Sodium channel protein type 8 subunit alpha mediates the voltage-dependent sodium ion permeability of excitable membranes. Adopting opened or closed conformation 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. It is a multi-pass transmembrane protein that is widely expressed in neurons of the central and peripheral nervous system. There are four named isoforms. 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 thought to be the voltage-sensors and are characterised by a series of positively charged amino acids at every third position.