Learning Objective

By the end of this lesson, learners should be able to identify the key components of the cardiac conduction system, describe their anatomical location and function, and understand clinical applications such as pacemaker therapy.

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Cardiac Conduction System

The cardiac conduction system is a collection of nodes and specialised conduction cells that initiate and coordinate the contraction of the heart muscle. It consists of:

• Sinoatrial node
• Atrioventricular node
• Atrioventricular bundle (bundle of His)
• Purkinje fibres

In this article, we shall look at the anatomy of the cardiac conduction system – its structure, function, and clinical correlations.

 

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Overview of Heart Conduction

The following sequence of electrical events occurs during one full contraction of the heart muscle:

1. An excitation signal (action potential) is created by the SA node.
2. The wave of excitation spreads across the atria, causing them to contract.
3. Upon reaching the AV node, the signal is delayed.
4. It is then conducted into the bundle of His, down the interventricular septum.
5. The bundle of His and the Purkinje fibres spread the wave impulses along the ventricles, causing them to contract.

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Components of the Cardiac Conduction System

Sinoatrial Node

The SA node is a collection of specialised pacemaker cells, located in the upper wall of the right atrium, at the junction where the superior vena cava enters.

The rate of impulses generated by the SA node is influenced by the autonomic nervous system:

• Sympathetic nervous system – increases SA node firing rate → increased heart rate.
• Parasympathetic nervous system – decreases SA node firing rate → decreased heart rate.

Atrioventricular Node

Located within the atrioventricular septum, near the opening of the coronary sinus, the AV node delays impulses by approximately 120ms to allow full atrial contraction before ventricular systole.

Atrioventricular Bundle (Bundle of His)

The bundle of His transmits impulses from the AV node to the Purkinje fibres. It descends through the interventricular septum and splits into:

• Right bundle branch – to the Purkinje fibres of the right ventricle
• Left bundle branch – to the Purkinje fibres of the left ventricle

Purkinje Fibres

The Purkinje fibres are a sub-endocardial network that rapidly transmits action potentials from the bundle of His to the ventricular myocardium, ensuring coordinated ventricular systole.

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Clinical Relevance

Artificial Pacemaker

Artificial pacemakers are small electrical devices implanted under the left clavicle, with leads connecting to the heart to monitor and correct the heart rhythm.

The most common indication is bradycardia, though they are also used for tachycardia, heart block, and other rhythm disorders.

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Activity