Comparison of functional characteristics of intradental A- and C-nerve fibres in dental pain.

The sensory nerve fibres in the pulp consist of myelinated A- and unmyelinated C-fibres which conduct nerve impulses. The A-fibres are larger in diameter and fast conducting. Most of the A-fibres are in the A-delta group, but also the existence of very fast A-beta fibres has been demonstrated. C-fibres are small and slow conducting. When natural stimuli such as heat, cold, drilling or drying of dentine with air blasts are applied on the tooth, the only sensation perceived seems to be that of pain. On the other hand perception of pain symptoms in clinical situations varies from sharp and piercing to dull and poorly localised. A- but not C-fibres respond to dentine stimulation, and therefore responsible for dentine sensitivity. When heat is applied to the tooth, there is a two-phase response. First there is an immediate A-fibre response followed by a C-fibre response. The A-fibres could be responsible for the sharp well localised pain in human subjects while C-fibres could be responsible for the dull pain radiating to other parts of the face. A-fibres are activated at a higher level of electrical stimulation than C-fibres. C-fibres, but not A-fibres respond to application of bradykinin and histamin. The functional differences in A- and C-fibres may be one explanation for the change in the pain symptoms which at the beginning of pulpitis are mediated by A-fibres and in advanced pulpitis by C-fibres.