عضلة القلب
Throughout the cardiac cycle, blood pressure increases and decreases. The movements of cardiac muscle are coordinated by a series of electrical impulses produced by specialised pacemaker cells found within the sinoatrial node and the atrioventricular node. Cardiac muscle is composed of myocytes which initiate their internal contractions without applying to external nerves—with the exception of changes in the heart rate due to metabolic demand.