A muscle spindle, or neuromuscular spindle, consists of 5 – 10 thin striated muscle fibers (intrafusal muscle fibers) (A1) which are surrounded by a fluid-filled connective-tissue capsule (A2). The fibers of the up to 10 mm 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 longi-tudinal orientation as the extrafusal fibers, stretching and shortening of the muscle af-fects 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 (A3) of an intra-fusal fiber contains several cell nuclei but no myofibrils; this part of the spindle is not contractile. Only the two segments (A4) that contain striated myofibrils are contractile. A thick sensory nerve fiber (A5) terminates at the central part; its terminal branches wind around the muscle fibers like spirals and form the annulospiral endings (AC6; B). A delicate sensory fiber (A7) attaches in an umbelliform fashion (flower-spray ending) (A8, D) at one side, or at both sides, of the annulospiral ending.
Both contractile polar segments are inner-vated by thin fusimotor fibers (γ-fibers) (A9). Their small motor end plates have only poorly developed subneural clefts; like the extrafusal muscle fibers, they are epilem-mal. The sensory annulospiral endings liebelow the basement membrane of the muscle fiber (C10) and, hence, are hypolem-mal. Theγ-fibers stem from small motorneurons in the anterior horn (γ-mo-toneurons); 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 im-pulses are transmitted not only via the spinocerebellar tracts to the cerebellum but also via reflex collaterals directly to the large anterior horn cells (α-motoneurons). Stimulation of the latter neurons during sudden stretching results in immediate muscle contraction (stretch reflex).
The muscle spindle contains two different types of intrafusal fibers: the nuclear chainfibers (EF11) and the nuclear bag fibers (EF12).Both types of fiber are innervated by an-nulospiral endings. Flower-spray endings are predominantly found at nuclear chain fibers. The thicker nuclear bag fibers re-spond to the ongoing stretching of the muscle, whereas the continuous state of muscle stretching is registered by the thin-ner nuclear chain fibers. Thus, muscle spindles transmit to the cerebellum not only information on the length of the muscle but also on the speed of muscle stretching.
Apart from tendon organs and muscle spindles, there are sensory end organs in joint capsules and ligaments (tension recep-tors) that constantly send information onmovement and posture of trunk and limbs to the cerebellum (anterior and posterior spinocerebellar tracts).