1.11. Unfolding of the myocardial band.
The diagram in Figure 1.13 details the different segments comprising the band, as well as the final insertions constituting the origin and end of the band, very close to each other in the intact heart.
Histological analysis of the myocardial band. The histological analysis sequence of the unfolded myocardial band demonstrates its linear orientation according to the segmental continuity of its spatial organization when the band is coiled, both in its internal and external surfaces (Figure 1.14). These orientations are identical in both surfaces (internal and external). And if two things are identical it means that they present a unique orientation.
Figure 1.12. Myocardial band unfolded in all its extension.
Figure 1.13. Descriptive diagram of the muscle band segments.
It can be seen that the myocardial structure is not a lattice but a band. The lattice concept used was developed due to the band folding resulting in overlapping segments, which are functionally independent and with friction between their surfaces. The sliding motion between segments implies that they have a lubricating system that facilitates autonomous motion with lower energy expenditure despite being overlapped, as will be analyzed in this chapter. This arrangement is essential to achieve myocardial torsion, a fundamental action of cardiac mechanics that would be impossible with the lattice structure (crisscross of myocardial fibers).
No segment of the band histological sequence explored in our investigations presents a lattice organization. As the external surface of the distal descending segment (Figure 1.14, lower panel) twists to become the ascending segment, the cardiomyocytes generate in the planimetric histological sections a different architecture in their orientation from that of the internal surface, only site (cardiac apex) where this situations occurs. The rest of the orientation is always parallel. In the apex, the spiral course of the myocardial fibers, which shift from the periphery towards the center, determine a torsion where the subepicardial fibers become subendocardial, overlapping like the tiles of a roof, as evidenced in this image.
5. Interpretation of the origin and end of the muscle band.
The cardiac fulcrum
As previously expressed, the opposite orientation and rotational movement of the left ventricular fibers, both at the basal and distal apical segment levels, explain Torrent Guasp’s myocardial band model. The author considered that the ventricular myocardium is formed by an assembly of muscle fibers coiled unto themselves as a rope (rope theory) (Figure 1.4) and flattened laterally to form a band, which by giving two spiral turns describes a helix limiting the two ventricles and defining their function. In turn, the phylogenetic study allows to theoretically understand, through the 600 million-year evolution of the circulatory system, that the ends of the band are located at the root of the great vessels, as it is formed from a loop of the primitive circulatory duct. (109)
Figure 1.14. Segment sequence from the myocardial band histological analysis.
The muscle shaping the right ventricle corresponds to the origin of the myocardial band (right segment), which begins both in the cardiac fulcrum as in the fibrous structures related with the pulmonary artery and the tricuspid annulus (pulmo-tricuspid cord) (Figures 1.15 to 1.17). The ascending segment, which is part of the autochthonous muscle bundles constituting the left ventricle, ends at the base of the aorta, and the cardiac fulcrum constitutes a solid point of attachment of the end of the myocardial band.
Figure 1.9 demonstrates that the aortic annulus is not continuous. Its circumference is interrupted between the ends of the trigones, in the region where the mitral valve anterior cusp is inserted. Our research has demonstrated that in the course of the aortic annulus septal segment, extending from the left to the right trigone, there is a thickening we have called cardiac fulcrum (below and in front of the right trigone) where most of the myocardial band is attached, since as any muscle needs a supporting point to develop the leverage required to fulfill its function.
The insertion of the myocardial fibers in the fibrous skeleton of the heart has been considered for three centuries. The development of the myocardial band reported in 1970 by Torrent Guasp (106-108) indicates in the anatomical research that it originates and ends at the root of the great vessels, but that the fibers do not insert in the atrioventricular annuli. The myocardium attaches to these annuli but does not insert in them. In our investigations we have not found insertion of cardiomyociytes in the collagen matrix of the trigones (Figure 1.39).
However, Torrent Guasp considered that the myocardial band lacked a fixed supporting point as those present in the muscular system to develop force. In this sense, he assumed that it behaved as the circular muscle of the arteries, finding support in its own chamber blood content (hemoskeleton). In our research, we have always considered that the myocardial band should have a fixed point of attachment to allow its helical rotation in order to achieve its motions and the essential muscle force of shortening-twisting and lengthening-untwisting. The study of a supporting point in the myocardial band finds correlation with an organic engine, such as the heart, which without a firm attachment to a resistant nucleus would lack the necessary mechanical faculties for its considerable power.
Figure 1.15. Detail of the pulmo-tricuspid cord between the tricuspid orifice and the pulmonary artery where the myocardial band begins. The circle shows the site of the cardiac fulcrum, which is visualized when the pulmonary artery and the pulmo-tricuspid cord are removed from their insertion at the beginning of the myocardial band unfolding (observe figure 1.27). The right coronary artery has been cut to reveal the cord trajectory (bovine heart).
This point of attachment implies, as in any muscle, its ability to achieve the necessary leverage and also to act as a bearing or pad, preventing the force of ventricular rotation, either by torque or torsion strain from transferring to the aorta, thus dissipating the energy produced by the helical muscle motion, and avoiding aortic constriction or bending during systolic ejection. (134)
Young bovine and human hearts (from spontaneous abortion fetuses, explanted adult hearts and cadaveric hearts retrieved from the morgue) were used to study the myocardial band insertion. The dissection was performed as described in Section 4: “Anatomy of the cardiac band. Dissection”, of this same chapter.
In anatomical investigations we have found in all the bovine and human hearts studied a nucleus underlying the right trigone, whose osseus, chondroid or tendinous histological structure depends on the specimen analyzed, and is oriented towards the muscle fibers of the ascending segment which intimately penetrate its structure to attach themselves. This point of attachment would serve to support both the origin and end of the myocardial band, as the fibers that initiate the right segment, origin of the myocardial band, attach to the anterior part of this nucleus as well as to the pulmo-tricuspid cord (Figure 1.16).
Figure 1.16. Descriptive photograph of the muscle bundles emerging from the cardiac fulcrum, which belong to the right segment forming the right ventricle (transverse section of a bovine heart).
This osseus, chondroid or tendinous attachment point is found in the vicinity of the tricuspid valve (right), the aorta (posterior) and the pulmo-tricuspid cord (anterior) (Figure
