Cold stimulation of teeth: a comparison between the responses of cat intradental A delta and C fibres and human sensation.

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Abstract
1. In nembutal-anaesthetized cats, the responses of intradental A.delta. 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 A.delta. and C fibres. The mean conduction velocity of A.delta. 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: 0.5-2.2 m/s). 3. In the intact tooth none of the identified A.delta. or C fibres showed any ongoing activity in the absence of intentional stimulation. 4. 84% of the A.delta. 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 A.delta. 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 = 0.89) was found between the initial dynamic discharge of responding A.delta. 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 A.delta. 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 = 0.6) with the static tooth temperature achieved. No discharge was observed as the temperature returned to its initial value. 9. For eleven cold-sensitive A.delta. and C fibres the receptive fields were determined by mechanical stimulation of the exposed pulp tissue. For A.delta. fibres the receptive 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 fifty-point 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 A.delta. and C fibres to cold stimulation of a tooth, it is concluded that A.delta. fibres with the properties found in the cat are probably responsible for the cold-evoked dental pain sensation in man.