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| ==The sinus node== | | ==The sinus node== |
| The 'ultimum moriens', the last part of the heart to stop beating when the organ is isolated from the body, first prompted Wenckebach to believe that this may also be the seat of the heart beat.1 The discovery of the sinus node in the heart of a mole culminated in a paper in 1907 in which Keith and Flack described 'a remarkable remnant of primitive fibres persisting at the sino•auricular junction in all mammalian hearts. These fibres are in close connection with the vagus and sympathetic nerves, and have a special arterial blood supply; in them the dominating rhythm of the heart is believed to normally arise'.2 The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.3 | | The 'ultimum moriens', the last part of the heart to stop beating when the organ is isolated from the body, first prompted Wenckebach to believe that this may also be the seat of the heart beat.<cite>1</cite> The discovery of the sinus node in the heart of a mole culminated in a paper in 1907 in which Keith and Flack described 'a remarkable remnant of primitive fibres persisting at the sino•auricular junction in all mammalian hearts. These fibres are in close connection with the vagus and sympathetic nerves, and have a special arterial blood supply; in them the dominating rhythm of the heart is believed to normally arise'.<cite>2</cite> The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.<cite>3</cite> |
| The sinus node predominantly occupies an antero-lateral location of the superior cavo-atrial junction within the terminal groove (Figure 11A). Only occasionally it is horseshoe-shaped draping over the right atrial summit. In most adult hearts it is shaped like a tadpole measuring about 3 mm in diameter at its widest part and 15 to 20mm in length. A tapering 'tail' of the node may be traced from the epicardium to pass intramyocardially toward the inferior part of the terminal crest. The sinus node is easily recognised by the light microscope at low magnification. It is made up of small cells grouped together in interconnecting fascicles set in a fibrous tissue matrix (Figure 11B). The fibrous matrix becomes more prominent with increasing age. At the margins of the node is a short transitional area where nodal cells merge into atrial myocardium. In places, discrete tongues of transitional cells are found which extend into the terminal crest and toward the myocardial sleeve of the superior caval vein. The blood supply to the node shows considerable variation. A main artery penetrating the length of the node is seen in some hearts. In others, the nodal substance is penetrated by ramifications of an artery approaching the node through one or both ends, there being variations in nodal approaches. Even the origin of the sinus node artery is diverse, arising from the right or left coronary artery at different locations. Collections of ganglion cells are usually observed in the epicardium and also in the environs of the sinus node. | | The sinus node predominantly occupies an antero-lateral location of the superior cavo-atrial junction within the terminal groove (Figure 11A). Only occasionally it is horseshoe-shaped draping over the right atrial summit. In most adult hearts it is shaped like a tadpole measuring about 3mm in diameter at its widest part and 15 to 20mm in length. A tapering 'tail' of the node may be traced from the epicardium to pass intramyocardially toward the inferior part of the terminal crest. The sinus node is easily recognised by the light microscope at low magnification. It is made up of small cells grouped together in interconnecting fascicles set in a fibrous tissue matrix (Figure 11B). The fibrous matrix becomes more prominent with increasing age. At the margins of the node is a short transitional area where nodal cells merge into atrial myocardium. In places, discrete tongues of transitional cells are found which extend into the terminal crest and toward the myocardial sleeve of the superior caval vein. The blood supply to the node shows considerable variation. A main artery penetrating the length of the node is seen in some hearts. In others, the nodal substance is penetrated by ramifications of an artery approaching the node through one or both ends, there being variations in nodal approaches. Even the origin of the sinus node artery is diverse, arising from the right or left coronary artery at different locations. Collections of ganglion cells are usually observed in the epicardium and also in the environs of the sinus node. |
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| ==The atrioventricular conduction system== | | ==The atrioventricular conduction system== |
| Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.4Tawara's monograph5 accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.6 This firmly estabIished the presence of an atrioventricular conduction system (Figure 10) and was subsequently confirmed by Keith and Flack in the same year.7 Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications. The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 12). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.8 The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 12D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 12C). The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 12B). The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 13).5 Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.9 | | Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.<cite>4</cite> Tawara's monograph<cite>5</cite> accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.<cite>6</cite> This firmly estabIished the presence of an atrioventricular conduction system (Figure 10) and was subsequently confirmed by Keith and Flack in the same year.<cite>7</cite> Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications. The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 12). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.<cite>8</cite> The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 12D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 12C). The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 12B). The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 13).<cite>5</cite> Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.<cite>9</cite> |
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| ==References== | | ==References== |
| <biblio> | | <biblio> |
| 1. Wenckebach KF. Beitrage zur Kenntnis der menschlichen Herzatigkeit. Archiv Anat u Physiol l907; 2:1. | | 1. Wenckebach KF. Beitrage zur Kenntnis der menschlichen Herzatigkeit. Archiv Anat u Physiol l907; 2:1. |
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| 2. Keith A, Flack M. The form and nature of the muscular connections between the primary divisions of the vertebrate heart. J Anat Physiol l907;41:172. | | 2. Keith A, Flack M. The form and nature of the muscular connections between the primary divisions of the vertebrate heart. J Anat Physiol l907;41:172. |
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| 3. Lewis T. Oppenheimer BS, Oppenheimer A. Site of origin of the mammalian heart beat: the pacemaker in the dog. Heart 1910;2:147. | | 3. Lewis T. Oppenheimer BS, Oppenheimer A. Site of origin of the mammalian heart beat: the pacemaker in the dog. Heart 1910;2:147. |
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| 4. His W Jr. Die Thatigkeit des embryonalen Herzens und deren Bedeutung für die Lehre von Herzbewegung beim Erwachsenen. Ar Med Klin Leip 1893:14. | | 4. His W Jr. Die Thatigkeit des embryonalen Herzens und deren Bedeutung für die Lehre von Herzbewegung beim Erwachsenen. Ar Med Klin Leip 1893:14. |
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| 5. Tawara S. Das Reizleitungssystem des Saugetierherzen. Gustav Fischer, Jena. 1906 | | 5. Tawara S. Das Reizleitungssystem des Saugetierherzen. Gustav Fischer, Jena. 1906 |
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| 6. Purkinje JE. Mikroskopisch neurologische Beobachtungen. Archiv Anat Physiol u Wiss Med I845;12:28I. | | 6. Purkinje JE. Mikroskopisch neurologische Beobachtungen. Archiv Anat Physiol u Wiss Med I845;12:28I. |
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| 7. Keith A, Flack M. The auriculo-ventricular bundle of the human heart. Lancet 1906:2:359. | | 7. Keith A, Flack M. The auriculo-ventricular bundle of the human heart. Lancet 1906:2:359. |
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| 8. Koch W. Der funktionelle Bau des menschlichen Herzens. Berlin: Urban v Schwarzenburg,1922:92. | | 8. Koch W. Der funktionelle Bau des menschlichen Herzens. Berlin: Urban v Schwarzenburg,1922:92. |
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| 9. Rosenbaum MB, Elizari MV, Lazzari JO. The hemiblocks. In: Tampa Tracings. Oldsmar, Fla. 1970. PMID: 5051397 | | 9. Rosenbaum MB, Elizari MV, Lazzari JO. The hemiblocks. In: Tampa Tracings. Oldsmar, Fla. 1970. PMID: 5051397 |
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| </biblio> | | </biblio> |
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| Figure Legends
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| Abbreviations
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| Ant= anterior; cs= coronary sinus; ICV= inferior caval vein; LA= left atrium; LAD= left anterior descending coronary artery; LIPV= left inferior pulmonary vein; LPA= left pulmonary artery; LSPV= left superior pulmonary vein; LV= left ventricle; Post= posterior; Pulm= pulmonary; OF= oval fossa; RA= right atrium, RIPV= right inferior pulmonary vein; RPA= right pulmonary artery; RSPV= right superior pulmonary vein; RV= right ventricle; SCV= superior caval vein; SMT= septomarginal trabeculation; TV= tricuspid valve;
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| Figure 1
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| The endocast is viewed from 5 different perspectives to demonstrate the spatial relationship between right (coloured blue) and left (coloured red) heart chambers and between atria and ventricles. The blue and white arrows represent the right and left ventricular outflow tracts respectively.
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| Figure 2
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| The long axis of the heart is at an angle to the long axis of the body. Approximately a third of the heart is to the right of the midline of the sternum and the remainder is to the left of the midline.
