sharing of sensory terminals between the dynamic bag1 and static bag2 fibers in the rat muscle spindle the nuclear bag1 intrafusal fiber mediates the dynamic velocity sensitivity whereas the nuclear bag2 and nuclear chain fibers mediate the static length sensitivity of muscle spindles to stretch the pattern of branching of primary and secondary afferents the distribution of their terminals to the 3 types of intrafusal fibers and the incidence of sensory crossterminals were determined by reconstruction of 4 spindles from serial 1micron and ultrathin transverse sections of rat extensor digitorum longus muscles a single primary afferent supplied each spindle and secondary afferents innervated intrafusal fibers in 3 spindles only static intrafusal fibers shared crossterminals of the secondary afferents in contrast the dynamic bag1 and static bag2 fibers of each spindle shared at least one terminal of the primary afferent crossterminals shared by the dynamic bag1 and static bag2 fiber parallel the presence of fusimotor gamma axons which coinnervate these types of intrafusal fibers in muscle spindles of rats consequently the greater degree of overlap of elements comprising the dynamic and static systems of spindles of the rat relative to that of the cat reduces the probability of generating a purely dynamic or purely static response to an applied stretch