Neuroscience Letters 368 (2004) 192–196 Effects of food consistency on the pattern of extrinsic tongue muscle activities during mastication in freely moving rabbits Makoto Inoue ∗ , Yohji Harasawa, Kensuke Yamamura, Sajjiv Ariyasinghe, Yoshiaki Yamada Division of Oral Physiology, Department of Oral Biological Science, Niigata University Graduate School of Medical and Dental Sciences, Niigata 951-8514, Japan Received 7 May 2004; received in revised form 9 June 2004; accepted 6 July 2004 Abstract The effects of physical characteristics of foods on the coordination of extrinsic tongue muscle activities during natural mastication were evaluated. Electromyograms of tongue-retractor (styloglossus, SG) and tongue-protractor (genioglossus, GG) muscles as well as the jaw- movement trajectories were recorded during raw rice and chow pellet chewing in the freely moving rabbit. Each masticatory cycle included a jaw closing (Cl) phase consisting of a fast-closing (FC) and a slow-closing (SC) phase, and a jaw opening (Op) phase. The duration of the Cl and SC phases was found to be much larger while the duration of the FC phase was much smaller during rice chewing than pellet chewing. The jaw movements during rice chewing had smaller amplitudes of the gape and lateral excursion of the jaw as compared with those during pellet chewing. The SG muscle had a double-peaked burst activity in each masticatory cycle with one peak during the Op phase (the SG1 burst) and the other during the Cl phase (the SG2 burst). They were significantly larger during pellet chewing as compared with rice chewing, but the duration of the SG2 burst was significantly longer during rice chewing than pellet chewing. The offset of the SG2 burst was delayed during rice chewing as compared with that during pellet chewing. There was little difference in the activity pattern of the GG burst between the foods. Our present results suggest that the SG muscle activity could be modified by the sensory feedback possibly to adapt to environmental demands during chewing. © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Styloglossus muscle; Genioglossus muscle; Mastication; Rabbit Masticatory movements are ascribed to the coordinated activ- ity of jaw, facial and tongue muscles [10,15]. In higher verte- brates, masticatory jaw and tongue movements are controlled by brainstem neural networks that provide initiation, rhythm generation and modulation of the masticatory motoneurons discharge [11,16]. Although, these networks are by them- selves capable of producing the coordinated rhythmic motor pattern, a fully balanced behavioral expression depends on a dynamic interplay between central and peripheral neuronal network mechanisms. Our group has recently reported that the activity patterns of jaw, tongue and suprahyoid muscles are modulated by the food consistency during natural mastication [4]. The ac- ∗ Corresponding author. Tel.: +81 25 227 2824; fax: +81 25 225 0281. E-mail address: inoue@dent.niigata-u.ac.jp (M. Inoue). tivity of the tongue-retractor muscle, but not of the tongue- protractor muscle, was increased with the hardness of foods during jaw closing. Furthermore, the activity on the chew- ing side was larger than that on the non-chewing side. The results suggest that the sensory feedback arising from the mechanoreceptors and/or jaw-closer muscle spindles may affect the activity of tongue-retractor and -protractor mus- cles differently when placing the foods to be squeezed and/or crushed in order to prepare a bolus for swallowing. The aim of this study was to record the jaw movements and the extrin- sic tongue muscle activities during natural mastication in the freely moving rabbit, and to analyze the modulatory effects of food consistency on the pattern of those muscle activities. The experiments were carried out on five adult male Japanese white rabbits weighing between 2.5 and 3.5 kg in accordance with the Guide for the Care and Use Commit- 0304-3940/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2004.07.043