myocardium /mī′ōkär″dē·əm/ [Gk, mys, muscle, kardia, heart] , a thick contractile middle layer of uniquely constructed and arranged muscle cells that forms the bulk of the heart wall. The myocardium contains a minimum of other tissue, except blood vessels, and is covered interiorly by the endocardium. The contractile tissue of the myocardium is composed of fibers with the characteristic cross-striations of muscular tissue. The fibers are about one third as large in diameter as those of skeletal muscle and contain more sarcoplasm. They branch frequently and are interconnected to form a network that is continuous except where the bundles and the laminae are attached at their origins and insertions into the fibrous trigone of the heart. Myocardial muscle contains less connective tissue than does skeletal muscle. Specially modified fibers of myocardial muscle constitute the conduction system of the heart, including the sinoatrial node, the atrioventricular (AV) node, the AV bundle, and the Purkinje fibers. Most of the myocardial fibers function to contract the heart. The metabolic processes of the myocardium are almost exclusively aerobic. Many key enzymatic reactions of the heart, such as the citric acid cycle and oxidative phosphorylation, take place in the highly concentrated myocardial sarcosomes. The process of oxidative phosphorylation produces adenosine triphosphate (ATP), the immediate energy source for myocardial contraction. Oxygen, which significantly affects ATP production and contractibility, is a critical metabolic component of the myocardium, which consumes from 6.5 to 10 mL/100 g of tissue per minute. Without this oxygen supply, myocardial contractions decrease in a few minutes. The myocardium maintains a relatively constant level of glycogen in the form of sarcoplasmic granules. Compare epicardium. See also cardiac muscle. −myocardial, adj.
