Motor coordination of masseter and temporalis muscle during mastication in mice
PURPOSE: Mice have been increasingly used to study the stomatognathic function, since genetically modified mice are expected to enhance our understanding of neural and motor disorders of the craniomandibular system. However, fundamental characteristics of jaw movements in mice have not been fully understood. In the present study, the jaw movement of mice during chewing foods of different textures was analyzed in relation to the coordination of the masseter and the temporalis. MATERIAL AND METHODS: At 11 weeks of age, electrodes were placed in the masseter and the temporalis for electromyography (EMG) recording, and jaw movement was measured using a three-dimensional jaw-tracking system. Hard food and soft food were tested. RESULTS: The jaw movement trajectories of mice in the sagittal plane showed antero-posterior excursions unlike those of human or other animals. Larger antero-posterior discrepancies between the opening and the closing paths were observed when chewing hard food, especially at the late-closing phase. The average amplitude of the temporalis was significantly larger at the late-closing phase when chewing hard food than soft food. Conclusion: Characteristic masticatory jaw movements in mice which include the divergence between jaw-opening and jaw-closing paths may be due to the earlier onset of the temporalis activity than that of the masseter. Also, anatomic factors such as the orientation of closing muscles and the shape of the temporomandibular joint may be attributed to the jaw trajectories specific to mice. During hard food chewing, the jaw closes at a more posterior position, which could extend the grinding path at the occlusal phase followed by anterior excursion, and consequently, the efficiency of grinding of food would be increased.