Synaptic basis of orbital cortically induced rhythmical masticatory activity of trigeminal motoneurons in immobilized...

Synaptic basis of the centrally induced masticatory rhythm was studied by intracellular recording from jaw closer and opener motoneurons during repetitive stimulation of the orbital gyrus (OG) of the encéphale isolé of immobilized cats, with the following results. (1) Repetitive OG stimulation induced a rhythmical alternation of hyperpolarizing and depolarizing potentials in jaw closer motoneurons. Rhythmical efferent burst discharges induced in the digastric nerve by OG stimulation coincided with the hyperpolarizing phase. The hyperpolarizing potential was reversed to a depolarizing potential by intracellular Cl- injection, while the depolarizing potential was not reversed to hyperpolarization, indicating that the hyperpolarizing and depolarizing potentials consisted at least mainly of IPSPs and EPSPs, respectively. (2) Repetitive OG stimulation induced rhythmical depolarizing potentials superimposed by spike bursts in jaw opener motoneurons, corresponding in time with the rhythmical efferent bursts in the digastric nerve. Synaptic activation noise was increased coincidentally with the depolarizing potential, indicating that EPSPs were involved in the generation of the depolarizing potential. No or little hyperpolarization if any was observed between successive depolarizing potentials. (3) Repetitive OG stimulation induced efferent burst discharges in the digastric nerve with the masticatory rhythm unaccompanied by any spike potential in the supratrigeminal neurons responsible for peripherally evoked inhibition of jaw closer motoneurons, indicating that the peripheral inhibitory mechanisms do not play an essential role in the central generation of the masticatory rhythm by OG stimulation.