Pathophysiology of SB: challenges from human and animal studies

Abstract: It becomes increasingly recognized that dentists can play roles in diagnosing and managing some sleep disorders, and their orodental consequences. Among them, sleep bruxism (SB) is a common sleep disorder, affecting approximately 20% in children and 10% of adults. However, physiological studies are limited in humans and animals. Polysomnographic studies in humans showed that rhythmic masticatory muscle activity (RMMA) occurred more frequently in patients with SB than in normal subjects. A majority of RMMA occurred during NREM sleep in association with transient arousals and cyclic sleep processes. To further understand the neurophysiological mechanisms of SB, the jaw motor activities were investigated in the naturally sleeping animals. During sleep, animals exhibited a variety of masseter and digastric contractions. Nonetheless, the RMMA episodes were found to occur during NREM sleep: the episodes were associated with cortical and cardiac activations as found in humans. In addition, animals showed phasic bursts of masticatory muscles, mimicking a coordinated pattern of chewing, during REM sleep. To investigate the excitability of masticatory central pattern generator (CPG), cortical descending tract, originating from cortical masticatory area, was electrically stimulated during wakefulness and natural sleep. Stimulations induced rhythmic masseter and digastric contractions during wakefulness and NREM sleep. However, the response threshold was higher and the response latency was longer during NREM sleep than during wakefulness. During REM sleep, stimulation induced rhythmic digastric muscle contractions. The response threshold was higher while response latency was similar in comparison to wakefulness and NREM sleep. These results suggest that distinct modulation of the excitability in masticatory CPG during sleep and wakefulness can underlie the increased occurrence of RMMA during NREM sleep in patients with SB. The above findings suggest that naturally sleeping animals can be used as an experimental model for investigating the pathophysiological mechanism of SB.