AV nodal conduction and pacemaker activity are under strict control by the autonomic nervous system. Due to the unique property of decremental conduction, the AVN protects the heart from an excessive ventricular rate during rapid atrial arrhythmias. On the other hand, the AVN is also an important source of brady- and tachyarrhythmias, and a target for various pharmacological and non-pharmacological arrhythmia therapies. Access to the complete content on Oxford Medicine Online requires a subscription or purchase.
Public users are able to search the site and view the abstracts for each book and chapter without a subscription. Please subscribe or login to access full text content. If you have purchased a print title that contains an access token, please see the token for information about how to register your code.
For questions on access or troubleshooting, please check our FAQs , and if you can't find the answer there, please contact us. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use for details see Privacy Policy and Legal Notice. Oxford Medicine Online. Publications Pages Publications Pages. Recently viewed 0 Save Search.
ESC CardioMed 3 ed. John Camm A. Thomas F. This electrical pulse travels down through the conduction pathways and causes the heart's lower chambers ventricles to contract and pump out blood. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. The ventricles then contract to push blood out into the blood vessels of the body. The original electrical impulse travels from the sinus node across the cells of your heart's right and left atria.
The signal travels to the AV node atrioventricular node. This node is located between the atria and the ventricles. In the AV node, the impulses are slowed down for a very short period. This allows the atria to contract a fraction of a second before the ventricles. The blood from the atria empties into the ventricles before the ventricles contract. Rather, they enter into contact with atrial working fibers after a small area composed of transitional cells.
In the SA node, Keith and Flack 5 distinguished between the sinus and working cells. Tawara 2 , however, indicated the difficulties he encountered in differentiating AV node cells from those of the bundle of His. He therefore proposed that the difference between them was purely anatomical. On the basis of this definition, the portion of the CS completely sheathed by the CFB is termed the penetrating bundle or bundle of His Figure 3a.
The atrial portion from the proximal conduction system to the bundle of His is called the AV node Figure 3b. This anatomical distinction is logical because the insulation of the penetrating bundle of His prevents it from making direct contact with the electrical activity of the afferent atrium.
Any atrial activity must therefore be previously directed through the AV node. Sagittal histological sections of the sinoatrial SA node of the human a;x10 and pig heart b;x40 stained with the van Gieson method. Sinus cells are characterized by being clearer and embedded in a greater amount of connective tissue red.
Note the shape of the compact AV node and the transitional cells TC in contact with the convex surface of the compact node. The intrinsic function of the SA node is to be the source of the cardiac impulse. The SA node in humans is an arched or fusiform structure.
Histologically it is composed of cells slightly smaller than normal working cells which are arranged in bundles. These mix together with no spatial order, stain weakly, and are embedded in a dense connective tissue matrix Figures 3 a and b.
With age, the amount of connective tissue increases with respect to the area occupied by the nodal cells. In addition, multiple radiations or extensions interdigitating with the working atrial myocardium have been described.
These penetrate intramyocardially into the terminal crest, and the superior and inferior vena cavae. The SA node is arranged around an artery known as the sinus node artery, which can run centrally or eccentrically inside the node. It has been suggested that the majority of these nerve fibers are parasympathetic, the sympathetic fibers being concentrated around the node's blood vessels.
The inherent function of the AV node is to delay the cardiac impulse. In humans, this node has a compact portion and an area of transitional cells.
The former is semi-oval and lies over the CFB Figure 3c. In the sections close to the base of the triangle of Koch, the compact part of the node divides into two extensions or prolongations. The artery vascularizing the AV node is usually found between these. The length of these extensions varies from one heart to another. They are surrounded by a greater quantity of connective cells than that covering the working cells, but they are not insulated from the adjacent myocardium.
Rather, they form a kind of bridge between the working and nodal myocardium, and collect electrical information from the atrial walls, transmitting it to the AV node. Controversy surrounds how the impulse from the SA node reaches the AV node. Some authors have suggested the existence of specialized tracts between them. The AV node continues distally with the penetrating bundle of His Figure 3d , although there are slight differences in terms of cellular arrangement between these two structures, including the arrangement of the bundle of His cells in a more parallel fashion.
The explanation for this might be morphological: the bundle of His starts to be surrounded by the connective tissue of the CFB, thus becoming a conducting tract that takes information to the ventricles. The AV node of the dog is smaller than that of humans, but has a longer penetrating bundle of His. In the rabbit, other authors 29 describe part of the bundle of His as though it formed part of the AV node, but this is a mistake Figures 4a-d. The most outstanding morphological difference between the AV node of the dog and those of the rabbit and humans is that the former is not covered by transitional cells.
