The connecting pathways between the sinus node and A-V node and between the right and the left atrium in the human heart

Abstract

In the human heart there are three connecting pathways between the sinus node and the A-V node, all of which contain Purkinje fibers but also many myocardial fibers without these characteristics. The anterior internodal tract passes from the sinus node to sweep anterior to the superior vena cava into Bachmann's bundle, where it divides to distribute to the left atrium and to curve back into the interatrial septum and descend to the A-V node. The middle internodal tract leaves the dorsal and posterior margins of the sinus node and courses behind the superior vena cava through the sinus intercavarum to the crest of the interatrial septum, and there descends into the A-V node, merging with fibers from the anterior tract as it approaches the node. The posterior internodal tract follows the crista terminalis from the sinus node to the Eustachian ridge and thence through the ridge to the posterior margin of the A-V node. Together, these three tracts, the two nodes, the ostia of both venae cavae, and the coronary sinus, comprise the general outline of the primitive sinus venosus. Reasons why the myocardium in this region may be anticipated to have specialized function are discussed. Although demonstration of internodal pathways does not establish the validity of the theory of circus movement, possible circles composed of limbs of the three internodal tracts lend anatomic support to such a theory. On the basis of anatomic considerations, conduction from the sinus node to the left atrium is believed to occur preferentially through Bachmann's bundle. An alternate interatrial pathway, which is much longer for the sinus impulse, spreads from the septal portion of the internodal tracts to the left atrium.