三さ神経運動ニューロンに対する大脳皮質性制御に関与する神経路ならびにそのシナプス機構

Abstract

The synaptic basis of the efferent discharge in the digastric nerveevoked by the stimulation of the cortical masticatory area (orbital gyrus) was analyzed firstof all by the intracellular recording from the anterior digastric motoneurons in the cats anesthetized with α-chloralose. Secondly, the role of the bulbar reticular formation in this cortical effect was studied by the transection of the brain stem at the level of the rostral border ofthe medulla oblongata. Thirdly, the synaptic mechanism responsible for the reversal of the orbital cortical effect on the masseteric monosynaptic reflex from depression to facilitation by the ponto-bulbar brain stem transection was analyzed by the intracellular recording from the jaw closer motoneurons. The following results were obtained: 1) Before the brain stem transection, the orbital cortical stimulation evoked excitatory postsynaptic potentials (EPSPs) in the anterior digastric motoneurons with a latency that was longer by one synaptic delay than that of the spike potentials evoked by the orbital cortical stimulation in the medial bulbar reticular neuronsforming direct excitatory synaptic linkages with the anterior digastric motoneurons. After the brain stem transection, the cortically evoked EPSPs were virtually abolished, but no inhibitory postsynaptic potential was revealed. Thus, the abolition of the cortically evoked efferentdigastric nerve discharge after the brain stem transection was due to the drastic reduction or even complete loss of the EPSPs (disfacilitation) in stead of the active inhibition of the anterior digastric motoneurons. 2) After the brain stem transection, stimulation of the orbital gyrus evoked EPSPs in the jaw closer motoneurons, indicating that the facilitation of the masseteric reflex was due to the EPSPs instead of the disinhibition of the jaw closer motoneurons. It was concluded that the orbital cortical excitatory effect on theanterior digastric motoneurons was mediated by the bulbar reticular formation, and the excitatory premotor neurons involved in this cortical effect were virtually all located in the bulbar reticular formation, while the excitatory premotor neurons projecting to the jaw closer motoneurons were located rostrally to the medulla oblongata.