The Conducting System of the Heart

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.

Impulses reaching the AV node from the atria are delayed a little as they pass through the trunk and crura. The impulses first reach the papillary muscles and their contraction closes both atrioventricular valves. Further rapid spreading of the impulses causesnsimultaneous contraction of both ventricles and blood is ejected in to the pulmonary trunk and aorta. Although the heart can beat by itself each stroke is initiated at the SA node, inherent activity is influenced by the autonomic nervous system. Sympathetic fibers to the heart carry impulses that produce an increase in the rate of impulses generated at the SA node. The heart beats faster. Para sympathetic activity on the other hand slows down the rate.

There is in fact a three-tier system of rhythmic activity in the heart. First, the muscle fibers themselves have a built-in capacity for contracting rhythmically. If the SA node is destroyed the muscle still contracts, although at a slower rate. The second tier of control is the SA node which under normal circumstances dictates the frequency of contraction.Finally the autonomic nervous system influences the rate according to the particular requirements of the body for blood. One must not of course forget the direct effects of noradrenalin from the adrenal gland.

Conducting system of the heart

Cardiac Branches and Cardiac Plexuses
The vagus gives of one superior cervical cardiac branch and an inferior cervical cardiac branch in the neck. These branches descend into the thorax and take part in performing the cardiac plexuses described below. Additional cardiac branches arise in the superior mediastinum and also from the recurrent laryngeal branches. The superficial cardiac plexus is located just below the arch of the aorta, close to the ligamentum arteriosum. It is formed by the inferior cervical branch of the left vagus nerve and the superior cervical branch of the left sympathetic trunk. The deep cardiac plexus is situated in front of the bifurcation of the trachea. It receives several branches from the right and left vagus nerves. They are right superior and inferior cervical cardiac branches, left superior cervical cardiac branch, branches from the right and left vagi arising in the thorax and branches from the right and left recurrent laryngeal nerves. The plexus also receives numerous cardiac branches from the right and left sympathetic trunks. Branches from the superficial and deep cardiac plexuses supply the heart.