"First, as a result of the depolarization, the internal Na⁺ gates snap shut and remain closed for about one-half of a millisecond.

This rapid decrease in Na⁺ permeability causes the membrane potential to begin to return to towards the K⁺ equilibrium potential.

The closing of these internal Na gates is what is responsible for the absolute and relative refractory period.

Until these gates begin to open the membrane will not respond to a depolarization"

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission