The Atrioventricular Node (AV Node).

The atrioventrcular node of the heart is made of specialized cells which conduct impulses coming from the SA node to the wall of the ventricles. It is situated between the atria and the ventricles in the left atrium. To be precise, it is situated in the posteroinferior region of the interatrial septum near the opening of the coronary sinus. It lies in the center of the Koch’s triangle formed by the septal leaflet of the tricuspid valve, the coronary sinus and interatrial septum.

Function of The Atrioventricular Node.

Movements of ions across the cell membranes of the myocytes (heart muscle cells) result in depolarization and repolarization of the cell membranes which cause contraction of the myocytes. The cardiac conduction system brings about the coordination of this cardiomyocyte activity.

The electric impulse which originates in the sinoatrial node spreads across the atria and travels to the atrioventricular node via the internodal pathways.

The AV node is also capable of generating its own impulses but does so at a slower rate than the SA node. Its intrinsic rate is 40 to 60 beats per minute. The conduction of the impulse from the AV node to its distal part which consist of the bundle of His and then to both the bundle branches and the Purkinje fibers and then to the ventricular fibers cause both the ventricles to contract in unison.

Picture showing AV node.

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Conduction Pathway in the Ventricles.

From the AV node, after a short delay, the impulse has to travel to ventricular myocardium to make the ventricles contract. It does so by traveling through the bundle of His to the left and right bundle branches and then to the Purkinje fibers and to the endocardium situated at the apex of the heart, from where it enters the ventricular myocardium. The ventricular contraction therefore begins at the apex of the ventricular myocardium and sweeps upwards pumping blood into the aorta to the left and the pulmonary artery to the right.

 About 400 million myocardial cells of the ventricles contract in less than one third of a second. There is a short delay before the AV node conducts the impulse downwards to the ventricular myocardium. This delay is to allow for ventricular filling before the ventricles contract.

Conduction Delay.

This is a very brief delay of 0.12 seconds in the cardiac cycle before the AV node transmits this signal to the bundle of His. This conduction delay is necessary as otherwise the atria and the ventricles would contract at the same time. This delay ensures that the atria have ejected the blood and filled the ventricles with blood before the ventricles contract. This delay is seen as the PR segment on ECG. This is called ventricular conduction delay.

Decremental Conduction.

This is an important property of the AV node by which the more rapidly it is stimulated, the more slower it conducts. This helps to prevent rapid contraction of the ventricles in case of atrial fibrillation and atrial flutter when the atria beat at a rapid and irregular rate. Know here that ventricular fibrillation is a life threatening condition and the importance of the decremantal conduction property of the AV node cannot be overstressed.

AV Node Ablation.

AV node ablation is an infrequently used procedure whereby the AV node is modifies to restore normal rhythmic contraction of the heart. In this procedure the AV node is cauterized or freezed using a catheter  to block or alter the electric conduction from the SA node. This is usually indicated in atrial fibrillation or flutter to prevent the higher frequency of atrial contractions from being transmitted to the ventricles. A typical AV node ablation also involves implanting an artificial and permanent pacemaker which helps to maintain regular heart rhythm.

Blood Supply of the AV Node.

Blood supply to the AV node can vary.

• It is usually from the posterior interventricular artery, which is the branch of the right coronary artery.
• In other individuals, the supply is still from the posterior interventricular artery but which branches out from the left coronary artery.

Nerve Supply of AV Node.

The AV node gets its nerves from the sympathetic and the parasympathetic nervous system which belong to the autonomic nervous system which is not under voluntary control and which regulates the organ function and homeostasis.

Related posts:

1. SA Node – Natural Heart Pacemaker.