Nepal Medical College

Depolarization starts from inner myocardium because purkinje fibers are in inner myocardium and depolarization spreads from inner to outer myocardium Repolarization starts from outer myocardium because outer myocardium has better blood supply. Inner capillaries are compressed during  depolarization (systole). Depolarization is fast and has Q- ventricular septal R- major ventricular and S- basal ventricular waves separetly and repolarization is slow process and represented by single T wave (as it is slow when basal part is still repolarizing major ventricular part start to repolarize and when major ventricular part and basal part is still repolarizing septal part start to repolarize , thus they overlap and is represented by single wave)

Why Av node is slow but Purkinje is fast? because AV node has more number of small cells while purkinje has less number of large cells so depolarization has to pass many membranes in AV node than Purkinje fibers AV node has less gap junction while purkinje has high number of gap junction AV node depolarization is by Ca2+ ions  but purkinje by Na+ ions which is fast AV node cells have less diameter which has high resistance AV node RMP is -60mV while purkinje is -90mV so cations move faster in purkinje fibers Source: Najeeb lecture videos