筋電図法による咀嚼筋の活動樣式に関する研究 (特に咬筋, 側頭筋, 顎二腹筋について)

抄録

Satisfactory application of the electromyographic method as a research tool has not been made as yet in the field of dentistry. Very few reports, therefore, are available for understanding the functions of orofacial muscles from this point of view.
The aim of this report is to describe the findings on the functions of masticatory muscles which were obtained through studying their myograms and, if necessary, the mandibular movements as well as the bite pressure recorded simultaneously.
The results and conclusoins are summarized as follows:
1) The discharge of the neuromuscular units composing the group of masticatory muscles was of a pattern similar to those of the limb muscles. In some units, the magnitude of spike discharge was measured a few milivolts. The spike duration was 2-5 milisec., a little shorter than that in the limb muscles and there were found some units with their discharge frequencies of more than 70 C.P.S.. From these results it is possible to consider that the innervation ratio for the masticatory muscles is smaller than that for the limb muscles.
2) In the rest position of the mandible, when the Frankfort plane of the subject was kept to be in paralell with the horizontal, no discharge was recorded from all the masticatory muscles under examination.
3) Determination was made of the rest position of the mandible through studying the myograms of the masseter, the temporal and the digastric muscles. The rest position thus obtained was 2.6 mm as to the masseter and the temporal, and 6.1 mm to the digastric muscle, having the clearance of 3.5 mm as their mean value. From this finding, it is probable that the rest position of the mandible can directly be determined by way of electromyography.
4) The followings are the findings on functions of muscles as to the mandibular movements which were obtained from their myograms in the movements in six directions (i.e., upward, downward; forward, backward; leftward, rightward) where the rest position was taken as starting position for the movements.
a) The elevation of the mandible is accomplished by the contraction of the masseter, the temporal and the internal as well as external pterygoid muscles of both sides.
b) The digastric muscles of both sides take parts in the forward and backward movements. The masseter and the internal pterygoid muscles participate in the forward movement, while the temporal and the external pterygoid in the backward.
c) To the lateral muandibular movement toward one side contribute the masseter and the internal pterygoid muscles of the same side but also the temporal and external pterygoid muscles of the other side and further the digastric muscles of both sides. Further investigation by way of electromyography is necessary to make more clearer the functions of the internal and the external pterygoid muscles as to the mandiblular movements.
5) The followings are the informations obtained from simultaneous recording of the mandibular movements with reference to the discharge of the muscles involved.
a) In biting, the superficial and the deep head of the masseter muscles function together, but in the case of the mouth closing the superficial alone.
b) The anterior, the middle and the posterior belly of the temporal muscle function together in biting, but in closing the mouth there functions the middle belly in chief.
c) The digastric and the sternohyoid muscles take part in the mouth opening. But the discharge in the sternohyoid muscles was not observed before the opening advanced to a great extent, so that the suprahyoid muscles seem to participate in the action of the mouth opening, partly aided by the infrahyoid muscles.
d) At the maximal opening of the mouth the masseter and the temporal muscles showed discharges which are considered to be elicited by their proprioceptive reflex. And this phenomenon is likely to indicate that there exist some muscle spindles in the masticatory muscles.