Sodium ion channels provide an integral service through the body, as they transmit depolarizing impulses at the cellular and intracellular level. This allows sodium ions to coordinate much more intensive processes such as movement and cognition. Sodium ion channels consist of various subunits, however, only the principle subunit is required for function. These sodium ion channels consist of four internally homologous domains, each of which containing six transmembrane segments and resembling a single subunit of a voltage-dependent potassium ion channel. The four domains fold together, forming a central pore. That central pore of the sodium ions dictates the selectivity of the channel: both ionic radius and ionic charge are key in channel selectivity.