title: Inhibition of spinal neuronal responses to noxious skin heating by stimulation of mesencephalic periaqueductal gray in the cat.
abstract: 1. Discharges of lumbar dorsal horn neurons were evoked by noxious radiant skin heating, and inhibition of the heat-evoked responses by stimulation of the mesencephalic periaqueductal gray was investigated in N2O-anesthetized cats. 2. Thirty-seven units selected on the basis of receiving afferent C-fiber input from the posterior tibial and/or superficial peroneal nerves responded vigorously to 50 degrees C heating of the plantar surface of the ipsilateral hindpaw. All discharges were inhibited by periaqueductal gray stimulation (PAGS) at current strengths of 300--900 microA; the mean threshold for inhibition was 167 microamperemeter. The mean frequency of the inhibited discharge was 39% of the control response. 3. Effective PAGS sites were distributed throughout the ventral PAG bilaterally. Stimulus current-distance estimates indicate that small (0.5--1.2 mm diameter) volumes of tissue within the PAG were stimulated. 4. A monotonic relationship between temperature and unitary discharge was found for skin heating from threshold to about 50 degrees C. PAGS resulted in a decrease in the slope of the curve plotting discharge against temperature, without altering the threshold. 5. Inhibition of the heat-evoked discharges rarely outlasted the PAGS. 6. Possible neural substrates for descending inhibition and correlates with neural mechanisms of analgesia are discussed.
mesh_terms: D000818:Animals; D001931:Brain Mapping; D002415:Cats; D004558:Electric Stimulation; D004594:Electrophysiology; D006358:Hot Temperature; D008636:Mesencephalon / Q000502:physiology*; D009433:Neural Inhibition; D010146:Pain / Q000503:physiopathology*; D011903:Raphe Nuclei / Q000502:physiology; D012867:Skin / Q000294:innervation; D013116:Spinal Cord / Q000502:physiology*