Functional differentiation of hypoglossal motoneurons during the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat.

Activity of hypoglossal motoneurons was studied in relation to the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat. Tongue protrudor and retrusor motoneurons were antidromically identified by stimulation of the medial and the lateral hypoglossal nerve branches, respectively. About 80% of protrudor motoneurons excited during the opening phase of the rhythmical jaw movements with or without prior excitation. Most of them did not respond to amygdaloid or cortical stimulation. Fifty-three percent of retrusor motoneurons were excited during the closing phase and 31% discharged around the peak opening. The majority of them and a few protrudor motoneurons responded to amygdaloid or cortical stimulation with similar latencies. Activities of the medial and the lateral hypoglossal branches corresponded with the discharge patterns of respective motoneurons. Tongue protrusion and retrusion appeared to slightly precede jaw opening and closing, respectively during the rhythmical jaw and tongue movements. This was probably due to activity of the brain stem pattern generator.