Research reportTongue and jaw muscle activities during chewing and swallowing in freely behaving rabbits
Introduction
Ingestion can be accomplished by the coordinated activities of the jaw, hyoid, tongue, and perioral muscles. It includes several motor behaviors such as mastication (chewing), food transportation, and swallowing (see Ref. [4], [16] for review). Although complete feeding sequences have been studied in humans with the use of videofluorography [6], [14], details in the burst timing of the participating muscles are still unclear. To understand the neural mechanism underlying the control of the oral motor behaviors, the activity of jaw and hyoid muscles have been studied during natural chewing and swallowing in animals [11], [12], [18], but the activity pattern of the tongue muscles and perioral muscles (e.g. facial muscles) during these orofacial movements have not been clarified due to methodological problems. Our objectives in the present study were to define the activity pattern of tongue muscles during chewing and swallowing and to elucidate the coordination among jaw, tongue, and hyoid muscle activities during these movements. To accomplish our objectives, jaw movements and the burst patterns and interactions of three muscle groups; jaw (masseter: Mas, digastric: Dig), extrinsic tongue (styloglossus: Sg, genioglossus: Gg), and hyoid (mylohyoid: Myl, thyrohyoid: Thy) muscles were studied during natural chewing and swallowing in the freely behaving animal.
Section snippets
Surgical preparation
The methods for animal preparation, recording and data analysis have been extensively detailed [12], [18], [19] and so only a brief description follows. The experiments were carried out in five male rabbits (Japanese white, 2.0–2.5 kg) in accordance with the Guide for the Care and Use of Laboratory Animal Care (NIH Publication #86-23, revised 1996). The animal protocols were approved by the Intramural Animal Care and Veterinary Science Committee of the Niigata University. Before the necessary
Properties of jaw movements and muscle activities during chewing cycles
Chewing cycles in the early stage of a masticatory sequence were characterized with simple open-close movements having little horizontal movement and smaller EMG activities in the jaw muscles compared those during the latter cycles. Here we termed the chewing cycles two-phase cycles that may be identical to the term ‘preparatory series’ [15] or ‘type I’ [13] chewing cycles named by other groups. The two-phase cycles were followed by the chewing cycles configured with larger movements in both
Discussion
The present study has documented the activity pattern of the jaw, hyoid and tongue muscles during natural chewing and swallowing. In the chewing cycles, Gg (main tongue protruder) was active synchronously with jaw-opening muscle, and Sg (tongue retractor) was active both in the opening and closing phases. In the swallowing cycles, dramatic changes were observed in the activity patterns of hyoid muscles (i.e. Thy and Myl). The duration of the swallowing cycles was significantly longer than that
Acknowledgements
The authors thank Professor B.J. Sessle for his help in reviewing this manuscript. The authors also thank Mr. H. Hirano and Mr. Y. Takahashi for their technical assistance. This study was partly supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture and Science of Japan (#11470389 to Y.Y.).
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