An enzyme tracer study of the organization of the somatic motor center for the innervation of different muscles of the tongue: evidence for two sources.

The hypoglossal nucleus has been described as the sole ipsilateral source of somatic motor innervation of the lingual and geniohyoideus muscles (Streeter, '40; Barnard, '40; Crosby et al., '62; Watkins, '78; Jenkins, '78). In this study microliter amounts of 30% solution of horseradish peroxidase (HRP) were injected into intact but surgically isolated canine genioglossus, hyoglossus, styloglossus, geniohyoideus, and intrinsic lingual muscles. All injections were confined to the left half of the tongue, and the right half served as control. The HRP injections resulted in ipsilateral labeling of hypoglossal neurons with the exception of the injections into the genioglossus muscle, which labeled this nucleus bilaterally. Surprisingly, a few neurons associated with the ventromedial aspect of the caudal pole of the ipsilateral facial motor nucleus also labeled after injections of lingual muscles. The somatotopic representation of the geniohyoideus and lingual muscles within the hypoglossal nucleus suggested that those muscles known to originate from the same or related primodia labeled in the same or closely related hypoglossal subnuclei. The presence of labeled neurons associated with facial motor nucleus supported the hypothesis (Langman, '75) of a possible dual origin of lingual muscles.