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  • Lateral jaw movement reflex induced by mechanical stimulation to the upper and lower incisors in the rabbit

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The reflex control mechanisms, which were changed accrding to the level of muscle activity before stimulation (i.e., background activity : BGA), from the incisors to the zygomatico-mandibular muscle was investigated in the decerebrated rabbits. Motor unit activities in this muscle were recorded by electromyogram. Mechanical stimulation was applied to the maxillary and the mandibular incisors independently or simultaneously. The results were as follows : 1. Excitatory renexes could be easily elicited from the zygomatico-mandibular muscle by the to contralateral, opposite to the recorded muscle, maxillary incisor and bilateral mandibular incisors. Inhibitory reflex could be easily induced in this muscle by the stimulation to the ipsilateral maxillary incisor. 2. Reversal levels calculated from BGA-response curves and reflex thresholds from the mandibular incisors were lower than those of contralateral maxillary incisor. 3. When the simultaneous stimuli were applied to the mandibular incisor and contralateral or ipsilateral maxillary incisor, reversal levels were higher than those of reflex response induced from the stimulation to the independent incisor. 4. Excitatory reflex was elicited by the ipsilateral maxillary incisor, while preceding stimulation was applied to the mandibular incisor. However excitatory response from mandibular incisor was depressed when preceding stimulation was applied to the ipsilateral maxillary incisor. It is suggested that positive feedback system functions on the lateral jaw movement to the ipsilateral side of occluded incisors, and that lateral jaw movements during mastication are controlled by integrative reflex mechanisms elicited from the maxillary and mandibular incisors.

本研究は、上下顎切歯からの末梢性入力による下顎側方運動の反射性制御の解析を目的に行った。除脳ウサギの上下顎切歯に、単独ないし、上下顎同時の圧刺激を加え、刺激前の筋の活動状態(BGA)に応じて変化する頬骨下顎筋の反射性応答を調べ、以下のことを明らかにした。1. 筋電図を記録している頬骨下顎筋からみて、反対側上顎切歯、両側下顎切歯単独刺激時には、主として興奮反射が引き起こされた。同側上顎切歯からは抑制反射だけが誘発された。 2. 反対側上顎切歯からの反射効果と下顎切歯からの効果を比較すると、反射効果が興奮から抑制に逆転するBGAの値(reversal level)ならびに、反射閾値が下顎切歯で低かった。 3. ともに興奮反射を生じやすい反対側上顎切歯と下顎切歯に同時に力を加えると、興奮反射が増強し、reversal levelが高くなった。下顎切歯と、抑制反射しか誘発しない同側上顎切歯に同時に力を加えても、下顎単独刺激時よりreversal levelが高くなり、下顎からの興奮効果が増強された。 4.下顎切歯刺激を先行させて、同側上顎切歯を刺激すると、単独刺激時とは反射効果が大きく異なり興奮反射が誘発された。逆に同側上顎切歯刺激を先行させると、下顎切歯からの興奮効果が生じ難くなった。以上のことから、頬骨下顎筋は上顎および下顎切歯から巧妙な反射性調節を受け、咬合圧が加わっている切歯と同側に下顎が偏位するポジティブフィードバック機構が働き、下顎側方運動が統合的に制御されていると考えられた。



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