The synapse is the site for interneuronal signalling in the nervous system. The adjectival form of synapse is synaptic. Synapses are either chemical or electrical. The CHEMICAL SYNAPSE consists of the PRESYNAPTIC TERMINAL, SYNAPTIC CLEFT and POSTSYNAPTIC neuron. The presynaptic terminal contains SYNAPTIC VESICLES with NEUROTRANSMITTERS. Upon the arrival of action potentials and the opening of CALCIUM (Ca2+) ion channels (see ACTION POTENTIAL; ION CHANNEL), these vesicles fuse with the cell membrane, and the transmitters are released into the synaptic cleft, which is about 10 nm in width. The released transmitters diffuse in the synaptic cleft and then bind to postsynaptic as well as presynaptic receptors (see RECEPTOR). In the postsynaptic membrane, binding of transmitters to IONOTROPIC receptors immediately opens ion channels, inducing a current flow and causing excitation or inhibition of the cell. In contrast, their binding to METABOTROPIC receptors is followed by a cascade of events leading to activation of SECOND MESSENGERS. This may or may not lead to induction of current flow, but could result in long-term changes in cellular functions. Morphologically, chemical synapses are defined by three criteria: (1) synaptic vesicles in the presynaptic profile; (2) pre- and postsynaptic densities; and (3) widening of the cell membranes to form the synaptic cleft.
Depending on the relative thickness of pre- and postsynaptic densities, synapses are either asymmetric or symmetric. Synapses can also be classified according to the pre- and postsynaptic components. AXODENDRITIC and AXOSOMATIC synapses are the most common. AXOAXONIC synapses are present in certain regions of the nervous system, and are thought to the anatomical substrate of presynaptic inhibition. Dendrites (see DENDRITE) may contain neurotransmitters and form DENDRODENDRITIC synapses, as in the OLFACTORY BULB and RETINA. TYPE I and TYPE II SYNAPSES have been identified by E.G.Grey. Type I synapses have round, clear synaptic vesicles and asymmetric densities, whereas type II synapses are associated with flattened vesicles with symmetric densities. It is often thought that type I synapses are excitatory whereas type II are inhibitory. In addition to chemical synapses, ELECTRICAL SYNAPSES are present in various regions of the nervous system. At electrical synapses, a current flows directly from one neuron to another in either bi- or unidirectional manner. The anatomical basis for electrical synapses is thought to be GAP JUNCTION channels, which are known to pass ions and other small molecules. Electrically coupled neurons can fire in synchrony, and such coupling is thought to play a role in stereotypic, all-or-none type behaviour in invertebrates, as well as brain maturation during development.
KAZUE SEMBA
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