The fibrous rings of the four valves of the heart are continuous with each other. They not only form the basis for the attachment of the corresponding valve cusps but also form an electrical barrier between the atrial and ventricular muscle of the heart. This fibrous network surrounding the valve openings is called the "fibrous skeleton" of the heart. Electrical impulses can spread through the heart muscle but not through the fibrous skeleton of the heart. Each beat of the heart is initiated in the right atrium at the upper end of the crista. The area where this occurs is called the sinoatrial node (SA node). From the SA node rhythmic impulses pass through the atrial musculature, causing them to contract and discharge blood into the ventricles. It is likely that impulses spread through the right atrial wall in several specialized bundles, both through to the left atrium and towards the fibrous skeleton of the heart. However this is not universally accepted. The impulses do not pass directly through the skeleton of the heart so the ventricles are relaxed while atrial contraction occurs. Impulses eventually reach the interatrial septal region near the opening of the coronary sinus. Here, just above this opening, close to the septal leaflet of the tricuspid valve, is another specialized group of myocytes called the atrioventricular node (AV node).
Impulses from the AV node travel onwards though the atrioventricular bundle of HIS. This short bundle pierces the fibrous skeleton and arrives in the region of the thinner membranous part of the interventricular septum. In the interventricular septum the bundle divides into right and left plura or bundle branches, which pass to the respective ventricles. The left crus supply the papillary muscles in the left ventricle and then spreads out as a network in the ventricular wall. The right crus take impulses to the septal and posterior papillary muscles of the right ventricle. It then proceeds in the septo marginal trabecula to the anterior papillary muscle. Eventually it terminates and sends out many branches to form the Purkinje network.