AbstractThe neuroanatomical organization of the dynamic (bag1) and static (bag2and chain) intrafusal systems was compared by light and electron microscopy of serial sections among 71 poles of muscle spindle in soleus (SOL), extensor digitorum longus (EDL), and lumbrical (LUM) muscles in the rat. Eighty‐four percent of 195 fusi‐motor (γ) axons to the spindles innervated either the dynamic bag1fiber or the static bag2and/or chain fibers. Sixteen percent of the γ axons coin‐nervated the dynamic and static intrafusal fibers. Some of these nonselective axons were branches of efferents that also gave rise to axons selective to either the dynamic or static types of intrafusal fibers in one or more spindles. Thus activation of individual stem γ efferents might not have a purely dynamic or purely static effect on the integrated afferent outflow from spindles of a hind‐limb muscle in the rat. In addition, primary afferents in all muscles had terminations that cross‐innervated the dynamic bag1and static bag2and/or chain intrafusal fibers in individual spindles, an arrangement that may enhance the mixed dynamic/static behavior of afferents when different intrafusal fibers are activated concurrently. Spindles of the slow SOL and fast EDL muscles had similar features, whereas differences were observed in the organization of the proximal (SOL and EDL) and distal (LUM) muscles. Spindles in LUM muscles had fewer static intrafusal fibers, a higher ratio of dynamic to static γ axons, and a higher incidence of skeletofusimotor (β) innervation to intrafusal fibers than spindles in the SOL or EDL muscles. Thus, the relative contribution of dynamic and static systems to muscle afferent outflow may differ among spindles located in different segments of the rat hindlimb. However, the dynamic and static intrafusal systems of spindle were less sharply demarcated in each of the three hindlimb rat muscles than in the cat tenu