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the contraction of a muscle or group of muscles in response to stimulation of the proprioceptors of the tendon of one of these muscles. The stimulation excites the muscle spindles, which transmit the excitation through quick-conducting nerve fibers to the motoneurons that innervate the given muscle. The reflex arc of a tendon reflex ends within a single segment of the spinal cord, and any excitation of the spinal cord affects the magnitude of the reflex. Consequently, disturbances of the central nervous system are reflected in changes in some tendon reflexes. An example is the patellar reflex—the extension of the leg at the knee when the patellar tendon is tapped.