1) Membrane-bound compartments which contain transmitter molecules. Synaptic vesicles are concentrated at presynaptic terminals. They actively sequester transmitter molecules from the cytoplasm. In at least some synapses, transmitter release occurs by fusion of these vesicles with the presynaptic membrane, followed by exocytosis of their contents.

2) 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.