8.10. Electrical Conduction in the Heart

Overview

Stage 1
The sinoatrial node (sometimes spelt ‘sinuatrial’ and abbreviated to ‘SA node’) is located close to the crista terminalis of the right atrium. It generates the action potential that will allow the heart to beat. The heart’s independence in generating its own action potentials means it is described as ‘myogenic’. The action potential travels via internodal pathways across the right and left atria until it reaches the next node…The action potential causes the atria to contract and empty their blood into the ventricles.

Stage 2
The next node is the atrioventricular node (abbreviated to AV node) and it is located in the lower part of the atrial septum. There is a slight delay in the conduction of the action potential across the AV node which allows time for the atria to fully empty before the ventricles are stimulated to contract. The AV node then conducts the action potential to…

Stage 3
The atrioventricular bundle (abbreviated to AV bundle, also called the bundle of His), which continues inferiorly through the membranous part of the interventricular septum. When it reaches the end of the membranous part and the beginning of the muscular part, the AV bundle divides into two branches, namely a left bundle branch and a right bundle branch for the respective ventricles…Note that at this stage, the interventricular septum is acting as an insulator to the action potential. This insulation allows the action potential to reach the apex of the heart and begin muscular contractions there, forcing blood up and out of the ventricles.*

Stage 4
Each bundle passes to the apex of the heart and then becomes continuous with the Purkinje plexus of that side (Purkinje plexus is made up of Purkinje nerve fibres), which travels superiorly through the muscular wall of the ventricle. The Purkinje fibres ensure the cardiac myocytes in the ventricular walls contract in time.

*The ventricular septum has to act as an insulator - if it didn't, the electrical conduction of the ventricle would begin superiorly and be directed inferiorly, meaning blood would be pushed towards the apex of the heart. This would be totally counter-productive, as we want the blood to be ejected away from the apex, in a superior direction, through the ascending aorta and pulmonary trunk.

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