Final answer:
The arrival of an action potential at the neuromuscular junction opens voltage-gated calcium channels, leading to neurotransmitter release.
Step-by-step explanation:
The propagation of an action potential to the axon terminal at the neuromuscular junction (NMJ) triggers the opening of voltage-gated calcium channels. When an action potential reaches the axon terminal, it causes these channels to open and allows calcium ions to enter the cell. The influx of calcium initiates a cascade that results in the fusion of synaptic vesicles with the presynaptic membrane and the release of neurotransmitters into the synaptic cleft.