XVIII.—The Conducting System of the Marsupial Heart

105

In 20 anesthetized dogs, close bipolar electrograms consisting of two deflections were recorded along the ligament of Marshall in the posterior left atrium. At the inferior portion of the ligament the deflections were separated by an interval of 10-15 msec and at the level of the left superior pulmonary vein by an interval of 60 msec or more. Histological and electrophysiological studies indicated that both deflections were associated with striated cardiac muscle; the first deflection was designated as a left atrial muscle potential and the second as a left atrial tract potential. Although the left atrial muscle potential was coincident with the middle portion of the P wave, the left atrial tract potential occurred during some part of the P-R segment. Conduction to the left atrial tract potential was altered, delayed, or completely blocked during retrograde conduction in the atrium, rapid pacing of the atrium (300-500/min), or local cooling at the point where the ligament of Marshall joined the inferior left atrium near the origin of the coronary sinus. Crushing Bachmann's bundle caused delay in conduction to the left atrial muscle but not to the left atrial tract. Left cardiac sympathetic nerve stimulation induced an atrial activation pattern consistent with pacemaker activity in the inferior left atrium, with inversion of the P wave in lead II. Pacing from the edge of the ligament of Marshall in the posterior left atrium produced the same change in P-wave morphology. These data indicate that the left atrial tract represents a terminal, insulated tract which is activated through an inferior interatrial pathway connecting the inferior right and left atria along the coronary sinus. The terminal end of the tract showed no reinsertion into atrial musculature. KEY WORDS bipolar electrogramsHis bundle recordings P waves left atrial rhythms ligament of Marshall atrial tracts cardiac sympathetic nerve stimulation cardiac pacing Bachmann's bundle• Recently there has been a revival of interest in the patterns of atrial activation based primarily on the work of James (1), who described histologically distinct atrial tracts running from the sinoatrial (SA) node to the atrioventricular (AV) node. In 1850 Marshall (2) described the prenatal development of an area of the left atrium running between the superior and inferior left pulmonary veins, which became a ligamentous fold. This structure, now referred to as the ligament

Section: Discussionmentioning

confidence: 99%

Inferior Interatrial Pathway in the Dog

Scherlag

Yeh

Robinson

1972

105

mentioning

confidence: 99%

“…6 In subsequent years, it was found that ganglia, nerve fibers, and nerve nets lie in close proximity to the AV node in the human heart 7 and that the terminations on nodal tissue vary from fine fibrils to complex reticular nets. 8 It should be noted, however, that all of these early works were based upon light microscopy and variable staining techniques and, thus, not all of these results were accepted conclusively.…”

mentioning

confidence: 99%

The influence of the parasympathetic nervous system on atrioventricular conduction.

Martin¹

1977

IN THIS brief review I will attempt to amalgamate current findings with important historical concepts about the influence of the parasympathetic nervous system on atrioventricular (AV) conduction. I first will describe the innervation of the AV nodal region. Then I will discuss the influence of acetylcholine and associated cholinergic agents, the effects of vagal stimulation, and the role of the vagus in the genesis of arrhythmias. Finally, I will describe briefly certain cardiovascular reflexes that affect AV conduction. I have not attempted to provide an exhaustive list of references; rather, the most relevant works were arbitrarily selected to illustrate specific points. I regret that many worthy contributions had to be omitted. Innervation of the AV NodeIt has been known since the early part of this century that the autonomic nervous system can affect AV conduction profoundly. "3 Anatomical studies during this era revealed that the AV node is richly innervated 4 -5 and that the density of nerve fibers in this region exceeds that in the myocardium. 6 In subsequent years, it was found that ganglia, nerve fibers, and nerve nets lie in close proximity to the AV node in the human heart 7 and that the terminations on nodal tissue vary from fine fibrils to complex reticular nets.8 It should be noted, however, that all of these early works were based upon light microscopy and variable staining techniques and, thus, not all of these results were accepted conclusively.Most of the definitive work in this area has been done within the past 10-20 years. I first will review the neural pathways to the AV node, and then describe the terminal innervation of the node. Although the focus here is on the vagus, the adrenergic innervation will also be mentioned briefly for completeness.Geis et al. 9 recently have delineated the various autonomic pathways to the AV node of the dog. They found that the left anterior limb of the ansa subclavia and the ventrolateral cardiac nerves innervated the AV node in each dog studied. The posterior ansa on the left side and both limbs of the ansa on the right side innervated the A V node in one-half to two-thirds of their dogs, whereas the innominate, the recurrent cardiac, the craniovagal, and the ventromedial cardiac nerves innervated the AV node in less than one-fourth of the animals. The right-sided sympathetic innervation of the AV node consisted of nerve fibers that accompanied the great vessels to the region where the inferior vena cava approximates the inferior border of the left atrium. The sympathetic innervation on the left side followed similar pathways, but additional fibers reached the node through the ventrolateral cardiac nerve. Both the left and right parasympathetic innervations traversed pathways along the superior left atrium and the region where the inferior vena cava lies close to the inferior margin of the left atrium.Thus it has been demonstrated that nerve fibers from both sides of both autonomic divisions project onto the AV node. The differential effects of neural act...

“…These data support the view, first, that the nodal fibers form a reticulum in which strands assume a whorled arrangement and, second, that the common bundle "arises from the centre of the main mass of the atrio-ventricular node, as the shaft fits into the knob of a walking stick." 11 The assumption is that the excitation process follows the course of these strands and that the latter form a continuum between the atrionodal junction on the one hand and the nodobundle junction on the other.…”

Section: Transmission Through the A-v Nodementioning

confidence: 99%

Potential Changes Attending the Excitation Process in the Atrioventricular Conduction System of Bovine and Canine Hearts

Pruitt

Essex

1960

Potential changes attending excitation of the atrioventricular node, the common bundle, the bundle branches, and initial ramifications of the conducting tissue in the subendoeardium of the bovine heart beating in situ have been recorded from unipolar exploring electrodes. Correlations between the form of nodal deflection and the location in the node of the electrode tip have been established. Prom right bundle branch and subendocardial action potentials, data were derived indicating a rate of transmission through the right branch of 2.02 M/sec. Certain observations on the effects of acute injury and severe hypoxia are recorded. The character of nodal deflections attending retrograde excitation is reviewed. In studies on canine hearts beating in situ, eleetrograms have been made from the regions in which the atrioventricular node, the common bundle, and bundle brandies are believed to lie. Deflections comparable to those recorded in bovine experiments were encountered and, by analogy with those bovine studies, certain morphologic correlations postulated. In general, canine nodal deflections are smaller than their bovine counterparts and lack the variety of form ascribed to potential changes in the several portions of the bovine node. In canine studies, deflections of atrial, atrionodal, and nodal type coexisted in recordings from a single needle electrode suggesting a degree of proximity between the sources of these potential changes not encountered in bovine experiments.PART I Observations on Bovine Hearts I N human and canine hearts, component parts of the atrioventricular conduction system can be identified only by painstaking dissection. In the bovine heart, these structures are morphologically distinct and offer certain advantages to study of potential changes attending their excitation. During the past 3 years, a series of studies has been carried on in our laboratory in an attempt to exploit this morphologic advantage. These studies will be summarized here.Meanwhile, widespread interest in the problem of atrioventricular excitation has found expression in a number of excellent Prom the Mayo Clinic and the Mayo Foundation,