In rats with a resting heart rate 10 times faster than that of dogs or humans , the AV node is proportionally comparable to that of the dog, but the CFB is smaller. This composite figure shows the atrioventricular AV node plus the bundle of His and its right and left bundle branches in the rabbit.
Horizontal bar in b represents 1 mm same for all images. Masson's trichrome staining. When the histological trajectory of the conduction system is followed towards the penetrating bundle of His, the latter is seen to turn towards the left in many human hearts, and emerge on the muscular crest of the interventricular septum.
Surrounded by connective tissue from the CFB, the length of the bundle of His can vary before splitting into the left and right bundle branches. The former branch cascades over the left side of the interventricular septum Figures 5a and c. The division of the bundle of His resembles a jockey squatting above the muscular crest of the interventricular septum Figure 5a. However, on occasions it is deviated towards the left Figure 5c. When this occurs, the right branch enters the interior of the septum musculature Figure 5b , appearing in the right ventricle in association with the insertion of the medial papillary muscle.
A indicates aorta; E, endocardium; TV, tricuspid valve. Along their proximal courses, the right and left bundle branches are covered by a fibrous lamina Figures 5b and d. As Tawara 2 showed Figure 6a , in humans the left branch is typically divided into three fascicles with extensive intercommunication. These fascicles become ramified in the ventricular apex, and extend to the interior of the two papillary muscles of the mitral valve, but also back along the ventricular walls toward the cardiac base.
More distally, in the apex of the ventricles of the human heart, it becomes almost impossible to trace the ramifications of the Purkinje fibers since these lose their fibrous coat and look much like the working myocardium. Subendocardial injections of Indian ink reveal the right and left bundle branches and the Purkinje network. Note in B the three fascicles of the left bundle branch arrows , and in C the moderator band MB. Note the elliptical arrangement of the network and offshoots from the edges that penetrate the myocardium arrows.
Note the difference in arrangement between the medial and deep layers of the left ventricle. Subendothelial injection of India ink is one of the methods used to observe these fibrous sheets and to demonstrate the subendocardial course of the right and left bundle branches and their ramifications in ungulate hearts Figures 6b and d.
Our studies on the hearts of sheep and calves show these to vary somewhat from human hearts. Calf hearts are more similar to human hearts in that the fascicles of the left bundle branch are usually three in number and originate in the upper part of the interventricular septum Figure 6b.
However, sheep hearts show only two fascicles, and these appear halfway down the length of the septal wall. In both sheep and calf hearts, small muscular trabeculae cross the ventricular cavity--the so-called «false tendon»--which inside them carry distal ramifications of the His branches towards the papillary muscles and the adjacent ventricular walls.
On the right side of the heart, the moderator band of both the sheep and calf heart is more slender than that of humans, but inside it always contains an offshoot of the right bundle branch Figure 6c. In ungulate hearts the subendocardial Purkinje network is elliptical in arrangement, both in the left and right ventricle Figure 6e.
In addition, from its contour arise branches that penetrate the ventricular walls, leading to new branches or anastamoses with other branches Figure 6e. However, intramural branches of the Purkinje network have not been demonstrated in the human heart.
A controversial point regarding the Purkinje network is the existence of transitional cells between the working ventricular myocardium and the Purkinje fibers. However, such cells have not been observed in the sheep heart. The spatial orientation of the working myofibrils in the ventricle walls determines the anisotropic nature of ventricular conduction Figure 6f.
Although differences exist between species, the structure of the nodes, as well as that of the remainder of the human AV conduction system, is similar to that of commonly used laboratory animals. The SA node, the structure that generates the cardiac impulse, is situated at one extreme of the right atrium. Impulses from it travel posteriorly in the atrial walls through an intricate but precise spatial arrangement of working atrial fibers until reaching the end of the atrium.
At this end, transitional cells of the AV node receive the impulse and delay it prior to its transmission via the bundle of His. The latter crosses the insulating fibrous plane between the atria and ventricles, and transmits the impulse via two branches the right and left bundle branches towards the corresponding ventricles.
Each of these branches is insulated by a connective sheath of working ventricular myocytes. This arrangement allows contact between the specialized and working myocytes only at the distal ramifications of the bundle of His. In this way, the AV conduction system, largely described by Tawara 2 nearly years ago, is structured to impart order to the transmission of cardiac impulses.
Knowing the structure and location of specific conductive tissue within the heart could help provide solutions to different disturbances in cardiac rhythm. Correspondence: Dr. Facultad de Medicina. E-mail: damians unex. Home Articles in press Current Issue Archive.
0コメント