Advanced - Cardiac Conduction

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Cardiac conduction in the adult heart begins in the sinoatrial (SA) node which is located at the junction between the SVC and the right atrium. The impulses generated here spread through the atria, initiating contraction. The impulses travel to the atrioventricular (AV) node which acts to slow the transmission of an impulse between the atria and ventricles. After this time lag, impulses travel to the ventricles via the common atrioventricular bundle (bundle of His) to the bundle branches in the IV septum. The branches split and terminate throughout the myocardium in a network of Purkinje fibres. The adult conduction system is shown below.

Diagram of the adult cardiac conduction system

Development of the conduction system

Cardiomyocytes in the caudal heart tube are the first to become electrically active and become the “pacemaker”. The SA node, which develops during the fifth week, initially develops in the sinus venosus and then is incorporated into the RA. The AV node arises slightly superior to the endocardial cushions. Fibres forming the bundle of His develop from fast-conducting ventricular myocardium while the SA and AV nodes are formed from the slow-conducting myocardium of the inflow tract and AV canal. Connective tissue grows in from the epicardium, forming the cardiac skeleton that separates conduction in the atria and ventricles.

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