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a specialized sensory nerve terminal that is located in the musculoskeletal system, which includes skeletal muscles, tendons, and ligaments. Proprioceptors react to the contraction and tension or the relaxation and stretching of muscles. They belong to the group of mechanoreceptors.
In addition to free nerve terminals, proprioceptors include the following: muscle spindles, which consist of several thin and intrafusal muscle fibers entwined by spiral-like nerve terminals; Golgi’s corpuscles, which are found in tendons; and Pacini’s corpuscles, which are concentrated in the fascia, tendons, and ligaments. The extent to which a spindle contracts is regulated by the gamma motor system (gamma efferent nerve fibers)—processes of special motor neurons of the spinal cord. Muscle spindles are excited by a lengthening of fibers, while Golgi’s corpuscles, or tendon organs, are excited by the contraction of fibers. Impulses from spindles that reach the central nervous system facilitate the contraction of a particular muscle and inhibit the contraction of its antagonist. Impulses from tendon proprioceptors exert the opposite effect. Signals on the movement of body parts originate in proprioceptors; these signals are the main regulators of information that arrives from other sensory organs.