Mechanism of motor coordination of masseter and temporalis muscles for increased masticatory efficiency in mice.
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.
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Yoshimi
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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http://orcid.org/0000-0002-3187-2798
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Koga
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Nakamura
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Fujishita
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Kohara
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Moriuchi
Yoshimi Skin Clinic, Nagasaki, Japan.
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Yoshimi
Division of Orthodontics, Department of Dentistry, Taipei Medical University Hospital, Taipei, Taiwan.
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Tsai
Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
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Yoshida
D000818:Animals; D001696:Biomechanical Phenomena; D019274:Botulinum Toxins, Type A / Q000494:pharmacology*; D003679:Deglutition / Q000502:physiology*; D004195:Disease Models, Animal; D004576:Electromyography; D008297:Male; D008409:Mastication / Q000502:physiology*; D008410:Masticatory Muscles / Q000473:pathology*; D051379:Mice; D008813:Mice, Inbred ICR; D009465:Neuromuscular Agents / Q000494:pharmacology*; D013704:Temporomandibular Joint / Q000473:pathology*
D009465:Neuromuscular Agents; D019274:Botulinum Toxins, Type A
botulinum toxin type A; electromyography; jaw relation record; masseter muscle; mastication; temporal muscle