Introduction
The parts of the heart normally beat in an orderly sequence: Contraction of the atria (atrial systole) is followed by contraction of the ventricles (ventricular systole), and during diastole, all four chambers are relaxed. The heartbeat originates in a specialized cardiac conduction system and spreads via this system to all parts of the myocardium. The structures that make up the conduction system are the sinoatrial node (SA node), the internodal atrial pathways, the atrioventricular node (AV node), the bundle of His and its branches, and the Purkinje system.
Origin & spread of cardiac excitation
In the human heart, the SA node is located at the junction of the superior vena cava with the right atrium. The AV node is located in the right posterior portion of the intertribal septum. There are three bundles of atrial fibers that contain Purkinje-type fibers and connect the SA node to the AV node: the anterior internodal tract of Bachman, the middle intermodal track of Wenckebach, and the posterior internodal tract of Thorez. Conduction also occurs through atrial myositis, but it is more rapid in these bundles. The AV node is continuous with the bundle of His, which gives off a left bundle branch at the top of the interventricular septum and continues as the right bundle branch. The left bundle branch divides into an anterior fascicle and a posterior fascicle.
Properties of cardiac muscle
The electrical responses of cardiac muscle and nodal tissue and the ionic fluxes that underlie them are discussed in detail and are briefly reviewed here for comparison with the pacemaker cells below. Myocardial fibers have a resting membrane potential of approximately –90 mV. The individual fibers are separated by membranes, but depolarization spreads radially through them as if they were a syncytium because of the presence of gap junctions.
Pacemaker potentials
Rhythmically discharging cells have a membrane potential that, after each impulse, declines to the firing level. Thus, this prepotential or pacemaker potential triggers the next impulse. At the peak of each impulse, IK begins and brings about repolarization. IK then declines, and a channel that can pass both Na+ and K+ is activated. Because this channel is activated following hyperpolarization, it is referred to as an “h” channel; however, because of its unusual (funny) activation, this has also been dubbed an “f” channel. As Ih increases, the membrane begins to depolarize, forming the first part of the prepotential. Ca2+ channels then open.
Bipolar leads
Bipolar leads were used before unipolar leads were developed. The standard limb leads—leads I, II, and III—each record the differences in potential between two limbs. Because current flows only in the body fluids, the records obtained are those that would be obtained if the electrodes were at the points of attachment of the limbs, no matter where on the limbs the electrodes are placed. In lead I, the electrodes are connected so that an upward deflection is inscribed when the left arm becomes positive relative to the right (left arm positive). In lead II, the electrodes are on the right arm and left leg, with the leg positive; and in lead III, the electrodes are on the left arm and left leg, with the leg positive.
Summary
Contractions in the heart are controlled via a well-regulated electrical signaling cascade that originates in pacemaker cells in the senatorial (SA) node and is passed via intermodal atrial pathways to the atrioventricular (AV) node, the bundle of His, the Purkinje system, and to all parts of the ventricle.
Most cardiac cells have an action potential that includes a rapid depolarization, initial rapid repolarization, a plateau, and a slow repolarization process to return to resting potential. These changes are defined by sequential activation and inactivation of Na+, Ca2+, and K+ channels.
