Distribution and response properties of cat SI neurons responsive to changes in tooth temperature

The activity of 214 tooth pulp-driven neurons (TPNs) in the primary somatosensory cortex (SI) activated by electrical stimulation of the canine tooth pulp was studied in anesthetized cats. These neurons were tested for their responses to thermal stimulation of the tooth pulp. One hundred fifty-five TPNs were not responsive to changes in tooth temperature (thermally insensitive, TINS-TPNs) and 59 TPNs were responsive (thermally sensitive, TS-TPNs: 38 TS-TPNs were heat sensitive and 21 were cold sensitive). TS-TPNs were also tested for responsiveness to mechanical and thermal stimulation of the skin, mucosa, or periodontal membrane. Each TS-TPN was classified on the basis of cutaneous, mucosal, or periodontal mechanical receptive-field properties as either low-threshold mechanoreceptive (LTM: 57%), wide dynamic range (WDR: 25%), nociceptive specific (NS: 10%) or pulp specific (PS: 8%). TS-TPNs were distributed in an upper bank of the orbital sulcus of SI. The majority were located in laminae III (32%) and IV (60%) of area 3b. Heat-sensitive LTM, WDR, PS TS-TPNs, and cold-sensitive LTM TS-TPNs were characterized by a rapid rise in firing rate during thermal stimulation of the tooth pulp. In contrast, heat-sensitive NS and cold-sensitive NS and PS TS-TPNs responded with a slow rise in firing frequency during thermal stimuli delivered to the tooth pulp. A linear regression analysis was applied to the stimulus-response functions of neuronal discharges of TS-TPNs. Fifty-six percent of heat-sensitive LTM, WDR, and cold-sensitive LTM TS-TPNs showed statistically significant relation (P < 0.5) between peak firing frequency and stimulus temperature and increasing firing frequency after increases in stimulus temperature, whereas heat-sensitive and cold-sensitive NS and PS TS-TPNs did not show a clear increase in firing frequency during the thermal stimulus. These findings suggest that heat-sensitive LTM and WDR TS-TPNs and cold-sensitive LTM TS-TPNs that showed high regression coefficients in stimulus-response function may be involved in encoding the intensity of noxious thermal stimulation of the tooth pulp.