Central connections between the facial and the hypoglossal nerve in rats

Background and objective. The perfect coordination and synchronization of h ypoglossal and facial muscles du ring chewing, swallowing, breathing,and vo calization requires particular concomitant activities of the facial muscles , in contrast no direct connection between the facial and hypoglossal nucle us on the level of the brain stem has been detected until now. Patients/methods. Facial and hypoglossal nuclei of rats were identified on the basis of their antidromic field potential recorded after peripheral sti mulation of the corresponding nerves. Stereotactically single or double flu orescence tracer injections (Biotin-Dextran, Fluorescine-Dextran, Rhodamine Dextran, Fluoro Gold) were placed into the nuclei. Results. Retrograde tracer injections into the facial nucleus consistently labeled smalt neurons in the hypoglossal nucleus. In reverse experiments th e injection of anterograde tracers into the hypoglossal nucleus labeled fin e caliber varicose nerve fibers, but no somata in the facial nucleus. Synch ronous injections of different tracers into the facial and hypoglossal nucl eus produced a small, but constant number of double-labeled cells in the pa rvocellular reticular formation. Conclusions. Both, hypoglossal interneurons projecting to the facial nucleu s and neurons of the parvocellular reticular formation double-projecting to the facial and hypoglossal nucleus might play an important role in coordin ated orofacial movements. Moreover, both populations of neurons might be re sponsible for the excellent postoperative results after hypoglossal-facial anastomosis.