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-anesthesized cats. 37 units selected on the basis of receiving afferent C-fiber input from the posterior tibial and/or superficial peroneal nerves responded vigorously to 50°C heating of the plantar surface of the ipsilateral hind-paw. All discharges were inhibited by periaqueductal gray stimulation (PAGS) at current strengths of 300-900 μA; the mean threshold for inhibition was 167 μA. The mean frequency of the inhibited discharge was 39% of the control response. 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. A monotonic relationship between temperature and unitary discharge was found for skin heating from threshold to about 50°C. PAGS resulted in a decrease in the slope of the curve plotting discharge against temperature, without altering the threshold. Inhibition of the heat-evoked discharges rarely outlasted the PAGS. Possible neural substrates for descending inhibition and correlates with neural mechanisms of analgesia are discussed.
|Original language||English (US)|
|Number of pages||11|
|Journal||Journal of Neurophysiology|
|State||Published - 1979|
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