The structure and innervation of the conductive system of the heart of the dog and rhesus monkey, as seen with a silver impregnation technique

Abstract

1. 1. A study of the impulse conducting system in the hearts of puppies and one rhesus monkey, prepared with the chloral hydrate formula of the Cajal silver nitrate technique, has confirmed the observations of previous investigators. In addition to the S-A and A-V nodes (Fig. 1), there is a distinct main bundle (Figs. 5, 6, and 7) which divides into right and left bundle branches. The latter and their ramifications are composed of swollen fibers, with small numbers of myofibrils, abundant sarcoplasm, and irregular nuclei. These fibers connect at various levels with the musculature of the ventricles (Fig. 9, right); in the puppy they resemble closely the typical Purkinje fibers of the calf, sheep, etc. (Fig. 9, left), whereas, in the monkey, they are less differentiated but nevertheless quite conspicuous (Fig. 10, A and B). 2. 2. The S-A and A-V nodes, respectively, are supplied by axons of neurons of the intrinsic cardiac ganglia (parasympathetic postganglionics), but the nerve terminals are much more numerous in the A-V node (cf. Fig. 2 with Fig. 4). Whether there is a sympathetic nerve supply to the nodes could not be ascertained because the technique used does not impregnate sufficiently the sympathetic postganglionics. 3. 3. The main bundle and bundle branches of the dog lack parasympathetic nerve endings. In the monkey these nerve terminals occur not only in the main bundle (Fig. 8), but also in the proximal portions of the right and left bundle branches. Parasympathetic nerve endings are absent in the ramifications of the bundle branches in the two species. 4. 4. The rich parasympathetic nerve supply to the A-V node, as compared with the innervation of the S-A node, may account for such phenomena as the production of A-V block through electrical stimulation of the vagus, the similar effect produced by morphine, by digitalis in cases of auricular fibrillation, etc., and, in general, the greater influence of vagal stimulation on the A-V node. 5. 5. The existence of a parasympathetic innervation of the ventricles, suggested by the effect of excitation of the vagus after the A-V bundle has been crushed, is questioned. The variable effects on the idioventricular rhythm reported by Jourdan and Froment cannot be explained on an anatomic basis. 6. 6. The anatomic basis for the inotropic effect of the vagus is indicated by the existence of parasympathetic nerve endings in the atrial and auricular musculatures.