Modifications of masticatory behavior after trigeminal deafferentation in the rabbit

Bilateral trigeminal deafferentation was performed in the rabbit in order to assess the role of orofacial inputs in regulation of the pattern of jaw movements during chewing. After bilateral combined section of the maxillary and inferior alveolar nerves, the animals did not eat food by themselves in the first postoperative week. However, they could chew and swallow when food was inserted into the mouth by an experimenter. The pattern of jaw movements and associated EMG activities of masticatory muscles during chewing were modulated remarkably by deafferentation. These modifications include 1) decrease in the horizontal excursions of the mandible at the power phase, 2) decrease in the maximum gape, 3) insufficient occlusion at the power phase (or increase in the minimum gape), 4) irregular patterns of jaw movements, 5) facilitation of the chewing rate, 6) increase in the number of chewing cycles in a masticatory sequence (the process from acceptance of food to swallowing), and 7) decrease in jaw-closing muscle activities. The findings indicate that deafferentation of the trigeminal sensory branches reduced masticatory force. On the other hand, no significant change was seen in the animals with disruption of cutaneous sensations of the face due to bilateral section of the infraorbital and mental nerves. Intraoral sensations rather than extraoral sensations may thus be important for regulation of masticatory force and jaw movements during chewing. Jaw movements during chewing were also analyzed in the animals with either bilateral ablation of the cortical masticatory area (CMA) or bilateral lesion of the ventral posteromedial nucleus (VPM) of the thalamus in order to examine whether profound effects of trigeminal deafferentation are produced via the transcortical loop. The animals with lesion of either the CMA or VPM demonstrated disturbances in feeding behavior, including the dropping of ingested food from the mouth, elongation of a masticatory process, reduction in the chewing efficiency, etc. However, the pattern of jaw movements during chewing were essentially similar to that in the preoperative period. These results do not necessarily deny a contribution of the CMA to regulation of jaw movements but suggest that the transcortical feedback loop via the CMA and thalamic VPM nucleus would not primarily be responsible for pattern formation of jaw movements during chewing in the rabbit. Probably, the sensory feedback via the transcortical loop may indirectly facilitate activities of the brain stem CPG, which facilitates the chewing rhythm or enables masticatory sequences to be conducted smoothly.