Selective reinnervation of fast-twitch and slow-graded muscle fibers in the toad

Abstract

The problem of selectivity during reinnervation of skeletal muscles was investigated in the toad. Nerves to the sartorius (homogeneous fast-twitch) and anterior semitendinosus (heterogeneous) muscles were transected and reunited so that each nerve was allowed to reinnervate either muscle. Experiments were performed 5–7 months after operation. Isometric tensions of the two muscles in response to nerve stimulation were recorded simultaneously. The results show that the low-threshold nerve fibers which normally innervate fast-twitch muscle fibers selectively reinnervated fast-twitch muscle fibers, while the high-threshold nerve fibers which normally innervate slow-graded muscle fibers in the anterior semitendinosus selectively reinnervated the slow-graded muscle fibers. Stimulation of the distal stumps of the sartorius nerves led to axon reflexes in the anterior semitendinosus with fast-twitch and slow-graded components. Motor unit studies confirmed the selective reinnervation of the two types of muscle fibers by the corresponding type of nerve fibers. Some axons reinnervated fast-twitch fibers in both muscles, and stimulation of identified axons led to significantly higher tensions in the original muscle. I suggest that nerve fibers have two types of specificities in their relation to muscle fibers during reinnervation, namely, type specificity and individual specificity, and that matching of nerve fibers with muscle fibers of the appropriate type may be important for the functional recovery of the reinnervated structure.