Pore-forming subunit of a voltage-gated sodium channel complex through which Na(+) ions pass in accordance with their electrochemical gradient (PubMed:2559760, PubMed:16303569, PubMed:28012039). Alternates between resting, activated and inactivated states (PubMed:28012039). Required for normal muscle fiber excitability, normal muscle contraction and relaxation cycles, and constant muscle strength in the presence of fluctuating K(+) levels (By similarity). Channel activity is regulated by the ancillary beta subunit SCN1B (PubMed:28012039). SCN1B strongly enhances the presence of the pore-forming alpha subunit at the cell surface (By similarity). Interaction with SCN1B is required for rapid channel inactivation and rapid recovery after inactivation, and prevents decrease of channel activity in response to repetitive, high-frequency depolarizations (PubMed:28012039). Potently inhibited by tetrodotoxin and saxitoxin (PubMed:16303569, PubMed:23077250). Component of a voltage-sensitive sodium channel complex that consists of a pore-forming alpha subunit and one or more regulatory beta subunits. Interacts with SCN1B (PubMed:28012039). Heterooligomer with SCN2B or SCN4B; disulfide-linked (By similarity). Interacts with the PDZ domain of the syntrophins SNTA1, SNTB1 and SNTB2 (By similarity). Interacts with the conotoxin GVIIJ (PubMed:24497506). Detected in skeletal muscle. The sequence contains 4 internal repeats, each with 5 hydrophobic segments (S1, S2, S3, S5, S6) and one positively charged segment (S4). Segments S4 are probably the voltage-sensors and are characterized by a series of positively charged amino acids at every third position. Phosphorylation at Ser-1321 by PKC in a highly conserved cytoplasmic loop slows inactivation of the sodium channel and reduces peak sodium currents. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.4/SCN4A subfamily.