The conducting system of the heart consists of two nodes—the sinoatrial node and the atrioventricular node—and a series of conduction fibers or bundles (tracts). Electrical impulses are generated at the sinoatrial (SA) node, a group of specialized nodal cardiac muscle cells located near the junction of the superior vena cava and the right atrium (see Fig. 13.5). Since the SA node has the fastest rate of depolarizations, it is referred to as the pacemaker of the heart. The pacemaker rate of the SA node is about 60 to 100 beats per minute. The SA node initiates an impulse that spreads along the cardiac muscle fibers of the atria and along internodal tracts composed of modified cardiac muscle fibers. The impulse is then picked up at the atrioventricular (AV) node and carried across the fibrous skeleton to the ventricles by the AV bundle (of His). The bundle divides into smaller right and left bundle branches and then into subendothelial branches, commonly called Purkinje fibers. The components of the conducting system convey impulses at a rate approximately four times faster than the cardiac muscle fibers and are the only elements that can convey impulses across the fibrous skeleton.
If the SA node fails to function (e.g., because of insufficient blood supply), then the area with the next fastest intrinsic rate of depolarization will take over. In this situation, the AV node will drive the heart contractions at a rate of about 50 beats per minute. In complete heart block when the conduction of electric impulses to the ventricles is interrupted, the ventricles will beat at their own rate of about 30 to 40 beats per minute, driven by depolarization of Purkinje fibers. Purkinje fibers have the slowest rate of intrinsic depolarization of the entire conducting system. The spread of electrical impulses through the myocardium can be monitored and recorded by the electrocardiogram (ECG). The ECG is obtained by placing electrodes at different points on the skin at specific distances from the heart. Electrodes record electrical activity of the heart by measuring voltage differences between different points. The coordinated spread of the electrical activity through the heart is responsible for the shape of the ECG waveform, careful analysis of which can provide information about heart rate, cardiac rhythm, conduction times through various parts of the heart, effects of electrolyte concentration, effects of cardiac medication, and location of pathologic (ischemic) damages in the heart.
The nodal cardiac muscle cells in both the SA and AV nodes are modified cardiac muscle fibers that are smaller than the surrounding atrial cardiac muscle cells. They contain fewer myofibrils and lack typical intercalated discs. The AV bundle, the bundle branches, and the Purkinje fibers are also composed of modified cardiac muscle cells, but they are larger than the surrounding ventricular muscle cells (Fig. 13.10 and Plate 32, page 432).
The terminal ramifications of the conducting system consist of Purkinje fibers.
Cardiac conducting cells that make up the bundle of His originate at the AV node, pass through the fibrous skeleton of