SUMMARY1. In nembutal-anaesthetized cats, the responses of intradental Ad and C fibres to rapid cooling of the crown of canine teeth were studied.2. Single-unit recordings were obtained from a total of eighty-six intradental Ad and C fibres. The mean conduction velocity of Ad fibres was 13-9 m/s (n = 43; range: 3-6-26-0 m/s), that of C fibres was 1-3 m/s (n = 43; range: 05-2-2 m/s).3. In the intact tooth none of the identified Ad or C fibres showed any ongoing activity in the absence of intentional stimulation.4. 84% of the Ad fibres (thirty-six out of forty-three) and 88% of the C fibres (thirty-eight out of forty-three) were excited by cold stimulation of the canine tooth they were innervating.5. For all cold-sensitive Ad fibres the responses to rapid lowering of the tooth temperature were rather uniform. After an initial high-frequency discharge during the most rapid change in temperature, the discharge rate fell as the rate of change of temperature became smaller, and firing stopped completely when the temperature had reached a steady level. No firing occurred as the tooth temperature returned to its initial value. 6. A good linear correlation (r = 089) was found between the initial dynamic discharge of responding Ad fibres and the maximum rate of change of temperature achieved in a particular experiment.7. The response behaviour of C fibres to rapid cooling of the tooth was also rather uniform but different from that of Ad fibres. For C fibres no initial dynamic response phase was observed. After a mean latency of 7-3 s the fibres began to discharge regularly at a low rate at a time when the change of tooth temperature was already small.8. The firing rate of the C fibres had a weak linear correlation (r = 06) with the static tooth temperature achieved. No discharge was observed as the temperature returned to its initial value.9. For eleven cold-sensitive Ad and C fibres the receptive fields were determined by mechanical stimulation of the exposed pulp tissue. For Ad fibres the receptive * Present address: Department of Physiology, University of Helsinki, Siltavuorenpenger 20 J, SF-00170 Helsinki, Finland.PHY 391 E. JYVASJARVI AND K.-D. KNIFFKI fields were located at the pulp-dentine border, those for C fibres were located much deeper in the pulp and tended to have higher mechanical thresholds.10. When a human tooth was stimulated by a similar cold stimulus as used in the animal experiments, the sensation evoked and rated by the subjects using a fiftypoint categorical division procedure was described as a sharp, shooting pain being localized strictly to the tooth tested. Sensations other than pain were not perceived.11. Comparing the pain ratings of human subjects with the response characteristics of feline intradental Ad and C fibres to cold stimulation of a tooth, it is concluded that Ad fibres with the properties found in the cat are probably responsible for the cold-evoked dental pain sensation in man.

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Cold stimuli of varying intensities were randomly applied to upper middle incisors of 12 healthy young subjects for a mean duration of 2 min by individually adapted thermodes the temperatures of which ranged from +30 degrees C to -30 degrees C. The subjects were asked to rate the magnitude of their pain sensations during application of the stimuli by means of a linear potentiometer according to a category scale. After each stimulus, they were asked to describe the quality of their pain sensations. Cold stimulation of the teeth evoked pain sensations were reproducible that in subsequent trials and could be graded according to stimulation intensity. Below certain individually different threshold thermode temperatures the onset of a stimulus was followed, after a short latency (1.6 +/- 1 sec), by a sharp and shooting pain sensation which immediately decreased after reaching its maximum value while the stimulus was still present. The mean maxima of the pain intensities were correlated to the thermode temperature. In general, this first pain component was followed by a second one (latency: 29.9 +/- 6.3 sec) with a lower threshold temperature, less of an increase in rate and lower magnitude. This was described as a dull, burning pain which was difficult to localize. The human pain ratings are compared to recordings of intradental nerve fibres in the cat and, under the assumption that the response behaviour of human pulpal nerve fibres is comparable to that of the cat, we hypothesize that the first pain component is evoked by intradental A delta fibres exhibiting their typical phasic response behaviour and firing during the initial steep temperature decrease. After some seconds, intradental temperature reached values sufficient to evoke C-fibre activity associated with the second pain component.