Final answer:
Calcium ions (Ca2+) are released in the axon terminal when an action potential arrives, which triggers synaptic vesicles to release neurotransmitters into the synaptic cleft.
Step-by-step explanation:
The ion released in the axon terminal with an incoming action potential, facilitating the release of neurotransmitters from the synaptic vesicles, is the calcium ion (Ca2+). When an action potential reaches the axon terminal, it causes depolarization of the presynaptic membrane, triggering voltage-gated Ca2+ channels to open and allowing Ca2+ ions to enter the neuron. This increase in intracellular calcium concentration initiates a cascade of events that promote the fusion of synaptic vesicles with the presynaptic membrane, resulting in the exocytosis of neurotransmitters into the synaptic cleft.