What action is directly responsible for the generation of an action potential at the sarcolemma?

Nervous stimulus action for muscle contraction

Explanation:

Nerve stimulation is sent to the sarcolemma, which are motor nerves at the neuromuscular junction (junction between the terminal part of a motor axon with a motor plate), after which nerve endings implant in the sarcolemma and form plaque on the surface of muscle fibers, which in turn transmit stimulus to the musculature.

Then, an electrical current (from the stimuli) is generated, it propagates through the muscle cell membrane, reaches the cytoplasm and triggers the muscle contraction mechanism.