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Anatomy of the Heart: Difference between revisions
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==The cardiac conduction system== | ==The cardiac conduction system== | ||
[[Image:Figure10.jpg|thumb|right]] | [[Image:Figure10.jpg|thumb|right|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.]] | ||
The full complement of the histologically specialised tissues making the conduction system of the heart comprises the sinus node and the atrioventricular system (Figure 10). The latter is made up of the atrioventricular node, the penetrating atrioventricular bundle and the ventricular bundle branches. The geometry of the right atrium is such that it is made up of bands of muscle which separate the orifices of the great veins and the oval fossa. The spread of excitation from the sinus to the atrioventricular node has been shown to spread preferentially along these broad bands of ordinary atrial myocardium. | The full complement of the histologically specialised tissues making the conduction system of the heart comprises the sinus node and the atrioventricular system (Figure 10). The latter is made up of the atrioventricular node, the penetrating atrioventricular bundle and the ventricular bundle branches. The geometry of the right atrium is such that it is made up of bands of muscle which separate the orifices of the great veins and the oval fossa. The spread of excitation from the sinus to the atrioventricular node has been shown to spread preferentially along these broad bands of ordinary atrial myocardium. | ||
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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.<cite>Wenckebach</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>Keith</cite> The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.<cite>Lewis</cite> | 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>Wenckebach</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>Keith</cite> The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.<cite>Lewis</cite> | ||
The sinus node predominantly occupies an antero-lateral location of the superior cavo-atrial junction within the terminal groove (Figure 11A). [[Image:Figure11A.jpg|thumb|right]]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. | The sinus node predominantly occupies an antero-lateral location of the superior cavo-atrial junction within the terminal groove (Figure 11A). [[Image:Figure11A.jpg|thumb|right|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.]]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). [[Image:Figure11A.jpg|thumb|right|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.]]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 atrioventricular conduction system== | ==The atrioventricular conduction system== | ||