Medullary cells of origin of vagal cardioinhibitory fibers in the pigeon. II. Electrical stimulation of the dorsal motor nucleus

Abstract

Abstract Stimulation of the peripheral end of the cervical vagus results in marked bradycardia, a secondary fall in blood pressure, and little change in respiration. Stimulation of the central end results in immediate apnea, a moderate fall in blood pressure, and little change in heart rate. Electrical stimulation of the dorsal medulla produces short‐latency bradycardia at points clustering in: (a) lateral dorsal motor nucleus, 0.5–1.0 mm rostral to the obex; (b) solitary tract; and (c) commissural nucleus of Cajal. Apnea frequently accompanies bradycardia except when elicited by stimulation of the dorsal motor nucleus. Hypotension always follows bradycardia, while hypertension accompanies tachycardia. These bradycardia responses are abolished by bilateral vagotomy and severly diminished when the ipsilateral vagus is cut or pharmacologically blocked. Beta‐sympathetic blockade has no effect on the occurrence of bradycardia responses. It is concluded that an efferent cardioinhibitory region in the pigeon is localized in the lateral dorsal motor nucleus 0.5–1.0 mm rostral to the obex and that bradycardia elicited from the solitary tract and commissural nucleus result from stimulating afferents. These results correlate nicely with the localization of the cells of origin of vagal cardioinhibitory fibers based on retrograde degeneration experiments, and together the anatomical and physiological findings indicate a concentration of these neurons in the ventrolateral aspect of the dorsal motor nucleus approximately three‐quarters of a millimeter rostral to the obex.