What are the 5 steps of heart conduction?

This article will discuss the components of the heart’s conduction system including their anatomy and clinical significance.

The heart has two main types of cells:1

• Conducting cells: generate and propagate electrical impulses.
• Contractile (muscle) cells: contract following receipt of electrical impulses. These cells can also propagate and, on occasion, generate electrical impulses.

Both conducting and specialised muscle cells form the hearts conduction system and orchestrate cardiac contraction. The electrical system is intrinsic to the heart meaning that contraction can persist in the absence of neuronal input.

Components of the cardiac conduction system

The cardiac conduction system involves the spread of electrical activity from the sinoatrial node, to the atrioventricular node, down the bundle of His and along the Purkinje fibres. As the electrical activity spreads along the heart’s conduction system it initiates myocardial contraction in the surrounding myocardial tissue.

The sinoatrial node (SAN)

• The sinoatrial node (SAN) is of a group of cells found high up in the right atrium close to its junction with the superior vena cava.1,4,5
• The SAN functions as the heart’s intrinsic pacemaker, regulating heart rate.
• The SAN spontaneously generates electrical impulses which are transmitted to the right and left atrium.
• These electrical impulses stimulate the atrial myocardium to contract.
• Atrial muscle conducts relatively fast (0.5m/sec).8

Atrioventricular node (AVN)

• The atrioventricular node (AVN) is a group of specialised cells situated in the atrioventricular septum just above the coronary sinus ostium.1,4,5
• The AVN receives electrical impulses from the atria and then transmits the electrical impulse from the atria to the ventricles.
• The AVN has a slower conduction velocity (0.05m/sec) than the atria, allowing maximal ventricular filling prior to contraction.8

The bundle of His

• The bundle of His is a collection of heart muscles cells specialised for electrical conduction. These cells receive input from the AVN.1,4,5
• The bundle of His branches into the left and right bundle branches which travel down the interventricular septum.
• Branches of the bundle of His propagate impulses to the left and right ventricles respectively.
• Each branch terminates as several Purkinje fibres.

Purkinje fibres

• Purkinje fibres are situated in the subendocardium.1,4,5
• They transmit the wave of electricity to the ventricular myocardium.
• This wave of electricity results in ventricular contraction.

Summary

Below is a summary of the key steps involved in the cardiac conduction cycle.

An artificial pacemaker is a medical device that is able to monitor and regulate heart rate and rhythm. The artificial pacemaker is able to generate electrical impulses that can trigger myocardial contraction, mimicking the function of the heart’s intrinsic conduction system.

Artificial pacemakers are often used to regulate heart rate and rhythm in patients with atrioventricular block and sinus node dysfunction.7

References

1. The University of Minnesota. Overview of Cardiac Conduction. Published in 2011. [LINK].
2. Madhero88. Cardiac conduction system. Licence: CC BY-SA. Available from: [LINK]
3. OpenStax CNX. Cardiac Muscle and Electrical Activity. Published in 2014. [LINK].
4. Park D, Fishman G. The Cardiac Conduction System. Published in 2011. [LINK].
5. National Heart, Lung, and Blood Institute. How the heart works. Published in 2011. [LINK].
6. Kalumet. Heart’s conduction system. Licence: CC BY-SA. Available from: [LINK]
7. Dalia T, Amr S. Pacemaker Indications. Published in 2020. [LINK].
8. Clinical ECG Interpretation e-Book. Clinical electrocardiography and ECG interpretation. [LINK].
9. Klabunde R. Normal Impulse Conduction. Published in 2012. [LINK].

The cardiac conduction system is a collection of nodes and specialised conduction cells that initiate and co-ordinate 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.

Overview of Heart Conduction

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

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

We will now discuss the anatomy of the individual components involved in the conducting system.

Components of the Cardiac Conduction System

Sinoatrial Node

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

These pacemaker cells can spontaneously generate electrical impulses. The wave of excitation created by the SA node spreads via gap junctions across both atria, resulting in atrial contraction (atrial systole) - with blood moving from the atria into the ventricles.

The rate at which the SA node generates impulses is influenced by the autonomic nervous system:

• Sympathetic nervous system - increases firing rate of the SA node, and thus increases heart rate.
• Parasympathetic nervous system - decreases firing rate of the SA node, and thus decreases heart rate.

Atrioventricular Node

After the electrical impulses spread across the atria, they converge at the atrioventricular node - located within the atrioventricular septum, near the opening of the coronary sinus.

The AV node acts to delay the impulses by approximately 120ms, to ensure the atria have enough time to fully eject blood into the ventricles before ventricular systole.

The wave of excitation then passes from the atrioventricular node into the atrioventricular bundle.

Atrioventricular Bundle

The atrioventricular bundle (bundle of His) is a continuation of the specialised tissue of the AV node, and serves to transmit the electrical impulse from the AV node to the Purkinje fibres of the ventricles.

It descends down the membranous part of the interventricular septum, before dividing into two main bundles:

• Right bundle branch - conducts the impulse to the Purkinje fibres of the right ventricle
• Left bundle branch - conducts the impulse to the Purkinje fibres of the left ventricle.

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Purkinje Fibres

The Purkinje fibres (sub-endocardial plexus of conduction cells) are a network of specialised cells. They are abundant with glycogen and have extensive gap junctions.

These cells are located in the subendocardial surface of the ventricular walls, and are able to rapidly transmit cardiac action potentials from the atrioventricular bundle to the myocardium of the ventricles.

This rapid conduction allows coordinated ventricular contraction (ventricular systole) and blood is moved from the right and left ventricles to the pulmonary artery and aorta respectively.

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Clinical Relevance: Artificial Pacemaker

An artificial pacemaker is a small electrical device commonly fitted to monitor and correct heart rate and rhythm. It is inserted into the chest under the left clavicle, with wires connected to the heart via the venous system.

The most common indication for a pacemaker is bradycardia. Once inserted, the pacemaker monitors the heart rate, and only fires if the rate becomes too slow. Pacemakers can also be used to treat some tachycardias, certain types of heart block and other rhythm abnormalities.

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