M01.05.014 The Conducting System of the Heart

The cardiac conduction system is a network of nodes and specialized conduction cells responsible for initiating and coordinating the contraction of the heart muscle. It ensures that the heart beats in a regular and synchronized rhythm, crucial for the effective pumping of blood throughout the body.

In this article, we will explore the components of the cardiac conduction system, their functions, and clinical correlations relevant to medical students.

Overview of the Heart Conduction Process

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

1. Sinoatrial (SA) Node: Initiates the electrical signal (action potential).
2. Atria Contraction: The signal spreads across the atria, leading to atrial contraction.
3. Atrioventricular (AV) Node: The signal is delayed at the AV node.
4. Bundle of His: The signal passes into the bundle of His and moves down the interventricular septum.
5. Purkinje Fibres: The signal spreads across the ventricles, causing ventricular contraction.

Components of the Cardiac Conduction System

1. Sinoatrial (SA) Node

• Location: Upper wall of the right atrium, near the superior vena cava entrance.
• Function: Acts as the heart’s natural pacemaker by spontaneously generating electrical impulses.
• Mechanism: The impulse spreads across the atria, causing atrial contraction (atrial systole) and pushing blood into the ventricles.
• Influence of the Nervous System:
  • Sympathetic Nervous System: Increases the firing rate of the SA node, raising heart rate.
  • Parasympathetic Nervous System: Decreases the firing rate of the SA node, lowering heart rate.

2. Atrioventricular (AV) Node

• Location: At the junction of the atrioventricular septum and near the opening of the coronary sinus.
• Function: Delays the electrical signal by approximately 120 milliseconds.
  • This delay ensures the atria have enough time to fully empty into the ventricles before ventricular contraction.

3. Atrioventricular Bundle (Bundle of His)

• Location: Continuation of the AV node’s specialized tissue.
• Function: Transmits the electrical impulse from the AV node to the Purkinje fibers.
• Structure: The bundle of His divides into two branches:
  • Right Bundle Branch: Carries the impulse to the right ventricle.
  • Left Bundle Branch: Carries the impulse to the left ventricle.

4. Purkinje Fibres

• Location: Found in the subendocardial surface of the ventricular walls.
• Function: Rapidly transmit the electrical impulse from the bundle of His to the ventricular myocardium.
  • This results in coordinated ventricular contraction (ventricular systole), pushing blood to the pulmonary artery and aorta.
• Characteristics: Purkinje fibers are rich in glycogen and have extensive gap junctions to allow for fast conduction of electrical signals.

Clinical Relevance

Artificial Pacemakers

• Indication: Commonly used to treat conditions like bradycardia (slow heart rate), certain types of heart block, and rhythm abnormalities.
• Procedure: The pacemaker is surgically implanted under the left clavicle, with leads connected to the heart via the venous system.
• Function: Monitors the heart rate and delivers electrical impulses if the rate becomes too slow, ensuring proper heart rhythm.

Points to Remember

• The SA node is the primary pacemaker of the heart, responsible for initiating the action potential.
• The AV node introduces a delay to ensure effective atrial contraction before ventricular systole.
• The bundle of His and Purkinje fibers ensures rapid transmission of the action potential to the ventricles, resulting in coordinated contraction.
• The sympathetic and parasympathetic nervous systems regulate the firing rate of the SA node to adjust the heart rate.
• Artificial pacemakers are essential in managing abnormal heart rhythms like bradycardia or heart block.

Table: Summary of the Cardiac Conduction System Components

This structure and function of the cardiac conduction system ensure that the heart beats in a coordinated manner, maintaining efficient blood circulation throughout the body.