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Events of muscle contraction

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"muscle contraction"[MeSH Terms] AND skeletal

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skeletal muscle contraction sliding filament excitation contraction coupling physiology

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diagram excitation-contraction coupling skeletal muscle T tubule sarcoplasmic reticulum calcium troponin cross bridge cycle

Events of skeletal muscle contraction

Excitation-contraction coupling flowchart
  1. Nerve impulse arrives at the neuromuscular junction.
    The motor neuron releases acetylcholine (ACh).
  2. End-plate potential and muscle action potential develop.
    ACh binds nicotinic receptors on the motor end plate, causing Na⁺ influx. If threshold is reached, an action potential spreads across the sarcolemma.
  3. Action potential travels down T-tubules.
    T-tubules conduct depolarization deep into the muscle fiber.
  4. Calcium is released from the sarcoplasmic reticulum (SR).
    T-tubule depolarization activates dihydropyridine receptors, which open ryanodine receptors in the adjacent SR. This releases Ca²⁺ into the sarcoplasm. In skeletal muscle, extracellular Ca²⁺ entry is not required for this step.
    Costanzo Physiology, 7th ed., p. 44.
  5. Ca²⁺ binds troponin C.
    The troponin complex changes its shape.
  6. Tropomyosin moves away from actin’s myosin-binding sites.
    This exposes the active sites on actin, permitting actin-myosin interaction.
  7. Cross-bridge cycle produces shortening.
    • Energized myosin head, carrying ADP + Pi, binds actin.
    • Pi, then ADP, are released.
    • The myosin head pivots in the power stroke, pulling thin filaments toward the center of the sarcomere.
    • A new ATP binds myosin, causing myosin to detach from actin.
    • ATP is hydrolyzed to ADP + Pi, re-cocking the myosin head.
    • The cycle repeats as long as Ca²⁺ and ATP are available.
  8. Sarcomere shortens.
    Thin filaments slide past thick filaments. The I band and H zone decrease, while the A band remains constant. The filaments themselves do not shorten. The actomyosin cycle converts ATP’s chemical energy into force.
    Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 816-818.
  9. Relaxation occurs.
    ACh is broken down by acetylcholinesterase, action potentials stop, and SERCA pumps Ca²⁺ back into the SR. Ca²⁺ dissociates from troponin, tropomyosin again blocks actin binding sites, cross-bridge cycling stops, and the muscle relaxes.

One-line sequence

Motor neuron AP → ACh release → muscle AP → T-tubules → SR Ca²⁺ release → troponin C → tropomyosin displacement → actin-myosin cross-bridges → power strokes → contraction → Ca²⁺ reuptake → relaxation.
This timing is consistent with the NCBI physiology overview: the muscle action potential occurs first, followed by intracellular Ca²⁺ rise, then mechanical contraction.
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