Difference Between SA Node and AV Node

The main difference between SA node and AV node is that the SA node generates cardiac impulses whereas the AV node relays and intensifies cardiac impulses. Furthermore, SA node is located in the right atrium, close to the point of entry of the superior vena cava while AV node is located in the right atrium, close to the opening of the coronary sinus.

SA node and AV node are two elements of the cardiac conduction system that controls the heart rate.

Key Areas Covered

1. What is SA Node
     – Definition, Location, Role
2. What is AV Node
     – Definition, Location, Role
3. What are the Similarities Between SA Node and AV Node
     – Outline of Common Features
4. What is the Difference Between SA Node and AV Node
     – Comparison of Key Differences

Key Terms

AV (Atrioventricular) Node, Pacemaker, Pacesetter, Right Atrium, SA (Sinoatrial) Node

What is SA Node

SA (sinoatrial) node is the group of cells that generate electrical impulses in the heart. It sends sparks electrical impulses within a set of intervals, establishing the rate of the heart as 60-80 beats per minute (bpm). The beats generated by the SA node are called the rhythm of the heart. When the body requires more oxygen while doing power-intensive physical activities, SA node increases the rate of the heart, pumping more blood to the tissues.

Figure 1: Electrical Conduction System of the Heart

SA node is made up of pacemaker cells surrounded by paranodal cells. The cells of the SA node are smaller than the cells that make up the atrium. Also, they encompass fewer mitochondria. Sympathetic nervous system slows down the rate of the impulses while parasympathetic nervous system accelerates the rate of impulses.  

What is AV Node

AV (atrioventricular) node is the group of cells located in the border between the right atrium and the right ventricle, which serves as the secondary contractile system of the heart. It is primarily induced by SA node. It relays on signals of the SA node and holds them for 0.09 seconds, which is the time required by the ventricles to be filled with the atrial blood. 

Figure 2: Cardiac Contraction

Then the cardiac impulses from the SA node are transmitted to the ventricles, contracting them. This chiefly means SA node sets the rhythm of the heartbeat while AV node sets the rhythm of the heart contraction. 

Similarities Between SA Node and AV Node

• SA node and AV node are two elements of the contractile system of the heart.
• They are located in the wall of the right atrium.
• Both are involved in maintaining the heart rate by inducing contraction.

Difference Between SA Node and AV Node

Definition

SA (sinoatrial) node refers to a small mass of tissue in the right atrium functioning as pacemaker of the heart by giving rise to the electric impulses that initiate heart contractions while AV (atrioventricular) node refers to a small mass of tissue that is situated in the wall of the right atrium and passes impulses received from the SA node to the ventricles.

Location

SA node is located in the superior lateral wall of the right atrium, close to the opening of the superior vena cava while AV node is located in the posterior septal wall of the right atrium, close to the opening of the coronary sinus.

Size and Shape

When comparing the size and shape, SA node is longer, flattened, and ellipsoidal while AV node is short and half-oval.

Function

While SA node generates cardiac impulses, AV node relays the cardiac impulses generated by SA node and intensifies them. This is the main difference between SA node and AV node.

Firing Rate

The firing rate of the SA node is 60-100 bpm while the intrinsic firing rate of the AV node is 40-60 bpm.

Transmission of the Impulses

SA node transmits cardiac impulses directly to the right and left atrium while AV node transmits cardiac impulses the right and left ventricles through the branches and the terminal strands of the bundle of His.

Significance

Whereas SA node serves as the pacemaker of the heart, AV node serves as the pacesetter of the heart.

Regulation

The autonomic nervous system regulates SA node while AV node is regulated by the SA node.

Conclusion

SA node is the primary element of the heart that produces cardiac impulses. Therefore, it is called the pacemaker of the heart. On the other hand, AV node is the secondary element of the heart, which relays on the signals of the SA node, intensifying them and transmitting them to the ventricles. The main difference between SA node and AV node is the function.

Reference:

1. Kashou AH, Kashou HE. Physiology, Sinoatrial Node (SA Node) [Updated 2017 Oct 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2018 Jan-. Available Here 
2. Klabunde, Richard E. “Normal Impulse Conduction.” Cardiovascular Physiology Concepts, Available Here 

Image Courtesy:

1. “ConductionsystemoftheheartwithouttheHeart-en” By Madhero88 (original files); Angelito7 (this SVG version); – ConductionsystemoftheheartwithouttheHeart.png & ConductionsystemoftheheartwithoutHeart.svg (CC BY-SA 3.0) via Commons Wikimedia  
2. “2019 Cardiac ConductionN” By OpenStax College – Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia