synapse, pl. synapses (sin´aps, sI-naps´; sI-nap´sez)
The functional membrane-to-membrane contact of the nerve cell with another nerve cell, an effector (muscle, gland) cell, or a sensory receptor cell. The s. subserves the transmission of nerve impulses, commonly from a variably large (1 to 12 mum), 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 s. occurs. In most cases the impulse is transmitted by means of a chemical transmitter substance (such as acetylcholine, gamma-aminobutyric acid, dopamine, norepinephrine) released into a synaptic cleft (15 to 50 nm wide) that separates the presynaptic from the postsynaptic membrane; the transmitter is stored in quantal form in synaptic vesicles: round or ellipsoid, membrane-bound vacuoles (10 to 50 nm in diameter) in the presynaptic element. In other s.'s transmission takes place by direct propagation of the bioelectrical potential from the presynaptic to the postsynaptic membrane; in such electrotonic s.'s ("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 s.), but in some s.'s synaptic vesicles occur on both sides of the synaptic cleft, suggesting the possibility of reciprocal chemical transmission. [syn- + G. hapto, to clasp]
axoaxonic s.the synaptic junction between an axon terminal of one neuron and either the initial axon segment or an axon terminal of another nerve cell.
axodendritic s.the synaptic contact between an axon terminal of one nerve cell and a dendrite of another nerve cell.
axosomatic s.the synaptic junction of an axon terminal of one nerve cell to the cell body of another nerve cell.pericorpuscular s;
electrotonic s.gap junction See also synapse.
pericorpuscular s.axosomatic s
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