The atrioventricular node (also known as the AV node) is a collection of specialized cardiac muscle cells, which are bundled together to form a node within the wall of the interatrial septum. It is an important component of the cardiac conduction system and is responsible for transmitting impulses that originate in the sinoatrial (SA) node to the ventricles of the heart. An important feature of the AV node is its ability to slightly delay electrical signals, thus coordinating the contraction firstly of the atria and secondly of the ventricles.
This article will cover the anatomy and function of the AV node.
|Structure||Node of specialized cardiac muscle cells|
|Location||Posteroinferior interatrial septum wall, within triangle of atrioventricular node (Koch’s triangle)|
|Function||Transmits impulses from atria to ventricles and coordinates their contraction|
|Blood supply||Atrioventricular nodal artery (branch of right coronary artery)|
|Innervation||Sympathetic and parasympathetic branches from cardiac plexus|
The atrioventricular node is an oblique, oval-shaped collection of cells located in the wall of the posteroinferior region of the interatrial septum, close to the coronary sinus. It lies within the triangle of atrioventricular node (or Koch’s triangle), which is the area bordered by the right atrioventricular valve anteriorly, the opening of the coronary sinus basally and the tendon of the inferior pyramidal space (tendon of Todaro) posteriorly.
The cells of the AV node are specialized cardiac muscle cells (cardiomyocytes), also known as nodal cardiac muscle cells, which are smaller than typical cardiomyocytes and lack intercalated discs. A special feature of nodal cells is the ability of self-excitation, through which they are able to independently produce spontaneous electrical impulses. Nodal cells transmit these impulses onto adjacent/perinodal cardiac myocytes through gap junctions. Important to note is that the electrical signals are delayed slightly within the AV node (by approx. 40 ms), which is due to the lower number of gap junctions present on these cells.
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The atrioventricular node is the second of several components of the cardiac conduction system. This intrinsic conduction system is responsible for generating impulses for the contractions of the heart and ensuring coordinated blood flow through the chambers of the heart. It consists of the following components:
- Sinuatrial (SA) node
- Atrioventricular (AV) node
- Atrioventricular (AV) bundle (of His)
- Right and left bundle branches
- Subendocardial branches of atrioventricular bundle (Purkinje fibers)
The first component, the SA node, acts as the pacemaker of the heart, as it sets the rhythm for the remaining components of the cardiac conduction system. These structures are all connected by specialized conduction fibers, which enable fast transmission of electrical impulses. The impulses spread firstly through the walls of the atria until they reach the AV node, where the signal is delayed slightly (by approx. 40 ms), before being passed onto the ventricles through the atrioventricular (AV) bundle (of His). The delay ensures sufficient time for the atria to empty before the contraction of the ventricles. The AV bundle can also be considered as a continuation of the AV node, stretching into the interventricular septum.
Although the AV node has the primary function of transmitting impulses into the ventricles of the heart, its nodal cells are also able to produce their own electrical impulses (self-excitation). When the SA node or the connected conducting fibers are dysfunctional and electrical signals are not transmitted further, the AV node is able to independently produce impulses in order to maintain the contractions of the ventricles. It is therefore often referred to as the secondary pacemaker of the heart.
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The atrioventricular node receives its arterial blood supply from the atrioventricular nodal artery, which is a large arterial branch in the interventricular septum, arising from the right coronary artery.
Although the heart contains its own intrinsic cardiac conduction system, it relies on external autonomic innervation received via the cardiac plexus to modulate its function. The parasympathetic portions of the cardiac plexus receive contributions from the vagus nerve only and are responsible for reducing the heart rate and force of contraction. The sympathetic part of the cardiac plexus is received via the cardiac nerves which originate from the cervical and upper thoracic sympathetic ganglia. Sympathetic input to the heart is responsible for increasing heart rate and force of contraction, such as during the ‘fight or flight’ response, which causes our heart to beat faster.
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Atrioventricular block (AV block)
A disruption or delay in the transmission of electrical impulses from the atria to the ventricles is referred to as atrioventricular or AV block. An AV block may occur as a result of a number of conditions affecting the AV node and/or the AV bundle (of His), including ischemia, compression or inflammation of these structures.
AV block can be classified according to the severity of the delay or disruption in conduction. A first-degree AV block describes an increased delay in conduction and results in a prolonged PR interval (> 200 ms) in an ECG - this interval represents the time between the depolarization of the atria and the depolarization of the ventricles. A second-degree AV block is defined as an increase in the PR interval with the occurrence of “dropped beats”. This phenomenon is the absence of a ventricular contraction following an atrial contraction, which is the result of inadequate conduction from the atria to the ventricles. A third-degree AV block, also called a complete AV block, occurs when there is a complete block of impulse transmission from the atria to the ventricles. In this case, the ventricular contractions are maintained by the AV node (secondary pacemaker), whereas the atria continue to contract to the rhythm of the SA node, so that these become completely disconnected and independent of one another.
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