Anatomy of the Heart: Difference between revisions

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 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.  

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

1. Wenckebach KF. Beitrage zur Kenntnis der menschlichen Herzatigkeit. Archiv Anat u Physiol l907; 2:1.

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.

3. Lewis T. Oppenheimer BS, Oppenheimer A. Site of origin of the mammalian heart beat: the pacemaker in the dog. Heart 1910;2:147.

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.

5. Tawara S. Das Reizleitungssystem des Saugetierherzen. Gustav Fischer, Jena. 1906

6. Purkinje JE. Mikroskopisch neurologische Beobachtungen. Archiv Anat Physiol u Wiss Med I845;12:28I.

7. Keith A, Flack M. The auriculo-ventricular bundle of the human heart. Lancet 1906:2:359.

8. Koch W. Der funktionelle Bau des menschlichen Herzens. Berlin: Urban v Schwarzenburg,1922:92.

9. Rosenbaum MB, Elizari MV, Lazzari JO. The hemiblocks. In: Tampa Tracings. Oldsmar, Fla. 1970. PMID: 5051397