alleryCalcium for muscle contraction is released from:
ABlood
BNucleus
CMitochondria
DSarcoplasmic reticulum (specialized ER of muscle cells)
Answer & Solution
Correct answer: D. Sarcoplasmic reticulum (specialized ER of muscle cells)
**SR:** Ca²⁺ store.
Depolarization of T-tubule (action potential along muscle membrane)
→ DHPR triggers RyR (ryanodine receptor) on SR
→ Ca²⁺ floods sarcoplasm.
Ca²⁺ binds troponin C
→ tropomyosin shifts
→ exposes myosin-binding sites on actin
→ cross-bridge cycling.
SERCA pumps Ca²⁺ back into SR for relaxation.
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