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Longitudinal dissociation of the aggregated specialized cardiomyocytes within the non-branching portion of atrioventricular conduction axis has proved a controversial topic for both morphologists and electrophysiologists. We have now used morphological methods, including three-dimensional assessment, to revisit, in human, canine, and bovine hearts, the presence or absence of interconnections between the aggregated cardiomyocytes making up the non-branching bundle. We analyzed three datasets from human and canine hearts, and two from bovine hearts, using longitudinal and orthogonal serial histological sections. In addition, we assessed three hearts using translucent India ink injected specimens, permitting assessment of the three-dimensional arrangement of the cardiomyocytes. Using the longitudinal sections, we found numerous oblique interconnections between the groups of specialized cardiomyocytes. When assessing orthogonal sections, we noted marked variation in the grouping of the cardiomyocytes. We interpreted this finding as evidence of bifurcation and convergence of the groups seen in the longitudinal sections. The three-dimensional assessment of the bovine material confirmed the presence of the numerous interconnections. The presence of multiple connections between the cardiomyocytes in the non-branching bundle rules out the potential for longitudinal dissociation., (© 2023 American Association of Clinical Anatomists and British Association of Clinical Anatomists.)

The cardiac conduction system (CCS) is crucial for regulating heartbeats; therefore, clinicians and comedicals involved in cardiovascular medicine treatment must have a thorough understanding of the CCS structure and function. However, anatomical education of the CCS based on actual dissection and observation is uncommon, although such educational methodology promotes three-dimensional structural understanding of the observed object. Based on previous studies, we examined the CCS structure in the heart of a swine (pig, Sus scrofa domestica) which has been used in the biological, medical and anatomical curricula as science teaching materials, by using macroscopic dissection procedures. Most CCS structures in a young pig heart were successfully identified and illustrated on a macroscopic scale. The atrioventricular bundle (His bundle) was located on the lower edge of the membranous interventricular septum and was clearly distinguished from the general myocardial fibres by its colour and fibre arrangement direction. Following the atrioventricular bundle towards the atrium or ventricle with properly removing the endocardium and myocardium, the atrioventricular node or the right and left bundles appeared respectively. In contrast, the sinoatrial node was not identified. The anatomy of the CCS in young pig hearts was essentially similar to that previously reported in humans and several domestic animals. Our findings of the CCS in young pig hearts are expected to be useful for medical and anatomical education for medical and comedical students, young clinicians and comedical workers., (© 2022 Wiley-VCH GmbH.)

Background: Subtle differences exist between dog and human, despite use of the dog as a model for cardiac surgical and electrophysiological research., Objective: The purpose of this study was to investigate the differences in the atrioventricular conduction axis and adjacent structures between dogs and humans., Methods: We prepared 33 human and 5 canine hearts for serial histologic sections of the atrioventricular conduction axis, making correlations with gross anatomic findings. We additionally examined and photographed 15 intact normal human hearts obtained from infants undergoing autopsy. Furthermore, we interrogated a computed tomographic dataset from a human adolescent and from 2 autopsied canine hearts, both with normal cardiac anatomy., Results: All canine hearts lacked an inferoseptal recess, with the noncoronary leaflet of the aortic valve and the right fibrous trigone having direct attachments to the septal surface of the left ventricular outflow tract. This correlated with an extensive nonbranching component of the ventricular conduction axis, which skirted half of the noncoronary aortic sinus. This anatomic arrangement was observed in 2 of 15 of autopsied infant hearts. In the human hearts with an inferoseptal recess, the relatively shorter nonbranching bundle is embedded within the fibrous tissue forming its right wall., Conclusion: We found a major difference between canine and the majority of human hearts, namely, the presence or absence of an inferoseptal recess. When this recess is absent, as in the canine heart and in some human hearts, a greater proportion of the atrioventricular conduction axis is found within the circumference of the subaortic outflow tract., (Copyright © 2021 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.)

Atrioventricular node is responsible for delaying the passage of the electrical impulse to ventricles in order to protect them from fast depolarizations coming from the atria. The importance of this study is to identify the morphological variations of the components of atrioventricular zone that affect the conduction system and its clinical relationship in different species of mammals. We analyzed ten human hearts, nine from horses, eight from pigs, and five from dogs without a clinical history of cardiac pathologies. Histological section thickness of 5 μm were obtained with a microtome and stained with hematoxylin-eosin and Masson's trichrome. We observed both an increase in collagen fibers and a decrease in the size of P cells (nodal pacemaker cells) within the atrioventricular node in dogs, horses and pigs in cases that presented cartilage in fibrous body. The percentage of fundamental substance in atrioventricular node was significantly higher in dogs and the percentage of collagen fibers was higher in pigs, both than in humans. The presence of cartilaginous metaplasia in cardiac fibrous skeleton from different species decreases the size of atrioventricular node and its cells and increases the percentage of collagen fibers within the node, which can reduce the transmission of the electrical impulse to ventricles and therefore predispose to the presentation of ventricular arrhythmias. Morphometric analysis has allowed us to objectively quantify each of the components of AV node and compare them in the different species.

