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Septomarginal trabecula or trabecula septomarginalis in latin, is a specialized and prominent muscular thickening of the cardiac tissue. It traverses the right ventricular chamber, and bridges the base of the anterior papillary muscle (the largest and most constant of the papillary muscles) with the inferior portion of the interventricular septum. The septomarginalis trabecula is part of the trabeculae carnae, a myriad of irregular muscular structures that are found in the walls of the inflow portion of the right ventricle.
The ‘moderator band’ is another synonym for the septomarginal trabecula. This wrongly named term originates from the old idea that believed this trabecula to be responsible of limiting the over distension of the ventricular chamber. It is also called ‘Leonardo’s cord’ by some, because of its presence in Leonardo da vinci’s anatomical illustrations of the heart.
The septomarginal trabecula acts as a support system for the septal surface of the heart. At the base, it divides into limbs that are continuous with the supraventricular crest. And towards the apex, it reinforces the anterior papillary muscle of the tricuspid valve. Moreover, it creates the anteroinferior border separating the superior smooth outflow tract, from the trabeculated inflow tract of the ventricle.
However, this trabecula’s true importance lies in its main function as part of the heart’s conducting system. It conveys a portion of the right branch of the AV bundle (atrioventricular bundle) called the crus dextrum. This branch is a round narrow group of muscle tissues that traverses the myocardium. It then runs subendocardially towards the apex of the ventricle, where it enters the septum towards the anterior papillary muscle on the opposite wall. This “shortcut” facilitates the time needed for conduction, which permits the coordinated contraction of the anterior papillary muscle.
Clinically, given the septomarginal trabecula’s consistent presence, doctors use it as a helpful landmark in situations where the ventricles are hard to identify (like in some forms of congenital heart diseases).
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