Chemical Synapses

Abstract

Abstract Chemical synapses are among the most elaborate junctions existing between two cells, enabling communication between neurons through chemical neurotransmission within milliseconds. This fast rate of transmission is achieved through three subsynaptic compartments; the presynaptic bouton, the synaptic cleft and the postsynaptic junction. The presynaptic bouton packages neurotransmitters into synaptic vesicles then releases them into the synaptic cleft. Release of synaptic vesicles occurs through several distinct stages, coordinated by a group of specialised proteins. The postsynaptic density (PSD) has evolved into a complex neurotransmitter reception apparatus, which enables the postsynaptic terminal to modulate the downstream response to neurotransmitters. Following activation of receptors on the postsynaptic membrane, neurotransmitters are taken back up into the presynaptic bouton and repackaged into synaptic vesicles (SVs). The synaptic cleft contains proteins that ensure that active zone and PSD remain in proximity. These proteins are also required during synaptogenesis to ensure that the synapse forms properly. Key Concepts: There are three major structural components that define the synapse: the presynaptic bouton (also known as presynaptic terminal), postsynaptic junction (also known as postsynaptic terminal) and the synaptic cleft. The presynaptic bouton is responsible for packaging neurotransmitters into synaptic vesicles, then releasing their contents into the synaptic cleft in response to calcium influx. Synaptic vesicles are released at a specialised site within the presynaptic bouton known as the active zone. Synaptic vesicles are released by a process known as exocytosis through several distinct steps involving specialised proteins that form a highly interactive and dynamic protein complex at the site of synaptic vesicle release. Interpretation of the presynaptic message takes place at the postsynaptic membrane through transmembrane proteins called receptors. The postsynaptic density is situated adjacent to the postsynaptic membrane within the postsynaptic terminal, juxtaposed to the presynaptic active zone. The postsynaptic density is composed of receptors, scaffolding and adhesion proteins, kinases and phosphatases, as well as cytoskeletal elements, which are linked together to form macromolecular complexes. The postsynaptic density of excitatory synapses is thicker (more pronounced) than the postsynaptic density of inhibitory synapses. The synaptic cleft is a narrow space, approximately 20–30 nm wide situated between the presynaptic and the postsynaptic plasma membranes. The synaptic cleft is composed of proteinaceous and carbohydrate‐rich cell adhesion molecules.