Neurotransmitters are chemical signaling molecules used in the nerve conduction and action potential propagation. The synthesis of the neurotransmitter is followed by the uptake of these substances in the synaptic vesicles with the help of a group of proteins known as transporters. Different neurotransmitters are synthesized differently, and stored in secretory granules called synaptic vessels.
When an action potential arrives at the axon terminal, the depolarization of the membrane causes the opening of voltage-gated Ca2+ channel. These voltage-gated calcium channels in the active zone cause an inward movement of calcium, flooding the cytoplasm of the cell as long as these channels are open. This increased concentration of calcium, therefore, causes neurotransmitters to be released from synaptic vesicles, through exocytosis. The membrane of the synaptic vesicle fuses with the presynaptic membrane at the active zone of the following neuron, allowing the contents of the vesicles to be released in the synaptic cleft.
The amount of neurotransmitter contained in a single vesicle is called a quantum. Each vesicle contains approximately the same number of neurotransmitter molecules which range upto several thousands. This quantal release of these neurotransmitters determines the size of the postsynaptic potential. In simpler terms, the post synaptic potentials (EPSP or IPSP) at a given synapse are quantized, meaning, many individual quantum contributes to the amount of released neurotransmitters during normal neurotransmission.