Coactivated rhythmical activity in the neck muscles during masticatory movements in the rabbit

It has been shown in humans that during mastication the head moves rhythmically and coordinately with jaw movements. A neurophysiological explanation for the mechanism of this finding has not been given so far, matter which became the objective of this study. Head movement and EMG of neck (sternomastoid and splenius capitis) muscles were recorded simultaneously in awake unrestrained rabbits during bread and raw rice chewing. Also, EMG activity was recorded in anesthetized rabbits from the sternomastoid and semispinalis capitis muscles during cortically evoked fictive mastication. In voluntary mastication, the rabbits moved their heads rhythmically in a cycle similar to their chewing cycle and in an opposite direction to the jaw movement. Rhythmical EMG activity was observed in the sternomastoid, which showed biphasic activity in the jaw closing and opening movements during bread chewing; however, during raw rice chewing it was active only in the jaw closing movements. The splenius muscle showed tonic activity and no rhythmic activity was observed regardless of the test food used. In cortically evoked fictive mastication the sternomastoid was activated only in the jaw closing movements. This activity increased when biting on wooden sticks between opposing molars took place. In the semispinalis capitis tonic activity was observed in both events. Rhythmical head movement coordinated with jaw movement was observed during mastication not only in humans but also in rabbits; this observation indicates that rabbits are appropriate animal models to study head movement during mastication. From the results of this experiment we suggest that the sternomastoid may be reflexly coactivated with masticatory muscles by peripheral inputs such as periodontal afferents and afferents from propioceptors regulating sense of position during jaw opening and closing movements.