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Calcium 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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