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 (at protein level) in myenteric sensory neurons. Expressed in small sensory neurons of the dorsal root ganglia (C-fiber neurons) and trigeminal ganglia. Expressed in dorsal root ganglia at 17 dpc onwards. Down-regulated after axotomy and up-regulated following hind paw inflammation. Down-regulated in vitro by electrical stimulation and by deprivation of NGF. 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.