Synaptic vesicles and exocytosis.

Neurons transmit information by releasing neurotransmitters from presynaptic nerve endings, In the resting stage , transmitters are stored in small organelles of uniform size and shape , the synaptic vesicles . When an action potential arrives in the nerve terminal , the membrane depolarizes and voltage-gated Ca2+ channels open . The resulting Ca2+ influx triggers exocytosis of synaptic vesicles , resulting in the release of neurotransmitter. The synaptic vesicle membrane is rapidly retrieved by endocytosis and reutilized for the formation of synaptic vesicles. These vesicles are reloaded with transmitter for another round of exocytosis . This cycle is repeated many times and can be studied in nerve terminals that have no connection with the neuronal cell body . Thus , the presynaptic compartment consists of an autonomous unit that contains all components required for repetitive exocytosis and membrane recycling . In contrast, release of neuropeptides involves large dense core vesicles that follow a different route; they will not be considered in this discussion (for review, see Thureson-Klein & Klein 1990) . Although the principles o f synaptic vesicle membrane traffic were worked out many years ago (reviewed by Ceccarelli & Hurlbut 1980) , it remains a major challenge to define the molecular events underlying the individual