The coupling of muscle excitation & muscle contraction involves the 2nd physiologic event. This process is best described by which of the following?
A) An action potential propagates along the motor axon innervating the muscle fiber.
B) The synaptic end bulb releases a neurotransmitter, causing the muscle to contract.
C) Ca2+ channels, stimulated by depolarizing T-Tubules, open and release Ca2+ ions to the sarcoplasm.
D) Crossbridge cycling & muscle contraction.

Excitation–contraction coupling is a link process which converts the electrical impulse to a mechanical response. It links the action potential of the sarcolemma to the muscle contraction.

In this process, when acetylcholine binds to the receptors it leads to the generation of the action potential. This action potential travels down via T-tubules from the sarcolemma to sarcoplasmic reticulum due to which calcium ions are released into the sarcoplasm. These calcium ions bind to the troponin which leads to the binding of the myosin to actin forming cross-bridge. This cross-bridge pulls the actin causing muscle contraction. This impulse generation and muscle contraction repeat again.

Thus, option D is the correct answer.