Microanatomy The myocardium is composed of cardiac myocytes enveloped in a dense extracellular matrix of collagen, the main structural protein of the heart. Cardiac myocytes account for 70-75% of the myocardium by cell volume but only 25-30% by cell number. Cardiac myocytes contain myofibrils that are composed of longitudinally repeating sarcomeres separated by Z bands (thickened and invaginated portions of the surface membrane called the sarcolemma). The sarcomeres occupy about 50% of the mass of cardiac myocytes. Thin filaments composed of actin are attached to each Z line and interdigitate with the thick filaments composed of myosin molecules.The thick and thin myofilaments slide past one another in a “ratchet-type”mechanism to generate force and shorten the myocyte.The myofilaments maintain a fixed length throughout contraction. Mitochondria compose about 20% of the cell volume and are the organelles in which adenosine triphosphate (ATP) is generated and located in close proximity to the myofibrils, as well as just beneath the sarcolemma. Platelike folds, or cristae, project inward from the surface membrane of the mitochondria and contain the respiratory enzymes necessary for energy production (Fig. 1).