The relationship between the discharge of intradental nerves and the rate of fluid flow through dentine in the cat

OBJECTIVE: To investigate further the relationship between dentinal tubular flow and the discharge evoked in intradental nerves. DESIGN: In anaesthetised cats, recordings were made of fluid flow through dentine during the application of hydrostatic pressure stimuli of 5 s duration in the range +500 to -500 mm Hg to exposed dentine and of the nerve impulses evoked by these stimuli. Single unit recordings were obtained from filaments dissected from the inferior alveolar nerve and multi-unit recordings, from the exposed dentine. RESULTS: Of 20 single units tested, 10 (conduction velocities: 2.4-36.2 m s(-1)) responded to negative pressures and four of these, also to positive pressures. None responded to only positive pressures. The pressure thresholds of the units (single and multi-unit preparations) ranged from -100 to -500 and +100 to +500. In terms of flow (measured 1 s after the start of a stimulus) the thresholds ranged from 0.4 to 2.2 nl s(-1) mm(-2) exposed dentine with outward flow, and 0.4-2.1 nl s(-1) mm(-2) with inward flow. The outward flow per tubule at the threshold of the most sensitive units was estimated to be 21 fl s(-1) and the corresponding mean velocity of the contents of the dentinal tubules at their pulpal ends, 27 microm s(-1). Although the thresholds to outward and inward flow were similar, with outward flow the mean discharge rate increased with stimulus intensity; whereas with inward flow few impulses were evoked and the number was little affected by the stimulus intensity. CONCLUSION: The transduction mechanism that generates impulses in hydrodynamic intradental afferents is much more responsive to outward than inward flow through the dentinal tubules, although the thresholds in both directions are similar.