EXCITATION OF HYPOGLOSSAL MOTONEURONS RESPONSIBLE FOR TONGUE PROTRUSIONS IS ASSOCIATED WITH PALATALLY INDUCED JAW-CLOSING REFLEX

The excitation of hypoglossal motoneurons innervating the genioglossus and geniohyoid muscles during transient jaw closing, the so-called ja w-closing reflex, was studied in cats. The application of diffuse pres sure stimulation to the posterior palatal surface produced the jaw-clo sing reflex, and it was found that mechanosensory inputs from the post erior palatal mucosa sent excitatory synaptic inputs to both genioglos sus and geniohyoid motoneurons. We demonstrated that, during the palat ally induced jaw-closing reflex, the tongue extended at jaw closure an d was still extended forward in the initial part of the opening phase. In five of 27 genioglossus motoneurons and nine of 23 geniohyoid moto neurons, the onset of burst was elicited before the onset of jaw closu re. The remaining cells produced the onset of burst in the closing pha se and in the initial part of the occlusal phase. However, the onset o f excitatory postsynaptic potentials was 75-180 ms (n=20), earlier tha n that of jaw closure. During the jaw-closing reflex, the genioglossus and geniohyoid motoneurons were excited during the same phase of jaw movements and there was no difference in the onset of firing between t he genioglossus and geniohyoid motoneurons. It is concluded that the e xcitation of the genioglossus and geniohyoid motoneurons may be associ ated with tongue protrusions during the palatally induced jaw-closing reflex. (C) 1997 IBRO.