Nerve cells, or neurons, are specialized to conduct impulses which affect specific cells over long distances.
Nerves are a collection of nerve cell axons which carries information away from the cell body. Calcium and potassium are very important in conducting the impulses down the axon. Axons can be extremely long and may be covered by a myelin sheath. Nerve cells receive impulses from the environment through dendrites. The cell can have many dendrites which connect to the cell body. The cell body contains a nucleus and cytoplasm. If an impulse is strong enough it will be passed on to the axon.
Myelin is produced by a Schwann cell, is high in fat, and appears white. Myelin may be wrapped around the axon, acting as an insulator over a wire, thus it affects the speed and efficiency with which impulses are sent. White matter is composed of axons covered by myelin; it gives the region a white appearance.
The axon ends at a synapse, which is a connection between nerve cells, or the nerve cell and other tissue. The nerve cell is not in physical contact with the other surface but stimulates the other surface by transmitting chemicals or electricity across the synapse. If the impulse is strong enough, the next nerve cell will pass on the impulse, or the tissue will respond. There are three types of nerve cells: sensory neurons, motor neurons, and interneurons.
Sensory neurons transmit sensory information to the brain.
Motor neurons transmit information away from the brain or spinal cord to the muscles or other tissue.
Interneurons connect motor and sensory neurons and are found exclusively in the central nervous system. These neurons play an important role in reflex arcs.
Reflex arcs are reflexes initiated entirely within the spinal cord and not the brain. The sensory input comes into the spine and synapse to an interneuron, which synapses with a motor neuron. This creates an almost instantaneous reaction. An example of this arc is when something hot is touched and one automatically pulls away from the hot object.
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Eckert, R. & D. Randall. 1983. Animal Physiology, second edition W. H. Freeman and Company, San Francisco. 830 pp.
Lockhart, R.D., G.F. Hamilton, et.al. 1974. Anatomy of The Human Body. Faber and Faber Limited. London. 697 pp.
Rahlman, J. & J.L. Smith. 1981. Ucla Kinesiology 14 Human Neuromuscular Anatomy. Academic Publishing Service. L A. 490 pp.
Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.
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