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Structure

Cardiac conduction system

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

Specialised myocardial cells generate and conduct the electrical impulse that coordinates the heartbeat: sinoatrial node, atrioventricular node, bundle of His, bundle branches and Purkinje fibres.

Terminologia Anatomica (Latin): Complexus stimulans cordis · Open in the 3D atlas

Location

The sinoatrial (SA) node lies at the junction of the superior vena cava and right atrium near the upper crista terminalis. The atrioventricular (AV) node lies in the triangle of Koch, and the bundle of His penetrates the central fibrous body to reach the crest of the muscular interventricular septum.

Structure

The AV bundle divides into a right bundle branch (running in the moderator band) and a left bundle branch that fans into anterior and posterior fascicles (and variably a septal fascicle), ending in the subendocardial Purkinje network. Atrial and ventricular myocardium are electrically separated by the fibrous skeleton except at the AV bundle. Intranodal and internodal pathways exist physiologically, though their anatomical detail is debated.

Blood supply

SA node artery: right coronary in about 60%, circumflex in about 40%. AV node artery: from the dominant coronary (right in most people) at the crux of the heart. The bundle and branches receive AV nodal and septal perforator branches of the LAD; the right bundle branch is mainly LAD-dependent.

Innervation

Rich sympathetic (beta-1) supply increases the rate of impulse generation and conduction speed; vagal supply (muscarinic) slows the SA node and, especially via the left vagus, the AV node. The nodes are therefore the main targets of autonomic control of heart rate.

Function

The SA node is the pacemaker, discharging at about 60 to 100 beats per minute at rest. The AV node delays conduction (about 0.1 s of the PR interval) to allow ventricular filling, and rapid His-Purkinje conduction produces coordinated ventricular contraction. Subsidiary pacemakers take over at slower rates if higher centres fail.

Clinical correlations

Sinus node dysfunction and AV block cause bradycardia and may need pacing; inferior infarction commonly affects the AV node because of its right coronary supply. Left or right bundle branch block appears as a widened QRS. Accessory pathways (for example Wolff-Parkinson-White) bypass the AV node and predispose to re-entrant tachycardias.

Common exam points

  • Conduction order: SA node, atrial myocardium, AV node, bundle of His, right and left bundle branches, Purkinje fibres.
  • AV nodal delay accounts for much of the PR interval; the node is in the triangle of Koch.
  • SA node artery: RCA in about 60%; AV node artery: dominant coronary at the crux.
  • Right vagus predominantly slows the SA node, left vagus the AV node.
  • Right bundle branch runs in the moderator band; left bundle branch has anterior and posterior fascicles.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)
  4. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)

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