Schematic illustration of the ultrastructure of a sectioned axo-dendritic synapse. A yellow pre-synaptic terminal bulb (bouton) is separated by a synaptic cleft from a dendrite (brown). Astrocyte processes (A) guard the margin of the synaptic cleft. Neurotransmitter molecules are contained within synaptic vesicles which release their contents into the synaptic cleft in response to Ca++ influx. Large secretory vesicles (arrow) are sometimes found in the terminal bulb.