Background: Reports of conduction abnormalities necessitating permanent pacemaker implantation due to atrioventricular node artery injury and increasing evidence of stenosis of the atrioventricular node artery in cases of sudden death are of unsolved clinical importance. Unfortunately, technical issues associated with physical and virtual dissections of the atrioventricular conduction axis make it difficult to accurately assess its arterial supply., Methods: We used a specialized dissection technique to gather anatomical information on the atrioventricular node artery and described them using attitudinally appropriate terminology., Results: The mean number of atrioventricular node artery branches was 1.6 in 103 submacroscopic examinations and 2.3 in 17 histological reconstructions. The artery had 5 origins in the modified AHA anatomy guidelines: distal RCA (#3), 10.4%; right posterior interventricular artery (#4PI), 7.3%; proximal RCA posterolateral branch (proximal #4PL), 76.8%; distal RCA posterolateral branch detouring the coronary sinus (distal #4PL), 1.8%; distal LCX (#13), 3.7%. Histological examination revealed that most atrioventricular node arteries immediately left the distal compact node (71.8%), suggesting that they supply mainly the proximal part of the AV conduction axis. The artery to the atrioventricular node tended to originate from the medial and atrial aspect of RCA posterolateral branch, and supplied adjacent structures within the inferior pyramidal space before entering the compact atrioventricular node., Conclusions: Based on the visualisation of the atrioventricular conduction axis and its arterial supply, we herein provide the 'gold standard' for understanding the origin, course and distribution of the artery to the atrioventricular node., (Copyright © 2018 Elsevier B.V. All rights reserved.)

We studied the morphology of the atrioventricular conduction system (AVCS) and Purkinje fibers of the yak. Light and transmission electron microscopy were used to study the histological features of AVCS. The distributional characteristics of the His-bundle, the left bundle branch (LBB), right bundle branch (RBB), and Purkinje fiber network of yak hearts were examined using gross dissection, ink injection, and ABS casting. The results showed that the atrioventricular node (AVN) of yak located in the right side of interatrial septum and had a flattened ovoid shape. The AVN of yak is composed of the slender, interweaving cells formed almost entirely of the transitional cells (T-cells). The His-bundle extended from the AVN, and split into left LBB and RBB at the crest of the interventricular septum. The LBB descended along the left side of interventricular septum. At approximately the upper 1/3 of the interventricular septum, the LBB typically divided into three branches. The RBB ran under the endocardium of the right side of interventricular septum, and extended to the base of septal papillary muscle, passed into the moderator band, crossed the right ventricular cavity to reach the base of anterior papillary muscle, and divided into four fascicles under the subendocardial layer. The Purkinje fibers in the ventricle formed a complex spatial network. The distributional and cellular component characteristics of the AVCS and Purkinje fibers ensured normal cardiac function., (© 2017 Wiley Periodicals, Inc.)

In this article, the authors outline the key components behind the automated generation of the cardiac impulses and the effect these impulses have on cardiac myocytes. Also, a description of the key components of the normal cardiac conduction system is provided, including the sinoatrial node, the atrioventricular node, the His bundle, the bundle branches, and the Purkinje network. Finally, an outline of how each stage of the cardiac conduction system is represented on the electrocardiogram is described, allowing the reader of the electrocardiogram to translate background information about the normal cardiac conduction system to everyday clinical practice., (Crown Copyright © 2016. Published by Elsevier Inc. All rights reserved.)

The electrical impulse for cardiac contraction is generated in the Sinoatrial node (SA node), subsequently spreads to the Atrioventricular node (AV node) and continues in the Atrioventricular bundle (AV bundle). The AV node may not always be present in different avian species and seems to differ in location and contents between species. In this study, the anatomy and histology of the AV node were studied five female adult ostriches (Struthio camelus). Routine paraffin sectioning and transmission electron microscopic method were performed. The study showed that in the ostrich, the AV node is located in the endocardium of the atrial surface of the right atrioventricular valve adjacent to the fibrous ring. The parenchyma of the AV node is formed by small specialized muscle fibres that are spread within a loose connective tissue network. The AV node is not covered by a connective tissue sheath and some arterioles are present. Nerve fibres are seen related to the node. Ultrastructurally, they stain lighter and contain fewer organized myofibrils than usual myocardial cells. The myofibril bundles run parallel to one another and have interspersed mitochondria, which display distinct cristae. The cells have a large euchromatic nucleus with a clear perinuclear area, and they connected by desmosomes. The ostrich is, thus, one of the birds that have the AV node, whose position varies from the other birds., (© 2011 Blackwell Verlag GmbH.)

