Question

◦ What is special about the cells of the conducting system of the heart? List the elements of the conducting system and describe its purpose.

Answer 1

The heart possesses the property of autorhythmicity, which means it generates its own electrical impulses and beats independently of nervous or hormonal control, i.e. it is not reliant on external mechanism to nitiate each heartbeat. However, it is supplied with both sympathetic and parasympathetic autonomic nerve fibres, which increase and decrease respectively the intrinsic heart rate. Furthermore, the heart responds to a number of circulating hormones, including adrenaline and throxine. Small groups of specialised neuromuscular cells in the myocardium initiate and conduct impulses, causing coordinated and synchronised contraction of the heart muscle.

Elements of conducting system of heart includes: 1. SA (Sinoatrial node), 2. Atrioventricular node (AV node),3. Atrioventricular bundle (AV bundle or bundle of His).

1. Sinoatrial node (SA node): It is a small mass of specialised cells located at the wall of the right atrium near the superior vena cava opening. The cells generates regular impulses because they are electrically unstable. This instability leads to depolarisation regularly, usually between 60 to 80 times per minute. However, it is followed by repolarisation and then immediately their instability leads them to discharge again, setting the heart rate. it is know to discharge faster than any other part of the heart, it normally sets the heart rate and is called pacemaker of the heart.

2. Atrioventricular node (AV node): It is located at the wall of the atrial septum near the atrioventricular valves. Normally, the AV node merely trasmits the electrical signals from the atria into the ventricles. The AV also has secondary pacemaker function ad takes over this role if there is any issue with the SA node.

3. Atriventricular bundle : It is a specialised fibres that originate from the AV node. The AV bundle crosses the fibrous ring that separates atria and ventricles then, at the upper end of the ventricular septum it divides into right and left bundle branches. Within the ventricular myocardium the branches break up into fine fibres, called the purkinje fibres. The AV bundle, bundle branches and purkinje fibres transmit electrical impulses from the AV node to the apex of the myocardium where the wave of ventricular contraction begins, then sweeps upwards and outwards, pumping blood into the pulmonary artery and the aorta.