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 voltage-sensitive sodium channel consists of an ion conducting pore forming alpha-subunit regulated by one or more beta-1 (SCN1B), beta-2 (SCN2B), beta-3 (SCN3B) and/or beta-4 (SCN4B). Beta-1 (SCN1B) and beta-3 (SCN3B) are non-covalently associated with alpha, while beta-2 (SCN2B) and beta-4 (SCN4B) are covalently linked by disulfide bonds. Interacts with NEDD4 and NEDD4L (By similarity). Interacts with FGF13 (PubMed:15282281). Interacts with FGF14, GBG3, GBB2 and SCN1B (By similarity). Interacts with the conotoxin GVIIJ (PubMed:24497506). Interacts with CALM1; the interaction modulates the inactivation rate of SCN8A (By similarity). Mainly localizes to the axon initial segment. Isoform 1 is highly expressed in brain, moderately in spinal cord, and at low levels in dorsal root ganglia, nodose ganglia and superior cervical ganglia. Not detected in sciatic nerve and non-neuronal tissues. Isoform 2 is hardly detectable, if at all, in brain, expressed at low levels in spinal cord and at highest levels in dorsal root ganglia. 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. May be ubiquitinated by NEDD4L; which would promote its endocytosis. Phosphorylation at Ser-1495 by PKC in a highly conserved cytoplasmic loop slows inactivation of the sodium channel and reduces peak sodium currents. May be due to competing donor splice site. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.6/SCN8A subfamily.