Myokard

Myokard, known in English as the myocardium, is the muscular tissue of the heart responsible for its contractile function. It forms the thick middle layer of the heart wall, lying between the endocardium and the epicardium. The myocardium is composed mainly of cardiomyocytes, which are short, branched, striated muscle cells connected by intercalated discs. These discs include gap junctions and desmosomes that enable rapid electrical coupling and mechanical cohesion, allowing synchronized contraction.

Cardiac muscle contracts rhythmically and involuntarily, driven by the heart’s intrinsic conduction system, which includes the sinoatrial node, atrioventricular node, and the His-Purkinje network. Calcium ions regulate contraction by triggering cross-bridge cycling in the sarcomeres. The myocardium’s contraction generates the systolic pump action, while its relaxation during diastole allows filling of the chambers.

The myocardium receives arterial blood from the coronary arteries. Perfusion predominantly occurs during diastole, when the ventricles are relaxed. The left coronary artery supplies most of the left ventricle and the interventricular septum, while the right coronary artery supplies the right ventricle and parts of the left ventricle. Venous drainage occurs through cardiac veins into the coronary sinus. Autonomic nervous system input modulates heart rate and contractility, with sympathetic stimulation increasing rate and force and parasympathetic activity reducing them.

Clinical relevance includes ischemic heart disease leading to myocardial infarction, myocarditis (inflammation of the myocardium), and cardiomyopathies such as dilated or hypertrophic forms, which affect myocardial structure and function. Developmentally, the myokard arises from cardiac mesoderm, and postnatally cardiomyocytes have limited regenerative capacity, with hypertrophy as a common adaptive response.