Origins of tooth pulp-evoked far-field and early near-field potentials in the cat.

Abstract

Electrical stimulation of the mandibular canine tooth pulp in barbiturate-anesthetized cats activated myelinated A-beta-fibers in the inferior alveolar (dental) nerve and evoked a complex pattern of short-latency potentials that were recorded epidurally from the cortex. Three far-field components ( FFPs designated I, IIa,b, and IIIa,b) and two early near-field components ( ENFPs designated P1 and N1) were identified from computer-averaged potentials recorded over the contralateral lateral sigmoid gyrus or ipsilateral presylvian /anterior coronal gyri. Several corroborative lines of evidence indicated that tooth pulp-evoked FFPs and ENFPs originate from the following generator sources: I, inferior alveolar (dental) nerve/semilunar (gasserian) ganglion; IIa,b, trigeminal lemniscal fibers from the principal (main) sensory nucleus; IIIa,b, thalamocortical fibers from the thalamic ventral posteromedial nucleus; P1, N1, cytoarchitectural area 3 on the banks of the coronal or orbital sulcus. The latencies and waveforms of volume-conducted, surface-recorded potentials were not significantly different from those recorded at putative generator sites along the trigeminal lemniscal projection system. The short latencies and distribution of surface- and depth-recorded tooth pulp-evoked potentials suggested bilateral, trisynaptic paths from peripheral nerve to cortical neurons. Amplitude measurements of FFPs and ENFPs recorded at the cortical, epidural surface were compared to calculated values based on a mathematical model of two concentric spheres of inhomogeneous media in which a single equivalent dipole is radially oriented at some distance from the center. The morphology and distribution of FFPs and ENFPs are discussed in relationship to source geometry. Surface- and depth-recorded tooth pulp-evoked potentials had recovery functions that were covariant with rates of stimulation. In addition, stimulation of putative generator sites along the trigeminal lemniscal system reproduced portions of the tooth pulp-evoked surface-recorded potentials. Selective ablation of trigeminal cortical area 3 abolished ENFPs and lesions of lemniscal brain stem sites abolished FFPs . Disruption of extralemniscal brain stem sites had no effect on the surface-recorded evoked potentials examined. These findings and other supporting evidence from previous studies imply that the projection of fast-conducting tooth pulp input to the cortex may subserve a nonnociceptive modality and participate in sensorimotor integration.