Theories of pain transmission through dentin

HYDRODYNAMIC THEORY OF DENTINAL PAIN PERCEPTIONThe pain sense in human teeth has some important differences from pain perception in other organs such as the skin: It is con-sidered to be the only sensation which can be elicited by any stimulus appliedto the teeth (excluding the periodontal receptors). Thus, there is no sense of warmth or cold, but only pain if the temperature exceeds certain limits -lower than 27o C or higher than 45oC (Matthews, 1977). While some authors have regarded the teeth as pressure-sensitive over a very high range of pressures, the most common sensation we usually experience is pain, whether with exposed dentin from enamel attrition or from a pulpal inflammation acting directly on nerve endings.Distribution of Pain Sensitivity in TeethMost dental pain researchers agree on at least two things: (1) the enamel of the teeth is completely insensitive, and (2) the dentin and pulp are sensitive to a variety of stimuli, including mechanical, thermal and chemical. The insensitivityof the enamel is not surprising, as that substance consists primarily of dry hydroxyapatite crystals with no demonstrable innervation. The pulp and dentin, on the other hand, are living structures which contain body fluids and identifiable nerve endings. One seemingly paradoxical finding is the sensitivity of the dentin-enamel junc-tion; although no nerve endings can be seen in this region of the tooth, when the junction is inadvertently touched by a dental drill, the pain reaction can be sudden and severe.Nerve Endings in Dentin
The sensory fibers in the pulp are branches of a plexus underlying the odontoblastic layer, the Plexus of Raschkow (Figure 1). The pulp nerves approach the horns of the pulp, then branch diffusely to form the plexus. Fibers arising from this network occasionally appear to cross the odontoblastic layer into the dentin. There has been some controversy over the years about whether or not nerve endings are present in the dentinal tubules. From light microscopic observations, R.W. Fearnhead (1963) stated that, "The question whether calcified dentin is innervated I now regard as settled. It is no longer a controversial topic. In suitable specimens nerve fibres can be demonstrated within the dentinal tubules". This is shown in thenext two figures.