Cardiac Impulses and Their Conduction

The heart has the unique property of rhythmicity. To maintain life, it must contract rhythmically; the average rate of contraction of the heart for an adult at rest is seventy times a minute. Proper function of the circulatory system also requires a well-coordinated contraction of the various parts of the heart. The origin of the cardiac impulses and their conduction are the functions of specialized cells distributed throughout the heart, consisting of the pacemaker (the initiator of cardiac action) and the conducting system (the distributor of the impulses through the heart). These cells are grouped together into three types of structures: two larger accumulations of cells, or nodes; nervelike conduits, or bundles , with branches; and the terminal portion of the branches, constituting a fine network on the inner surface of the ventricles.

The uppermost node, the sinoatrial node (S-A node ), is located at the junction of the superior vena cava and the right atrium. The lower node, the atrioventricular node (A-V node ), is located in the lower septal wall of the right atrium. From the lower portion of the A-V node emerges the bundle of His , entering the junction between the two atria and the two ventricles, where it divides into two principal branches: the left-bundle branch and the right-bundle branch, which run down either side of the ventricular septum. The branches subdivide into smaller and smaller branches, eventually forming the fine network, or Purkinje network , located

Figure 9. The heart with its front wall removed, shown to
indicate the principal parts of the conduction system.

in the endocardial layer of the ventricles, where it comes in contact with muscle cells to be stimulated. A diagram of the conducting system is shown in figure 9.

Specialized cells of the S-A node generate electrical potential. When that potential reaches a certain level, it is discharged, activating the conducting system throug which an electrical impulse travels, producing contraction of the cardiac muscle. The discharge of electrical potential in specialized cells is called depolarization . Immediately after depolarization the cell begins rebuilding electrical potential (repolarization ) for the next impulse. This process is analogous to discharging and recharging a battery. All specialized cells have the capacity of generating an electrical impulse as well as carrying it rapidly throughout the conducting system. However, impulse formation in cells other than those of the S-A node is suppressed by the function of the S-A node, which as the primary pacemaker has the fastest rate of discharge. Its impulse directly stimulates the atria to contract, then speeds through the atrium to the A-V node, where it slows considerably. When the impulse reaches the junction between the A-V node and the bundle of His, it passes quickly through the remainder of the conducting system down to the Purkinje system, through which it stimulates the ventricles to contract. Slow conduction through the A-V node is essential for appropriate coordination of atrial and ventricular contractions, which should be separated by an interval of about 0.16 seconds to facilitate flow of blood from the atria to the ventricles.

Since all cells within the conducting system are capable of impulse formation, they serve as standby pacemakers activated only when the primary pacemaker fails to discharge. The lower portion of the A-V node, at its junction with the bundle of His, is the secondary pacemaker . Its rate of discharge, 50 times a minute, is slower than that of the primary pacemaker. Lower divisions of the conducting system, including the Purkinje network, represent the third line of defense against failure of the impulse to reach the ventricles. The discharge rate of this tertiary pacemaker is 30 to 40 times a minute. It is most frequently activated when the connection between the A-V node and the Purkinje system is interrupted, in which case the atria may contract at a fast rate, obeying the primary pacemaker, while the ventricles contract more slowly, activated by the tertiary pacemaker (see chap. 6).

The heart muscle, like other muscle tissue, has the ability to contract (that is, reduce its length), thereby exerting a considerable force. Since the heart muscle is globular in shape and envelopes a cavity filled with blood, its contraction expels most of the contents of the cavity. Muscle cells, accepting the stimulus from the conducting system, also discharge electrical potential while contracting (being depolarized) and are recharged (repolarized) during relaxation. However, these cells under normal conditions are not capable of impulse formation.

The primary pacemaker, the S-A node, is under the control of the autonomic nervous system (the part of the nervous system unresponsive to a person's will) through fibers connecting it with both divisions of the autonomic nervous system: sympathetic nerve fibers can quicken impulse formation; parasympathetic nerve fibers can slow it. This nervous control permits necessary adjustments in cardiac function, such as accelerating the heart rate during exercise and slowing it afterward. The secondary pacemaker is less efficiently regulated by the autonomic nervous system. The tertiary pacemaker has no significant connections with that system: its rate of discharge remains the same under all conditions.