The Cardiac Conduction System: How the Heart Beats in Sync

The Cardiac Conduction System: How the Heart Beats in Sync

(This article is part of the original post and is written in Spanish. Sistema específico de conducción cardíaca)

Introduction

The heart must contract in a synchronized manner—from the atria to the ventricles—with an appropriate heart rate.

We refer to the group of phenomena related to the contraction and relaxation of the heart chambers as mechanical phenomena.

The efficiency of these mechanical phenomena depends on electrical phenomena, which are driven by the cardiac conduction system.

This system functions like a network of electrical cables that generate and conduct impulses.

We must remember that the heart contains two major types of cells: cardiomyocytes and cardionector cells. The latter form the basis of the conduction system. While cardiomyocytes are the predominant cell type, cardionector cells make up only about 1% of the total population. These cells are smaller and contain very few contractile filaments. However, they possess a special property called automaticity, which enables them to generate electrical impulses through spontaneous depolarizations of their resting membrane.

Electrical Pathways from the SA Node to the AV Node

There are two main pathways that conduct the electrical impulse from the sinoatrial node (SA node) to the atrioventricular node (AV node):

• The internodal pathways, which directly connect the SA node to the AV node.

• The Bachmann's bundle, which transmits the impulse from the SA node (located in the right atrium) to the left atrium, allowing both atria to contract simultaneously.

Functions of the Atrioventricular Node (AV Node)

Once the impulse reaches the AV node, this structure plays three critical roles in the cardiac conduction system:

1. It conducts the electrical impulse to the ventricles, forcing it to pass through the bundle of His.
   This process involves the fibrous skeleton of the heart, a structure that acts as an electrical insulator. It prevents direct conduction from the atria to the ventricles, ensuring that all impulses pass through the specialized conduction system.

2. It slows down the electrical signal before it reaches the ventricles. This delay is important for two reasons:

   • a) In cases of atrial arrhythmias, such as atrial fibrillation, the AV node filters the rapid impulses, protecting the ventricles from excessive stimulation and reducing the risk of tachycardiomyopathy.

   • b) The delay gives the atria enough time to contract, allowing the ventricles to fill more completely with blood. This increases ventricular preload by about 20%, improving cardiac output.

3. It serves as a secondary pacemaker. If the SA node fails, the AV node can take over and generate impulses on its own, although at a slower rate of 40 to 60 beats per minute.

Ventricular Conduction: His Bundle, Bundle Branches, and Purkinje Network

After the electrical signal reaches the atrioventricular node (AV node), it is transmitted to the bundle of His, which then divides into two main branches:

• The right bundle branch, which conducts the impulse to the right ventricle (RV).

• The left bundle branch, which conducts the impulse to the left ventricle (LV).

The left bundle branch further divides into two subdivisions:

• The anterior fascicle, which conducts the impulse to the anterosuperior Purkinje fibers of the LV.

• The posterior fascicle, which transmits the impulse to the posteroinferior Purkinje fibers of the LV.

The Purkinje network, made up of Purkinje cells, conducts the electrical impulse very rapidly from the subendocardium to the subepicardium.

Interestingly, ventricular repolarization occurs in the opposite direction, from subepicardium to subendocardium, mainly due to the coronary artery blood supply.

Finally, the ventricular myocardium depolarizes from cardiomyocyte to cardiomyocyte, thanks to gap junctions that allow them to function as a functional syncytium.

Thanks to the Purkinje fibers and the AV node, which delays the SA node's impulses, the ventricles are activated just after the atria, but in a coordinated and timely manner.
As a result, the heart contracts from apex to base, meaning the blood is ejected upward, from the bottom of the heart to the outflow tracts.

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