Voltage-gated sodium channels are essential for generation and propagation of action potentials in excitable neurons. Sodium channels and potassium channels account for almost all of the currents in axon membranes. An action potential along the axon opens the Na+ channel allowing free Na+ to enter the fiber and depolarizes the membrane; repolarization is then activated by K+. Na+ channels in the brain are characterized into three subtypes, brain type I, type II and type III, based on function and structure. These subtypes consist of a principal alpha subunit (with variable isoforms) and two auxiliary subunits termed β1 and β2.