What is the refractory period of a nerve action potential?

The refractory period is composed of two parts. The first part is the absolute refractory period in which another action potential cannot be stimulated in the nerve. In the relative refractory period that follows an action potential can occur but a greater depolarising stimulus than usual is required. The reason for the refractory period is due to the time and voltage characteristics of the Na+ and K+ channels that govern the membrane potential. The absolute refractory period is due to the inactivation of sodium channels where sodium ions cannot enter the channel to depolarise the membrane and occurs after the initiation of action potential spike. During the relative refractory period, sodium channels return to a state where they can conduct sodium but the potassium channels are delayed in closing leading to hyperpolarisation. Therefore more depolarising stimulus is necessary in the relative refractory period.