Neural cell adhesion molecule (N-CAM) is elevated in adult avian slow muscle fibers with multiple terminals

Many adult avian muscles contain two types of muscle fiber: those that receive innervation at single focal terminals and those with multiple terminals. The muscles of the syrinx, the vocal organ of birds, are such mixed muscles. To study this heterogeneity of fiber type and innervation, we combined immunocytochemistry to classify muscle fibers with techniques to visualize neuromuscular junctions. One monoclonal antibody, S58, directed against a slow class of myosin, labels only fibers that have multiple terminals. We also examined the distribution of immunoreactivity for neural cell adhesion molecule (N-CAM), which has been suggested to play a role in innervation of muscle and formation of neuromuscular junctions. S58-positive fibers have elevated N-CAM staining, indicating that multiple innervation of a fiber is correlated with the fiber's expression of high levels of N-CAM immunoreactivity. Most, and perhaps all, fibers that have multiple terminals also contain abundant N-CAM immunoreactivity. This suggests that N-CAM may play a role in the maintenance of multiterminal innervation in adult innervated muscle.