Final answer:
Calcium ions that enter the axon terminal upon an action potential trigger synaptic vesicles to release neurotransmitters by causing changes on proteins in these vesicles and their fusion with the presynaptic membrane.
Step-by-step explanation:
The calcium that flows into the axon terminal causes changes on proteins in the synaptic vesicles and merging on the presynaptic membrane. When an action potential reaches the axon terminal, it triggers voltage-gated Ca2+ channels to open, leading to an influx of calcium ions.
These ions then associate with proteins on the synaptic vesicles, facilitating the vesicles to fuse with the presynaptic membrane. The fusion process causes the vesicles to release their neurotransmitter molecules into the synaptic cleft, enabling communication between neurons.