8
The Ca?* released from the sarcoplasmic reticulum begins the contraction process by
EXPLANATION
Synaptic Transmission, Neurotransmitters, Exc
- Excitation-Contraction Coupling in Striated Muscle: Depolarization of T-tubules activates dihydropyridine receptors (DHPR), opening ryanodine receptors (RyR1) to release $Ca^{2+}$ from the sarcoplasmic reticulum.
- Molecular Trigger: Free cytosolic $Ca^{2+}$ binds to Troponin C (TnC), inducing a conformational shift that pulls tropomyosin out of the myosin-binding groove on F-actin filaments, thereby exposing actin cross-bridge binding sites to allow myosin head binding and power-stroke execution (Option 3).
Correct option: (3) Exposing actin filament cross-bridge binding sites
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