Muscle spindles are nerve endings located within muscle tissue that monitor the length of a muscle as it contracts and elicit a reflex contraction (stretch or myotactic reflex) when the muscle is stretched beyond a certain limit. The gamma loop system consists of gamma motor neurons that innervate the small, intrafusal muscles within the muscle spindle organ and serves to adjust the tension on the spindle structure, thereby regulating its sensitivity. The Golgi tendon organ also monitors muscle length, but it elicits reflexes (the reverse myotactic reflex) that are opposite those of the muscle spindle.
Basic spinal motor programs are built in part from combinations of myotactic and reverse myotactic reflexes along with reciprocal reflexes resulting from linkages between synergistic and antagonistic motor units.
Cortical and brain stem regions provide motor instructions to alpha and gamma motor neurons in order to achieve more complex volitional and nonvolitional movements.
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