SYNAPSE

also called neuronal junction the site of transmission of electric nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell (effector). A synaptic connection between a neuron and a muscle cell is called a neuromuscular junction (q.v.). Two types of synapses can be distinguished, the chemical and the electric. At a chemical synapse, each ending, or terminal, of a nerve fibre (presynaptic fibre) swells to form a knob-like structure that is separated from the fibre of an adjacent neuron, called a postsynaptic fibre, by a microscopic space called the synaptic cleft. The typical synaptic cleft is about 0.02 microns wide. The arrival of a nerve impulse at the presynaptic terminals causes the movement toward the presynaptic membrane of membrane-bound sacs, or synaptic vesicles, which fuse with the membrane and release a chemical substance called a neurotransmitter (q.v.). This substance transmits the nerve impulse to the postsynaptic fibre by diffusing across the synaptic cleft and binding to the receptor molecules on the postsynaptic membrane. The chemical binding action alters the shape of the receptors, setting off a series of reactions that open channel-shaped protein molecules. Electrically charged ions then flow through the channels into or out of the neuron. This sudden shift of electric charge across the postsynaptic membrane changes the membrane's electric polarization, producing what is called the postsynaptic potential (q.v.), or PSP. If there is a large enough net flow of positively charged ions into the cell, then the PSP is said to be excitatory; that is, it can lead to the generation of a new nerve impulse, called an action potential (q.v.). Once they have been released and have bound to postsynaptic receptors, neurotransmitter molecules are immediately deactivated by enzymes in the synaptic cleft; they are also taken up by receptors in the presynaptic membrane and recycled. This mechanism creates a series of brief transmission events, each one taking place in only 0.5 to 4.0 milliseconds (five ten-thousandths to four one-thousandths of a second). Whether a nerve impulse is transmitted at the chemical synapse depends upon the particular neurotransmitter and the postsynaptic receptors. A single neurotransmitter may elicit different responses from different receptors. For example, norepinephrine, a transmitter common in the autonomic nervous system, binds to some receptors that excite nervous transmission and to others that inhibit it. The membrane of a postsynaptic fibre has many different kinds of receptors, and some presynaptic terminals release more than one type of neurotransmitter. Also, each postsynaptic fibre may form hundreds of competing synapses with many neurons. These variables account for the complex responses of the nervous system to any given stimulus. The synapse, with its chemical transmitter substance, acts as a physiological valve, directing the conduction of nerve impulses in regular circuits and preventing random or chaotic stimulation of nerves. Electric synapses allow direct communications between neurons whose membranes are fused by permitting ions to flow between the cells through channels called gap junctions. Found in invertebrates and lower vertebrates, gap junctions allow faster synaptic transmission as well as the synchronization of entire groups of neurons. Gap junctions are also found in the human body, most often between cells in most body organs and between glial cells of the nervous system. Chemical transmission seems to have evolved in large, and complex vertebrate nervous systems, where transmission of multiple messages over longer distances is required.