The Heart's Electrical System: Anatomy and Function

Five components of electrical conduction make the heart beat

The electrical system of the heart is critical to how it functions. It controls the electrical impulses that cause your heart to beat and their conduction, which organizes the beating of your heart.

The movement of electrical signals across the heart is what is traced on an electrocardiogram (EKG). The EKG is also what allows irregularities in the heart's electrical system, and with them any related symptoms and medical conditions, to be assessed.

This article explains five components of the cardiac conduction system and how they work. It also discusses how heart disease or a heart attack can cause serious disruptions in the system.

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

An illustration of the electrical system of the heart

Illustration by Mira Norian for Verywell Health

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 that originates in the sinus node of the heart

Illustration by Mira Norian for Verywell Health

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

The AV node is highlighted blue in the heart's electrical system

Illustration by Mira Norian for Verywell Health

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 AV conduction system including the AV node, the "his bundle", right and left bundle branches (RBB and LBB), and the split of the LBB into left anterior fascicle (LAF) and the left posterior fascicle

Illustration by Mira Norian for Verywell Health

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

The heart and an electrical signal spreading across the ventricles

Illustration by Mira Norian for Verywell Health

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

Senior man with chest pain

AsiaVision / Getty Images

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: In second degree AV block, some of the electrical impulses from the atria reach the ventricles, but some are blocked and never reach the ventricles. Depending on the underlying cause of this condition, a pacemaker may need to 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.

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.

Other Heart Electrical System Problems

  • Sinus bradycardia ("brady" means slow) is the most common cause of a low heart rate and is caused by the SA node firing at a reduced rate.
  • Sinus tachycardia ("tachy" means fast) refers to a rapid heart rate and can be caused by the SA node firing at an increased rate.

A Word From Verywell

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.

Having a conduction disorder, in which the heart's electric pathway is disrupted, increases the risk of major complications, including cardiac arrest. Fortunately, there are ways to reduce your risk of a conduction disorder or related complications, including regular exercise and a heart-healthy diet.

13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Heart Association. How the healthy heart works.

  2. Mercy Health. Sinus tachycardia.

  3. American Heart Association. What is an arrhythmia?.

  4. Glover BM, Brugada P. Clinical handbook of cardiac electrophysiology. Springer.

  5. Achilles JP, Withrow GW. The cardiac pump. In: Cardiovascular Physiology. 2013;2013(10):55-90. doi:10.1016/B978-0-323-08697-4.00004-6

  6. Klabunde, RE. Cardiac cycle - atrial contraction (phase 1). Cardiovascular Physiology Topics.

  7. John Hopkins Medicine. Anatomy and Function of the Heart's Electrical System.

  8. ACLS Medical Training. The Basics of ECG.

  9. Johns Hopkins Medicine. Heart block.

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  13. National Heart, Lung, and Blood Institute. Conduction disorders.

Additional Reading
  • Fogoros RN, Mandrola JM. Disorders of the heart rhythm: Basic principles. In: Fogoros’ Electrophysiologic Testing. Wiley Blackwell, 6th edition.

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.