Jaw-tongue reflex: Afferents, central pathways, and synaptic potentials inhypoglossal motoneurons in the cat

The tongue position is reflexively controlled by the jaw position (the jaw- tongue reflex). The purpose of this study was to clarify the mechanism of t his reflex in terms of afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat. Intracellular recordings from hypog lossal motoneurons revealed that electrical stimulation of the temporalis m uscle nerve evoked excitatory and inhibitory post-synaptic potentials in hy poglossal motoneurons. The threshold of temporalis muscle nerve stimulation for evoking the synaptic potentials was higher than 2.0 times the nerve th reshold. The amplitude of the potentials increased with stimulus intensity up to 5.0 times the nerve threshold. Punctate light pressure applied to the temporalis muscle induced a tonic depolarizing potential in hypoglossal mo toneurons on which action potentials as well as depolarizing synaptic activ ation noise were superimposed. On the other hand, electrical stimulation of the temporalis muscle during jaw-opening could slightly inhibit the electr omyographic activities in the genioglossus and styloglossus muscles. Lesion s including the Probst's tract at the level caudal to the trigeminal motor nucleus abolished both excitation and inhibition in hypoglossal motoneurons induced by tonic depression of the lower jaw, but exerted no effects on ei ther the tonic stretch reflex or the trigemino-hypoglossal reflex. In contr ast, lesions including the trigeminal spinal tract produced no changes in e ither excitation or inhibition of hypoglossal motoneurons induced by tempor alis muscle afferents, whereas the excitation of hypoglossal motoneurons wa s abolished by the lesions. We conclude that the group II muscle spindle af ferents from the temporalis muscle are primarily responsible for evoking th e jaw-tongue reflex.