Activation of heat-sensitive nerve fibres in the dental pulp of the cat

Abstract

We have recorded responses of inferior alveolar nerve fibres to heating of the intact enamel of the canine tooth crown in anaesthetized cats. After identification of intradental nerve units by monopolar electrical stimulation, the tooth was heated with an electrothermal stimulator (Peltier element). The rate of temperature change in the tooth was considerably slow ( < l°C/sec). Responses of 37 heat-sensitive units were recorded. They were all quite slowly conducting (CV = 1.7 ± 0.7 (S.D.) m/sec). Only 8 fibre units with conduction velocity below 3.5 m/sec did not respond to heating. The mean threshold temperature was 43.8 ± 3.4 (S.D.)°C. Nerve activity appeared as irregular bursts of action potentials. When heating was repeated at short intervals (2–3 min), an elevation in the thresholds was noticed. After cooling or a recovery period of about 10 min the thresholds for heating returned towards the initial ones, but they still remained somewhat elevated. This change in thresholds might have been due to heat induced injury in pulp tissue. When heating was stopped the activity ceased with declining temperature regardless of the temperature reached during the stimulation. Spontaneous firing never occurred. Not one of the units with a conduction velocity above 3.5 m/sec (n = 28,CV = 13.2 ± 7.1 (S.D.) m/sec) were activated by heating. Cooling of the tooth did not induce responses in any of the recorded units. Only 3 of 10 slowly conducting units fired, when pulp was mechanically irritated. On the other hand 11 of 14 fast conducting units were mechanosensitive. It is concluded that there exist differences in heat sensitivity of fast and slowly conducting pulp nerve units in the cat. The possible activation of slowly conducting intradental nerve fibres also in man might be significant in mediation of pain sensations induced by heating of the tooth crown.