Changes in Feeding Behavior and Jaw Muscle Activity During Mastication in Rats with Prolonged Inflammation of the Temporomandibular Joint Region-Longitudinal Pathological, Behavioral and Electromyographical Studies

Rats were injected with the complete Freund's adjuvant in the left temporomandibular joint (TMJ) region to induce inflammation of long duration. Control animals received the injection of saline. Feeding behavior and jaw muscle activity during a masticatory sequence of a pellet were recorded photographically and electromyographically on the 5th, 10th, 15th and 20th day following the day of inoculation and compared with the data from control animals. Progression of inflammation of the TMJ and elbow joint regions on both sides was also examined pathologically in the same time course. The relationship between pain processes in the TMJ region and jaw muscle dysfunction was discussed. During this study, The Ethical Guidelines for Investigations of Experimental Pain in Conscious Animals, published by the Committee for Research and Ethical Issues of the International Association for the Study of Pain, have been followed ; particularly to exclude pain arising from installing the EMG electrodes directly in muscle tissue, the chronic EMG electrode technique was used. The following results were obtained : 1) On the 5th day the TMJ and masseter muscle on the injection side had been slightly inflammed ; the masseter muscle was nearer to the injection site than any other muscle. Rats seemed to have itching in the skin overlying the injection site and to masticate a pellet on the posterior teeth contralateral to inoculation, with the head slightly tilted. 2) On the 10th day the inflammation spreaded far to the ipsilateral temporal, internal and external pterygoid muscles. On the 15th day the feature became more pronounced and most marked on the 20th day. The intensity of pain arising from the affected TMJ region increased with the progress of inflammation. 3) Throughout the experimental period no inflammatory signs were observed in the TMJ and its surrrounding tissues on the non-injection side. No signs were also observed in the anterior digastric muscles and elbow joint regions on both sides. 4) Chewing EMG bursts recorded from the right and left masseter, temporal, and digastric muscles were analyzed with respect to amplitude, duration and cycle time of individual bursts and compared statistically between experimental and control groups. Effects of the inflammation were most conspicuous on the amplitude of all muscles, whereas no effects were on the cycle time. 5) On the 5th day no inflammatory signs were seen in both the temporal and digastric muscles. However, there was a significant increase in the amplitude of the above two muscles on both sides. This increase was considered as resulting from excitatory nociceptive reflex caused bilaterally by mastication with the TMJ being inflamed. 6) After the 10th day the amplitude of the masseter muscle was on the decrease on the injection side, but was on the increase on the non-injection side. From the above results it is suggested that the pain caused by inflamed TMJ provokes the jaw muscles to excitatory reflex action, whereas the pain arising from inflamed jaw muscles provides themselves with inhibitory reflex action.