Question

which of the following events triggers the subsequent steps of excitation - contraction coupling?
propagation of an action potential along the sarcolemma and t tubules
release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum
release of acetylcholine from axon terminals at the neuromuscular junction
binding of calcium ions to troponin, which removes the blocking action of tropomyosin
check answer

Explanation:

Excitation - contraction coupling in muscle cells begins with the propagation of an action potential. This action potential travels along the sarcolemma (the muscle cell membrane) and down the T - tubules (invaginations of the sarcolemma). This electrical signal then triggers the release of calcium ions from the sarcoplasmic reticulum. The release of acetylcholine is part of the neuromuscular junction communication to start the action potential, but it is not the trigger for excitation - contraction coupling itself. Binding of calcium to troponin is a later step in the contraction process after the excitation (action potential propagation) has occurred.

Answer:

propagation of an action potential along the sarcolemma and T tubules