Response properties of periodontal mechanosensitive neurones in the rat trigeminal sensory complex projecting to the posteromedial ventral nucleus of the thalamus

Unitary discharges from periodontal mechanosensitive (PM) neurones responding to mechanical stimulation of the tooth were recorded from the trigeminal sensory complex in the rat brainstem. Of the PM units recorded, 22% were activated by antidromic stimulation of the contralateral (20%) or ipsilateral (2%) posteromedial ventral nucleus of the thalamus. Although thalamic-projecting neurones were recorded extensively throughout the trigeminal sensory complex, they originated most often in the region from the caudal main sensory nucleus to the rostral subnucleus oralis of the trigeminal spinal tract nucleus. The response latencies of the rostral nucleus units to orthodromic stimulation of peripheral receptive fields and antidromic stimulation of the thalamus were significantly shorter than those of the caudal nucleus units. More than half were single-tooth units originating from incisor teeth. They responded continuously when pressure was applied to the tooth. The magnitude of the response varied with the direction of the stimulus. Maximal responses were obtained when the stimulus was applied labiolingually or vice versa. The threshold for mechanical stimulation of the tooth was less than 0.05 N. The rostrocaudal distribution and response properties of thalamic-projecting PM neurones were very similar to those of non-thalamic-projecting PM units that were not activated by antidromic stimulation of the thalamus.