The demand for the use of mice as animal models for elucidating the pathophysiologies and pathogeneses of oral motor disorders has been increasing in recent years, as more and more kinds of genetically modified mice that express functional disorders of the stomatognathic system become available. However, the fundamental characteristics of mouse jaw movements during mastication have yet to be fully elucidated. The purpose of this study was to investigate the roles of the masseter and temporalis muscles, and the mechanisms of motor coordination of these muscles for increasing masticatory efficiency in the closing phase in mice. Twenty-two male Jcl:ICR mice were divided into control (n = 8), masseter-hypofunction (n = 7) and temporalis-hypofunction groups (n = 7). Botulinum neurotoxin type A (BoNT⁄A) was used to induce muscle hypofunction. The masticatory movement path in the horizontal direction during the occlusal phase became unstable after BoNT⁄A injection into the masseter muscle. BoNT⁄A injection into the temporalis muscle decreased antero-posterior excursion of the late-closing phase corresponding to the power phase of the chewing cycle. These results suggest that the masseter plays an important role in stabilizing the grinding path, where the food bolus is ground by sliding the posterior teeth from back to front during the occlusal phase. The temporalis plays a major role in retracting the mandible more posteriorly in the early phase of closing, extending the grinding path. Masticatory efficiency is thus increased based on the coordination of activities by the masseter and temporalis muscles.
It has been suggested that feeding a soft diet could possibly inhibit normal development of the masticatory function. However, the consequences of such changes in the alimentary habits have yet to be fully clarified. Therefore, the aim of this study was to determine whether a soft diet prevents the development of masticatory function and whether a critical period for programming the masticatory system exists. To examine these hypotheses, we used a three-dimensional jaw-movement tracking device and jaw muscle electromyography (EMG) to analyse masticatory function changes in mice. Jcl:ICR mice were divided into three groups, with the normal group fed a hard diet, the hypofunctional group fed a soft diet, and the rehabilitation group first fed a soft diet that was then changed to a hard diet. Our results showed that the excursion and duration of late-closing phase (occlusal phase) of the chewing cycle and EMG activity in the masseter muscle were not only reduced in the hypofunctional but also in the rehabilitation group as compared with the normal group. These results suggest that optimisation of the chewing pattern and acquisition of appropriate masticatory function are impeded by feeding a soft diet during the animal's growth period and that no catch-up effect of the masticatory function is observed when there is a prolonged period of time prior to changing the diet from soft to hard. In conclusion, masticatory function can only be fully developed through a learning process such as exposure to chewing various kinds of foods with different food textures.
Background
Mastication is one of the most fundamental functions for the conservation of life. The demand for devices for evaluating stomatognathic function, for instance, recording mandibular movements or masticatory muscle activities using animal models, has been increasing in recent years to elucidate neuromuscular control mechanisms of mastication and to investigate the etiology of oral motor disorders. To identify the fundamental characteristics of the jaw movements of mice, we developed a new device that reconstructs the three-dimensional (3D) movement trajectories on an arbitrary point on the mandible during mastication.
Methods
First, jaw movements with six degrees of freedom were measured using a motion capture system comprising two high-speed cameras and four reflective markers. Second, a 3D model of the mandible including the markers was created from micro-computed tomography images. Then, the jaw movement trajectory on the certain anatomical point was reproduced by integrating the kinematic data of the jaw movements with the geometric data of the mandible.
Results
The 3D movements at any points on the mandible, such as the condyle, molar, and incisor during mastication, could be calculated and visualized with an accuracy > 0.041 mm in 3D space. The masticatory cycle was found to be clearly divided into three phases, namely, the opening, closing, and occlusal phases in mice.
Conclusions
The proposed system can reproduce and visualize the movements of internal anatomical points such as condylar points precisely by combining kinematic data with geometric data. The findings obtained from this system could facilitate our understanding of the pathogenesis of eating disorders or other oral motor disorders when we could compare the parameters of stomatognathic function of normal mice and those of genetically modified mice with oral behavioral dysfunctions.
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