The Heart's Electrical System: Anatomy and Function

Where Do Electrical Impulses Start in the Heart?

The heart generates its own electrical signal. This electrical signal is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The right atrium is one of four heart chambers and related valves which include two atria at the top of the heart with two ventricles at the bottom.

From the sinus node, the electrical signal spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract. This pushes their load of blood into the right and left ventricles, the bottom two chambers of the heart.

The electrical signal then passes through the AV node to the ventricles, where it causes the ventricles to contract in turn.

The cardiac electrical signal controls the heartbeat in two ways. First, since each electrical impulse generates one heartbeat, the number of electrical impulses determines the heart rate. In normal sinus rhythm, that rate will be between 60 and 100 beats per minute.

The sinus node signal also controls electrical conduction of the heart's steps as it "spreads" across the heart. It causes the cells of heart muscle to contract in the correct sequence and ensures regular, efficient, and coordinated heartbeats.

Components of the Cardiac Conduction System

Figure 1: The components of the heart's electrical system including the sinus node (SN) and atrioventricular node (AV node) are illustrated here. You can see the two atria and the two ventricles. Separating them is a layer of fibrous tissue, labeled AV disc. This tissue keep the electrical signal passing through the AV node.

In this figure:

• SN = sinus node
• AVN = AV node
• RA = right atrium
• LA = left atrium
• RV = right ventricle
• LV = left ventricle
• TV = tricuspid valve (the valve that separates the right atrium from the right ventricle)
• MV = mitral valve (the valve that separates the left atrium from the left ventricle)

The Heart's Electrical Signal Spreads Across the Atria

Figure 2: The electrical impulse originates in the sinus node. From there, it spreads across both atria (indicated by the blue lines in the picture), causing the atria to contract. This is referred to as atrial depolarization.

As the electrical impulse passes through the atria, it generates the so-called "P" wave on the EKG. (The P wave is indicated by the solid red line on the EKG off to the left side).

The Heart's Electrical Signal Reaches the AV Node

Figure 3: When the wave of electricity reaches the AV disc, it is stopped, except in the AV node. The impulse travels through the AV node at a slow, controlled rate toward the ventricles. The solid red line on the EKG in this figure indicates the PR interval.

The Heart's Electrical Signal Passes to the Ventricles

Figure 4: The specialized AV conduction system consists of the AV node (AVN), the "His bundle," and the right and left bundle branches (RBB and LBB). The AV node conducts the electrical impulse to the His bundle. The His bundle passes the signal to the right and left bundle branches.

The right and left bundle branches, in turn, send the electrical impulse to the right and left ventricles, respectively. The figure also shows that the LBB itself splits into the left anterior fascicle (LAF) and the left posterior fascicle (LPF).

Because the impulse travels only very slowly through the AV node, there is a pause in the electrical activity on the EKG, referred to as the PR interval. (The PR interval is illustrated on the EKG in Figure 3.) This "pause" in the action allows the atria to contract fully, emptying blood into the ventricles before the ventricles begin to contract.

Problems anywhere along the heart's electrical pathway can cause abnormalities in the heart rhythm. For example, AV block (heart block) is one of the two major causes of a low heart rate (bradycardia). Third-degree heart block is the most severe type and usually requires a pacemaker.

There are different degrees of block in the heart's electrical system, and different ways to fix the electrical problems with the heart.

Bundle branch block occurs in either the right bundle branch or left bundle branch, with those in the left bundle branch usually most serious. Bundle branch blocks may occur for no apparent reason but often occur when the heart is damaged due to a heart attack or other cardiac conditions.

The Heart's Electrical Signal Spreads Across the Ventricles

Figure 5: This figure shows the electrical impulse spreading throughout the right and left ventricles, causing these chambers to contract. As the electrical signal travels through the ventricles, it generates the “QRS complex” on the EKG. The QRS complex is indicated by the solid red line on the EKG to the left.

In this manner, the electrical system of the heart causes the heart muscle to contract and send blood to either the organs of the body (via the left ventricle) or to the lungs (via the right ventricle).

Bottom Line

From the initiation of a heartbeat in the SA node, through contraction of the ventricles, the cardiac electrical system causes the heart to contract in a coordinated manner, maximizing the efficiency of the beating heart.