The Conducting System
The cardiac conducting system consists of a network of conducting fibres situated within the inner aspect of the cardiac chambers. The fibres are responsible for transporting the signals from pacemaker cells in a rapid manner through both atria and ventricles, so that a coordinated contraction can be achieved.
When this system becomes damaged both tachy and brady arrhythmias may occur. The arrhythmia that develops will depend on which portion of the pathway becomes damaged. The animation below shows how the electrical signal propagates through the conducting system, and what the resultant ECG changes look like.
Componants of the Electrical Conducting System
And their function
Sinoatrial Node
The sinoatrial node is the heart’s natural pacemaker. It is located in the right atrium near the opening of the superior vena cava. The SA node generates electrical impulses that initiate each heartbeat and set the heart rate. These impulses cause the atria to contract and push blood into the ventricles.
The SAN usually depolarises at a rate of 60-100bpm which sets the resting heart rate. A small bundle called bachman's bundle, transmits the signals through to the left atrium whilst inter-nodal pathways help propagate the signal through to the AV node.
Atrioventricular Node
The atrioventricular node is located between the atria and ventricles, near the centre of the heart. It receives the electrical signal from the atria and briefly delays it. This delay allows the ventricles time to fill with blood before they contract.
Problems with the AV node can lead to both tachy and brady arrhythmias. For example, if a re-entry circuit forms within the node then re-entry tachycardias can develop. but if the node becomes damaged from ischaemia then heart blocks can occur.
Bundle of His
The Bundle of His carries the electrical impulse from the AV node into the interventricular septum. It is the only normal electrical connection between the atria and ventricles and ensures the signal is transmitted efficiently to the lower chambers.
Bundle branches
The right and left bundle branches extend from the Bundle of His down either side of the interventricular septum. They conduct the electrical impulse toward the apex of the heart, ensuring both ventricles receive the signal almost simultaneously.
Purkinje Fibres
The Purkinje fibres are a network of fine fibres that spread throughout the ventricular muscle. They rapidly distribute the electrical impulse across the ventricles, causing a strong, coordinated ventricular contraction that pumps blood to the lungs and the rest of the body.
Together, these components ensure the heart beats in a regular, organised pattern—first contracting the atria, then the ventricles—to maintain effective blood circulation.
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