In the orofacial area of the first somatosensory cortex (SI), we recorded single unit activity from 699 neurons in 11 awake cats. Fifty-two percent (362/699) were mastication-related neurons (MRNs) showing activity related to some aspects of masticatory movements. MRNs were divided into three types by their activity patterns: (1) the rhythmical type, showing rhythmical bursts in pace with the masticatory rhythm; (2) the sustained type, showing a sustained firing during the period of taking food and (3) the transient (biting) type, showing intense discharges in coincidence with biting hard food. MRNs had mechanoreceptive fields in the perioral, tongue, periodontal and mandibular regions. The activities of perioral rhythmical-MRNs, mandibular transient-MRNs, tongue rhythmical-MRNs and periodontal transient-MRNs were correlated with food texture, while perioral rhythmical-MRNs, perioral sustained-MRNs and tongue sustained-MRMs were not. Both facial and intraoral MRNs were scattered throughout the facial and intraoral projection areas in SI. These findings provide evidence that the orofacial SI monitors masticatory movements for food ingestion.

Electrophysiological studies using the single neuron recording technique have led to the hypothesis that nociceptive neurons in the medullary dorsal horn (MDH) and the first segment of the spinal cord (C1) encode the stimulus intensity of noxious stimuli applied to the tooth pulp. The present study utilized the Fos protein technique in combination with electrical and chemical stimulation of the tooth pulp to test this hypothesis. Upper canine tooth-pulp stimulation with intensities just above the threshold stimulus intensity for evoking the jaw-opening reflex (JOR) did not produce a clear expression of Fos protein-like immunoreactive (LI) cells in the MDH and C1 of cats. Fos protein-LI cells were mainly found in the superficial laminae (laminae I-II) of the MDH and C1 after tooth-pulp stimulation of 200% of the JOR threshold intensity. When higher intensities (400-600% of the JOR threshold intensity) or mustard oil were applied, Fos protein-LI cells were also found in laminae III-IV as well as in laminae I-II. The number of Fos protein-LI cells significantly increased when 600% of the JOR threshold intensity or mustard oil was applied. Furthermore, the rostro-caudal distribution of Fos protein-LI cells was greater following increases in stimulus intensities and the greatest after mustard oil application. These data suggest that the change in number and spatial arrangement of nociceptive neurons in the MDH and C1 reflect changes in the encoding of the stimulus intensity applied to the tooth pulp.

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The purpose of this study was to determine the role of somatosensory cortex (SI) in the control of orofacial movements during eating. We identified perioral and tongue projection regions of the cat SI and destroyed cells in one region by injecting kainic acid. The effects on orofacial behavior were then studied over a period of 4-6 weeks. Cats with unilateral lesions in the perioral region (PL-cats) dropped food from the contralateral side of the mouth in the early phase. Failure in erection of the contralateral whisker hairs during masticatory movements and delay of the masticatory start were observed throughout the experimental period. Furthermore, in the late phase, PL-cats showed prolongations of the masticatory and food intake periods, which were accompanied by the increase in the number of swallows and chewing cycles. Cats with unilateral lesions in the tongue region (TL-cats) showed the prolongation of the masticatory period in the early phase, which was accompanied by the increase in the number of swallows and chewing cycles. TL-cats did not show the prolongation of the food intake period and failure in erection of the contralateral whisker hairs. In both PL- and TL-cats, masticatory and swallowing rhythms were normal.

This study was undertaken to investigate patterns of neuronal activity during mastication in