synapse

synapse The junction between two adjacent neurons (nerve cells), i.e. between the axon ending of one (the presynaptic neuron) and the dendrites of the next (the postsynaptic neuron). In chemical synapses the swollen tip of the axon of the presynaptic neuron, called the synaptic knob, contains vesicles of neurotransmitter substance. At a synapse, the membranes of the two cells (the pre- and postsynaptic membranes) are in close contact, with only a minute gap (the synaptic cleft) between them. A nerve impulse is transmitted across the synapse by the release from the presynaptic membrane of neurotransmitter, which diffuses across the synaptic cleft to the postsynaptic membrane. This triggers the propagation of the impulse from the dendrite along the length of the postsynaptic neuron. Most neurons have more than one synapse. Much less common are electrical synapses, in which ions flow directly from one neuron to another via gap junctions. These are found, for example, in the heart and in certain parts of the vertebrate central nervous system. Such an arrangement ensures virtually instantaneous transmission of impulses. See also excitatory postsynaptic potential.