The heart is able to contract on its own because it contains specialized cardiac muscle tissue that spontaneously forms impulses and transmits them to the myocardium to initiate contraction. This
specialized tissue forms the conduction system of the heart, which consists of the sinoatrial node, atrioventricular node, AV bundle, bundle branches, and ventricular fibers. Observe the location of the conduction system and its parts in.

The sinoatrial node (SA node) is located in the right atrium at the junction of the superior vena cava. It is known as the pacemaker of the heart because it rhythmically forms electrical impulses to initiate each heartbeat. The impulses are transmitted to the myocardium of the atria, where they produce a simultaneous contraction of the atria. The flow of impulses causes contraction of the atria from superior to inferior, forcing blood into the ventricles. At the same time, the impulses are carried to the atrioventricular node (AV node), which is located in the right atrium near the junction with the interventricular septum.

There is a brief time delay as the impulses pass slowly through the AV node, which allows time for the ventricles to fill with blood.

From the AV node, the impulses pass along the AV bundle (bundle of His), a group of large fibers that divide into left and right bundle branches extending inferiorly to the interventricular septum and superior to the lateral walls of the ventricles. The smaller ventricular (Purkinje) fibers arise from the bundle branches and carry the impulses to the myocardium of the ventricles, where they stimulate ventricular contraction. The distribution of the ventricular fibers causes the ventricles to contract from the apex superiorly so that blood is forced into the pulmonary trunk and aorta.

Electrocardiogram

The origination and transmission of impulses through the conduction system of the heart generate electrical currents that may be detected by electrodes placed on the body surface. An instrument called an electrocardiograph is used to transform the electrical currents picked up by the electrodes into a recording called an electrocardiogram (ECG or EKG).

A normal ECG of five cardiac cycles and an enlargement of a normal ECG of one cardiac cycle. Note that an ECG consists of several deflections, or waves. These waves correlate with the flow of impulses during particular phases of the cardiac cycle.

An electrocardiogram has three distinct waves: the P wave, QRS complex, and T wave. The P wave is a small wave. It is produced by the depolarization of the atria. The QRS complex is produced by the depolarization of the ventricles. The greater size of the QRS complex is due to the greater muscle mass of the ventricles. The last wave is the T wave, which is produced by the repolarization of the ventricular myocardium. The repolarization of the atria is not detected because it is masked by the stronger QRS complex. An ECG provides important information in the diagnosis of heart disease and abnormalities. In reading an ECG, physicians pay close attention to the height of each wave and to the time required for each wave.