What was the question for this implicit rationale, and corresponding answer?
The relevant information is: When a motor neuron generates an action potential, it travels rapidly along the nerve until it reaches the neuromuscular junction, where it initiates an electrochemical process that causes acetylcholine to be released into the space between the presynaptic terminal and the muscle fiber.
 The answer: the neuromuscular junction
The question: When a motor neuron generates an action potential, it travels rapidly along the nerve until it reaches the neuromuscular junction, where it initiates an electrochemical process that causes acetylcholine to be released into the space between the presynaptic terminal and the muscle fiber. The acetylcholine molecules then bind to nicotinic ion-channel receptors on the muscle cell membrane, causing the ion channels to open. Calcium ions then flow into the muscle cell, initiating a sequence of steps that finally produce muscle contraction.

Answer this question: where is acetylcholine released from to stimulate muscle contraction?