Types of Synapses-Chemical and Electrical
There are two major types of synapses: (1) the chemical synapse and (2) the electrical synapse.
Almost all the synapses used for signal transmission in the central nervous system of the human being are chemical synapses. In these, the first neuron secretes atits nerve ending synapse a chemical substance called a neurotransmitter (or often called simply transmitter substance), and this transmitter in turn acts on recep-tor proteins in the membrane of the next neuron to excite the neuron, inhibit it, or modify its sensitivity in some other way. More than 40 important transmitter substances have been discovered thus far. Some of the best known are acetylcholine, norepinephrine, epinephrine, histamine, gamma-aminobutyric acid (GABA), glycine, serotonin, and glutamate.
Electrical synapses, in contrast, are characterized bydirect open fluid channels that conduct electricity from one cell to the next. Most of these consist of small protein tubular structures called gap junctions that allow free movement of ions from the interior of one cell to the interior of the next. Only a few examples of gap junc-tions have been found in the central nervous system. However, it is by way of gap junctions and other similar junctions that action potentials are transmitted from one smooth muscle fiber to the next in visceral smooth muscle and from one cardiac muscle cell to the next in cardiac muscle.
“One-Way” Conduction at Chemical Synapses. Chemicalsynapses have one exceedingly important characteris-tic that makes them highly desirable for transmitting most nervous system signals: they always transmit the signals in one direction: that is, from the neuron that secretes the transmitter substance, called the presynaptic neuron, to the neuron on which the trans-mitter acts, called the postsynaptic neuron. This is the principle of one-way conduction at chemical synapses,and it is quite different from conduction through elec-trical synapses, which often transmit signals in either direction.
Think for a moment about the extreme importance of the one-way conduction mechanism. It allows signals to be directed toward specific goals. Indeed, it is this specific transmission of signals to discrete and highly focused areas both within the nervous system and at the terminals of the peripheral nerves that allows the nervous system to perform its myriad func-tions of sensation, motor control, memory, and many others.
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