Identification and characterization of afferent periodontal C fibres in the cat

The presence of afferent periodontal C fibres was studied in pentobarbitone-anaesthetized cats. 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 260 periodontal nerve fibres, 142 (55%) were classified as C fibres according to their conduction velocities (≤ 2.5 m/sec) as determined by electrical stimulation of the periodontal ligament (c.v.p). The mean (± S.D.) c.v.p was 1.2 ± 0.6 m/sec (n = 142; range: 0.3–2.5 m/sec). In addition, the axonal conduction velocity of 14 periodontal C fibres was determined by bipolar electrical stimulation of the trunk of the inferior alveolar nerve (c.v.n). On average the c.v.n was 42% higher than the c.v.p; the mean value was 1.7 ± 0.8 m/sec (n = 14; range: 0.6–3.9 m/sec). Nevertheless, the classification of nerve fibres based on c.v.p proved to be reliable; only 1 fibre had a c.v.p < 2.5 m/sec and a c.v.n > 2.5 m/sec and was therefore reclassified as an Aδ fibre. The responses of 30 electrically identified periodontal C fibres were tested by mechanical, thermal and chemical stimuli applied to the periodontal space. Thirteen of 19 periodontal C fibres tested responded to a strong mechanical force applied to the tooth from different directions while none could be activated by slight touch. A rudimentary directional sensitivity was observed. Eleven of these 13 responsive fibres were also activated by heat and/or cold and/or chemical stimulation. Fifteen of 25 periodontal C fibres tested responded to heat (thermode contact temperature +80°C), and 14 of 19 C fibres tested responded to cold (thermode contact temperature −80°C) stimuli applied to the alveolar bone overlying the periodontal ligament; 9 fibres responded to both types of thermal stimuli. Ten of 20 periodontal C fibres tested were activated by a saturated solution of potassium chloride applied locally to the periodontal ligament; 9 of these responded also to thermal stimuli. The receptive fields of 4 periodontal C fibres determined by probing of the thin bone layer covering the periodontal ligament with a sharp dental probe were located in the lower half of the root area of the tooth. The observed polymodal response behaviour of periodontal afferent C fibres, in particular their high threshold to mechanical stimulation, suggests that they may play a role in periodontal nociception.