What causes the refractory period in sodium channels after an action potential has been initiated? The sodium channel CANNOT be restimulated during this period.

Explanation:

This is because once a Voltage Gated Sodium Channels closes up after depolarization, they can not be reopened.The  transmission  of action potential is such a way that  they are propagated one after the other in a forward direction, and never backwards. Thus once a sodium channel depolarizes,it closes up,  to under go a shot rest.No matter the strength of the next stimulus it will not open. Then the channel ahead of it opens to depolarise  to conduct action  potential, this again after shut opening closes up for the channel ahead to open.  This short period of rest when the sodium channels closes up  is called refractive period.  However after sometime of rest the rested sodium channels open, from the inactivated stage to conduct action potential.