Internodal tracts (heart, anatomy)

The interatrial or internodal conduction tracts are bands of specialised myocytes which are believed to lie between the sinuatrial node and the atrioventricular node. Previously, conduction between the two nodes was thought to occur through direct stimulation and conduction between normal atrial myocytes. The latter theory does not explain why the SAN impulses arrive at the AVN more quickly than would be expected by just atrial myocyte conduction.

However, ultrastructural evidence for discrete bands of rapidly-conducting myocytes is sparse. Where clear differentiation has been made, the myocytes appear to resemble the Purkinje cells of the ventricles. Three discrete internodal tracts have been reported:

• anterior:
  • leaves anterior surface of SAN
  • passes posteromedially to interatrial septum and then inferiorly
  • splits into two bundles:
    • one passes to left atrium
    • the other passes posterior to torus aorticus to reach AVN
• middle:
  • leaves the posterior and superior surface of SAN
  • passes inferiorly and medially around orifice of inferior vena cava
  • reaches interatrial septum where it passes inferiorly to AVN
• posterior:
  • leaves the posterior and inferior surface of the SAN
  • passes inferiorly through the crista terminalis
  • passes medially around valve of the inferior vena cava
  • reaches posterior surface of AVN