When a sufficiently large stimulus reaches the membrane, an action potential results. Channels open and Na+ floods in, following its concentration gradient. The membrane is now depolarized, which causes the opening of K+ channels and potassium diffuses out. The membrane is now depolarized, but sodium and potassium have switched positions. During the short time the sodium-potassium pump requires to restore the ions to their original positions, the neuron is refractory; it cannot fire again.