The specialized cardiac conduction system (CCS) consists of the sinoatrial node (SAN) and the atrioventricular (AV) conduction system (AVCS), which includes proximal (AV node, bundle of His and bundle branches) and distal (Purkinje fibers) components. In four CCS marker mice [two transgenic (cGATA6|lacZ, CCS|lacZ) and two targeted gene knock-in (minK|lacZ, Hop|lacZ)] the expression of the lacZ gene (beta-gal) has been reported to mark portions of the proximal and distal AVCS; the expression of this marker in the adult SAN is unknown. The primary objective of this study was to analyze the utility of these marker mice in the identification of the SAN. Intercaval and interventricular septal regions, containing all the components of the CCS, were freshly dissected from adult mice based on the anatomical landmarks and sectioned. Immunohistochemical characterization was performed with SAN markers (Cx45, HCN4), compared to the reporter expression (beta-gal) and markers of the working myocardium (Cx40 and Cx43). In all four of the CCS marker mice, we found that beta-gal expression is consistently observed in the proximal and distal AVCS. However, the presence of lacZ gene expression in the working myocardium outside the CCS and/or the absence of this reporter expression in the SAN prevent the effective use of these mice to identify the SAN, leading us to conclude that none of the four CCS marker mice we studied specifically mark the SAN.

Scaling can be defined as the adjustment of a structure, a function, or an organ to the size of the mammalian body. An example is the size of the heart in relation to the size of the body. The duration of the PR interval on the electrocardiogram (atrioventricular delay) in relation to the size of the heart is a perplexing example of scaling. During evolution, mammalian species changed their shape, size, and function while adapting to the habitat in which they had to live and survive. This review deals with the problem of the apparent mismatch in scaling of the atrioventricular delay (PR interval) in relation to the size of the mammalian heart from mouse to whale.

Concomitant with the development of catheter ablation techniques for the treatment of atrial arrhythmias, there has been renewed interest in the morphologic arrangement of the cardiac conduction system. The first descriptions of the anatomy of the nodes and atrioventricular conduction system appeared nearly 100 years ago. Since then the subject has been controversial, possibly because of the early researchers' imprecise knowledge of histology. The components and structure of the specific conduction system in humans are similar to those found in commonly used laboratory animals. The conduction system is composed of specialized myocytes. Its atrial components, the sinus node and the atrioventricular node, are in contact with atrial myocardium. The His bundle penetrates the right fibrous trigone, then divides into two specialized ventricular bundle branches (right and left), which also are surrounded by a fibrous sheath that separates the specialized myocytes from the ordinary myocardium. Only at the distal ramifications of the bundle branches do the fibrous sheaths disappear, allowing continuity with the ventricular myocardium. Knowledge of the specialized myocardium can help in the development of potentially useful therapies for some forms of cardiac arrhythmia.

Although we soon will be marking the 100th anniversary of the discovery of the atrioventricular (AV) node, the mysteries of this most complex of all parts of the conduction system of the heart remain. We are still battling controversies related to the precise morphology of the AV node and its atrial approaches. We are still debating the exact reentrant pathways of the AV nodal reentrant tachycardia. We are still uncertain if the so-called dual AV nodal electrophysiology encompasses two or more pathways, and what exactly makes these pathways in the absence of distinct insulated cables between the atrium and the AV node. It may be just surprising, in view of the above limitations, that current level of knowledge has nevertheless made possible some of the most spectacular successes in the modern cardiac electrophysiology. Thus, the cases of typical AVNRT are cured with a very high rate of success by radiofrequency ablations, increasing the quality of life of thousands of patients. AV nodal modifications are being performed to slow the ventricular rate during atrial fibrillation, although more progress is needed in this endeavor. The goal of the present review is to outline the major anatomic and electrophysiologic efforts in understanding the mechanisms underlying the dual pathway AV nodal propagation and to trace some novel approaches that promise to widen the horizon of the experimental and clinical fields.

Although knowledge of its existence has barely exceeded one century, the anatomic pathway for conduction of the cardiac impulse from atria to ventricles has been the source of much controversy, controversy that continues to the present day. This article describes how attention to the original descriptions, along with adherence for sensible definitions for the histological distinction of conduction tissues, could have defused many of these disagreements. One of the problems with the initial descriptions was that they were written in German. Even this is no longer a problem, since the major text devoted to the topic has recently been translated into English, and is now available for all to study. This key work was the monograph produced by Sunao Tawara, a Japanese pathologist who studied in the first decade of the 20th century in the laboratory of Ludwig Aschoff. Tawara showed that the atrial impulse was conducted to the ventricular myocardium by a solitary pathway of specialized myocardium. He likened this pathway to a tree, with its roots in the base of the atrial septum, and its branches straddling the crest of the muscular ventricular septum. Significantly, he showed that the pathway became insulated as it crossed from atrial to ventricular musculatures, and suggested that the point at which the muscle became engulfed in fibrous tissue should be taken as the boundary between the "knoten," or atrioventricular node, and the penetrating bundle described earlier by Wilhelm His. The authors' findings are entirely compatible with the account given by Tawara. Like Tawara, they recognize atrial transitional cells, a compact node, the penetrating bundle, and the ventricular conduction pathways as the segments of the conduction axis which can be considered to represent the specialized atrioventricular junction.

