Webster, Douglas B. Department of Otorhinolaryngology and Biocommunication, Louisiana State University Medical Center, Baton Rouge, Louisiana.
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A nerve cell; the functional unit of the nervous system. A neuron is a specialized, basic nerve cell that transmits and receives information, in the form of electrical impulses, throughout the body. It is the fundamental component of the nervous system. Structurally, the neuron is made up of a cell body (soma) and one or more long processes—a single axon and dendrites (see illustration). The cell body contains the nucleus and usual cytoplasmic organelles with an exceptionally large amount of rough endoplasmic reticulum, called the Nissl substance (or Nissl body) in the neuron. The longest cell process is the axon, which is capable of transmitting propagated nerve impulses. There may be none, one, or many dendrites composing part of a neuron. If there is no dendrite, it is a unipolar neuron; with one dendrite, it is a bipolar neuron; if there is more than one dendrite, it is a multipolar neuron. The dendrites are shorter and more branched than the axon. Dorsal-root spinal ganglia (groups of nerve cell bodies) and most cranial nerve ganglia have unusual pseudounipolar neurons. In pseudounipolar neurons, which are always sensory, a single process leaves the soma and then bifurcates, sending a long peripheral process to skin, muscle, or viscera and sending a central process into the spinal cord or brain. Both processes can conduct nerve impulses. In most neurons, only the axon propagates nerve impulses; the dendrites and somas are also irritable, but they do not propagate nerve impulses. See also: Autonomic nervous system; Brain; Central nervous system; Endoplasmic reticulum; Ganglion; Nerve; Nervous system (invertebrate); Nervous system (vertebrate); Neurobiology; Neuronal replacement therapy; Spinal cord
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