Mediates the voltage-dependent sodium ion permeability of excitable membranes. Assuming opened or closed conformations in response to the voltage difference across the membrane, the protein forms a sodium-selective channel through which Na(+) ions may pass in accordance with their electrochemical gradient. 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. Available data suggest that activation and inactivation gates are located near the cytoplasmic surface of the membrane. It is hypothesized that residues 802-806, 847-857, 894-910, and 942-955 might, in conjunction with the positively charged residues of S4, act as a voltage sensor involved with the activation gate. Belongs to the sodium channel (TC 1.A.1.10) family.