Background: Direct 3D analysis (ie, stereotaxic analysis of 3 planes) has shown that the atrioventricular (AV) node (AVN) is continuous with only specialized myocardium of the proximal AV bundle (PAVB) and distal AV bundle (DAVB) or His bundle. The purpose of the present study was to determine whether the PAVB, AVN, and DAVB possess histological features distinct from each other and from the ordinary myocardium., Methods and Results: A protocol that preserves the cytoplasmic and interstitial integrity of the tissue and permits serial sections of the AV junction region to be made in 3 orthogonal planes showed that the PAVB, AVN, and DAVB are characterized by myocardium aggregated into fascicles containing approximately 8 myofibers. Myofibers within the fascicles are coiled or spiraled about each other; and spiraling is most compact in the PAVB. Collagen encases individual fascicles and segregates primary fascicles into secondary fascicles. Fascicles, and not myofibers, are in parallel array in the PAVB, interwoven in the AVN, and parallel in the DAVB. Narrow junctions of parallel fascicles separate the AVN from the PAVB and DAVB. Myocytes, which are largest in DAVB, possess clear perinuclear regions; thin finger-like end processes, which are most numerous in the AVN; uniform, delicate cross-striations; and intercalated disks, which are broader in the PAVB and form short stacks in the AVN. Sheaves of nerve terminals are found, including boutons as in skeletal muscle [corrected]., Conclusions: The PAVB, AVN, and DAVB have distinct histological features. Collagen septation of primary and secondary fascicles presents natural barriers within the tissues and to surrounding myocardium and structures. These findings confirm that the AV junction region contains a specialized conduction system that is anatomically isolated from ordinary myocardium.

There is little detailed knowledge of the architecture of the AV junction region, the cytoarchitecture of the AV node or of its atrial connections. In the present study, the gross anatomy and topography of intracardiac structures in 21 adult canine hearts were photographically compared in whole and dissected hearts and tissue blocks and serial histologic sections made in three orthogonal planes. There are seven major new findings: 1) A coronary sinus fossa exists at the crux of the heart. It separates the right medial atrial wall (MAW) superoposterior region from the left atrium, its floor is the coronary sinus, and it carries the medial atrionodal bundle and proximal AV bundle on its right wall. 2) The posterior MAW forms two isolated bridges of myocardium as it surrounds the coronary sinus ostium, is isolated from the sinus venarum with crista terminalis and interatrial septum-by the floor of the inferior vena cava, and the narrow bridges link the posterior atrial wall to the mid MAW. 3) The tendon of Todaro has both epicardial and endocardial exposures, terminates in the superoposterior MAW and its medial aspect is adjacent sequentially to the medial atrionodal bundle and proximal AV bundle. 4) Only ordinary myocardium contacts the anulus fibrosus. 5) The ventricular septum's shoulder is humped shape posteriorly, is completely overlaid by anular myocardium and the medial leaflet and is joined by struts of papillary muscle. 6) The membranous septum joins the anterior ventricular septum to the crista supraventricularis, forms part of the posterior noncoronary and right aortic valve sinus walls and encases the right bundle branch. 7) The specialized conduction tissues, the superior, medial and lateral atrionodal bundles, the proximal AV bundle, AV node, distal AV bundle and right bundle branch are subjacent to MAW epicardium outside the right atrium, share regular intracardiac relationships with topographic landmarks and the medial atrionodal bundle, terminal superior atrionodal bundle, the proximal AV bundle and AV node are aligned to the medial leg of Koch's triangle. Thus, atrial myocardium of the AV junction region is that of the MAW. The floor of the inferior vena cava forms a natural barrier to impulse transmission along the full extent of the posterior MAW. The specialized tissues are outside of the MAW. Anatomic landmarks form reliable topographic landmarks for the specialized AV junction region tissues. A knowledge of the association of the specialized conduction tissues with specific regions of the MAW is useful in localizing the tissues and along with the coronary sinus fossa provides several extracardiac approaches., (Copyright 1999 Wiley-Liss, Inc.)

Current nomenclature for the atrioventricular (AV) junctions derives from a surgically distorted view, placing the valvar rings and the triangle of Koch in a single plane with antero-posterior and right-left lateral coordinates. Within this convention, the aorta is considered to occupy an anterior position, although the mouth of the coronary sinus is shown as being posterior. Although this nomenclature has served its purpose for the description and treatment of arrhythmias dependent on accessory pathways and atrioventricular nodal reentry, it is less than satisfactory for the description of atrial and ventricular mapping. To correct these deficiencies, a consensus document has been prepared by experts from the Working Group of Arrhythmias of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. It proposes a new anatomically sound nomenclature that will be applicable to all chambers of the heart. In this report, we discuss its value for description of the AV junctions, establishing the principles of this new nomenclature.

