A horseradish peroxidase study of rat lingual motoneurons with axons passing through the cervical nerve.

The peripheral course of axons of rat lingual motoneurons was studied by HRP injection into the hypoglossal nerve in combination with transecting of the hypoglossal and/or cervical nerve components of the hypoglossocervical plexus. Furthermore, soma sizes of labeled lingual motoneurons were compared in transverse section with those of labeled geniohyoid and thyrohyoid motoneurons, which are situated adjacent to the lingual motoneurons. We found that axons of the majority of lingual motoneurons lying in the main hypoglossal nucleus passed through the hypoglossal nerve throughout their course to the tongue. In a remaining small number of lingual motoneurons lying in a medial portion of the ventromedial subnucleus in the caudal fourth of the main hypoglossal nucleus, their axons passed through the first cervical nerve to the upper root of the ansa cervicalis to the hypoglossal nerve and then to its medial branch. The labeled lingual motoneurons with axons passing through the cervical nerve were intermingled with those whose axons passed through the hypoglossal nerve. The latter motoneurons, however, diminished in number while being traced caudally, and finally in the most caudal main hypoglossal nucleus the former motoneurons occupied a major part of this nucleus. The lingual motoneurons with axons passing through the cervical nerve were smaller in soma size than those with axons passing through the hypoglossal nerve. These two types of lingual motoneurons were both smaller in soma size than the geniohyoid and thyrohyoid motoneurons, and their soma shape was not as flat as that of the latter types of motoneurons.