Myokardium

Myokardium, the muscular tissue of the heart, is the middle layer of the heart wall situated between the endocardium and the epicardium. It is mainly made up of cardiomyocytes arranged in spiral and longitudinal bundles that form the walls of the atria and ventricles. The left ventricle contains the thickest layer of myocardium, reflecting its role in pumping blood to the systemic circulation.

Histology and cellular features: Cardiomyocytes are striated, usually with a single central nucleus, and connected by intercalated discs that contain gap junctions and desmosomes. This arrangement enables rapid electrical spread and synchronized contraction. The myocardium is rich in mitochondria and is supported by a dense capillary network and connective tissue that organize the muscle fibers.

Function and regulation: The myocardium contracts in response to electrical impulses generated by the heart’s conduction system, including the sinoatrial node and atrioventricular node, with impulse propagation through the His-Purkinje system. Contraction relies on calcium-dependent excitation-contraction coupling and substantial energy supply from mitochondria. The autonomic nervous system modulates heart rate and contractile force, but the contraction is intrinsic to the myocardium.

Blood supply and clinical relevance: The myocardium is supplied by the coronary arteries, primarily the left coronary artery (its left anterior descending and circumflex branches) and the right coronary artery. Coronary veins drain to the coronary sinus. Ischemia or infarction can damage the myocardium, as in myocardial infarction, and myocarditis is inflammation of the muscular layer. Chronic changes, such as hypertrophy or dilatation, can impair function and contribute to heart failure.