Concomitant with the development of catheter ablation techniques for the treatment of atrial arrhythmias, there is renewed interest in the morphologic arrangement of the cardiac conduction system. In this article, we revisit the anatomy of the specialized tissues, making special reference to the descriptions given at the time of their discovery. According to criteria for histologic distinction of morphologically specialized tracts set nearly 100 years ago, the penetrating bundle (of His) and the ventricular bundle branches are tracts of specialized cells encased by insulating sheaths of fibrous tissue. In contrast, the sinus and AV nodes are recognized histologically but are not insulated from the working atrial myocardium. At its distal extent, the AV node is distinguished from the penetrating bundle not so much by cellular characteristics, but by the presence of a fibrous collar that surrounds the specialized cells. At the atrial part, a zone of histologically transitional cells interposes between the compact node and the working atrial myocardium. Transitional cells enter the triangle of Koch to join the compact node from superiorly, inferiorly, posteriorly, and from the left. Transitional cells of the sinus node, in contrast, are limited to short tongues that interdigitate with musculature of the terminal crest. Apart from a variable extension of its tail, there are no prominent histologically discrete extensions from the sinus node into the working atrial musculature. The internodal myocardium does not contain discrete conducting tracts comparable with the ventricular bundle branches. Preferential conduction more likely reflects the arrangement of the working internodal cells and their related cellular properties.

The mechanisms responsible for atrioventricular (AV) delay remain unclear, in part due to the inability to map electrical activity by conventional microelectrode techniques. In this study, voltage-sensitive dyes and imaging techniques were refined to detect action potentials (APs) from the small cells comprising the AV node and to map activation from the "compact" node. Optical APs (124) were recorded from 5 x 5 mm (approximately 0.5-mm depth) AV zones of perfused rabbit hearts stained with a voltage-sensitive dye. Signals from the node exhibited a set of three spikes; the first and third (peaks I and III) were coincident with atrial (A) and ventricular (V) electrograms, respectively. The second spike (peak II) represented the firing of midnodal (N) and/or lower nodal (NH) cell APs as indicated by their small amplitude, propagation pattern, location determined from superimposition of activation maps and histological sections of the node region, dependence on depth of focus, and insensitivity to tetrodotoxin (TTX). AV delays consisted of tau 1 (49.5 +/- 6.59 ms, 300-ms cycle length), the interval between peaks I and II (perhaps AN to N cells), and tau 2 (57.57 +/- 5.15 ms), the interval between peaks II and III (N to V cells). The conductance time across the node was 10.33 +/- 3.21 ms, indicating an apparent conduction velocity (theta N) of 0.162 +/- 0.02 m/s (n = 9) that was insensitive to TTX. In contrast, tau 1 correlated with changes in AV node delays (measured with surface electrodes) caused by changes in heart rate or perfusion with acetylcholine. The data provide the first maps of activation across the AV node and demonstrate that theta N is faster than previously presumed. These findings are inconsistent with theories of decremental conduction and prove the existence of a conduction barrier between the atrium and the AV node that is an important determinant of AV node delay.

Introduction: The usually accepted definition of the dual pathway electrophysiology requires the presence of conduction curves with a discontinuity ("jump"). However, AV nodal reentrant tachycardia has been observed in patients with "smooth" conduction curves, whereas discontinuity of the conduction curve does not guarantee induction of stable reentry. We hypothesize that the duality of AV nodal conduction can be revealed by careful choice of stimulation sites during the generation of AV nodal conduction curves., Methods and Results: In 21 rabbit heart atrial-AV nodal preparations, programmed electrical stimulation with S1-S2-S3 pacing protocol was applied either posteriorly at the crista terminalis input site (CrT) or anteriorly at the lower interatrial septum input site (IAS), or (in 8 preparations with surgically divided input sites) at both. We found that in intact preparations with "smooth" conduction curves, pacing at long coupling intervals produced shorter AV nodal conduction times from the IAS (56 +/- 9.8 msec vs 69 +/- 10.1 msec; P < 0.01). At short coupling intervals, in contrast, shorter conduction times were obtained from the CrT (173 +/- 21.8 msec vs 188 +/- 22.8 msec; P < 0.01). This resulted in a characteristic crossing of the superimposed IAS and CrT conduction curves. After division of the inputs, the IAS site had rapid conduction to the His bundle but a longer refractory period, whereas the CrT site had long conduction times and shorter refractory periods. Wavefronts entering the AV node from these two inputs can summate, resulting in improved conduction., Conclusion: Pacing protocols designed to accentuate the asymmetry between the AV nodal inputs can help to reveal the functional difference between the dual pathways and thus to better assess the properties of AV nodal conduction.

To trace anatomical structures that might be associated with the dual atrioventricular (AV) nodal pathway and to investigate the morphologic characteristics of the cells that form these pathways, we examined serial sections of the AV junctional area with a light microscope and reconstructed them 3-dimensionally with a computer. Twelve hearts were obtained at autopsy from patients who had not shown AV conduction disturbances or supraventricular tachycardia before death. The method of Lev et al was used to prepare serial sections. Fascicles of atrial muscle contiguous with the AV node were examined with a light microscope and were classified into 3 groups, on the basis of morphologic characteristics and myocyte diameter. A computer was used to reconstruct 3-dimensionally the course of the fascicles and surrounding structures. At the border of the AV node and bundle of His relatively large myocytes extended directly into the AV bundle from the anterosuperior interatrial septum. Morphologically, the course was considered to be consistent with the fast pathway. In contrast, small cells that entered the AV node from the inferoposterior interatrial septum resembled sinus node cells with few myofibrils and a winding shape. These cells extended from the coronary sinus ostium to the tricuspid valve annulus and are thought to make up the slow pathway.

