ca²⁺ has exposed actin sites; the cocked myosin head binds actin → forms the cross-bridge.
2. Power stroke: Pi is released → myosin head pivots, pulling actin toward the M-line; then ADP is released. This is the force-generating step.
3. Rigor state: Myosin remains tightly bound to actin (low-energy, ADP-free).
4. Detachment: A new ATP molecule binds the myosin head → reduces myosin's affinity for actin → cross-bridge detaches.
5. Re-cocking (re-energising): ATP is hydrolysed to ADP + Pi → myosin head returns to its cocked, high-energy position, ready for another cycle.

Flow: Cock (ATP hydrolysis) → bind actin → Pi release + power stroke → ADP release (rigor) → new ATP binds → detach → re-cock.

High-yield: ATP is required for DETACHMENT of myosin from actin, not for the power stroke itself. When ATP is absent (death), myosin stays bound → rigor mortis. The power stroke is driven by release of Pi, not by ATP binding.

High-yield: Rigor mortis = persistent actin–myosin binding due to ATP depletion after death; begins ~2–4 h, maximal ~12 h, resolves ~24–48 h as autolysis degrades proteins.

Mnemonic for cycle (force then release): "Cock, Cross, Crunch, Cleave" — Cock the head, Cross-bridge to actin, Crunch (power stroke), Cleave off with new ATP.

Role of calcium, troponin and tropomyosin

At rest, tropomyosin physically covers the myosin-binding sites on actin, preventing contraction. The troponin complex has three subunits — a classic table question:

Troponin subunit	Binds	Function
Troponin C (TnC)	Calcium	Ca²⁺ binding triggers conformational shift
Troponin T (TnT)	Tropomyosin	Attaches the complex to tropomyosin
Troponin I (TnI)	Actin (inhibitory)	Inhibits actin–myosin interaction at rest

Steps: Ca²⁺ released from sarcoplasmic reticulum (SR) → binds Troponin C → conformational change pulls tropomyosin off the actin active sites → myosin heads bind exposed actin → cross-bridge cycling and contraction.

High-yield: Calcium binds Troponin C in skeletal/cardiac muscle. (In smooth muscle there is no troponin — Ca²⁺ binds calmodulin → activates myosin light-chain kinase (MLCK) → phosphorylates myosin. This contrast is frequently tested.)

High-yield: Cardiac Troponin I and Troponin T are the most specific serum markers of myocardial infarction.

Excitation–contraction (E-C) coupling

This links the surface action potential to internal Ca²⁺ release — a very high-yield mechanistic chain.

Sequence:

1. Action potential at the neuromuscular junction (ACh → nicotinic receptors → end-plate potential → muscle AP).
2. AP propagates along the sarcolemma and down the T-tubules (transverse tubules, invaginations of the sarcolemma).
3. T-tubule depolarisation is sensed by the dihydropyridine receptor (DHPR) — a voltage sensor (an L-type Ca²⁺ channel).
4. In skeletal muscle, the DHPR is mechanically coupled to the ryanodine receptor (RyR1) on the SR → opens RyR → Ca²⁺ released from SR into cytosol. (No extracellular Ca²⁺ needed for skeletal contraction.)
5. Ca²⁺ binds Troponin C → contraction.
6. Relaxation: Ca²⁺ is pumped back into SR by SERCA (sarco-endoplasmic reticulum Ca²⁺-ATPase); stored bound to calsequestrin in SR lumen.

Feature	Skeletal	Cardiac
DHPR–RyR coupling	Mechanical (direct)	Calcium-induced calcium release (CICR)
Extracellular Ca²⁺ needed?	No	Yes (essential trigger)
RyR isoform	RyR1	RyR2
Functional syncytium	No	Yes (gap junctions)

High-yield: In skeletal muscle, E-C coupling is mechanical (DHPR directly opens RyR1) and does NOT require extracellular calcium. In cardiac muscle it depends on CICR and extracellular Ca²⁺ — this is why cardiac muscle is sensitive to plasma Ca²⁺ and calcium-channel blockers.

High-yield: A triad in skeletal muscle = 1 T-tubule + 2 terminal cisternae of SR, located at the A-I junction. Cardiac muscle has dyads (1 T-tubule + 1 SR cistern, at the Z-line).

Length–tension relationship

Describes how the passive, active, and total tension of a muscle vary with sarcomere length — frequently tested as a graph-interpretation question.

• Active tension is maximal at the optimal sarcomere length (~2.0–2.2 µm), where there is maximal overlap between actin and the cross-bridge-bearing region of myosin (maximum number of cross-bridges).
• Overstretched muscle → fewer overlapping cross-bridges → active tension falls.
• Over-shortened (below ~1.65 µm) → thin filaments collide / overlap, thick filaments crumple against Z-line → tension falls.
• Passive tension rises steeply with stretch (mainly due to titin).
• Total tension = active + passive.

High-yield: Maximum active tension occurs at optimal length (Lo) because of maximal cross-bridge overlap. This is the structural basis of the Frank–Starling law in the heart (preload → optimal sarcomere length → stroke volume).

Force–velocity relationship

• The velocity of shortening is inversely related to the afterload (force).
• Maximum velocity (Vmax) occurs at zero load.
• At maximum load, velocity = 0 (an isometric contraction — tension develops, no shortening).
• Isotonic contraction = constant tension, muscle shortens (lifting a constant weight).

Contraction type	Length	Tension	Example
Isometric	Constant	Increases	Pushing a wall; holding a posture
Isotonic	Decreases (concentric)	Constant	Lifting a weight
Eccentric	Increases (lengthening)	Active	Lowering a weight, walking downstairs

Motor units and recruitment

A motor unit = one alpha motor neuron + all the muscle fibres it innervates.

