Electrical Conduction and the Heart

The electrical conduction system of the heart, which produces the electrical activity measured by the electrocardiogram, is composed of the sinoatrial (SA) node, the internodal and interatrial conduction tracts, the atrioventricular (AV) junction (consisting of the atrioventricular node and the bundle of HIS), the right and left bundle branches, and the Purkinje fibers.

Conduction system of heart

The sinoatrial node, located at the junction of the superior vena cava and the right atrium, is the area of the heart, which, under normal conditions, depolarizes most rapidly. The specialized tissue in the sinoatrial node consists of cells that generate electrical current automatically and regularly. Thus, the sinoatrial node is often referred to as the pacemaker of the heart. Each electrical impulse travels from the sinoatrial (SA) node through the right atrium via three internodal atrial conduction tracts to the atrioventricular (AV) node, located at the proximal part of the atrioventricular junction.

The interatrial conduction tract, also referred to as Bachmann’s bundle, branches off one of the internodal atrial tracts and extends across the atria, conducting the impulse from the right to left atrium.

Once the wave of depolarization reaches the AV node, there is a delay that allows the atria to contract, thus emptying blood into the ventricles before the ventricles are stimulated to contract.During this time, electrical activity moves very slowly from the atrium into and through the AV node and into the proximal portions of the ventricular conduction system, the bundle of His, and the bundle branches. Depolarization spreads along the ventricular conduction system (the distal bundle branches and Purkinje fibers) first through the septum, then through the apex and into the bulk of the left and right ventricular walls. The muscle cells of the heart form a syncytium, i.e., they are joined in a way that enables electrical activity to move quickly and easily from one cell to the next. The depolarization of each cardiac cell acts as an electrical impulse on adjacent cells,causing them to depolarize. It is this propagation of electrical impulse from cell to cell that produces waves of depolarization that can be measured as an electric current flowing in the direction of depolarization.

As the cells repolarize, another electric current is produced, similar to the first but moving in the opposite direction. The magnitude and direction of the electrical activity occurring during depolarization and repolarization can be detected by electrodes attached to the skin; this information is then amplified and displayed on the ECG as waves and complexes.

Although the heart has four chambers, it can be considered to be having only two, from the electrical point of view. The two atria as one unit and the two ventricles as another electrical unit,separated by a non-conducting fibrous body through which the HIS bundle penetrates forming the only electrical connection between the two sets of contractile masses.