The atrioventricular node artery is a major contributor to the arterial supply of the atrioventricular conducting pathway and is an important vessel in the pathogenesis of heart block. The terminal ramifications of this artery were studied in detail by serial sectioning techniques in 50 human infant hearts. The artery provided branches to the posterior interventricular septum in all hearts (100%) and to the interatrial septum in 22 hearts (44%). The vessel supplied the atrioventricular node in 45 hearts (90%) but it terminated before reaching the node in five (10%). It supplied the penetrating bundle in 32 hearts (64%). Alternative sources of arterial supply to the atrioventricular conducting pathway include: the first septal branch of the left anterior descending coronary artery; the descending septal artery; and anterior atrial branches. Although the arterial supply to this region of the heart is variable, it is possible to make hypothetical predictions of conducting tissue involvement in myocardial infarction of various types.

The morphology of the human atrioventricular node, atrioventricular bundle and bundle branches is described. A block of tissue bounded by the ostium of the coronary sinus, the pars membranacea, the septal leaflet of the tricuspid valve and the atrial and ventricular septa is removed. The block is then sectioned serially from the right endocardial surface in the frontal plane of the heart. Sectioning in this way produces fewer sections than from techniques previously described. Outlines of the atrioventricular node, atrioventricular bundle and proximal bundle branches are digitally registered and stored in a computer. Three dimensional reconstructions of the structures are then generated by computer and displayed on an oscilloscope so that the entire three dimensional image can be rotated in any plane. Stereoscopic image pairs are produced to assist perception of the shape of the atrioventricular node, bundle and branching patterns of the bundles. This technique is unique in that it describes a method from which a relatively small number of histologic sections are generated permitting not only a complete histologic examination, but also a study of the morphology of the area.

The atrioventricular conduction system in three avian species has been studied by light and electron microscopy. A morphologically definable atrioventricular node was not found in any of these. The atrioventricular bundle is a well-defined structure, the proximal portion of which is in direct continuity with the atrioventricular ring, located in the arterial sheet of the muscular valve of the right atrioventricular opening. In the zone of transition between atrioventricular ring and bundle the compactness of the bundle is loosened, but the fibers do not establish continuity with the atrial fibers. The ring consists of Purkinje-like fibers, 10-15 microns in diameter, and (peripherally) small 3-5-microns-diameter junctional fibers which are in continuity with the common atrial fibers. In the muscular atrioventricular valve the fibers of the ring are insulated from the ventricular myocardium by a connective tissue sheet of the annulus fibrosus. It is suggested that in the avian heart the atrioventricular ring may fulfill a role similar to that of the atrioventricular node of mammals.

A study has been conducted into the morphological arrangement of the atrioventricular junctional area of human heart. The area was investigated in infantile, child, young adult, middle-aged, and old nodes. Although marked individual and ageing variations were observed, a general pattern of nodal architecture could be distinguished. The junctional area was therefore divided into four areas: (1) a transitional zone, (2) the compact node, (3) the penetrating bundle, and (4) the branching bundle. The transitional zone was intermediate between atrial myocardium and compact nodal specialized myocardium, but itself exhibited specialized characteristics. An important connection of the zone was to the myocardium of the left side of the interatrial septum. The compact node was itself composed of two segments which approached each other anteriorly from the mitral and tricuspid extremities of the septal anulus fibrosus. The junction of compact node and penetrating bundle could not be distinguished using cytological criteria. It was arbitrarily defined as the last point to make contact with transitional cells. Tissue distal to this was considered as penetrating bundle. A bypass tract was defined as any fiber contacting the bundle distal to this point, but such tracts were not observed in normal hearts. The branching bundle originated at the point of bifurcation of the penetrating bundle.

The interrelationships of different components of atrioventricular junction of the rat heart were examined on serial sections. The results are the following: 1) The atrioventricular node of the rat is situated at the level of membraneous septum separating left ventricular cavity from the right atrium. This particular localization may be of functional significance since at this level both the state of systemic arterial circulation and venous return, can be integrated. 2) Apart from the node of Aschoff-Tawara adjacent to the upper edge of muscular septum, an accessory atrioventricular node is regularly present close to the aortic root. 3) In the proximity of both atrioventricular nodes we found vascular lacunae containing abundant mastocytes and chromaffin cells. These pictures seem to suggest the existence of local chemoregulation and transmitter liberation. 4) His bundle is apposed to subendocardial fibrous lamina running from mitral valve to the cartilaginous ring of the aortic root. This association of specialized cells with their connective environment reminds the organization of different mechanoreceptors described in literature.

