The CAZ is seen under the electron microscope to be a dark (electron dense) area close to the membrane. The active zone is the region in the presynaptic bouton that mediates neurotransmitter release and is composed of the presynaptic membrane and a dense collection of proteins called the cytomatrix at the active zone (CAZ).
When an action potential reaches the presynaptic bouton, the contents of the vesicles are released into the synaptic cleft and the released neurotransmitter travels across the cleft to the postsynaptic neuron (the lower structure in the picture) and activates the receptors on the postsynaptic membrane. As shown in the adjacent diagram, a synapse consists of the presynaptic bouton of one neuron which stores vesicles containing neurotransmitter (uppermost in the picture), and a second, postsynaptic neuron which bears receptors for the neurotransmitter (at the bottom), together with a gap between the two called the synaptic cleft (with synaptic adhesion molecules, SAMs, holding the two together ). Two neurons make near contact through structures called synapses allowing them to communicate with each other. The active zone or synaptic active zone is a term first used by Couteaux and Pecot-Dechavassinein in 1970 to define the site of neurotransmitter release.
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