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| Figure 3
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| A. Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
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| B. The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.
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| Figure 4
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| A. This frontal view shows the right and left surfaces of the heart. The left anterior descending coronary artery buried in epicardial fat marks the plane of the ventricular septum.
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| B. The obtuse and acute margins of the ventricles are demonstrated in this apical view.
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| Figure 5
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| A. This right lateral view shows the right atrium dominated by its large, triangluar shaped appendage. The dots mark the terminal groove. The arrow indicates the crest of the appendage.
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| B. The lateral wall of the appendage incised and flipped backward to show the pectinate muscles and the thin, membrane-like atrial wall between the muscle bundles. The terminal crest (dots) marks the border between the pectinated appendage and the smooth-walled venous sinus. The oval fossa is surrounded by its muscular rim. The smooth-walled vestibule leads to the tricuspid valve orifice.
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| Figure 6
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| A. This view from the left-lateral aspect shows the finger-like left atrial appendage with the left atrium situated posteriorly. The left ventricle tapers to a rounded apex.
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| B. This section through the aortic root and mitral valve displays the left atrial aspect of the septum enface. The crescentic edge (arrow) of the fossa valve has not sealed completely resulting in a PFO. The asterisk marks the location of the transverse pericardial sinus.
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| Figure 7
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| A. The right ventricle is opened to show the septum and the muscular crest separating tricuspid from pulmonary valves. The moderator band (open arrow) extends from the foot of the septomarginal trabeculation to the free wall of the right ventricle. Coarse trabeculations fill the apical component.
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| B. This close-up view of the tricuspid valve at the commissure between septal and antero-septal leaflets shows the annulus (broken line) crossing the membranous septum (dots) dividing it into atrioventricular(av) and interventricular(iv) components.
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| Figure 8
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| A. The left ventricle is opened through its outflow tract into the aortic valve. The aortic valve leaflets are in fibrous continuity with the anterior leaflet of the mitral valve. The fibrous continuity is expanded at the right and left fibrous trigones. The right trigone(asterisk) is the landmark for the atrioventricular conduction bundle. Note how the thickness of the left ventricular wall diminishes remarkably at the apex (open arrow).
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| B. This dissection shows the central location of the aortic valve. L, N and R are the left-coronary, non-coronary and right-coronary aortic sinuses respectively.
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| Figure 9
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| Diagram showing the right (RCA) and left (LCA) coronary arteries and their main ventricular branches. The left anterior descending (LAD) and posterior descending (PDA) coronary arteries mark the anterior and posterior margins of the ventricular septum.
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| Figure 10
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| The cardiac conduction system. Normally, the insulating fibro-fatty tissue plane at the atrioventricular junction prevents atrial myocardium from contacting ventricular myocardium. The penetrating bundle is the only muscular bridge.
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| Figure 11.
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| A. The sinus node (dotted shape) is superimposed onto the terminal groove in this picture of the right atrium viewed from the right side. The arrows indicate the sectioning plane of the histological section shown in B.
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| B. This section from an infant heart is stained in Masson’s trichrome stain that colours myocardium red and fibrous tissue blue. The sinus node is readily identifiable by its composition of small myocytes in a fibrous matrix.
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| Figure 12.
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| A. This view of the right atrium and right ventricle shows the anterior and posterior borders of the triangle of Koch (broken lines) that mark location of the atrioventricular node and bundle (orange shapes). The arrows B, C, D indicate the cuts made through the conduction system as shown on the histologic sections.
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| B, C and D are step sections stained with Masson’s trichrome technique and displayed in similar orientation tracing the atrioventicular conduction system from the AV node (AVN) that adjoins the central fibrous body (cfb), to the penetrating His bundle (H), and the branching bundle (BB) dividing into the left (LBB) and right (RBB) bundle branches.
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| Figure 13
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| This picture from Tawara’s monograph (1906) shows the tree-fascicular arrangement of the left bundle branch in man.
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| [Tawara S 1906 Das Reizleitungssystem des Säugetierherzens. Eine Anatomisch-Histologische Studie Über das Atrioventrikularbündel und die Purkinjeschen Fäden. Gustav Fischer, Jena.]
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