The membrane of an axon is also packed with gated ion channels that open and close during an action potential At resting potential, the gated channels are closed If a stimulus changes the distribution of charge across the membrane sufficiently the gated sodium channels open Movement of sodium ions across the membrane makes the inside of the cell more positive This reversal of the charge distribution causes the gated sodium channels to close and the gated potassium channels to open As potassium ions move out of the cell the original charge difference is re-established across the membrane closing the gated potassium channels This sequence of events is called an action potential The sodium-potassium pump restores the distribution of ions back to their previous levels at resting potential