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.