Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 13 - The Peripheral Nervous System and Reflex Activity - Review Questions - Page 522: 23


A flexor action of a muscle is a contraction that decreases the angle between two bones at a joint . This is the response that takes place if a limb suddenly pierced by a sharp object or comes in contact with a very hot one. A flexor reflex that produces such an effect is the withdrawal reflex. Its effect is to remove the injured member from the vicinity of the nociceptive agent. A flexor reflex is ipsilateral, polysynaptic, and protective.

Work Step by Step

A crossed extensor reflex includes both flexor and extensor responses. In the classical case one steps on a sharp object ( tack, needle broken glass) ; nociveptive implsese are transmitted by sesory nervesto the spinal cord . The sensory neurons synapse with interneurons which synapse with motor neurons which inhibit limb extenders but excite limb flexors. The consequence is that the flexor muscles contract, the limb is lifted and withdrawn from the injurious object. Simultaneously, the sensory neurons cross and synapse with other interneurons on the other side of the spinal cord ( contralateral side). From these interneurons impulses go out by motor neurons to inhibit flexors in the opposite (uninjured limb) and stimulate extensors to contract. The contralateral limb is thus stiffened and prepared to support the entire weight of the body being shifted to it.
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