This protein 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 sodium ions may pass in accordance with their electrochemical gradient. It is a tetrodotoxin-resistant sodium channel isoform. Also involved, with the contribution of the receptor tyrosine kinase NTRK2, in rapid BDNF-evoked neuronal depolarization (By similarity). The voltage-resistant sodium channel consists of an ion conducting pore forming alpha-subunit regulated by one or more auxiliary subunits SCN1B, SCN2B and SCN3B. Expressed in the dorsal root ganglia (C-fiber neurons), spinal cord, trigeminal ganglia, testis, ovary, uterus and small intestine. Expressed in embryo at 15 dpc onwards. 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. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.9/SCN11A subfamily.