Electromyography of the mandibular postural position in the rhesus monkey (Macaca mulatta).

The mandibular postural position is attained when all of the muscles involved in mandibular movement show no other activity than that which is necessary to hold the mandible in a balanced position against gravity. Mandibular position is influenced by the fusimotor system through psychic input and through such stimuli from peripheral receptors as those of the temporamandibular joint, periodontal ligaments, gingiva, tongue, and palate. Mandibular position also is related to the maintenance of the pharyngeal airway. The postural position of the mandible in man is maintained by tonic activity in the elevator musculature opposing gravitational forces. The purpose of this study was to determine the relative participation of the orofacial muscles of the rhesus monkey in the maintenance of mandibular posture. Thirty-three Macaca mulatta, aged six months to eight years, were monitored in 113 electromyographic recording sessions. Bipolar needle electrodes were placed in the anterior and posterior portions of the temporal muscle, the anterior portion of the superior head of the masseter muscle, the orbicularis oris muscle, the two heads of the lateral pterygoid muscle, and in the suparhyoid muscle group. Posture, oral reflexes, and random jaw movement were studied while each animal was isolated in a sound-