title: Cardio-inhibitory mechanism in the gigantocellular reticular nucleus of the medulla oblongata.
abstract: A cardio-inhibitory mechanism was localized in the ventral part of the medullary gigantocellular reticular nucleus (GRN) in chloralose--urethane anesthetized cats. Stimulation of this mechanism produced an average 58.9% reduction of the heart rate (calculated from 55 responsive points having more than 40% reduction) associated mostly with hypotension, or no change or occasionally a slight increase of the arterial blood pressure. Midcollicular decerebration did not affect this bradycardia. The bradycardia following GRN stimulation of either side by a pair of symmetrically placed electrodes was reduced slightly but equally by: (1) sectioning either side of the vagus nerve; (2) hemisection at a level 4 mm rostral of the obex on either side; or (3) partial destruction of the dorsal motor (DM) and solitary (SN) nuclei on either side. Additional section of the vagus nerve on the opposite side completely abolished the bradycardiac response. Besides, the GRN bradycardia was also slightly but equally attenuated by making a midline bisection in a length extending from 10 to 4 mm rostral of the obex. Additional section of the vagus nerve on either side abolished completely only the bradycardia following electrical stimulation of the GRN on the same side, while that following electrical stimulation of the GRN on the opposite side remained unaffected. On the other hand, the GRN bradycardia was not affected by simply making a caudal midline bisection in a length extending from 3 or 4 mm rostral to 2 mm caudal of the obex. The results suggest: (1) the ventral part of the GRN is a cardio-inhibitory mechanism independent of the higher center; (2) the efferent pathway descends both ipsilaterally and contralaterally and makes synaptic relay in the areas of DM and/or SN and finally exits via the vagus nerves; (3) the fibers decussate rostral to the level 4 mm rostral of the obex; and (4) both ipsilateral and contralateral descending fibers appeared to exert the same degree of suppressive influence in the heart rate.
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002415:Cats; D004837:Epinephrine / Q000494:pharmacology; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D008526:Medulla Oblongata / Q000502:physiology*; D009433:Neural Inhibition / Q000187:drug effects; D009434:Neural Pathways / Q000502:physiology; D009474:Neurons / Q000502:physiology; D011311:Pressoreceptors / Q000502:physiology; D012018:Reflex / Q000502:physiology; D014630:Vagus Nerve / Q000502:physiology
chemical_list: D004837:Epinephrine