The release of chemical messenger from synaptic vesicles is under the influence of these ion(s).

Step-by-step solution

When an impulse arrives at the synaptic knob of the axon, it depolarizes the presynaptic membrane and increases its permeability to calcium ions ( Ca ²⁺ ) . Ca ²⁺ ions from the synaptic cleft pass into the synaptic knob via voltagegated channels. Sudden rise in the cytosolic concentration of 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.