Presence of the rat lingual motoneurons with axons passing through the ansa cervicalis

The peripheral course of axons of the rat lingual motoneurons was studied by HRP injection to the hypoglossal trunk in combination with sectionings of the hypoglossal or cervical nerve components of the hypoglossocervical plexus. Furthermore, the sizes of the labeled lingual motoneurons were compared in cross sections with those of the labeled geniohyoid (GH) and thyrohyoid (TH) motoneurons, which are located close to the lingual motoneurons. Axons of the lingual motoneurons, lying in the main hypoglossal nucleus, mostly pass through the hypoglossal nerve to reach the tongue, while those of a few lingual motoneurons, lying at the medial portion of the ventromedial subnucleus in the caudal fourth of the main hypoglossal nucleus, pass through the first cervical nerve to the upper root of the ansa cervicalis before joining the hypoglossal nerve. The lingual motoneurons passing through the cervical nerve are intermingled with those passing through the hypoglossal nerve. The latter motoneurons, however, diminish in number while traced caudally, and finally in the caudalmost part of the main hypoglossal nucleus the motoneurons passing through the cervical nerve come to f orm the major part of this nucleus. The size and degree of flatness of the labeled lingual motoneurons are smaller than those of the labeled GH and TH motoneurons. The lingual motoneurons passing through the cervical nerve are smaller in size than those passing through the hypoglossal nerve. This difference between motoneurons passing through the hypoglossal nerve and those passing through the cervical nerve is similar to that in the GH motoneurons.