Voltage-gated sodium channels are selective ion channels that regulate the permeability of sodium ions in excitable cells. During the propagation of an action potential, sodium channels allow an influx of sodium ions, which rapidly depolarize the cell. The (alpha) subunits of sodium channels type I and III are predominantly expressed in neuronal cell bodies and proximal processes, while type II(alpha) subunits are more abundant along axons. The (beta)1 subunit of sodium channel type I is expressed in brain, skeletal and cardiac muscle. In the brain, (beta)1 and (beta)2 are highly expressed in Purkinje cells, and (beta)1 is also expressed in the pyramidal cells of the deep cerebellar nuclei. Impaired voltage-gated sodium channels lead to a number of diseases including myotonia.