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During muscle contraction, the immediate trigger for actin-myosin binding is the release of:
AAcetylcholine into the synaptic cleft area
BSodium ions into the muscle cytoplasm area
CCalcium ions (Ca²⁺) from sarcoplasmic reticulum
DPotassium ions out of the muscle cell area
Answer & Solution
Correct answer: C. Calcium ions (Ca²⁺) from sarcoplasmic reticulum
Action potential → Ca²⁺ released from SARCOPLASMIC RETICULUM into sarcoplasm → Ca²⁺ binds TROPONIN → tropomyosin shifts off actin's myosin-binding sites → myosin heads attach + pull actin (power stroke). Ach triggers AP but Ca²⁺ is the immediate intracellular trigger.
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