ABSTRACT
Most neurotransmitter release occurs by transmitter-loaded synaptic vesicles fusing with the presynaptic membrane so that the contents of the vesicle are discharged into the synaptic cleft. In vesicular release, neurotransmitter is secreted in discrete packets or quanta. Each quantum represents the release of the contents of a single vesicle, about 4000 molecules of transmitter. Direct evidence for the role of calcium is provided by calcium imaging, a technique which makes visible how Ca2+ signals spread in time and space through cells. Exocytosis from small clear synaptic vesicles (SSVs) involves several linked steps, most of which need calcium. The releasable pool is located at the active zone and can take part in repeated cycles of exocytosis and endocytosis at low neuron firing frequencies. The guanosine 5'-triphosphate-bound form of dynamin requires calcium, so the same increase in nerve terminal Ca2+ concentration responsible for exocytosis also enables endocytosis.
