1. The functional membrane-to-membrane contact of the nerve cell with another nerve cell, an effector (muscle or gland) cell, or a sensory receptor cell. The synapse subserves the transmission of nerve impulses, commonly from a variably large (1–12 mcm), generally knob-shaped or club-shaped axon terminal (the presynaptic element) to the circumscript patch of the receiving cell's plasma membrane (the postsynaptic element) on which the synapse occurs. In most cases, the impulse is transmitted by means of a chemical transmitter substance (such as acetylcholine, γ-aminobutyric acid, dopamine, or norepinephrine) released into a synaptic cleft (15–50 nm wide) that separates the presynaptic from the postsynaptic membrane; the transmitter is stored in quantal form in round or ellipsoid, membrane-bound synaptic vesicles (10–50 nm in diameter) in the presynaptic element. In other synapses, transmission takes place by direct propagation of the bioelectrical potential from the presynaptic to the postsynaptic membrane; in such electrotonic synapses (“gap junctions”), the synaptic cleft is no more than about 2 nm wide. In most cases, synaptic transmission takes place in only one direction (“dynamic polarity” of the synapse), but in some synapses synaptic vesicles occur on both sides of the synaptic cleft, which suggest the possibility of reciprocal chemical transmission.
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