Fig. 1. The structure of the active zone. An electron micrograph of a Drosophila NMJ [adapted from Atwood (Atwood, 2006)] shows electron-dense active zones, synaptic vesicles and mitochondria (m). The schematized active zone on the right shows the cycling of synaptic vesicles (white circles) at the active zone. Synaptic vesicle endocytosis occurs peripheral to the active zone, and synaptic vesicle exocytosis and neurotransmitter release occur within the active zone.