The structure indicated is the atrioventricular node.
The conduction system of the heart is responsible for the synchronised contraction of the atria and ventricles and is comprised of the following main structures:
- Sinoatrial node
- Atrioventricular node
- Atrioventricular bundle of His
- Right and left bundle branches
- Purkinje fibres
The atrioventricular node is located centrally in the Triangle of Koch, between the atria and ventricles in the infero-posterior portion of the interatrial septum close to the opening of the coronary sinus. The triangle of Koch has the following boundaries:
- Ostium of coronary sinus
- Tendon of Todaro
- Anterior-septal leaflet commissure
The atrioventricular nodes receives signals from the sinoatrial node. The impulses from the atrioventricular node then pass to the Bundle of His and then follow the left and right bundle branches through the interventricular septum and ultimately to the Purkinje fibres which propagate the signal into the ventricles.
The atrioventricular node delays the impulse generated by the atria, which allows the atria time to contract and propel blood into the ventricles before ventricular contraction.