The myocardium is a muscle tissue composed of cardiomyocytes, including specifically differentiated cardiomyocytes and general working cells of the atrial and ventricular myocardium. The myocardium has specially differentiated cardiomyocytes, such as the sinus node, intra-atrial bundle, atrioventricular junction, atrioventricular bundle, and Purkinje fibers, which make up the heart’s pacemaker conduction system. In addition the myocardium has generalized working cells of the atrial and ventricular myocardium. Specially differentiated cardiomyocytes make up the pacemaker conduction system and are the functional basis for cardiac rhythmic activity, whereas generalized atrial and ventricular myocyte working cells are the functional basis for cardiac diastolic and contractile activity. Cardiomyocytes are non-renewable cells, and necrosis of cardiomyocytes can occur during extreme ischemia and hypoxia, commonly seen in people with acute myocardial infarction, which can lead to compromised cardiac function.