When the action potential reaches the nerve terminal it causes the let go of a chemical neurotransmitter by the synaptic vesicles. Mammalian nervous system employs many substances as neurotransmitters. These have the amino acids glycine and glutamate, acetylcholine, the biogenic amines such as dopamine and epinephrine and several of little peptides like as the enkephalins.  For instance, acetylcholine is stored in synaptic vesicles a specialized form of secretory vesicle and is released into the synaptic cleft by exocytosis in a Ca2+   -dependent manner in figure. the acetylcholine molecules then circulate across to the plasma membrane of the post-synaptic cell where they bind to specific receptor.

    figure:   Propagation of the action potential along an axon