MUSCLE-FIBER TYPE CORRELATES WITH INNERVATION TOPOGRAPHY IN THE RAT SERRATUS ANTERIOR MUSCLE

Previous studies have reported that motoneurons from the sixth spinal nerve (C6) innervate the majority of muscle fibers in the rat serratus anterior (SA) muscle. The seventh spinal nerve (C7) innervates a limi ted number of SA fibers, increasing caudally. This topographic map is partially reestablished following denervation. in the present study, m uscle fibers of the SA were stained with monoclonal antibodies for the muscle-specific fast myosin heavy chain (F-MHC) and slow myosin heavy chain (S-MHC) proteins. We found that the majority of fibers in the S A muscle stained for F-MHC antibody, and the percentage of muscle fibe rs staining for S-MHC antibody increased caudally. When newborn SA mus cles were denervated and then reinnervated by the entire long thoracic (LT) nerve or only the C6 branch to the LT nerve, the reinnervated mu scle had the normal proportion of muscle fibers expressing S-MHC prote in. However, if the LT nerve was crushed and only C7 motoneurons allow ed to reinnervate the SA muscle, a greater percentage of muscle fibers stained for S-MHC antibody than normal. We conclude that there is a c orrelation between muscle fiber type and innervation topography in the SA muscle of the rat. (C) 1996 John Wiley & Sons, Inc.