The Electrical Activity of Heart

The Atria and the ventricles are the chambers of the heart which contract and relax during each cardiac cycle. The contraction of these chambers is controlled by wave of depolarization which travels through muscular walls of these chambers. The contraction is followed by relaxation of the chambers which is also known as repolarization.

The ECG machine records both of these phases of electrical activity. The atrial depolarization is triggered in SA node of the heart which is also known as sinoatrial node. The special conducting system of heart starts from the SA node which is located in right atrium. The atria and ventricles are separated by fibrous non conducting tissue. The electrical impulse can only travel through specialized conducting tissue from upper to lower chambers. In a normal heart electrical impulse can only travel from atria to ventricles through AV node. The AV node is located in the interatrial septum near the opening of coronary sinus. The depolarizing signal gets slightly delayed in AV node. This gives some time to ventricles to fill with blood pushed by atrial contraction. The depolarizing current further travels through bundle of His. The bundle of His is located in Interventricular septum which further divides into right and left budle branches. Th right and left budle branches supply right and left ventricles respectively.

The first part of ventricular muscle mass to depolarize is interventricular septum and the movement of depolarization is always from left to right bundle branch. The wave of depolarization always spreads from endocardium to epicardium of ventricular wall while repolarization takes place in the opposite direction that is from epicardium to endocardium.