Figure 1. Close-up schematic view of a chemical synapse. Only membrane-bound organelles are indicated. Neurotransmitter (pale green) is stored at the presynaptic nerve terminal (top part) in synaptic vesicles (small green circles). The vesicles are clustered in the vicinity of the plasma membrane. A docking step moves vesicles to the membrane where there might be additional reactions required, called priming, before the vesicle can fuse upon calcium (yellow) entry. The vesicle membrane is retrieved via endocytosis and recycled back into the cluster. Grey arrows indicate alternate endocytic pathways. A similar recycling of vesicles has been postulated in the postsynaptic compartment (bottom part) where neurotransmitter receptors are inserted and retrieved as one mechanism to regulate synaptic strength.

Figure 2. Upon fusion of the vesicle with the plasma membrane the two membranes become continuous, thus creating a pathway for the transmitter to escape into the synaptic cleft.