Location of motoneurons supplying upper neck muscles in the chicken studied by means of horseradish peroxidase.

The distribution of motoneurons innervating the upper cervical muscles, biventer cervicis, splenius capitis, complexus, rectus capitis dorsalis, rectus capitis lateralis, and rectus capitis ventralis in the chicken was examined by retrograde transport of horseradish peroxidase. Labeled motoneurons supplying upper neck muscles ranged from 30 to 60 micron in diameter and were located within two subnuclei in the brainstem. The rostrocaudal distributions of motoneurons projecting to individual cervical muscles ranged from 3 to 9 mm in length, both rostral and caudal to the obex. Detailed analysis of the data showed that the more dorsally positioned subnucleus projected mainly to the hypaxial muscles, i.e., the rectus capitis ventralis and lateralis, whereas the ventral subnucleus supplied chiefly the epaxial muscles, i.e., the biventer cervicis and splenius capitis. The complexus and rectus capitis dorsalis were innervated by both of these subnuclei. Historically these dorsal and ventral subnuclei, respectively, have been called the nucleus hypoglossus ventralis and the nucleus hypoglossus ventralis ventrolateralis. In view of the observation that these nuclei do not undergo retrograde degeneration following section of the hypoglossal nerves, this older nomenclature is misleading. In agreement with other authors, we suggest that these motoneuron groups should be collectively referred to as the nucleus supraspinalis.