Cardiac muscle is a muscle tissue composed of cardiac muscle cells. The broad definition of cardiac myocytes includes the specially differentiated cardiac myocytes that make up the sinoatrial node, intraatrial bundle, atrioventricular junction, atrioventricular bundle (i.e., Heath bundle), and Purkinje fibers, as well as the general working cells of the atrial and ventricular muscles. The first five species comprise the pacing and conduction system and contain few or no myogenic fibers, so none of them has a contractile function; however, they are autoregulatory and conductive and are the functional basis of the heart’s autoregulatory activity; the latter two species are contractile and are the functional basis of the heart’s diastolic activity. Myocardial dystrophy is most often seen in cardiomyopathies and other conditions. Cardiomyopathy is a myocardial disease of unknown cause and is generally thought to be related to viral infections, autoimmune reactions, genetics, drug intoxication and metabolic abnormalities. It does not include atopic cardiomyopathies of definite etiology or secondary to systemic diseases.