Synaptic bases of cortically-induced rhythmical hypoglossal motoneuronal activity in the cat

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Intracellular recordings were made from hypoglossal motoneurons during cortically-induced fictive mastication in paralyzed encéphale isolé cats. Repetitive stimulation of the masticatory area of the cerebral cortex induced rhythmical tongue movements coordinated with jaw movements. After the animal was immobilized, the cortical stimulation still induced rhythmical burst activities in the hypoglossal nerve and the digastric nerve. The burst activities in the medial and lateral branches of the hypoglossal nerve alternated rhythmically, and were in and out of phase with the burst activities of the digastric nerve, respectively. All hypoglossal motoneurons showed rhythmical intracellular potentials during repetitive cortical stimulation. The rhythmical depolarizing potentials superimposed by spike bursts appeared in phase with rhythmical bursts in either the lateral or medial branch of the hypoglossal nerve. No hyperpolarization was present between consecutive depolarizing potentials. Synaptic activation noise increased coincidentally with the depolarizing potential, indicating that EPSPs were involved in the generation of the depolarizing potential. No evidence was obtained for the existence of IPSPs during the inter-depolarizing phase by intracellular current injection. It was concluded that rhythmical bombardment of excitatory impulses to hypoglossal motoneurons was responsible for the rhythmical activity induced by repetitive stimulation of the cortical masticatory area.