Final answer:
Synaptic vesicles in the presynaptic terminal release acetylcholine, opening ligand-gated Na+ channels and causing an excitatory postsynaptic potential (EPSP) in the muscle cell.
Step-by-step explanation:
Each presynaptic terminal contains synaptic vesicles, which contain the neurotransmitter acetylcholine that, when released, cause ligand-gated Na+ channels to open in the postsynaptic membrane. This causes an excitatory postsynaptic potential (EPSP) in the muscle cell. When an action potential reaches the axon terminal, it triggers voltage-gated Ca2+ channels to open, allowing calcium ions to enter the cell. This influx of calcium leads to the fusion of synaptic vesicles with the presynaptic membrane and the subsequent release of acetylcholine into the synaptic cleft. The acetylcholine then binds to ligand-gated Na+ channels on the postsynaptic membrane, resulting in Na+ ions entering the muscle cell and causing depolarization, known as an EPSP, which triggers muscle contraction.