Heart contraction results from a series of electrical potential changes, or depolarization waves, that travel through the heart preliminary to each beat. The heart’s ability to beat is intrinsic. It does not depend on impulses from the nervous system to start its contraction, and will continue to beat even when all nerve connections are severed. The intrinsic conduction system, or nodal system, of the heart consists of specialized no contractile myocardial tissue. This system ensures that the heart muscle depolarizes in an orderly and sequential manner from atria to ventricles and that the heart beats as a coordinated unit. The conduction of impulses through the heart generates electrical currents that spread throughout the body. These impulses can be detected on the body’s surface and recorded with an electrocardiograph. The graphic recording of the electrical changes, depolarization and repolarization, that occur during the cardiac cycle is called and electrocardiogram (ECG).