Two preferential conducting pathways within the bundle of His of the dog heart.

An isolated preparation formed exclusively of the specific conducting tissue of the A. V. node, bundle of His and bundle branches was dissected from the dog heart and studied using the microelectrode technique. The preparation, being completely isolated from the normal neighboring conducting tissues, can be turned over or twisted, so as to stimulate and record either separately or simultaneously the electrical activity of the dorsal and ventral surfaces of the bundle of His. The preparation showed spontaneous activity and maintained its physiological properties for long periods of time. Morphological and electrophysiological evidence was found to differentiate two functional strands within the common trunk of the bundle of His. These two strands appear in different planes: The anterior or ventral strand starts in the superior part of the atrioventricular node and continues to form the right bundle branch; the posterior or dorsal strand begins in the inferior part of the A. V. node and runs underneath the ventral strand to form later the left bundle branch. The two strands can be separated from each other so as to have two functional preparations: one formed of the superior part of the A. V. node, the anterior strand and the right bundle branch; the other constituted by the inferior part of the A. V. node, the posterior strand and the left bundle branch. Some of the electrophysiological properties of both strands are similar, except for the conduction velocity, which appears to be faster in the posterior strand than in the anterior. Transversal propagation occurs between the two strands and is slower than the longitudinal propagation that takes place along the parallel fibers of each strand. The presence of cellular transversal bridges between the strands assures the activation of the two strands as if they were a single conducting cable. These characteristics are discussed in relation to some disturbances in propagation.