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. Two neurons make near contact through structures called synapses allowing them to communicate with each other. 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). 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. 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). The CAZ is seen under the electron microscope to be a dark (electron dense) area close to the membrane. Proteins within the CAZ tether synaptic vesicles to the presynaptic membrane and mediate synaptic vesicle fusion, thereby allowing neurotransmitter to be released reliably and rapidly when an action potential arrives. Function The function of the active zone is to ensure that neurotransmitters can be reliably released in a specific location of a neuron and only released when the neuron fires an action potential. As an action potential propagates down an axon it reaches the axon terminal called the presynaptic bouton. In the presynaptic bouton, the action potential activates calcium channels (VDCCs) that cause a local influx of calcium. The increase in calcium is detected by proteins in the active zone and forces vesicles containing neurotransmitter to fuse with the membrane. This fusion of the vesicles with the membrane releases the neurotransmitters into the synaptic cleft (space between the presynaptic bouton and the postsynaptic membrane). The neurotransmitters then diffuse across the cleft and bind to ligand gated ion channels and G-protein coupled receptors on the postsynaptic membrane. The binding of neurotransmitters to the postsynaptic receptors then induces a change in the postsynaptic neuron. The process of releasing neurotransmitters and binding to the postsynaptic receptors to cause a change in the postsynaptic neuron