Tooth pulp input to the shell region of nucleus ventralis posteromedialis of the cat thalamus

Abstract

A population of neurons in the somatosensory part of the nucleus ventralis posteromedialis (VPM proper) that responded to electrical stimulation of the tooth pulp were studied in cats under urethan-chloralose anesthesia. Two classes of units responsive to electrical stimulation of the contralateral canine tooth pulp were identified. One class was responsive only to tooth pulp stimulation and these units were designated as tooth pulp specific (TPS) units. The other class of units responded to mechanical stimulation of the contralateral trigeminal integument in addition to tooth pulp stimulation. Their receptive field characteristics identified them as wide dynamic range (WDR) units responsive to tooth pulp stimulation. Both classes of units were located in the shell region of the caudal VPM proper; TPS units were coexistent with trigeminal nociceptive specific (NS) units and were found in the dorsomedial as well as ventromedial parts of the NS zone. WDR units responsive to electrical stimulation of the tooth pulp were located in the dorsomedial as well as ventromedial parts of WDR zone, a narrow band, approximately 300 micron wide, just in front of the NS zone. Tooth pulp units in the dorsomedial shell region of the VPM proper responded to the maxillary canine tooth pulp, whereas those in the ventromedial shell region responded to the mandibular canine tooth pulp. Some tooth pulp units in these two regions were responsive to stimulation of both maxillary and mandibular canine teeth. Both TPS and WDR units were antidromically excited by electrical stimulation of the SI area of the somatosensory cortex. Cooling the dorsolateral surface of the caudal medulla oblongata reversibly blocked tooth pulp evoked responses of TPS and WDR units. Trigeminal tractotomy just above the level of the obex irreversibly abolished tooth pulp-evoked responses of TPS and WDR units. These findings suggested that TPS neurons in the marginal layer of the trigeminal subnucleus caudalis and WDR neurons in the lateral part of the subnucleus reticularis dorsalis relay afferent impulses derived from the tooth pulp to the shell region of the VPM proper.