Atrioventricular Node - Function

Function

Contraction of myocytes (heart muscle cells) requires depolarization and repolarization of their cell membranes. Movement of ions across cell membranes causes these events. The cardiac conduction system (and AV node part of it) coordinates myocyte mechanical activity. A wave of excitation spreads out from the sinoatrial node through the atria along specialized conduction channels. This activates the AV node. The atrioventricular node delays impulses by approximately 0.12s. This delay in the cardiac pulse is extremely important: It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract. This also protects the ventricles from excessively fast rate response to atrial arrhythmias (see below).

The AV node receives two inputs from the atria: posteriorly, via the crista terminalis, and anteriorly, via the interatrial septum.

AV conduction during normal cardiac rhythm occurs through two different pathways:

• the first “pathway” has a slow conduction velocity but shorter refractory period
• the second “pathway” has a faster conduction velocity but longer refractory period.

An important property that is unique to the AV node is decremental conduction, in which the more frequently the node is stimulated the slower it conducts. This is the property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter.

The AV node's normal intrinsic firing rate without stimulation (such as that from the SA node) is 40-60 times/minute.