Medullary sites involved in hypothalamic inhibition of reflex vagal bradycardia in the cat

Abstract

In 9 chloralosed spinal cats (C 7) electrical stimulation of the posteromedial hypothalamus (PMH) on either side was found to inhibit the vagal bradycardia elicited by stimulation of the right nucleus of tractus solitarius (NTS) and right dorsal nucleus medullae oblongatae centralis (d-MOC) but not the bradycardia elicited from the right n. ambiguus (NA). In 21 chloralosed spinal cats, field potentials were recorded at 405 sites in the right and left medulla during stimulation of the ipsilateral and contralateral PMH. The field potentials were located in the inferior olivary n., lateral reticular n., NTS, ventral and d-MOC, NA, parahypoglossal area, and dorsal longitudinal fasciculus. The field potentials had a peak latency of 19–53 msec and did not follow frequencies greater than 26 Hz, were not affected by muscle paralysis, but were abolished by barbiturate and by asphyxia. Ipsilateral medullary field potentials were abolished by ipsilateral hemitransection at the pontomedullary junction whereas contralateral field potentials were reduced by ipsilateral transection and abolished by contralateral transection. It is concluded that the inhibitory effect of PMH stimulation on reflex vagal bradycardia is mediated by crossed and uncrossed pathways which alter the electrical activity of neurons in discrete medullary structures.