Inhibition of styloglossus motoneurons during the palatally induced jaw-closing reflex

The inhibition of hypoglossal motoneurons innervating the styloglossus musc le during transient jaw closing, the so-called jaw-closing reflex, was stud ied in cats. The application of diffuse pressure stimulation to the posteri or palatal surface produced the jaw-closing reflex and inhibitory postsynap tic potentials in the styloglossus motoneurons, indicating that mechanosens ory inputs from the posterior palatal mucosa sent inhibitory synaptic input s to styloglossus motoneurons. We also demonstrated that, during the palata lly induced jaw-closing reflex, the tongue extended at jaw closure and was still extended forward in the initial part of the opening phase. In all of 22 styloglossus motoneurons studied, the depression of firing was elicited after the onset of jaw closure. In 14 of 22 styloglossus motoneurons, the d epression of firing was elicited in the closing phase, and in the remaining cells it was elicited in the occlusal phase. By increasing the intracellul ar concentration of chloride ions, the inhibitory postsynaptic potential el icited in the styloglossus motoneuron converted to a depolarizing potential . It is concluded that the inhibition of styloglossus motoneurons may be in volved in the maintenance of tongue protrusions during the palatally induce d jaw-closing reflex, and that inhibitory postsynaptic potentials evoked in the styloglossus motoneurons are partly due to a chloride-dependent inhibi tory postsynaptic potential. (C) 1999 IBRO. Published by Elsevier Science L td.