The release of chemical messenger from synaptic vesicles is under...

When an impulse arrives at the synaptic knob of the axon, it depolarizes the presynaptic membrane and increases its permeability to calcium ions $\left(\mathrm{Ca}^{2+}\right) . \mathrm{Ca}^{2+}$ ions from the synaptic cleft pass into the synaptic knob via voltagegated channels. Sudden rise in the cytosolic concentration of $\mathrm{Ca}^{2+}$, causes the release of a chemical, called neurotransmitter substance, from small synaptic vesicles present there into the synaptic cleft by exocytosis through the presynaptic membrane.