Specialized Conduction System of Heart
The specialized heart tissues include the sinoatrial (SA) node, atrioventricular (AV) node, common atrioventricular bundle or bundle of His, right and left bundle branches, and peripheral ramifications of these bundle branches, which make up the subendocardial and intra- myocardial Purkinje network. In addition, other fiber groups in the atria meet some of the histologic and electrophysiologic criteria for specialization. These tissues constitute Bachmann’s bundle and the inter- nodal conducting paths of the right atrium.
The body of the SA node is in the
wall of the right atrium, at the junction between the atrium proper and the superior
vena cava. At the lower end, the nodal fibers change and form the common
bundle. The common bundle divides into right and left bundle branches, which
extend subendocardially along both septal surfaces. The left bundle branch
rapidly subdivides, forming a broad sheet of fascicles sweeping over the left
interventricular septal surface. The right bundle branch extends for a distance
without subdivision; one branch usually passes through the moderator band,
and other parts extend over the endocardial surface of the ventricle.
Peripherally, both bundle branches subdivide and form the subendocardial
network of Purkinje fibers, which extend a variable distance into the
ventricular walls and are in direct continuity with fibers of the ventricular
muscle.
In definitive histologic studies of
the human atrium, James demonstrated the
existence of three discrete internodal paths and the relationship of one of
these to Bachmann’s bundle. The anterior internodal tract leaves the
head of the sinus node and spreads to the left, dividing to form two branches:
One extends along the dorsal aspect of the interatrial band to ramify over the
left atrium. This subdivision constitutes the specialized fibers of Bachmann’s
bundle. The other branch curves across the interatrial septum to the region of
the AV node, where it merges with fibers from other nodal tracts. The middle
internodal tract leaves the posterodorsal margin of the sinus node and
crosses the interatrial septum to merge at the AV node with other specialized
atrial fibers. This tract corresponds to the bundle described by Wenckebach. The posterior internodal
tract extends from the tail of the sinus node along the crista terminalis,
through the eustachian ridge, the right superior margin of the AV node. A
description of the interconnections of internodal tracts with the atrium and AV
node follows.
Physiologic evidence suggests that
the spread of the sinus impulse to the left atrium and from the sinus node to
the AV node normally depends primarily on activation of the anterior internodal
tract and Bachmann’s bundle. The physiologic significance of these tracts is also described here.
The only normal anatomic communication
between the atria and ventricles of the mammalian heart is the atrioventricular
node with the common bundle of His. On the atrial side, the AV node
communicates with the atrium through the branched and interweaving fibers of
the internodal tracts and perhaps through connections with ordinary atrial
musculature. In addition, in studies of the canine AV node, fiber tracts appear
to bypass the nodal body and connect with distal portions close to the junction
of nodal fibers and the common AV bundle. Similar “bypass” fibers can be
demonstrated in studies of the human AV node.
