Synaptic basis of cortically induced rhythmical activity of hypoglossal motoneurons in cats

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Synaptic basis of the rhythmical activity induced by repetitive stimulation of the masticatory area of the cerebral cortex was studied by intracellular recording from hypoglossal motoneurons in cats, with the following results: (1) Repetitive cortical stimulation induced rhythmical tongue movements coordinated with rhythmical jaw movements. After the animal was paralyzed, the cortical stimulation still induced a rhythmically alternating efferent burst activity in the medial and lateral branches of the hypoglossal nerve; the bursts in the medial and lateral branches were in and out of phase with that of the digastric nerve, respectively. (2) In all of the recorded 36 hypoglossal motoneurons, repetitive cortical stimulation induced a rhythmical depolarizing potential superimposed by spike bursts corresponding with the rhythmical efferent burst in the hypoglossal nerve. No hyperpolarizing potential was present between the consecutive depolarizing potential. (3) Synaptic activation noise was increased coincidentally with the depolarizing potential, indicating that EPSPs were involved in the generation of the depolarizing potential. By intracellular application of either DC current or Cl-, no evidence was obtained for existence of IPSPs during the inter-excitatory phase.It was concluded that rhythmical bombardment of excitatory impulses to hypoglossal motoneurons was responsible for their rhythmical activity induced by repetitive stimulation of the cortical masticatory area.