Identification and characterization of afferent periodontal A delta fibres in the cat.

1. The presence and responsiveness of afferent periodontal A delta fibres was studied in pentobarbitone-anaesthetized adult cats. 2. Extracellular single fibre recordings were made from fine nerve filaments split from the proximally cut end of the inferior alveolar nerve. Periodontal nerve fibres were identified by constant current stimulus pulses applied via platinum wire electrodes inserted into the periodontal space of the lower canine tooth. 3. Of a total of 252 periodontal nerve fibres, 97 (37%) were classified as A delta fibres according to their conduction velocities (CV) (> 2.5 m s-1, < 30 m s-1) as determined by electrical stimulation of the periodontal ligament. The mean (+/- S.D.) conduction velocity was 11.0 +/- 7.7 m s-1 (n = 97; range: 2.6-28.2 m s-1). 4. A good exponential correlation (r = 0.85) was found between the electrical thresholds of the A delta fibres and their conduction velocities. 5. For four A delta fibres a complete stimulus-duration curve was determined. It followed rather well the I = I0/(1-et/tau) law, where I represents the stimulus amplitude, t the stimulus duration, I0 the rheobasic current and tau the time constant. 6. In the intact tooth none of the identified periodontal A delta fibres showed any ongoing activity in the absence of intentional stimulation. 7. The responses of sixteen electrically identified periodontal A delta fibres were tested by mechanical, thermal and chemical stimuli applied to the periodontal space. Seven of nine periodontal A delta fibres tested responded to mechanical forces applied to the tooth from different directions of which none could be activated by slight touch. A rudimentary directional sensitivity was seen. When a human tooth was stimulated by a mechanical stimulus of similar strength the sensation evoked was described as a dull, poorly localized pain. 8. Six periodontal A delta fibres were activated by heat and/or cold and/or chemical stimulation. Two of eight periodontal A delta fibres tested responded to heat and four of six A delta fibres tested responded to cold stimuli applied to the alveolar bone overlying the periodontal ligament; none of them responded to both types of thermal stimuli. Two of seven periodontal A delta fibres tested were activated by a saturated solution of potassium chloride applied locally to the periodontal ligament; two of these responded also to cold. 9. The response behaviour of periodontal afferent A delta fibres observed in this study suggests that they may play a role in periodontal nociception.