Myocardium is the muscular tissue of the heart. Other tissues are the endocardium (inner lining, effectively a specialised endothelium) and the pericardium (a connective tissue layer around the heart).
The myocardium is composed of specialized cardiac muscle cells which do not resemble muscle tissue elsewhere in the body. Cardiac muscle has the property of contraction, like other muscles, and also electrical conduction, like nerves.
The blood supply of the myocardium is by the coronary arteries. If these arteries are occluded by atherosclerosis and/or thrombosis, this can lead to angina pectoris or myocardial infarction.
Certain virusus lead to inflammation of the myocardium, or myocarditis.
Failure of the heart to contract properly (for various reasons) are termed heart failure, generally leading to fluid retention , oedema, pulmonary oedema, renal insufficiency, hepatomegaly, a shortened life expectancy and decreased quality of life.