absolute refractory period and the relative refractory period

Action potentials have both absolute and relative refractory periods. During the absolute refractory period, the inactivation gate is closed and will not be reset until the membrane has repolarized sufficiently. During the relative refractory period, many, but not all, Na+ channels have been reset. Additionally, the membrane is typically hyperpolarized below the resting membrane potential. This means it would take a stronger than normal stimulus to bring the potential to threshold and recruit enough Na+ channels to generate a new action potential. When action potentials are traveling along, the depolarization spreads in both directions (electrotonically), but can only activate channels that are not in the absolute refractory period (i.e., in front of the AP.) Channels in the relative refractory period can be opened, but by the time the current spreads to them, it has decayed enough that it is no longer able to be the stronger than normal stimulus required to generate a new AP. Thus, the AP can move only in the forward direction. The frequency of action potentials is also limited by the absolute refractory period. It is impossible for another action potential to be started until the channels have reset from the first one; thus, there is a limit to the frequency (number of action potentials per unit time).