Mediates the voltage-dependent sodium ion permeability of excitable membranes. Assuming opened or closed conformations in response to the voltage difference across the membrane, forms a sodium-selective channel through which Na(+) ions may pass in accordance with their electrochemical gradient (By similarity). May contribute to the regulation of serotonin/5-hydroxytryptamine release by enterochromaffin cells (By similarity). In pancreatic endocrine cells, required for both glucagon and glucose-induced insulin secretion (By similarity). Heterooligomer of a large alpha subunit and 2-3 smaller beta subunits. Heterooligomer with SCN2B or SCN4B; disulfide-linked. Interacts with NEDD4L (By similarity). Interacts with the conotoxin GVIIJ (PubMed:24497506). Interacts with the spider beta/delta-theraphotoxin-Pre1a (PubMed:25784299, PubMed:28428547). Interacts with the spider RTX-VII toxin (AC P0DL75) (PubMed:25784299). 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-1452 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. Nav1.3/SCN3A subfamily.