Flat epitheliod cells form lamellae around the axon terminal. The capsule is continuous with the perimysium of muscle. Within the fluid-filled capsule are intrafusal muscle fibers. The central region of these fibers is filled with nuclei and is noncontractile. This region is contacted by afferent nerve endings.
Polar regions have myofilaments and are contacted by efferent nerve fibers.
Stretching of the muscle activates afferent fibers which make monosynaptic contact with motor neurons that supply the muscle. Gamma efferents increase the sensitivity of the spindle. The capsule encloses small areas of muscle-tendon junctions. Free nerve endings intertwine among collagen bundles. Afferent fibers are activated by tension on muscle-tendon created by strong muscle contraction and inhibits the motor neurons that supply the muscle.