These research projects examine the manner in which nerves influence the selective gene expression in muscles to produce fiber differentiation. One project is designed to clarify the nature of the signals emitted by the nerve. To ascertain the ability of the nerves to control the biochemical and contractile properties of muscle fibers by trophic influences other than the electrical excitatory activity, we are testing the muscles of a chronic cat preparation in which the motoneurones to hindlimb muscles are surgically deprived of their inputs, and thus remain silent, though intact. We are assessing the potency of these electrically silent nerves to effect the usual chemical and physiologcal transformations of fast and slow muscles after a nerve cross. In another project we are temporally mapping and correlating the conversions of several biochemical and contractile characteristics of fast and slow muscles after cross-reinnervation in rats. Finally, through studies of isoactin in nerve-crossed rats, we are studying the origin of the altered types of fibers appearing in the formerly fast and slow muscles. We are finding out whether the fibers are new ones developing from satellite cells, or rather pre-existing fibers which have changed their characteristics.