What causes the increase in cytosolic calcium that triggers the fusion of secretory vesicles to the membrane and the release of neurotransmitters into the synaptic cleft?

The change in conformation of voltage-gated calcium channels in the axon terminal membrane causes the increase in cytosolic calcium that triggers the fusion of secretory vesicles to the membrane and the release of neurotransmitters into the synaptic cleft.

Explanation:

As the calcium concentration becomes less in the cytosol, it allows calcium ions and functions it as a secondary messenger in the calcium signaling process. During the action potential, or any nerve impulse, that arrives at the terminal of axon, the activation of voltage-gated calcium channel is carried. This activation is done in the cell membrane. Calcium ion has the key responsibility for the triggering of synaptic vesicles to release the neurotransmitters.