Discussion
Action potential at sarcolemma of the muscle fiber spreads in T-tubules releasing .......................... from reticulum.
Answer: C
The action potential releases Ca++ from the sarcoplasmic reticulum.
In excitation-contraction coupling the action potential travels along the sarcolemma and down the transverse (T) tubules into the fibre. This triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium binds to troponin, which moves tropomyosin away from the binding sites on actin, so the myosin heads can form cross-bridges and the muscle contracts.
ATP powers the cross-bridge cycle and ADP is its product, but neither is released from the reticulum. Chloride ions play no part in triggering contraction.
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