Interactions between neural and hydrodynamic mechanisms in dentine and pulp.

Evidence is presented that the rate of inward diffusion of chemicals through exposed dentine is affected by the rate of outward flow of fluid through the dentinal tubules. Such a flow has been demonstrated in cats. The flow rate appears to depend upon the pulpal tissue-fluid pressure; flow increased during pulp vasodilatation and decreased, even reversing in direction, during vasoconstriction. Pulp vasodilatation can be produced by stimulating intradental afferent nerves, including some of those that seem to be excited by displacement of tubule contents (i.e. by a hydrodynamic mechanism). Thus, when dentine is exposed and these afferents are stimulated they will help to protect the pulp by producing reflex vasodilatation, which will decrease the rate of diffusion of toxins from the mouth into the pulp. The relation between the rate of flow through dentine and the discharge evoked in intradental nerves was investigated in cats. Single fibres were more sensitive to outward than to inward flow. The flow rates required to excite the pulp afferents were greater than those observed during even maximal pulpal vasodilatation.