AVN (anatomy)

The atrioventricular node is a specialised collection of conducting cells which provide the only link between the atria and the ventricles for the passage of a wave of depolarisation to trigger contraction.

It is oval and about 0.6 by 0.3cm in size. It is within the subendocardial layer of the heart wall of the interatrial septum. In relation to the right atrium, its position is within the triangle of Koch. In relation to the left atrium, its position is next to the base of the mitral valve annulus. Anteriorly and inferiorly, the node is continuous with the atrioventricular bundle as it runs posterior to the central fibrous body.

The AVN is thought to be depolarised either by a conduction wave spreading from adjoining atrial contractile myocytes or via specialised conducting pathways - the internodal tracts. The cells of the node are responsible for the slowing of conduction between atria and ventricles. This functions to permit sequential atrial then ventricular contraction. The node is susceptible to neural modulation: sympathetic fibres increase the speed of conduction, parasympathetic fibres have the opposite effect.

Histologically, the key elements of the AVN are:

• Purkinje cells:
  • sited on the outside of the node
  • conduct relatively rapidly to the inner cell groups
• stellate nodal myocytes; a smaller population than in the SAN
• transitional myocytes:
  • conduct slowly
  • largest component of the node
  • irregularly arranged superiorly but form discrete tracts inferiorly where it extends into the atrioventricular bundle