大脳皮質の顎反射変調作用とその発現機序

BILATERAL INFLUENCES OF THE FRONTAL CEREBRAL CORTEX ON THE JAW REFLEXES AND THEIR POSSIBLE MECHANISMS IN RABBITS

I．はじめに

　三叉神経運動核の支配をうける咬筋（閉口筋）および顎二腹筋，顎舌骨筋（開口筋）は顔面や口腔・咽頭などにある種々の感覚受容器の興奮により，さまざまな反射反応を惹起する。咬筋の収縮，すなわち閉口反射は自己受容器に連絡する感覚ニューロンが運動ニューロンにひとつのシナプスを介して興奮を伝達することによつておこり，顎二腹筋，顎舌骨筋の収縮すなわち開口反射は口腔粘膜や歯根膜の受容器からの2ないし3個のシナプスを含む反射経路によつて伝えられる。顎反射を惹起するような感覚刺激はまた，咀嚼，嚥下，嘔吐などのように比較的複雑な反応をも発現させる^4）。このような反射現象は大脳皮質の影響を強くうけ^6），とくに前頭葉前外側部に相当する領域に頻数電気刺激をあたえると，律動性の咀嚼や嚥下運動が哺乳類の各種動物で惹起される^4,12）。この事実はヒトにおいても脳手術の経過中に確認されている^7）。最近，Sessleによりこのような大脳皮質作用の種類，程度，時間経過などについての分析が多数の三叉神経運動ニューロンについて行なわれているが^10,11），本報告では大脳皮質作用の左右差，すなわち三叉神経からみて大脳皮質が同側であるか反対側であるかにより，作用にどのような相違があるかに．主眼をおいて解析した結果と，これについての考察の概要を述べる。

In 25 lightly anesthetized (ether inhalation) and motoparalysed (Flaxedil, 3mg/kg, i. v.) adult rabbits, action potentials were recorded from the masseteric or the mylohyoid nerve bundle when the potentials were produced by a single stimulus to the mesencephalic tract of the trigeminal nerve or to the inferior dental nerve ipsilaterally. The reflex potential of the masseteric nerve is known to be conveyed monosynaptically and achieves the proprioceptive jaw closing reflex, while the reflex potential of the mylohyoid nerve is produced poly-synaptically and contributes to the exteroceptive (nociceptive) jaw opening reflex. Reflex stimuli were delivered with various delays after the con-ditioning stimuli applied to the ipsilateral or contra-lateral frontal cortex. The reflex potential of the masseteric nerve was inhibited for the period begining at 3-4 msec and lasting to 80-120 msec after the stimulus. No significant difference was noted in the appearance of these effects when the conditioning stimulus was applied to the ipsi or contralateral cortex. However, the reflex potentialof the mylohyoid nerve received a bilaterally different influence from the cortex. Namely, a stimulus to the ipsilateral cortex produced no or a scare change, whereas that to the contralateral cortex brought about a remarkable inhibition begining at a few msec and lasting till 60-80 msec after the cortical stimulus. From these, bilateraldifferences in the functional connection can be postulated mainly between the frontal cerebral cortex and the arc of the jaw opening reflex. A possible neural arrangement and mode of connection for such differences have been discussed in the relevance of some physiological function of the jaw.