Figure 2. Action potential. Depolarization is a graded phenomenon until threshold is reached. At that point, voltage-gated sodium channels are opened and the movement of sodium rapidly depolarizes the neuron. The voltage-gated sodium channels are then rapidly inactivated and voltage gated potassium channels are opened, which drives the cell back toward the potassium equilibrium potential (near -90 mV). Until the cell repolarizes, the voltage-gated sodium channels are inactivated and another action potential cannot be generated (absolute refractory period). Until the voltage gated potassium channels are inactivated it is harder (but not impossible) to generate an action potential (relative refractory period).