Physiological study of effects of occlusal function on head posture and autonomic function

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Firstly, rhythmical head movements associated with the masticatory movement and its neuromuscular control mechanism were investigated in rabbits. Head movements and EMG activities of neck muscles (sternomastoid, SM ; and semispinalis capitis, SS) were recorded in awake unrestrained rabbits during chewing. The rabbits moved their heads cyclically and with the same rhythmical characteristics of the masticatory jaw movement but in the opposite direction. The SM (head flexor muscle) showed biphasic activities in the jaw closing and opening phases respectively. The SS (head extensor muscles) were tonically active. The SM and SS EMG activities were recorded during cortically evoked rhythmical jaw movements in anesthetized animals with the head fixed. The SM was active only in the jaw closing phase, the activity was enhanced by tooth contact. The SS was tonically active regardless of tooth contact. These results suggest that the head flexor muscle may contribute to the head movement after loading the tooth by reflex action, and the head extensor muscles may play a role on fixing the head with tonic activity.Secondary, the effects of afferent signals from the periodontal mechanoreceptors and muscle spindles of jaw-closing muscles on adrenal nerve activity were examined using anesthetized rats. The activity of adrenal nerve increased when pressure stimulation was applied to the upper incisors. When the jaw was opened passively, there was no significant change in the adrenal nerve activity. Placing a wooden-stick between the opposing incisors during cortically evoked rhythmical jaw movements markedly facilitated the nerve. activity. After denervation of the periodontal ligament, the facilitation of adrenal nerve activity due to application of the wooden-stick was not observed. These results indicate that afferent signals from periodontal mechanoreceptors facilitate adrenal nerve activity and that signals from muscle spindles do not affect the activity.