Introduction

The cardiac conduction system is a complex of specialized cardiac muscle cells that generate and transmit electrical impulses throughout the heart. These impulses coordinate the heart’s rhythmic contractions during each cardiac cycle.

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Atrial Pathways

Normal cardiac rhythm is established by the sinoatrial (SA) node, a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium near the orifice of the superior vena cava. The SA node has the highest inherent rate of depolarization and is known as the heart’s pacemaker. It initiates the sinus rhythm or normal electrical pattern, followed by heart contraction.

This impulse spreads from its initiation in the SA node throughout the atria through specialized internodal pathways to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the atrioventricular node. The impulse travels between these two nodes in approximately 50 milliseconds (msec). The relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply by following the cell-by-cell pathway through the contractile cells of the myocardium in the atria.

In addition, a specialized pathway called Bachmann’s bundle or the interatrial band conducts the impulse directly from the right to the left atrium.

Regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node.

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Atrioventricular (AV) Node

The atrioventricular (AV) node is a second clump of specialized myocardial conductive cells located in the inferior portion of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node.

A critical pause occurs before the AV node depolarizes and transmits the impulse to the atrioventricular bundle. This delay in transmission is partially attributable to the small diameter of the node’s cells, which slows the propagation of the impulse. Also, conduction between nodal cells is less efficient than between conducting cells. These factors mean the impulse takes approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction, which pumps blood into the ventricles before the impulse is transmitted to the ventricular cells.

With extreme stimulation by the SA node, the AV node can transmit impulses at a maximum rate of 220 per minute. This establishes the typical maximum heart rate in a healthy young individual. Damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood.

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Ventricular Pathways

Arising from the AV node, the atrioventricular bundle, or bundle of His, proceeds through the interventricular septum before dividing into two atrioventricular bundle branches, commonly called the left and right bundle branches.

The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. The left bundle branch is also considerably larger, as the left ventricle is significantly larger than the right. Portions of the right bundle branch are found in the moderator band and supply the right papillary muscles.

Both bundle branches descend and reach the apex of the heart, where they connect with the Purkinje fibers. In a healthy heart, the entire process of impulse transmission from the bundle of His to the Purkinje fiber network typically takes 30-50 milliseconds.

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The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart.

The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all ventricular muscle cells in about 75 ms. Since the electrical stimulus begins at the apex, the contraction also begins at the apex. It travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom.

This allows blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. The total time elapsed from initiating the impulse in the SA node until the ventricles depolarize is approximately 225 ms.

Conduction System Activity

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References and Attributions

OpenStax

Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Reference: “Cardiac Muscle and Electrical Activity“

Wikipedia, the free encyclopedia.

Reference: “Electrical Conduction System of the Heart“