Myocardium

The myocardium is the muscular middle layer of the heart wall (covered by the visceral layer of the serous pericardium (epicardium), and lined by the endocardium).

The myocardium is composed of cardiomyocytes arranged in a highly organized, three-dimensional meshwork (vortex of heart). These cardiomyocytes are supported by a collagenous fibrous matrix with epimysial, perimysial, and endomysial components, which provide structural integrity and facilitate force transmission during contraction.

Within the ventricular walls, cardiomyocytes are not organized as discrete muscle bundles or a single continuous band, but rather as a syncytial continuum forming a complex mesh. The orientation of myocytes changes gradually across the wall: subendocardial fibers are arranged in a right-handed helix, while subepicardial fibers form a left-handed helix, with a smooth transmural transition. This helical architecture underlies the heart’s twisting motion during systole and diastole, contributing to efficient ejection and filling.