The functional subregions of the rabbit atrioventricular (AV) node have been examined in detail, but it has not been possible to subject the normal canine AV node to such rigorous scrutiny. Arterial perfusion and beveled microelectrodes have now allowed us to explore the anatomic and functional regions of the isolated canine AV node. The atria and AV junctional tissues of 20 young dogs were excised and selectively perfused with physiological solution through the nutrient arteries to the sinus node, AV node, and His bundle. AV node action potentials had shapes similar to those of the rabbit AV node and were conducted slowly (56 +/- 7 mm/s); slowest conduction (16 +/- 14 mm/s) was observed in the distal AV node where diastolic depolarization was always recorded and where maximum upstroke velocity of the action potential was slowest. From subsequent anatomic studies of the same preparations we found that the proximal region of the canine AV node was predominantly composed of interwoven slender transitional cells and that the distal region contained bundles of transitional cells and small but conspicuous aggregations of P-cells. The proximal portion of the canine AV node appeared to be organized for triage of atrial input during sinus rhythm. Electron microscopic examinations revealed numerous relatively long (greater than 1.0 micrometers) gap junctions between proximal AV node cells. The large gap junctions may provide an anatomic basis for electrotonic interaction during summation and conduction of AV node inputs. The distal AV node is the site of slowest AV conduction and also where most forms of AV junctional rhythm originate.

The atrioventricular (AV) node of the golden hamster is situated unusually high in the interatrial septum when compared to other species such as the rat. 2 main cell types, characterized by electron-lucent or electron-dense cytoplasm respectively, are found in the node; although both types contain numerous myofilaments these are irregularly arranged and sarcomeric banding is poor. A third variety comparising transitional cells, with features intermediate between the main nodal cells and general atrial myocardial cells, are found at the periphery of the node. Similar electron-lucent and electron-dense cells are also found in the bundle and the mean diameter of bundle cells increases as one passes from the node to the bundle bifurcation. In the node, specific heart granules (SHG) identified by ANP-28 immunoreactivity are found only in transitional cells and even here they are very sparse, unlike general atrial myocytes in which they are plentiful. Numerous nerve varicosities are present throughout the node and bundle and 5-hydroxydopamine (5-OHDA) labelling demonstrates that most of them have features of either noradrenergic or cholinergic terminals; a few non-cholinergic, non-adrenergic varicosities are also present.

There is a widely recognized relationship between the atrioventricular conduction bundle and the margins of the ventricular septal defects according to whether the defect is perimembranous or muscular. We have shown that this relationship may in turn be markedly influenced by the precise topography of the trabecula septomarginalis. Fifteen hearts, each with a ventricular septal defect, were studied by serial sectioning. Four hearts had an isolated ventricular septal defect in the setting of normal chamber connections and relations. One heart had a truncus arteriosus, seven hearts had Fallot's tetralogy, and three hearts exhibited complete transposition. In those hearts with outlet defects in which a bar of muscle separated the rim of the defect from the position of the conduction bundle, deficiency of the posterior limb of the trabecula septomarginalis permitted the right-sided margin of the conduction axis to "surface" in directly subendocardial position. Moreover, in some of these outlet defects, the conduction axis in the region of the branching bundle was exposed directly upon the crest of the muscular septum. The position of the medial papillary muscle complex was the best landmark to this potential danger area. In some of the examples of Fallot's tetralogy, we observed a firm muscle bar overlying the posteroinferior margin of the defect, which nonetheless was perimembranous. This protected the nonbranching component of the conduction axis in the anticipated danger area. Paradoxically, a deficiency in the trabecula septomarginatis then permitted the branching bundle to sit directly astride the septum along the inferior rim of the defect. We noted particularly marked variability of the atrioventricular conduction axis among the hearts with tetralogy. Our results suggest that close inspection during the operation may reveal whether or not a well-developed trabecula septomarginalis is present and permit conclusions to be drawn concerning the precise position of the atrioventricular conduction axis.

The work deals with the geometry of the structure of different sections of the atrio-venticular (A-V) node of the sheep's heart and measurement of the diameter of fibres in its three zones: atrio-nodal (A-N), the node proper (N) and transition of the node into His bundle (NH). The revealed difference in the diameter of contacting fibres of N and NH areas under worse conditions of conducting may contribute to block of excitation. The geometrical features of A-N and N areas contribute to prolonged delay of excitation and geometrical parameters of the site of contact between N and NH zones to its block.

To determine the architecture of the atrioventricular (AV) junctional region, structures in atrial preparations were correlated to those in serial sections made either parallel or perpendicular to the long axis of the AV node (AVN)/AV bundle complex. The results demonstrated the following for the first time: 1) A right medial atrial wall (MAW) extends anteriorly from the interatrial septum, superior to the interventricular septum (IVS). 2) An atrial interventricular septum (A-IVS) groove is located between the base of the MAW and the crest of the IVS. 3) Three atrionodal bundles converge to form a proximal AV bundle (PAVB), which in turn is contiguous with the AVN. The atrionodal bundles are associated with the MAW or the superomedial and inferolateral margins of the coronory sinus. Terminal portions of the atrionodal bundles and the PAVB reside within the A-IVS groove. The AV bundle was termed distal (DAVB) to avoid confusion. 4) The location of the AVN/DAVB complex topographically is deep to the apex of the septal cusp of the tricuspid valve subjacent to the MAW. Intracardially, the AVN/DAVB complex is within the central fibrous body. Significantly, this study resulted in the first unequivocal demonstration of discrete bundles of myocardial fibers associated with the atrial end of the AV node. Moreover, it appears likely that the atrionodal AV bundles are continuous with the sinoatrial nodal extensions, thereby forming internodal tracts.

