Structure of the nodes and the atrioventricular conduction system. Studies in which the histological techniques employed were similar to those used by Tawara 2 and later workers such as Davies 16 and Truex et al 17 to mention just a few have shown that the CS of humans is arranged in a manner quite like that of other mammals with slight variations between species and between hearts.
Simply put, specialized myocytes stand out from working myocytes when viewed under the light microscope, and can be «followed» from one histological section to the next. In his monograph, Robb 8 preferred to define the conductive tissue with the term «connecting» rather than «conducting» system, because histological preparations better define cell morphology than function.
He also observed differences in the texture of the specialized myocardium depending on the freshness of autopsy material and the fixing and staining methods used.
Tawara 2 was aware of this and pointed out the heterogeneity of specialized myocyte morphology even in histological sections of the heart. Within a given species, the most obvious differences are related to the age of the individuals examined. However, no specific marker has been found that that can highlight this tissue in adult humans. 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. It is believed by many that there are three preferential anatomic conduction pathways from the sinoatrial node to the atrioventricular node [1,18]. In general, these can be considered as the shortest electrical routes between the nodes. Note that there are microscopically identifiable structures, appearing to be preferentially oriented fibers, that provide a direct node-to-node pathway.
In some hearts, pale staining Purkinje-like fibers have also been reported in these regions. More specifically, the anterior tract is described as extending from the anterior part of the sinoatrial node, bifurcating into the so-called Bachmann's bundle which importantly delivers impulses to the left atrium and with a second tract that descends along the interatrial septum that connects to the anterior part of the atrioventricular node.
The middle or Wenckebach's pathway extends from the superior part of the sinoatrial node, runs posteriorly to the superior vena cava, then descends within the atrial septum, and may join the anterior bundle as it enters the atrioventricular node.
The third pathway is described as being posterior Thorel's which, in general, is considered to extend from the inferior part of the sinoatrial node, passing through the crista terminalis and the Eustachian valve past the coronary sinus to enter the posterior portion of the atrioventricular node. In addition to excitation along these preferential conduction pathways, general excitation spreads from cell to cell throughout the entire atrial myocardium via the specialized connections between cells, the gap junctions, that typically exist between all myocardial cell types see below.
It then follows that towards the end of atrial depolarization, the excitation reaches the atrioventricular node via the aforementioned atrial routes, with the final result being excitation of the atrioventricular node.
Further, these routes are known as the slow or fast pathways, which are considered to be functionally and anatomically distinct. The slow pathway typically crosses the isthmus between the coronary sinus and the tricuspid annulus; it has a longer conduction time, but a shorter effective refractory period. The fast pathway is commonly a superior route, emanating from the interatrial septum, and has a faster conduction rate but, in turn, a longer effective refractory period.
Though the primary function of the atrioventricular node may seem simple, that is to relay conduction between the atria and ventricles, its structure is very complex [1]. As a means to describe these complexities, mathematical arrays and finite element analysis models have been constructed to elucidate the underlying structure-function relationship of the node [19]. In general, the atrioventricular node is located in the so-called floor of the right atrium, over the muscular part of the interventricular septum, inferior to the membranous septum: i.
Following atrioventricular nodal excitation, the slow pathway conducts impulses to the His bundle, indicated by a longer interval between atrial and His activation. Currently, there is interest in the ability to place pacing leads to preferentially activate the bundle of His; in such approaches, ultrasound or other imaging modalities are used to map the electrical characteristic His potentials to position the pacing leads [20].
After leaving the bundle of His, the normal wave of cardiac depolarization spreads first to both the left and right bundle branches; these pathways rapidly and simultaneously carry depolarization to the apical regions of both the left and right ventricles see Figure 1. Finally, the signal broadly travels through the remainder of the Purkinje fibers and ventricular myocardial depolarization spreads. In certain pathological conditions, direct accessory connections from the atrioventricular node and the penetrating portion of the bundle of His to the ventricular myocardium have been described [21].
Yet, the function and prevalence of these connections, termed Mahaim fibers, is poorly understood. A rare bundle of Kent, an additional aberrant pathway when present, exists between the atria and ventricles and is associated with the clinical manifestation of ventricular tachycardias also known as Wolff-Parkinson-White syndrome.
The atrioventricular node AVN and the surrounding area is a crucial part of the cardiac conduction system. It consists of specialized tissue located at the base of the atrial septum within the triangle of Koch.
The inherent physiological function of the AVN is to delay cardiac impulse propagation between the atria and the ventricles, and to function as a backup pacemaker in the setting of sinoatrial node dysfunction or advanced atrioventricular AV block.
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.
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