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Comparison of responses of cutaneous nociceptive and nonnociceptive brain stem neurons in trigeminal subnucleus caudalis (medullary dorsal horn) and subnucleus oralis to natural and electrical stimulation of tooth pulp.

The activity of 160 single neurons excited by electrical stimulation of the canine tooth pulp was studied in the subnucleus caudalis (medullary dorsal horn) and the subnucleus oralis of the trigeminal (V) spinal tract nucleus in chloralose-anesthetized cats to test the effects of natural as well as electrical stimulation of the tooth pulp. The neurons were functionally classified on the basis of their cutaneous receptive-field properties as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), or nociceptive specific (NS). The orofacial receptive-field properties and responses evoked by electrical stimulation of the tooth pulp indicated that the oralis and caudalis neurons examined had characteristics typical of those previously documented for oralis LTM neurons and for caudalis LTM, WDR, and NS neurons. Each neuron was also tested with cold and warm stimulation of the canine tooth, and some neurons were also tested for responsiveness to thermal stimulation of the premolar tooth or to mechanical and chemical stimuli delivered to the dentine of the canine tooth. Although all the neurons could be excited by electrical stimulation of the pulp, we found that the only neurons that consistently responded to thermal pulp stimuli were those located in the V subnucleus caudalis. Moreover, only those caudalis neurons that had been functionally classified as nociceptive (4 WDR and 21 NS neurons) showed this responsiveness. Heating of the canine or premolar tooth excited 24 of these 25 nociceptive neurons; cooling activated only 3, and none of the small number of neurons tested with mechanical and chemical stimulation of the dentine was excited. The response of the nociceptive neurons to heating of the tooth contrasted with the responses of the same neurons to pinching and heating of their cutaneous receptive field.(ABSTRACT TRUNCATED AT 400 WORDS)