Response properties of slowly and rapidly adapting periodontal mechanosensitive neurones in the primary somatosensory cortex of the cat.

Periodontal mechanosensitive neurones in the primary somatosensory (SI) cortex are classified as either slowly or rapidly adapting. The responses of cortical neurones and their projection pathways were studied using mechanical and electrical stimulation of the teeth and electrical stimulation of the thalamic posteromedial ventral (VPM) nuclei and contralateral SI cortex. A total of 247 periodontal mechanosensitive units were recorded from the SI cortex in 35 anaesthetized cats, distributed mainly in area 3b: 14% were slowly adapting and 86% rapidly adapting units; 62% of the slowly adapting and 9% of the rapidly adapting units were single-tooth units sensitive to stimulation of only one tooth. The incidence of slowly adapting units with an ipsilateral receptive field was almost equal to that of slowly adapting units with a contralateral receptive field, and more than half of the units were directionally selective to mechanical tooth stimulation. The majority of rapidly adapting units had their receptive field in the contralateral teeth and were directionally non-selective to tooth stimulation. The latencies of the cortical responses of the slowly and rapidly adapting units were 7.3 and 10.7 ms, respectively, on electrical stimulation of the contralateral teeth, and 1.8 and 2.0 ms, respectively, on electrical stimulation of the ipsilateral VPM nucleus. From these findings, it is inferred that slowly adapting neurones are useful for discriminating the tooth stimulated, the stimulus direction, the stimulus intensity and the change of pressure applied to the tooth, while rapidly adapting neurones could function to signal initial contact with food or the opposing teeth.