Action-potential mechanism. (A) On the initiation of an action potential, the membrane potential moves from the resting potential upward toward the Na⁺ equilibrium potential and then downward toward the K⁺ equilibrium potential. (B) The currents through the Na⁺ and K⁺ channels underlying the action potential. (C) The states of the Na⁺ and K⁺ channels during the action potential.