What Controls the Exocytosis of Neurotransmitter?

Abstract

Before commencing our answer to the question posed in the title, we present a sketch of some required terms and concepts for those unfamiliar with basic neurobiology. The most evident features of a neuron (nerve cell) are a cell body together with a long axon. See Figure 1. Extending from the cell body are a number of finger-like processes called dendrites that usually transmit the input into the cell. The axon branches into many fibres at the end of which are typically located small button-like structures called nerve endings. If the input to a cell is sufficiently excitatory (i. e. if the depolarizing voltage change exceeds a certain threshold) an electrical wave called an action potential is generated, which travels down the axon and passes into each nerve ending. The resulting depolarization brings about the exocytosis (secretion) of special chemicals called neurotransmitters that are believed to be located in spheroidal vesicles (diameter 100–500Å) close to the membrane of the nerve ending at specific release sites. Opposite these sites on the membrane of the adjacent postsynaptic cell is a special active zone with receptors that reversibly bind the released neurotransmitter after it traverses an intercellular gap of width approximately 500Å. The two special release and receptor regions together with the gap constitute a synapse. The average number of vesicles released by an action potential (or by a similar artificial stimulus) is the quantal content, called here total release. Spontaneous release of individual vesicles also occurs, at a low rate.