Figure 16-11 Pathway of synaptic vesicle movement in the nerve terminal. Synaptic vesicles accumulate neurotransmitters (NT) by active transport (stage I) and then move to the plasma membrane (stage II), where they become docked at the active zone (stage III). Calcium (Ca²⁺ ) influx after membrane depolarization triggers synaptic vesicle exocytosis and release of neurotransmitters (stage IV), after which the empty synaptic vesicles are endocytosed by clathrin-coated pits (stage V) and recycled (stage VI) by means of an endosomal intermediate (stage VII). Stages V and VII have not been definitely proved but are probable on the basis of morphologic observations. (From Südhof TC, Jahn R: Proteins of synaptic vesicles involved in exocytosis and membrane recycling. Neuron 6:665, 1991.)