Projection of tooth pulp afferents to the cat trigeminal nucleus caudalis.

Unit activity was recorded extracellularly from cat medullary neurons following electrical stimulation of the canine tooth pulp. Response characteristics of the neurons quickly stabilized at specific suprathreshold stimulus intensities but such properties as spike latency, interspike interval and spike density varied systematically as intensity was raised to maximally effective values. Receptive fields were principally unilateral. The majority included both canines and extended into other oro-facial areas. Suppression of a pulpal response could be effected by preceding tooth stimulation with a conditioning stimulus applied to some other point in the receptive field of the responding cell at an appropriate interstimulus interval. In contrast, a pulpal response could be enhanced by presenting two stimuli successively to the same canine at such intervals. Similar enhancing effects followed simultaneous stimulation of spatially segregated loci in a field. The pulp-responsive neurons were localized histologically in, or in the immediate vicinity of, the nucleus caudalis of the spinal trigeminal complex where the possibility of their existence has been questioned previously. Most of the cells were situated along the ventromedial border of the nucleus, a region reported to contain other pain-related neurons with trigeminal fields.