Synapse

Within the nervous system, nerve impulses travel from neuron to neuron along complex nerve pathways. The junction between the parts of two such neurons is called a "synapse." Actually, these cells, called "presynaptic" and "postsynaptic neurons," are not in direct contact at the synapse. There is a gap called a "synaptic cleft" between them, and for an impulse to continue along a nerve pathway it must cross this space. The typical one-way transmission from axon to cell body is due to the fact that axons usually have rounded "synaptic knobs" at their presynaptic terminals, which the cell bodies lack. These knobs contain numerous membranous sacs, called "synaptic vesicles," and when a nerve impulse reaches a knob, some of the vesicles respond by releasing a substance which diffuses across the synaptic cleft. If a sufficient amount of the substance (called "neurotransmitter") is released, the membrane is stimulated, and a nerve impulse is triggered.