The Reflex Arc and Reflexes


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A reflex arc represents the simplest type of nerve pathway found in the nervous system. This pathway begins with a receptor at the end of a sensory nerve fiber. The sensory fiber leads into the central nervous system and may communicate with one or more interneurons. Some of these interneurons, in turn, communicate with motor neurons, whose fibers lead outward to effectors.

Thus, when a sensory receptor is stimulated by some kind of change occurring inside or outside the body, nerve impulses may pass through a reflex arc, and, as a result, effectors may respond. Such an automatic, subconscious response is called a reflex.

Most reflexes demonstrated in this lab are stretch reflexes. When a tendon is stretched by a tap over its tendon, stretch receptors called muscle spindles are stretched within the muscle, which initiates an impulse over a reflex arc. The stretched muscle responds by contracting to resist or reverse further stretching. These stretch reflexes are important to maintain proper posture, balance, and movements. Observations of many of these reflexes in clinical tests on patients may indicate damage to a level of the spinal cord or peripheral nerves of the particular reflex arc.

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