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reflex arc[′rē‚fleks ‚ärk]
a group of nerve structures involved in reflex action. The term “reflex arc,” or “nervous arc,” was introduced in 1850 by the British physician and physiologist M. Hall, who was describing the anatomic elements of a reflex.
A reflex arc includes (1) receptors, or nerve endings that respond to stimulation; (2) afferent (centripetal) nerve fibers, or the processes of receptor neurons that transmit impulses from sensory nerve endings to the central nervous system; (3) a nerve center, that is, neurons that sense excitation and transmit it to effector neurons through the appropriate synapses; (4) efferent (centrifugal) nerve fibers that transmit excitation from the central nervous system to the periphery; and (5) an effector organ whose activity changes as a result of a reflex.
The simplest two-neuron, or monosynaptic, reflex arc consists of receptor and effector neurons separated by a synapse. A multineuron, or polysynaptic, reflex arc consists of a receptor neuron, several internuncial neurons, and an effector neuron, all of which are separated by synapses. A reflex arc does not completely reflect the structure of a reflex because of the proven existence of reverse afference, that is, excitation that informs a nerve center about the condition of an effector organ.
V. G. ZILOV