Jaw-Closing Muscle Activities with the Mandible in Rest Position

¹ Department of Oral Physiology, Osaka University Dental School, Osaka, Japan

Jaw muscle activities with the mandible at rest were analyzed by an electromyographic method in man and in the cat. Spontaneous EMG activities were recorded, from the anterior superficial belly of the masseter muscle in man, and from the masseter and temporalis muscles in the cat. In the latter instance, four different types of activity were observed. Regular and continuous activity was recorded mainly from the anterior superficial belly and all of the deep belly of the masseter muscle and from the lower and medial parts of the temporalis muscle. Irregular and intermittent activity was recorded mostly from the medial and posterior parts of both bellies of the masseter muscle and also from the lower part of the temporalis muscle. The third type was recorded mostly from the upper part of the superficial belly only during the expiratory phase. Finally, a long-lasting and low-amplitude burst was recorded from only the medial part of the superficial belly.

Most of the first type of EMG activity was inhibited by stretch of the digastric and contralateral muscles. In addition, spontaneous EMG activities in the posterior belly of the temporalis muscle were inhibited by stretch of the anterior belly and vice versa.