---
title: Cardiac conduction system
description: How an electrical impulse starts in the sinoatrial node and travels through the atria, the AV node, the bundle of His, its branches and the Purkinje fibres, so that the heart contracts in the right order.
type: anatomy
aliases:
  - conduction system
  - electrical system of the heart
  - SA node
  - AV node
  - bundle of His
  - Purkinje fibres
sources:
  - tier: reference
    org: statpearls
    title: Physiology, Cardiac
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK526089/
  - tier: reference
    org: statpearls
    title: Atrioventricular Node
    year: 2026
    url: https://www.ncbi.nlm.nih.gov/books/NBK557664/
  - tier: textbook
    title: Anatomy and Physiology 2e (OpenStax), Chapter 19.2 Cardiac Muscle and Electrical Activity
    year: 2022
    isbn: 9781711494067
    url: https://openstax.org/books/anatomy-and-physiology-2e/pages/19-2-cardiac-muscle-and-electrical-activity
sources_checked: 2026-10-11
review: null
---

Every heartbeat is set off by an electrical signal, and that signal starts within the heart itself.<Cite n={1} /> It arises in the <Structure id="sa-node">sinoatrial (sinus) node</Structure> in the right atrium and always follows the same route: the <Structure id="av-node">atrioventricular (AV) node</Structure>, the <Structure id="bundle-of-his">bundle of His</Structure>, the right and left bundle branches, and the <Structure id="purkinje-fibers">Purkinje fibres</Structure>.<Cite n={[1, 3]} /> This route is called the cardiac conduction system. Thanks to it, the atria contract first and the ventricles afterwards.<Cite n={[1, 3]} />

<Diagram name="conduction-system">
  <p slot="scheme">The impulse starts in the sinoatrial node, the heart's pacemaker, in the upper back wall of the right atrium.<Cite n={3} /> It spreads across the atria and passes into the left atrium along a separate bundle; this excitation shows on the ECG as the P wave, and the atria contract just after it.<Cite n={3} /> The impulse can reach the ventricles only through the AV node: everywhere else, connective tissue stops it.<Cite n={3} /> In the AV node the impulse is delayed, so the atria have time to finish contracting.<Cite n={[1, 3]} /> It then runs down the bundle of His in the interventricular septum<Cite n={1} />, along the left and right bundle branches to their own ventricles<Cite n={3} />, and through the Purkinje fibres, which spread it quickly through the ventricles; contraction starts at the apex and moves towards the base.<Cite n={3} /> Excitation of the ventricles shows on the ECG as the QRS complex.<Cite n={3} /></p>
</Diagram>

## How it is built

<Patient>

The **sinoatrial node** (sinus node) is a cluster of specialised cells in the upper part of the right atrium.<Cite n={[1, 3]} /> These cells produce electrical impulses on their own, without any outside signal, which is why the sinus node is called the heart's pacemaker: normally it sets the heart rate.<Cite n={[1, 3]} />

The **AV node** (atrioventricular, meaning between the atria and the ventricles) lies in the lower part of the right atrium, next to the wall between the chambers.<Cite n={[2, 3]} /> It is the only "gateway" for the impulse from the atria to the ventricles: everywhere else they are separated by connective tissue, which does not let the impulse through.<Cite n={3} />

The **bundle of His** runs from the AV node down the wall between the ventricles.<Cite n={1} /> It divides into the **right and left bundle branches**, which lead into a branching network of **Purkinje fibres** running through the walls of the ventricles.<Cite n={[1, 3]} />

</Patient>

<Student>

The sinoatrial (SA) node is located in the superior and posterior walls of the right atrium.<Cite n={3} /> Its cells have the highest inherent rate of depolarisation in the heart, which is why it acts as the pacemaker.<Cite n={3} />

The impulse is carried from the SA node to the AV node by the internodal pathways (anterior, middle and posterior), although their importance is debated: the impulse would reach the AV node simply by passing cell to cell through the atrial myocardium. It reaches the left atrium through a separate interatrial band (Bachmann's bundle).<Cite n={3} /> The intercalated discs between myocardial cells contain many gap junctions that allow ions to pass between cells; this helps the cells contract in synchrony.<Cite n={3} />

