Medical Definition of synaptic vesicle
: a small secretory vesicle that contains a neurotransmitter, is found inside an axon near the presynaptic membrane, and releases its contents into the synaptic cleft after fusing with the membrane When a nerve impulse moves down the axon of a neuron and arrives at an axon terminal, it stimulates synaptic vesicles in the terminal to discharge neurotransmitters.—Rita L. Atkinson et al., Introduction to Psychology, 9th edition, 1987
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