The cardionectors of 32 patients who had died at different ages without clinical or morphological indications of heart disease were examined. The autopsy was performed 8 to 24 h after death. Histochemical and histomorphometric tests were used to detect age-related changes in the conduction system (sino-auricular and AV nodes). The proportion of connective tissue increases in the cardionector and diminishes in the muscle tissue with increasing age, the changes being more pronounced in the sino-auricular than in the AV node. In the age group over 60, for instance, the proportion of connective tissue by volume is over 120% higher, in the sino -auricular node and 90% higher in the AV node than in the age group under 10. The correlation coefficient between age and proportion of connective tissue is also higher in the sino-auricular node (0.65) than in the AV node (0.53).

The arterial blood supply to the sino-atrial (S-A) and atrio-ventricular (A-V) nodes was studied in 63 baboon hearts (Papio cynocephalus and Papio hamadryas). The arterial coronary injection was made with latex neoprene (23 hearts), a latex and lead oxide mixture (30 hearts) and polyester resin (10 hearts) for dissection, radiographic and corrosion cast studies, respectively. The S-A artery is a long recurrent left coronary branch (92.1%) or an ascendant less long right coronary branch (7.9%). The A-V arteries were 2 or 3 small branches of the interventricular posterior artery originating from the right or the left with the same frequency as in man. The percent distribution of the other coronary branches was similar to that observed in man.

Cannulation of the atrioventricular (AV) nodal artery for selective perfusion of the AV node is a useful physiological method for evaluating the direct effects of pharmacological agents on the AV node. However, previous reports have not included a detailed description of the technique for AV nodal artery cannulation. Furthermore, successful cannulation is dependent on familiarity with the anatomical variations of the AV nodal artery [i.e., the most superior posterior septal artery (PSA)], which supplies blood to the AV nodal region and the posterior descending artery (PDA). The purpose of this report is to describe in detail the technique for cannulation of the AV nodal artery as well as the common anatomical variations of this artery. The anatomy of the PDA and PSA was studied at postmortem examination with ink injection in 30 dogs. Verification of the anatomical location of the AV nodal artery was aided by the induction of transient AV nodal conduction block following intracoronary administration of acetylcholine in the beating heart, as was done in previous studies. Two main variations and two subtypes of PDA anatomy and three main variations of AV nodal artery were found. Based on the present findings, an improved technique for cannulation of the AV nodal artery was established. Using this technique, we achieved a high rate of successful cannulation.

The atrioventricular node of Tawara was dissected macroscopically under dissecting microscope with scalpel and microforceps. The node is a flat and fan-shape mass of the specialized cardiac muscular tissue, which is sandwiched between the floor of the right atrium and the right fibrous trigone as an epi- or extra-myocardial structure, in the center of the heart as a whole. The node is situated in the apical (anterior) angle of the triangle of Koch, and does not show any individual variation in its location, occupying the same site topographically. Orifice of the coronary sinus is not proper to indicate the location of the node, because orifice of the coronary sinus changes its position individually. Clinically, the location of the node can be presumed correctly by identifying the important anatomical landmarks; postero-inferior angle of the membranous portion of the interventricular septum, commissure between the septal and the accessory (or anterior) cusps of the tricuspid valve, attachment line of the septal cusp, and the tendon of Todaro.

According to recent developments the atrial pacemaker area and the right atrium show a peculiar morpho-functional organization, i.e.: 1) The pacemaker area is formed of clusters of cells containing relatively few myofibrils and showing embryologic characteristics. Such cells are known as nodal cells and between these and the atrial muscles are in general situated transitional cells. Each cluster is separated from the other by collagenous boundaries. The resistance of the membranes to the current flow seems to be relatively low between the cells of the same cluster but the collagenous boundaries are, according to TRAUTWEIN e UCHIZONO (1963), very poor conductors. The pacemaker activity seems to originate inside the various clusters. 2) The functional relationships between the sinoatrial node and the atrioventricular node as well as the interatrial relationship would take place through preferential pathways. These pathways corresponding approximately to the tracts described by JAMES (1966) (anterior, posterior and middle internodal tracts) and to the interatrial or Bachmann bundle, seems to show a higher velocity conduction. In general the fibres of which the tracts are composed are neither morphologically nor functionally isolated from the atrial muscle. The functional consequences of the above mentioned nodal and atrial organization seems to be: a) The possible conditioning of the pacemaker functions by the various clusters activity i.e. the dominance of one cluster over another. b) The shifting of the pacemaker activity from one cluster to anothr due to the arrival of nervous stimuli or chemical substances, etc. According to some Authors as a consequence of the shift the pacemaker area can sometimes move out side the nodal tissue and settle inside an area belonging to the internodal pathways. c) Another consequence of the shift can be the different involvement of the conducting pathways which can lead to a change in the dynamics of the atrial invasion by the excitement.