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.

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 a normal sinus rhythm, that rate will be between 60 and 100 beats per minute.

The sinus node signal also controls electrical conduction as it "spreads" across the heart. It causes the cells of the heart muscle to contract in the correct sequence, and ensures regular, efficient, and coordinated heartbeats. For this reason, the sinus node is often referred to as the heart's "natural pacemaker."

Components of the Cardiac Conduction System

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 the AV disc. This tissue keeps 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

The electrical impulse originates in the sinus node. From there, it spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract. This is referred to as atrial depolarization, an action that pushes blood into the right and left ventricles, the bottom two chambers of the heart.

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 in the left side of the image above.

The Heart's Electrical Signal Reaches the AV Node

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, so there is a pause in the electrical activity on the EKG, referred to as the PR interval. This is shown by the solid red line on the EKG in this figure.

This pause allows the atria to contract fully, emptying blood into the ventricles before the ventricles begin to contract.

The Heart's Electrical Signal Passes to the Ventricles

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).

The Heart's Electrical Signal Spreads Across the Ventricles

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 the organs of the body (via the left ventricle) and to the lungs (via the right ventricle).

Conduction Disorders and Treatments

Heart block is a conduction disorder in which the heart's electrical signals are unable to move from the atria to the ventricles. This interference prevents the atria from telling the ventricles when to contract and pump blood.

In most cases of heart block, the electrical signals are weakened but do not stop completely. There are three degrees of heart block severity:

• First degree heart block: The least severe degree of heart block, in which the electrical signals are slowed but still reach your ventricles. Treatment may not be needed.
• Second degree heart block: The heart's electrical signals are irregular, causing the heart to occasionally skip a beat. A pacemaker may be considered.
• Third degree heart block: The most severe degree of heart block, in which electrical signals completely fail to reach the ventricles. If this happens, a person's pulse may dramatically slow, or there may be no pulse at all. A pacemaker is almost always required.

Another type of conduction disorder, bundle branch block, occurs when a blockage in the right or left bundle branch causes one ventricle to contract slightly slower than the other. This results in an irregular heartbeat (arrhythmia).

Bundle branch block by itself often does not require treatment. When it does, treatment involves managing the underlying health condition, be it heart disease, high blood pressure, a congenital (present at birth) heart defect, or something else.