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Simplified diagram of a muscle spindle.
muscle spindles specialized sensory structures within skeletal muscles, consisting of small intrafusal muscle fibres which do not contribute to load-bearing or power generation, but participate in control of the working (extrafusal) fibres. The intrafusal fibres are innervated towards their ends by gamma motor neurons, but are also invested more centrally by both primary and secondary sensory endings; these signal changes in length, and also rates of change in length of the muscle, to the central nervous system. The gamma motor neurons control the sensitivity of the sensory endings to stretching. They also compensate for the shortening of muscle spindles during extrafusal contraction by causing additional intrafusal contraction; this ensures that sensory feedback continues during muscle contractions. The overall system mediates both the stretch reflex and, in voluntary movement, appropriate adjustment of force-generation to load, provided the load is not varying too rapidly. See also gamma motor system, tendon jerk reflex.