Localized myocardial responses to stimulation of small cardiac branches of the vagus.

Direct electrical excitation of small cardiac branches from the thoracic vagus elicited highly localized and differential responses from individualized segments of the myocardium. For example, small nerves from the vagus at the level of the superior pulmonary veins frequently induced moderate inhibition in contractile force of the ipsilateral atrium with little or no influences elsewhere. Branches from more rostral levels of the thoracic vagus induced changes in atrial contractility, with or without changes in sinoauricular (SA) nodal discharge rates, and often with partial or complete artioventricular (AV) nodal blockade.Excitation of individual, small vagal branches sometimes initiated acceleration in artrial rate and augmentation in atrial contractile force concurrently with complete AV nodal blockade. The negative dromotropic response was eliminated by atropine, leaving only the positive chronotropic and inotropic changes, thus revealing the intermingling of both sympathetic and parasympathetic components even in these small branches. There are frequently in excess of 20 small branches from the vagal trunk between the level of the caudal cervical ganglion and the superior pulmonary vein on each side which will induce highly selective changes in cardiac function upon stimulation. Inhibitory branches are particularly concentrated in the region of the recurrent laryngeal nerve on either side.