Final answer:
When an action potential reaches the axon terminal of a motor neuron, voltage-gated calcium channels open, allowing the release of neurotransmitter molecules.
Step-by-step explanation:
When an action potential reaches the axon terminal of a motor neuron, voltage-gated calcium channels open. The influx of calcium ions into the cell initiates a signaling cascade that leads to the fusion of synaptic vesicles with the presynaptic membrane. These vesicles contain neurotransmitter molecules, which are then released into the synaptic cleft to transmit the signal to the next neuron or muscle cell.