The muscle stretch (myotatic) reflex is the stereotyped contraction of a muscle in response to stretch of that muscle. The stretch reflex is a basic reflex that occurs in all muscles and is the primary mechanism for regulating muscle tone. Muscle tone is the tension present in all resting muscle. Tension is controlled by the stretch reflexes.
The best example of a muscle stretch or deep tendon reflex is the knee-jerk reflex. Tapping the patellar ligament stretches the quadriceps muscle and its muscle spindles. Stretch of the spindles activates sensory endings (la afferents), and afferent impulses are transmitted to the cord. Some impulses from stretch receptors carried by la fibers monosynaptically stimulate the alpha motoneurons that supply the quadriceps. This causes contraction of the muscle and a sudden extension of the leg at the knee. Afferent impulses simultaneously inhibit antagonist muscles through interneurons (in this case, hamstrings).
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