Muscle activities are differently modulated between masseter and neck muscle during sleep–wake cycles in guinea pigs

Sleep bruxism is a sleep-related movement disorder characterized by an exaggerated jaw motor activity during sleep. Currently, the magnitude of jaw motor activation in normal sleep remains poorly understood. In this study, we aim to assess the state-dependent changes in the magnitude of electromyographic activities of the jaw-closing masseter muscle in comparison with those of a neck muscle (specifically, the obliquus capitis) during sleep–wake cycles in guinea pigs. These electromyographic activities were integrated for 10-s epochs during wakefulness, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The masseter activity per epoch was found to be five times lower in both sleep stages while the neck muscle activity also decreased to 30% in NREM sleep and was lowest (16%) in REM sleep. In the periods without motor activity, masseter tone did not differ between the three states, whereas neck muscle tone decreased from wakefulness to NREM sleep and further to REM sleep. Moreover, in the epochs with masseter activation, the neck muscle activity did not increase during sleep. These results suggest that masseter activity decreases but is occasionally activated during sleep, and that state-dependent changes in electromyographic activity can be differently modulated in time and intensity between the masseter and the obliquus capitis.