Masticatory muscle activities during rhythmic jaw movement associated with tooth contact in lightly anesthetized rats

Rhythmic jaw movements were observed in lightly anesthetized rats when a bitable object was placed between the maxillary and mandibular incisors. To elicit the jaw movements, an initiating stimulus was required. A successful stimulus was to push on the mandible at the incisors in both opening and posterior directions. This may be a suitable movement for studying mastication because of the following properties: (i) The rhythmic jaw movements could not continue without tooth contact. (ii) The timing pattern of the electromyographic activity of the masseter and digastric muscles and the tooth contact was similar to that of human mastication. (iii) Frequency of the movements was independent of hardness of the object placed between the incisors, though oral stimulation easily stopped the movement as did a noxious stimulus applied around the oral cavity. We conclude that peripheral activation, which may be generated by tooth contact, is as essential to the rhythmic jaw movement as central activation.