The paradox of pain from tooth pulp: low-threshold "algoneurons"?

Normally, pain is felt when signals generated by noxious stimuli in thinly myelinated Ador unmyelinated C-nociceptive afferents reach a conscious brain. We propose that the intrinsic innervation of teeth may be a unique exception. Teeth are densely innervated by primary sensory neurons located in the trigeminal ganglion (TRG). There are many Adand C-fibers in the tooth pulp, with the Ad axons ending mostly in the inner third of the dentinal tubules and the C-fibers ending mostly in the pulp itself [5,6,16,21,34,36]. This, coupled with the fact that tooth pulp stimulation primarily evokes pain, has lead to the doctrine that Adand C-nociceptors dominate intrinsic tooth innervation. Among the few Ab-fibers present, some might be nociceptive [12], but most are thought to be low-threshold mechanoreceptors (LTMs) that subserve ‘‘prepain,’’ the nonpainful tingling reported when the tooth crown is stimulated electrically at liminal current strength [13,47]. However, as most dental patients can testify, weak mechanical stimuli such as air puffs and water spray evoke intense pain when directed at exposed dentin [10]. As these stimuli do not activate nociceptors or evoke pain on skin or gingiva, there must be something different about teeth. Is mechanically evoked dentinal pain indeed due to impulses in afferent nociceptors?