The rabbit digastric muscle has a single belly that opens and retracts the mandible. It does not contain connective tissue partitions, and all fibers arise from the same tendon and insert into a single broad site. Historically, it was assumed that the muscle functioned as a single unit. Since we had preliminary evidence that this might not be the case, we carried out five small studies in rabbits. First, we showed that electromyographic (EMG) activity varies between recording sites within the muscle during the masticatory cycle induced by repetitive stimulation of the sensorimotor cortex. We found that EMG activity in the caudal region sometimes began before the anterior EMG during mastication when the jaw swung to the side of the muscle, but the two regions became active at the same time during other patterns. We next showed that separate branches of the mylohyoid nerve enter the anterior, intermediate and caudal regions of the digastric. However, a separate study showed that the motor endplates were distributed across a continuous sheet, consistent with a single anatomical partition. We then stimulated single nerve branches to deplete glycogen. By comparing the optical density of fibers labeled by the periodic acid-Schiff method for glycogen, we were able to show that the three branches innervate separate regions of the muscle. Finally, we applied either FluoroGold or Fast Blue dyes to the central cut ends of the branches to label the cell bodies of the three pools of motoneurons. These were found within the middle and caudal thirds of the trigeminal motor nucleus, but there appeared to be no spatial separation of the three pools or double labeling of cells. We conclude that the digastric muscle contains two and possibly three functional subregions. The fact that the motoneurons are intermingled suggests that the distribution of motor commands to the three pools is not based on their location.

In this study, we focused on the relationship between the morphological features of the articular eminence and the presence of the anterior disc displacement on MR image in youngsters. We classified the morphology of the articular eminence into four categories, flattened, sigmoid, box and deformed on MR image. The results of this study indicated that morphological features of the articular eminence on serial MR images were related to the articular disc configuration, which were the presence of the anterior disc displacement in the intercuspal position, the levels of the anteriorly displaced disc in the intercuspal position and the presence of the reductions of the anteriorly displaced disc in the maximum opening mouth position. There were statistically significant differences in the distribution of the morphology of the articular eminence between normal disc position and anterior disc displacement in all of three sagittal planes. There were statistically significant differences in the morphology of the articular eminence among the levels of the anteriorly displaced disc in central and lateral planes. There was statistically significant difference in the distribution of the morphology of the articular eminence between presences of reduction of the anteriorly displaced disc in the central plane. We concluded that the morphological feature of the articular eminence continuously changes as dental arch develops in the time with temporal instability of mixed dentition, and seems to have positive correlation to the anterior disc displacement. having the joint clicking. Although it has been a controversial question for a long time, etiological factors causing the anterior disc displacement suggested by some authors are absence of posterior teeth, malocclusion, occlusal disharmony, jaw trauma, luxation of the joint, mechanical differences in the morphology of the joint and steep articular eminence 1-8). In the time with mixed dentition period, some of these etiological factors seem to become remarkable in orofacial region in juvenile subject because the developmental changes both the craniomandibular structure and the dentition continuously occurs. In this study, we focused on the relationship between the morphological features of the articular eminence and presence of the anterior disc displacement on MR image in youngsters.

I-cell disease is a rare autorecessive metabolic disorder that is classified as one of the lysosomal storage diseases. Gingival enlargement is a representative oral manifestation of patients with I-cell disease. This report describes a case with a satisfactory prognosis after gingivectomy for gingival enlargement accompanying I-cell disease in a 2-year-old boy. The chief complaints were both eating disorders and night terrors accompanied by gingival enlargement of both the maxillary and mandibular alveolar gingivae, especially in the region of the first deciduous molars. Although it was supposed that there were various hazards associated with gingivectomy caused by aortic incompetence, sigmoidal spinal curvature and hypertrophy of the laryngopharynx, gingivectomy with an internal bevel incision was performed under general anesthesia to improve the chief complaints. A piece of the enlarged gingivae dissected during the operation was examined histopathologically, and the histopathological diagnosis was gingival hyperplasia. The patient's signs and symptoms of the eating disorders and night terrors have improved since the gingivectomy operation. Recurrence of the gingival enlargement has not been identified up to 18 months after the surgery. It is therefore concluded that gingivectomy for gingival enlargement accompanying I-cell disease was effective for this 2-year-old boy.