Absolute refractory period is due to:
Inactivated gates of sodium are closed during depolarization and Remain closed until repolarization no action potential can occure untill inactivated sodium gate are opened Ref:Guyton and Hall textbook of medical physiology 12th edition,page number:70
Opening of calcium channels
Opening of potassium channals
Closure of Active gates of sodium channals
Closure of inactive gates of sodium channals
General physiology