Activation of G-proteins and formation of second messengers require several seconds and cannot account for the rapid transfer of information seen at neural synapses or the neuromuscular junction (see Chapter 6). Rapid transfer of information leading to electrical changes in membranes (depolarization or hyperpolar-ization) is mediated by a special group of receptors found in nerves and muscles. These receptors are bifunctional. They have both recognition and effector domains and do not require the intercession of transducer molecules. A good example is the nicotinic receptor for the neurotransmitter acetylcholine. This receptor is a large complex that spans the thickness of the membrane. It is also an ion channel comprised of five subunits, the products of four genes. When a molecule of acetylcho-line binds to each of the two a subunits, an allosteric shift in configuration opens the channel for about a millisecond to allow passage of both sodium and potassium ions. Initially, influx of sodium exceeds the efflux of potassium, so that the membrane depolarizes rapidly, causing the muscle to contract. Other examples of transmitter-gated ion channels are the receptor channels in the central nervous system that open to admit sodium when stimulated by glutamate or serotonin and those that open to admit chloride when stimulated by glycine or y-aminobutyric acid (GABA).
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