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 IIalpha subunits are more abundant along axons. The beta1 subunit of sodium channel type I is expressed in brain, skeletal and cardiac muscle. In the brain, beta1 and beta2 are highly expressed in Purkinje cells, and beta1 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.