Role of intradental A- and C-type nerve fibres in dental pain mechanisms.

The arousal of the two components of pain (the first rapid or sharp pain and the second dull pain) are considered to be related to activation of A delta- and C-type nociceptive primary afferents, respectively. The same dichotomy of pain sensations may also exist in teeth, although due to the short distance between the site of stimulation and the brain the two sensations might not be as clearly separated as in stimulation of, for example, the extremities. The sensations evoked by stimulation of human teeth vary according to the type of the stimuli applied. Low-intensity electrical stimulation is able to induce non-painful (prepain) sensations. At high current intensities pain is evoked. Drilling, probing and air-drying of exposed dentin induce only pain. Most studies also indicate that thermal stimulation only induces painful sensations. The quality of dental pain can vary. Typically, dentinal stimulation of teeth with healthy pulps induces sharp pain. On the other hand intense heat stimulation can result in dull pain which radiates to a wider area of the face and jaws. This component of the stimulus-induced pain seems to share some characteristics of toothache associated with painful pulpitis. Single fibre recordings of intradental nerve activity in experimental animals have shown that in addition to A-fibres a considerable number of C-type primary afferents innervate the dental pulp. This is in accordance with the results of neuroanatomical studies, which indicate that 70-80% of pulpal axons in human, monkey, dog, and cat teeth are unmyelinated. Intradental A- and C-fibre groups seem to be functionally different and can be activated separately by certain external stimuli. Comparison of the response characteristics of the pulp nerve fibres and the sensations induced from human teeth indicate that: 1) A-fibres are responsible for the sensitivity of dentine and thus for the mediation of the sharp pain induced by dentinal stimulation, 2) Prepain sensations induced by electrical stimulation result from activation of the lowest threshold A-fibres some of which can be classified as A beta-fibres according to their conduction velocities. Comparison of the responses of the A beta- and A delta-fibres indicate that they belong to the same functional group, 3) Intradental C-fibres are activated only if the external stimuli reach the pulp proper. Their activation may contribute to the dull pain induced by intense thermal stimulation of the tooth and to that associated with pulpal inflammation.