Nerve branching in rat molars

Experiments were carried out in rats to evaluate procedures that might be used in recording intradental nerve activity from dentin in man (JDR 85B #1078, 2006). Objectives: to evaluate methods for isolating a small area of dentin for recording action potentials, to determine what proportion of intradental nerve fibres branch and innervate the dentin of more than one cusp, and to determine if it is possible to record unitary action potentials from one cusp that are evoked by electrical stimulation of another cusp of the same tooth. Methods: Recordings were made from a lower first molar in anesthetized rats. A cavity (diam: 0.2mm, depth: 0.3mm) was cut into dentine with a hand-held drill at the tip of each of 4 cusps, and filled with a Ag/AgCl mixture that was packed onto the end of a silver wire lead (diam: 0.1mm). Electrical stimuli were applied to the cut inferior alveolar nerve (IAN) or a cusp electrode. Results: Of several types of electrode tested for recording from dentin, the above provided good signal:noise ratios, was stable, and was simple and quick to produce. Of 17 single unit responses evoked by IAN stimulation, 3 were recorded from all 4 cusps, 0 from 3, 9 from 2, and 5 from just 1. Responses recorded from the same unit at different sites had different waveforms and latencies. Latencies ranged from 0.36 to 11.2ms (conduction distance: 20mm). Responses of 12 units could be recorded from 1 to 3 cusps during electrical stimulation of another cusp. Latencies ranged from 0.18 (minimum detectable) to 6.1ms. Conclusions: In a rat molar, a high proportion of fibres that innervate the dentin of one cusp also innervate the dentin of another cusp. Responses can be recorded from one of these branches in response to electrical stimulation of another.