The AV node lies within the triangle of Koch at the base of the right atrium.<Cite n={2} /> Histologically, it consists of small ovoid and spindle-shaped nodal myocytes embedded in a fibrous, collagen-rich matrix, with fewer gap junctions than working myocardium.<Cite n={2} /> In right-dominant circulation, the AV nodal artery most commonly arises from the right coronary artery.<Cite n={2} /> The compact AV node continues distally into the penetrating bundle of His.<Cite n={2} />

The bundle of His passes through the interventricular septum and divides into two bundle branches; the <Structure id="left-bundle-branch">left bundle branch</Structure> is considerably larger than the right and has two fascicles.<Cite n={3} /> Purkinje fibres conduct the impulse rapidly.<Cite n={3} /> Some fibres of the <Structure id="right-bundle-branch">right bundle branch</Structure> run in the moderator band to the papillary muscles of the right ventricle. As a result, they receive the impulse at approximately the same time and begin to contract just before the rest of the ventricular myocardium; this is believed to allow tension to develop on the chordae tendineae before right ventricular contraction.<Cite n={3} />

</Student>

<HeartModel
  highlight={[
    'sa-node',
    'av-node',
    'bundle-of-his',
    'right-bundle-branch',
    'left-bundle-branch',
    'purkinje-fibers',
  ]}
/>

## How it works

<Patient>

The sinus node sends out an impulse, which spreads across both atria: they contract and pump blood into the ventricles.<Cite n={3} /> The impulse then reaches the AV node, where it is briefly held up.<Cite n={[1, 3]} /> This pause matters: it gives the atria time to finish contracting and fill the ventricles.<Cite n={[2, 3]} /> Without the pause, the atria and ventricles would contract at the same time and blood would not flow properly through the heart.<Cite n={1} /> The AV node also limits how many very rapid impulses get through from the atria to the ventricles.<Cite n={2} />

After the pause, the impulse travels quickly along the bundle of His, the bundle branches and the Purkinje fibres, and the ventricles contract.<Cite n={[1, 3]} /> Contraction starts at the apex of the heart and moves towards the base, pushing blood out of the ventricles into the aorta and the pulmonary trunk.<Cite n={3} />

If the sinus node stops working, the AV node can take over setting the rhythm, but more slowly: about 40–60 beats per minute.<Cite n={[1, 2, 3]} />

**How this shows on an ECG.** The small P wave reflects excitation of the atria, the large QRS complex reflects excitation of the ventricles, and the T wave reflects their recovery (repolarisation).<Cite n={3} /> There is more about the waves in the [ECG](../../diagnostic/ecg/) article.

</Patient>

<Student>

The delay in the AV node is approximately 100 ms.<Cite n={3} /> It is partly due to the small diameter of the nodal cells and to conduction between nodal cells being less efficient than between conducting cells.<Cite n={3} /> This physiological conduction delay optimises ventricular filling and preserves atrioventricular synchrony, on which cardiac output and mechanical efficiency depend.<Cite n={2} />

The AV node also limits the transmission of excessively rapid atrial impulses to the ventricles.<Cite n={2} /> This is particularly important in supraventricular tachyarrhythmias such as atrial fibrillation.<Cite n={2} />

Pacemaker hierarchy: if a higher level fails, a lower one takes over, but at a lower inherent rate — the AV node at 40–60 per minute, the bundle of His at approximately 30–40, the bundle branches at 20–30 and the Purkinje fibres at 15–20.<Cite n={3} />

On the ECG, atrial repolarisation occurs during the QRS complex, which masks it.<Cite n={3} /> The PR interval starts at the beginning of the P wave and ends at the beginning of the QRS complex.<Cite n={3} />

</Student>

## In disease

<Patient>

For a rhythm disorder involving the atria, see the article on [atrial fibrillation](../../disease/atrial-fibrillation/).

</Patient>

<Student>

Rhythm and conduction disorders are covered in the disease articles, for example [atrial fibrillation](../../disease/atrial-fibrillation/).

</Student>

<Quiz />