Final answer:
Acetylcholine binds to receptors at the neuromuscular junction, leading to the opening of Na+ channels, depolarization of the postsynaptic membrane, and initiation of an action potential for muscle contraction.
Step-by-step explanation:
The binding of acetylcholine to its receptor at the neuromuscular junction causes the opening of a channel for both Na+ and K+. When acetylcholine is released at the synapse between a nerve and muscle, it causes postsynaptic Na+ channels to open. The Na+ enters the postsynaptic cell, causing the membrane to depolarize. This process leads to the creation of an action potential which ultimately triggers muscle contraction.