Sharing of sensory terminals between the dynamic bag 1 and static bag 2 fibers in the rat muscle spindle

Abstract

The nuclear bag 1 intrafusal fiber mediates the dynamic (velocity) sensitivity, whereas the nuclear bag 2 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 cross-terminals were determined by reconstruction of 4 spindles from serial 1-μm 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 cross-terminals of the secondary afferents. In contrast, the dynamic bag 1 and static bag 2 fibers of each spindle shared at least one terminal of the primary afferent. Cross-terminals shared by the dynamic bag 1 and static bag 2 fiber parallel the presence of fusimotor (γ) 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 in 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.