Muscle Contraction: Step-by-Step Molecular Mechanism
- Action potential arrives at motor endplate → ACh release → sarcolemma depolarization.
- Ca²⁺ release from sarcoplasmic reticulum via ryanodine receptors.
- Ca²⁺ binds troponin C → tropomyosin shifts → exposes myosin-binding sites on actin.
- Cross-bridge cycling: Myosin heads bind actin → power stroke (ADP/Pi release) → ATP binds → detachment → ATP hydrolysis → re-cocking.
- Relaxation: Ca²⁺ pumped back into SR (SERCA) → tropomyosin blocks sites → cross-bridges detach.
Summed sarcomere shortening = muscle contraction.
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