Quantitative studies on motoneurons. II. Spatial and dimensional organization of hypoglossal motoneurons in the boa constrictor,Constrictor constrictor

The spatial distribution and sizes of degenerating hypoglossal neurons were studied following the removal of the genioglossus or hyoglossus muscles or of unilateral glossectomy in common boas with survival times of 7, 14, 21 or 40 days. The spatial organization of this nucleus differs from that of most cranial nerve nuclei in lacking a subnuclear organization: genioglossus and hyoglossus motoneurons are spread evenly throughout the nucleus and intrinsic tongue neurons show only a slight tendency to accumulate in the caudal half of the nucleus. The genioglossus and tongue neurons probably serve both ipsilateral and contralateral muscles with the ipsilateral innervation being largest; the hyoglossus motoneurons probably innervate only ipsilateral muscles. Complementary analyses of the transverse sectional areas of both degenerating and non-degenerating neurons indicates that, although their size ranges overlap, genioglossus motoneurons are small and tongue and hyoglossus motoneurons are progressively larger.