Wall of the heart
The wall of the heart consists of three distinct layers—the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer). Coronary vessels supplying arterial blood to the heart penetrate the epicardium before entering the myocardium. This outer layer, or visceral pericardium, consists of a surface of flattened epithelial (covering) cells resting upon connective tissue.
The myocardial layer contains the contractile elements of the heart. The bundles of striated muscle fibres present in the myocardium are arranged in a branching pattern and produce a wringing type of movement that efficiently squeezes blood from the heart with each beat. The thickness of the myocardium varies according to the pressure generated to move blood to its destination. The myocardium of the left ventricle, which must drive blood out into the systemic circulation, is, therefore, thickest; the myocardium of the right ventricle, which propels blood to the lungs, is moderately thickened, while the atrial walls are relatively thin.
The component of the myocardium that causes contraction consists of muscle fibres that are made up of cardiac muscle cells. Each cell contains smaller fibres known as myofibrils that house highly organized contractile units called sarcomeres. The mechanical function arising from sarcomeres is produced by specific contractile proteins known as actin and myosin (or thin and thick filaments, respectively). The sarcomere, found between two Z lines (or Z discs) in a muscle fibre, contains two populations of actin filaments that project from opposite Z lines in antiparallel fashion and are organized around thick filaments of myosin. As actin slides along crossbridges that project from myosin filaments at regular intervals, each myosin is brought into contact with an adjacent myosin filament. This process shortens the muscle fibre and causes contraction (see muscle).
Interaction between actin and myosin is regulated by a variety of biological processes that are generally related to the concentration of calcium within the cell. The process of actin sliding over myosin requires large amounts of both calcium and energy. While the contractile machinery occupies about 70 percent of the cardiac cell volume, mitochondria occupy about 25 percent and provide the necessary energy for contraction. To facilitate energy and calcium conductance in cardiac muscle cells, unique junctions called intercalated discs (gap junctions) link the cells together and define their borders. Intercalated discs are the major portal for cardiac cell-to-cell communication, which is required for coordinated muscle contraction and maintenance of circulation.
Forming the inner surface of the myocardial wall is a thin lining called the endocardium. This layer lines the cavities of the heart, covers the valves and small muscles associated with opening and closing of the valves, and is continuous with the lining membrane of the large blood vessels.
