The myocardium is the muscle layer of the heart, responsible for the heart's pumping action, which supplies the entire body with blood. The myocardium consists of cardiac muscle, a type of muscle unlike any other muscle in the body. Cardiac muscle combines features of skeletal muscle, which controls voluntary body movement, and smooth muscle, which controls the movement of all body organs other than the heart.
The myocardium is the middle layer of the cardiac wall; the outermost layer is the epicardium, while the innermost is the endocardium. The epicardium consists mostly of connective tissue and serves to protect the inner structures of the heart. The endocardium is a thin layer of epithelial cells, similar to that which lines the inside of blood vessels.
The cardiac muscle that makes up this structure is involuntary, like the smooth muscle in the body's other organs. Involuntary muscle is not under conscious control, and contrasts with voluntary skeletal muscle, which is attached to the skeleton and used for skeletal movement like walking and standing. Cardiac muscle is more similar in structure, however, to skeletal muscle than to smooth muscle. Both cardiac muscle and skeletal muscle are striated, meaning the muscle fibers are arranged into parallel bundles, and have alternating thick and thin protein filaments. Striated muscle is better suited to brief, intense contractions than smooth muscle.
While skeletal muscle fibers are arranged into regular, non-branching bundles, the muscle fibers of the myocardium branch at irregular angles, and connect to other muscle cells at junctions called intercalated discs. The cells that make up cardiac muscle are called cardiac myocytes, or cardiomyocytes. They also differ from skeletal muscle cells in that they require extracellular calcium for contraction to take place.
The contractions of the myocardium are responsible for pumping oxygenated blood throughout the body, providing the body with the oxygen and other nutrients it needs to function properly. The heart muscle also pumps deoxygenated blood into the lungs so that it can be oxygenated again. After the blood has delivered oxygen throughout the body, deoxygenated blood returns to the heart, which in turn pumps the blood into the lungs. After the blood is reoxygenated in the lungs, it returns to the heart to be pumped throughout the body once again. Like all body tissues, the myocardium itself requires a blood supply in order to function; the coronary arteries supply the heart muscle with blood.