Muscle spindles are sensorimotor organs located within skeletal muscle. Each muscle spindle is made up of three to five specialized muscle cells known as intrafusal cells. These cells, bundled together by a sheath of connective tissue, lay alongside the rest of the skeletal muscle fibers. When intrafusal cells detect a change in muscle length, they reflexively stimulate a muscle contraction to prevent over-stretching and muscle fiber damage. This is known as the stretch reflex, and is most commonly illustrated by the classic knee-tap reflex test performed during physical exams.
Intrafusal cells are similar in structure to standard, or extrafusal, muscle cells. Both are long, roughly cylindrical cells made up predominantly of sarcomeres. They differ in that the center of an intrafusal cell lacks the contractile element myosin at its center, so that contraction only occurs at the ends. The passive central area is wrapped in specialized nerve endings termed annulospiral and flower-spray endings. These nerve endings enter the spindles and branch throughout the intrafusal cells, wrapping each cell in a spiral arrangement, or spreading out on the fiber surface like a spray of flowers.
When the ends of the intrafusal cells contract, or when the muscle belly itself lengthens, it creates a passive stretch in the center of the muscle spindles. The nerve endings detect this stretch, and signal the motor neurons of the adjacent extrafusal muscle fibers to contract. Upon contraction, the muscle fibers shorten, halting the stretch of the muscle spindles. This causes the muscle spindles to cease the contraction signal, relaxing the muscle.
During physical exams, a doctor will test the stretch reflex as an indicator of overall nervous system functioning. The well-known tap on the knee briefly pulls on the patellar tendon, a cord of connective tissue that runs from the quadriceps to the tibia just below the knee. This passively stretches the muscle fibers of the thigh, activating the stretch response from the intrafusal fibers within the quadriceps. Stretching of the fibers stimulates the reflexive contraction of the same muscle, resulting in the little kick associated with the test.
The knee-tap test, or patellar tendon test, acts as a sampling of the nervous system as a whole. By testing the stretch reflex, doctors can assess various neural and muscular systems. This includes the functioning of motor neurons, the inhibitory or excitatory influence from higher nervous system levels, and the efficacy of the muscle spindles, as well as the activation of the muscles themselves. Testing the performance of one reflex assists doctors in finding clues as to the presence of various central or peripheral nervous system disorders.