• Innervation ratio (fibres per neuron) is small in muscles needing fine control (extraocular ~1:3–10, hand muscles) and large in big postural muscles (gastrocnemius ~1:1000–2000).
• Recruitment: graded force is achieved by (1) recruiting more motor units and (2) increasing firing frequency.
• Size principle (Henneman): small (slow, fatigue-resistant) motor units are recruited first, larger (fast, fatigable) units later, as force demand rises.
• Summation: repeated stimuli before relaxation → temporal (wave) summation → with high frequency → tetanus (fused, maximal tension). Treppe (staircase) = progressive increase in force on repeated stimulation due to rising cytosolic Ca²⁺.

Muscle fibre types (table beloved by examiners)

Feature	Type I (slow oxidative)	Type IIa	Type IIx/IIb (fast glycolytic)
Contraction speed	Slow	Fast	Fastest
Myosin ATPase	Low	High	High
Main energy source	Oxidative (aerobic)	Mixed	Glycolytic (anaerobic)
Mitochondria/capillaries	Many	Intermediate	Few
Myoglobin → colour	High → red	Red-pink	Low → white
Fatigue resistance	High	Intermediate	Low
Example	Postural, marathon	—	Sprint, power

Mnemonic: "One slow red ox" — Type One = Slow, Red, Oxidative.

Energy sources and muscle fatigue

ATP is regenerated by three systems in temporal sequence:

1. Immediate: Creatine phosphate (phosphocreatine) via creatine kinase → fastest, lasts seconds.
2. Short-term: Anaerobic glycolysis → lactate, seconds to ~2 min.
3. Sustained: Oxidative phosphorylation → minutes to hours, requires O₂.

Mechanisms of muscle fatigue (peripheral): accumulation of inorganic phosphate (Pi) (impairs cross-bridge force + SR Ca²⁺ release — considered the major cause), ADP accumulation, reduced SR Ca²⁺ release, glycogen depletion, and acidosis (lactate/H⁺, contribution now thought modest). Central fatigue (CNS drive) also contributes.

High-yield: The chief biochemical contributor to high-intensity muscle fatigue is accumulation of inorganic phosphate (Pi), not lactic acid as classically taught.

Clinical correlation (links physiology to disease)

These appear as clinical-vignette stems testing the underlying mechanism.

• Malignant hyperthermia: mutation in RYR1 (ryanodine receptor) → uncontrolled SR Ca²⁺ release triggered by volatile anaesthetics (halothane) and succinylcholine → masseter rigidity, hyperthermia, rising end-tidal CO₂, rhabdomyolysis. Treatment: dantrolene (blocks RyR1, inhibiting SR Ca²⁺ release). Highly testable.
• Duchenne muscular dystrophy: dystrophin absence → membrane fragility, ↑↑ CK, calf pseudohypertrophy, Gower's sign; X-linked recessive; frameshift mutation.
• Myasthenia gravis: autoantibodies to postsynaptic nicotinic ACh receptors → fatigable weakness; a NMJ (not contractile-apparatus) disorder.
• Lambert–Eaton syndrome: antibodies to presynaptic voltage-gated Ca²⁺ channels → reduced ACh release; improves with activity.
• McArdle disease (Type V glycogenosis): myophosphorylase deficiency → exercise intolerance, second-wind phenomenon, no rise in lactate on ischaemic exercise test.
• Rigor mortis: ATP depletion (above) — forensic mechanism question.

Recently asked / exam angle

• "During muscle contraction which band remains constant?" → A band (I band and H zone shorten).
• "ATP is directly required for which step?" → Detachment of myosin head from actin (and re-cocking via hydrolysis); rigor mortis if absent.
• "Calcium binds to which protein in skeletal muscle?" → Troponin C (vs calmodulin in smooth muscle).
• "Skeletal muscle E-C coupling — extracellular calcium required?" → No (mechanical DHPR–RyR1 coupling); cardiac requires it (CICR).
• "Drug of choice in malignant hyperthermia?" → Dantrolene (acts on RyR1).
• "Triad location in skeletal muscle?" → A-I junction; cardiac has dyads at the Z-line.
• "Largest protein / responsible for passive tension?" → Titin.
• "Sliding filament theory — what slides?" → Thin (actin) filaments slide over thick (myosin); filament lengths unchanged.
• Force–velocity graph: Vmax at zero load, velocity zero at isometric maximum.
• Size principle: smallest motor units recruited first.

Rapid revision

1. Sarcomere = Z-line to Z-line; A band constant, I band and H zone shorten on contraction.
2. Sliding filament theory: filaments slide, do not shorten.
3. Power stroke = Pi release; ATP binding causes detachment; ATP hydrolysis re-cocks the head.
4. No ATP → rigor mortis (myosin stays bound to actin).
5. Calcium binds Troponin C → tropomyosin uncovers actin sites → contraction.
6. Troponin: C = Calcium, T = Tropomyosin, I = Inhibitory (actin).
7. Skeletal E-C coupling is mechanical (DHPR→RyR1); no extracellular Ca²⁺ needed; cardiac uses CICR + extracellular Ca²⁺.
8. SERCA pumps Ca²⁺ back into SR (relaxation); calsequestrin stores it; calmodulin/MLCK run smooth muscle.
9. Triad (A-I junction) = skeletal; dyad (Z-line) = cardiac.
10. Optimal sarcomere length (~2.0–2.2 µm) = max cross-bridge overlap = max active tension; titin = passive tension.
11. Type I = Slow, Red, Oxidative, fatigue-resistant; Type IIx = fast, white, glycolytic.
12. Malignant hyperthermia = RYR1 mutation, triggered by halothane/succinylcholine, treated with dantrolene.