The refractory periods are due to the inactivation property of voltage-gated sodium channels and the lag of potassium channels in closing. Voltage-gated sodium channels have two gating mechanisms, the activation mechanism that opens the channel with depolarization and the inactivation mechanism that closes the channel with repolarization. While the channel is in the inactive state, it will not open in response to depolarization. The period when the majority of sodium channels remain in the inactive state is the absolute refractory period. After this period, there are enough voltage-activated sodium channels in the closed (active) state to respond to depolarization. However, voltage-gated potassium channels that opened in response to repolarization do not close as quickly as voltage-gated sodium channels; to return to the active closed state. During this time, the extra potassium conductance means that the membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire. In other words, because the membrane potential inside the axon becomes increasingly negative relative to the outside of the membrane, a stronger stimulus will be required to reach the threshold voltage, and thus, initiate another action potential. This period is the relative refractory period.