Tattersall_1986_J.Neurophysiol_56_1411

Single-unit electrical activity has been recorded from 95 viscerosomatic neurons in the T9 and T11 segments of the cat's spinal cord. These neurons were excited by electrical and/or natural stimulation of visceral and somatic afferent fibers. The excitatory and inhibitory effects on these neurons of volleys in somatic and visceral afferent fibers and of electrical and chemical stimulation of the nucleus raphe magnus (NRM) and adjacent areas of the reticular formation (Ret. F.) have been studied. Electrical stimulation of the splanchnic nerve produced, after the initial excitation of the neurons, a period of inhibition lasting for up to 1 s. This inhibition reduced the responsiveness of the neurons to all inputs, somatic and visceral, and was still present after spinalization of the animals with cold block, which indicates a segmental organization of the inhibition. Electrical stimulation of afferent fibers within the somatic receptive field of the neurons produced, after the initial excitation, a period of inhibition similar to that induced by visceral afferent volleys. During this period of inhibition all inputs to the neurons were reduced. Reversible spinalization of the animals with cold block did not abolish this inhibition. On the basis of the effects of reversible spinalization on the visceral input to viscerosomatic neurons, two types of neurons were distinguished: 1) neurons whose visceral responses increased in the spinal state (neurons under tonic descending inhibition) and 2) neurons whose visceral responses were decreased or abolished in the spinal state (neurons subject to descending excitation). Neurons under tonic descending inhibition were inhibited by electrical stimulation of locations within the NRM and Ret. F. This inhibition lasted for less than 100 ms and could be evoked at intensities of stimulation of 100 microA or less. Neurons under descending excitation were also inhibited by electrical stimulation in the NRM and Ret. F. but, in addition, the inhibition was preceded by an excitation in 75% of these neurons. Chemical stimulation with DL-homocysteic acid (DLH) of locations within the NRM and Ret. F. was used to activate cell bodies, but not axons, located in these brain stem sites. The only effect observed following injections of DLH into the NRM and Ret. F. was inhibition of viscerosomatic neurons including those with descending excitation as well as those with descending inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)