The study identifies a calcineurin-dependent transcriptional pathway that controls the expression of muscle-specific genes in skeletal muscle fibers. Calcineurin, a calcium-regulated serine/threonine phosphatase, is activated by tonic motor nerve activity, which maintains higher intracellular calcium levels in slow-twitch fibers compared to fast-twitch fibers. This activation leads to the dephosphorylation and nuclear translocation of NFAT proteins, which bind to specific DNA sequences in the promoters of slow-fiber-specific genes, such as myoglobin and troponin I. The interaction between NFAT and other transcription factors, including MEF2, is crucial for the activation of these genes. Inhibition of calcineurin activity with cyclosporin A in intact animals promotes a shift from slow to fast fiber types, suggesting that this pathway is essential for maintaining the specialized characteristics of different muscle fiber types. The findings provide a molecular basis for how motor nerve activity influences the differentiation and specialization of skeletal muscle fibers.The study identifies a calcineurin-dependent transcriptional pathway that controls the expression of muscle-specific genes in skeletal muscle fibers. Calcineurin, a calcium-regulated serine/threonine phosphatase, is activated by tonic motor nerve activity, which maintains higher intracellular calcium levels in slow-twitch fibers compared to fast-twitch fibers. This activation leads to the dephosphorylation and nuclear translocation of NFAT proteins, which bind to specific DNA sequences in the promoters of slow-fiber-specific genes, such as myoglobin and troponin I. The interaction between NFAT and other transcription factors, including MEF2, is crucial for the activation of these genes. Inhibition of calcineurin activity with cyclosporin A in intact animals promotes a shift from slow to fast fiber types, suggesting that this pathway is essential for maintaining the specialized characteristics of different muscle fiber types. The findings provide a molecular basis for how motor nerve activity influences the differentiation and specialization of skeletal muscle fibers.