Primary somatosensory cortical neuronal activity during the monkey's detection of the perceived change in tooth-pulp stimulus intensity. Koichi Iwata, Yoshiyuki Tsuboi and Rhyuji Sumino. Departments of Physiology and Division of Pathophysiology in the Dental Research Center, School of Dentistry, Nihon University, Kanda-surugadai, Chiyoda-ku, Tokyo 101, Japan.
APStracts 4:370N, 1997.
ABSTRACT
In order to elucidate the functional properties of primary somatosensory cortical neurons for the perception of tooth-pulp sensation, neuronal activity was recorded from the primary somatosensory cortex (SI) in awake behaving monkeys. Monkeys were trained to detect changes in tooth-pulp stimulus intensity applied to the upper canine or incisor tooth pulp. Stimulus intensities applied to the tooth pulp were multiples of the threshold intensity for the jaw opening reflex (1.0 T) elicited by tooth-pulp stimulation. When monkeys pressed a button, baseline electrical pulses (V1: 0.5 T, 1.0 T, 2.0T or 3.0T) were applied to the tooth pulp. After 4 to 8 seconds, a V2 stimulus (0.3 T, 0.5 T, 1.0 T or 2.0 T) was added to V1. Percent escapes at V1 stimulus intensity of 0.5 T and 1.0 T were about 10%, 22% at 2.0 T and 40% at 3.0 T (total of 1997 trials). A total of 862 single units were recorded from the SI. Thirty seven SI neurons responded to electrical stimulation of the tooth pulp (tooth-pulp-driven neurons: TPNs), 139 SI neurons responded to tactile stimulation of the lateral face area, 90 to upper lip and 99 to lower lip, 44 to tongue and 102 to periodontal membrane, whereas 351 SI neurons were not responsive to tactile stimulation of the orofacial regions. Thirty of 37 TPNs were recorded long enough to test with V1 stimuli ranging from 0.5 T to 3.0 T. Eleven of 30 TPNs linearly increased their firing frequency following increases in stimulus intensity (encoding TPNs), whereas 19 did not (non-encoding TPNs). Mean first spike latency of encoding TPNs was 24.8 ñ 1.7 ms (n=11), that of non-encoding TPNs was 23.6 ñ 1.5 ms (n=19) and that of unclassified TPNs was 24.7 ñ 3.7 ms (n=7). TPNs were distributed in the areas 1-2, 3a and 3b within the oral projection area and the transition zone between the face and oral projection areas of the SI. All of them received inputs from the intraoral structures, facial skin or both. The firing frequency of 8 encoding and non-encoding TPNs were correlated with detection latency at stimulus intensities of 0.5 T and 1.0 T. On the other hand, when the baseline stimulus was increased to 2.0 T and 3.0 T, the discharge of most TPNs did not increase in firing frequency with the reduction in detection latency. These results indicate that the discharge rates of some SI TPNs are correlated with detection latency at near-noxious threshold and noxious stimulus intensities. These findings suggest that some TPNs are involved in the sensory-discriminative aspect of tooth-pulp sensation in the near-pain threshold and pain ranges. 

Received 12 September 1997; accepted in final form 9 December 1997.
APS Manuscript Number J755-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 January 1998