Unsolved Questions on the Anatomy of the Ventricular Conduction System

Abstract

We reviewed the anatomical characteristics of the conduction system in the ventricles of human and ungulate hearts and then raised some questions to be answered by clinical and anatomical studies in the future. The ventricular conduction system is a 3-dimensional structure as compared to the 2-dimensional character of the atrial conduction system. The proximal part consisting of the atrioventricular node, the bundle of His and fascicles are groups of conducting cells surrounded by fibrous connective tissue so as to insulate from the underlying myocardium. Their location and morphological characters are well established. The bundle of His is a cord like structure but the left and right fascicles are broad at the proximal and branching at the distal part. The more distal part of fascicles and Purkinje system are linear networks of conducting cells at the immediate subendocardium but the intra-mural network is detected at the inner half of the ventricular wall. The papillary muscle also harbors Purkinje system not in the deeper part. It is hard to recognize histologically in human hearts but conducting cells as well as Purkinje cells are easily recognized in ungulate hearts. Further observation on human and ungulate hearts with myocardial infarct, we could find preserved Purkinje system at the subendocardium in contrast to the damaged system at the deeper myocardium. Further studies are necessary on the anatomical characteristics of this peripheral conduction system so as to correlate the clinical data on hearts with ventricular arrhythmias.

Keywords: Heart conduction system; Myocardial infarction; Purkinje fibers; Tachycardia, ventricular.

Figure 1. (A) Low magnification of AV node and neighboring myocardium of atrial and ventricular septa in a human heart (bar=600 μm). Yellow arrows delineate the compact nodal cells and black arrows demarcate transitional zone between the node and atrial myocardium. (B–E) Histology of myocytes of ventricular myocardium (B, 1 in A), compact zone (C, 2 in A), transitional zone (D, 3 in A) of AV node, and atrial muscle (E, 4 in A), respectively (bars of B–E=100 μm).

AV = atrioventricular; RA = right atrium.

Figure 2. Morphological variation of the left ventricular septal surface in human hearts as seen from the left after incision of the LV between 2 major papillary muscles and incision at the center of the anterior leaflet of mitral valve. (A) Fine and regular trabeculae throughout the whole wall. (B) Fine trabeculae admixed with a few coarse trabeculae. (C) Coarse and regular medium sized trabeculae. (D) Prominent big trabeculae and deep inter-trabecular spaces.

LV = left ventricle.

Figure 3. Histological sections of the ventricular septum reveal the bundle of His, left and right bundles and peripheral branches of the bundle in a human heart. (A) Scanning view of the ventricular septum. Bar represents 5 mm. (B, C) Low and high magnification of the bundle of His (1 in Figure 3A) showing tightly bound small spindle cells in thick fibrous sheath. (D, E) Low and high magnification of the proximal right bundle showing some vacuolated conducting cells in the myocardium (2 in Figure 3A). Conduction cells often show perinuclear clearing (arrows). (F–M) Low and high magnification of distal left bundle and Purkinje fibers (F and G, 3; H and I, 4; J and K, 5; L and M, 6 in Figure 3A). Distal part of the left bundle branches shows bundles of 5–10 cells covered by fibrous sheath. Purkinje cells are peripheral part of the bundle branches showing ground glass appearing cytoplasm rather than fibrillary cytoplasm (bars in B, D, F, H, J, and L represent 400 μm and in C, E, G, I, K, and M represent 200 μm).

LV = left ventricle; MS = membranous septum; RA = right atrium; RV = right ventricle; TV = tricuspid valve.

Figure 4. Histological sections of the apical part of the LV reveals the outer compact layer and the inner trabecular layer of the myocardium in a human heart. This particular case had old myocardial infarct and subendocardial fibrosis is noted. (A) Scanning view of the ventricular wall (bar=5 mm). (B, C) Low and high magnification of one of trabeculae (1 in Figure 4A) showing a few Purkinje cells at the endocardial zone of the trabeculae. (D, E) Other deeper part of the endocardium (“2” in Figure 4A) showing similar Purkinje cells at the endocardium. (F, G) The interstitium deep in the compact myocardium (“3” in Figure 4A) does not show Purkinje cells (bars in B, D and F represent 400 μm and in C, E, and G represent 200 μm).

LV = left ventricle.

Figure 5. Scanning view of histological sections of AV node area of a goat heart. (A) The middle of the AV node shows collection of nodal cells (arrow) at the surface of the right atrial wall. (B) Anterior end of the AV node (arrow) is buried underneath the fibrous annulus of the TV near the “MS”. There is a mass of myocardium (delineated with 9 arrows) in a goat heart at the sub-aortic zone of the LV where MS separates LV and the RA in human. (C) More anteriorly the myocardial tissue still present (delineated with arrows) so that the bundle of His (arrow) is still in the myocardium. (D) The bundle of His bifurcates into the right and left bundles (arrows) underneath the myocardial tissue (delineated with arrows) which is the location of the MS in human. The right bundle runs within the myocardium of the septum but the left bundle runs to arrive at the left ventricular surface of the septum (double arrow) so that it is at the subendocardial location like the left bundle in human (bars represent 4 mm).

AV = atrioventricular; LV = left ventricle; MS = membranous septum; RA = right atrium; TV = tricuspid valve.

Figure 6. Low and high magnification of the AV node (A, B) and left and right bundles (C, D) in a goat heart. AV node is a collection of specialized cells and nerve endings (arrows in B). The cells of the left and right bundles (arrows in C) are basically same cells to the node, being characterized by ground glass cytoplasm devoid of myofilaments (D). Double arrow indicates the endocardial location of the left bundle after the intramyocardial course of the proximal part of the bundle in this goat heart.

AV = atrioventricular.

Figure 7. Purkinje system of the LV in the goat heart. (A, B) Purkinje cells (arrows) are located in the superficial layer of the subendocardium (Masson's trichrome stain). Bars represent 300 μm. (C, D) Magnification of the Purkinje system shows intimate attachment of cells without reticulin fibers (C, Reticulin stain). The cytoplasm free of myofilament (D, Masson's trichrome stain). Bars in C and D represent 100 μm. (E–H) Purkinje cells are also seen at the inter-fascicular interstitium of the myocardium. (E and G, Masson's trichrome stain; F and H, Reticulin stain). Bars represent 300 μm.

LV = left ventricle.

Figure 8. Purkinje system in goat heart with myocardial infarct. (A) Low magnification of the left ventricular wall shows extensive transmural loss of myocytes and fibrosis in the myocardium. The Purkinje system remains at the endocardial surface of the wall (arrow) (bar=600 μm). Masson's trichrome stain. (B) Magnification of the endocardial part of the section (A) shows details of the Purkinje system (arrows) (bar=300 μm). Masson's trichrome stain. (C) Reticulin staining of the same area of the section (B) shows extensive deposition of reticulin fibers at the zone of myocardial loss. The absence of intercellular reticulin fibers at the Purkinje system is also noted (bar=300 μm).