Muscle Contraction Biochemistry
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Direct answer
Calcium release flips the thin filament's switch: it binds troponin C, tropomyosin rolls clear of the myosin-binding sites on actin, and each myosin head that finds a site hydrolyses one ATP, delivers a power stroke, and lets go only when a fresh ATP binds. The ratchet needs ATP twice over — for motion and for detachment — which is why ATP depletion after death freezes muscle in rigor. In skeletal muscle the calcium signal itself is mechanical: the dihydropyridine receptor senses voltage and wrenches open the ryanodine receptor (RYR1), without needing extracellular calcium; cardiac muscle instead requires a small inward calcium current through L-type channels to trigger RYR2 (calcium-induced calcium release). Biochemistry meets genetics at dystrophin, the largest human gene, whose loss causes Duchenne muscular dystrophy.
What you must remember
- Cross-bridge cycle: ATP binds myosin → detachment; ATP hydrolysis cocks the head (ADP and Pi retained); calcium exposes the site → binding; Pi and ADP release → power stroke; without ATP there is no detachment — rigor mortis sets in within a few hours of death and passes with autolysis.
- One ATP per stroke: every cross-bridge cycle consumes exactly one ATP; force comes from thousands of heads working asynchronously, not from large individual excursions.
- Thin filament regulation: troponin C binds calcium, troponin I inhibits actin-myosin interaction, troponin T anchors tropomyosin; smooth muscle uses none of these — calcium-calmodulin activates myosin light-chain kinase instead.
- Coupling difference: skeletal DHPR is a voltage sensor mechanically coupled to RYR1; cardiac contraction depends on extracellular calcium entry triggering RYR2 — why calcium and digoxin change cardiac but not skeletal contractility.
- Relaxation is active: SERCA pumps calcium back into the sarcoplasmic reticulum (buffered by calsequestrin); phospholamban restrains SERCA until phosphorylated by protein kinase A — the biochemical site of beta-adrenergic inotropy.
- Malignant hyperthermia: RYR1 gain-of-function under halothane or succinylcholine — sustained calcium release, rigidity, hyperthermia, rhabdomyolysis; dantrolene blocks RYR1 release.
- Dystrophin: about 2.4 million base pairs at Xp21; out-of-frame deletions abolish the protein (Duchenne — calf pseudohypertrophy, Gower's sign, creatine kinase in the tens of thousands), in-frame deletions leave a shorter working protein (Becker).
One beat, step by step
Walk a ventricular myocyte through systole. Depolarisation opens L-type channels, the entering calcium meets RYR2, and the sarcoplasmic reticulum floods the cytosol; troponin C saturates and cross-bridges fire. For diastole, SERCA sweeps the calcium back, and the adrenaline of exercise phosphorylates phospholamban through protein kinase A, releasing SERCA's brake and shortening relaxation. Now contrast the skeletal myocyte at the neuromuscular junction: the action potential dives down the T-tubule, and the dihydropyridine receptor — a voltage sensor, not a channel of consequence here — physically pulls RYR1 open. No extracellular calcium is required, which is why a skeletal muscle twitches in a calcium-free bath while a cardiac one lies silent. After death, when ATP is gone and the sarcoplasmic reticulum has leaked its store, every head locks to actin: rigor mortis, the same chemistry read by a forensic surgeon estimating the time since death.
Where students slip
Attributing relaxation to calcium "running out" — relaxation costs ATP through SERCA; it is work, not decay. Confusing troponin C with troponin T (the cardiac isoform measured in infarction) trips one-mark questions. The reliably asked comparison is calcium source: skeletal from the sarcoplasmic reticulum alone, cardiac from extracellular trigger plus reticular amplification. And smooth muscle deserves its own line — no troponin, calmodulin-myosin light-chain kinase regulation, and latch bridges that hold tone with minimal ATP expenditure, the reason sphincters stay closed economically.
Frequently asked questions
What triggers the power stroke?
Release of inorganic phosphate and ADP from the actin-bound myosin head after calcium has exposed the binding site.
Why does ATP depletion cause rigor mortis?
Without ATP, myosin heads cannot detach from actin, so cross-bridges lock until autolysis breaks the complex.
How does skeletal differ from cardiac excitation-contraction coupling?
Skeletal muscle couples the dihydropyridine receptor mechanically to RYR1 with no calcium influx needed, whereas cardiac muscle requires extracellular calcium entry to trigger calcium-induced calcium release through RYR2.
Which protein is defective in Duchenne muscular dystrophy?
Dystrophin, linking the actin cytoskeleton to the dystroglycan complex; its absence makes fibres fragile and leaky, hence very high creatine kinase.
How does dantrolene treat malignant hyperthermia?
By blocking ryanodine receptor-1 calcium release from the sarcoplasmic reticulum, breaking the cycle of sustained contracture and heat production.