A muscle spindle, or neuromuscular spindle, consists of 5 or 10-thin striated muscle fibers (intrafusal muscle fibers) which are surrounded by a fluid-filled connective-tissue capsule. The fibers of the up to 10mm long spindles are arranged in parallel with the other fibers of the muscle (extrafusal muscle fibers) and attach either to the tendons of the muscle or to the connective-tissue poles of the capsule. As the intrafusal fibers lie in the same longitudinal orientation as the extrafusal fibers, stretching and shortening of the muscle affects them in the same way. The number of spindles within individual muscles is quite variable. Muscles participating in delicate and precise movements (finger muscles) possess a large number of spindles, whereas muscles for simple movements (trunk muscles) contain far fewer spindles.
The central equatorial part of an intrafusal fiber contains several cell nuclei but no myofibrils; this part of the spindle is not contractile. Only the two segments that contain striatedmyofibrils are contractile. A thick sensory nerve fiber terminates at the central part; its terminal branches wind around the muscle fibers like spirals and form the annulospiral endings. A delicate sensory fiber attaches in an umbelliform fashion (flower-spray ending) at one side, or at both sides, of the annulospiral ending. Both contractile polar segments are innervated by thin fusimotor gamma fibers. Their small motor end plates have only poorly developed subneural clefts; like the extrafusal muscle fibers, they are epilemmal. The sensory annulospiral endings lie below the basement membrane of the muscle fiber and, hence, are hypolemmal. The gamma-fibers stem from small motor neurons in the anterior horn (gamma-motoneurons); impulses from these neurons cause contraction of the polar segments of the intrafusal fiber. This results in stretching of the equatorial segment and does not only stimulate the annulospiral ending but alters the sensitivity of the spindle as well.
The muscle spindle is a stretch receptor, which is stimulated when the muscle is stretched but becomes inactive when the muscle is contracted. Upon stretching the muscle the impulse frequency increases with the change in muscle length. This way, the spindles transmit information on the prevailing length of the muscle. The impulses are transmitted not only via the spinocerebellar tracts to the cerebellum but also via reflex collaterals directly to the large anterior horn cells (alfa-motoneurons). Stimulation of the latter neurons during sudden stretching results in immediate muscle contraction.