Which of the following events triggers the subsequent steps of excitation-contraction coupling?

a. release of acetylcholine from axon terminals at the neuromuscular junction

b. binding of calcium ions to troponin, which removes the blocking action of tropomyosin

c. release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum

d. propagation of an action potential along the sarcolemma and T tubules

Excitation-contraction coupling refers to the series of events that begins with the excitation of the sarcolemma in response to stimulation by a neurotransmitter, and results in the release of calcium ions from the sarcoplasmic reticulum.

Hence, option D is the 1st process that starts the excitation-contraction coupling.

d. propagation of an action potential along the sarcolemma and T tubules

Explanation:

Excitation-contraction coupling occurs in the body as an electrical signal to the muscles, which must capture this signal and perform muscle contraction.

After this signal is emitted, an action potential is generated that will act on the neuromuscular junction and spread quickly and coordinated by the sarcolemma, reaching the T tubules.

In the T tubules, receptors of dihydropyridine receptors are located, which will continue the process, until muscle contraction is performed.