In vertebrates, the synapses between motor neurons and muscle cells are always excitatory; that is, motor end plates always re spond to acetylcholine by depolarizing the postsynaptic membrane. Synapses between neurons, however, are not always excitatory.
Recall that a neuron may have many dendrites. Axon terminals from many other neurons may form synapses with those dendrites and with the cell body. The axon terminals of different presynaptic neurons may store and release different neurotransmitters, and the plasma membrane of the den-drites and cell body of a postsynaptic neuron may have receptors for a variety of neurotransmitters. Thus, at any one time, a postsynaptic neuron may receive a variety of different chemical messages. If the postsynaptic neuron's response to a neurotransmitter is depolarization, as at the neuromus-cular junction, the synapse is excitatory; if its response is hy-perpolarization, the synapse is inhibitory.
How do inhibitory synapses work? In vertebrates, the two most common inhibitory neurotransmitters are gamma-aminobutyric acid (GABA) and glycine. The postsynaptic membranes at inhibitory synapses that bind these neuro-transmitters have receptors that are chemically gated chloride channels. When these channels bind their neurotrans-mitter and open, they hyperpolarize the postsynaptic membrane. Thus the release of neurotransmitter at an inhibitory synapse makes the postsynaptic cell less likely to fire an action potential.
Neurotransmitters that depolarize the postsynaptic membrane are excitatory; they bring about an excitatory postsynaptic potential (EPSP). Neurotransmitters that hyperpolar-ize the postsynaptic membrane are inhibitory; they bring about an inhibitory postsynap-tic potential (IPSP).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.