Topographical representation of shoulder motor nuclei in the cat spinal cord as revealed by retrograde fluorochrome tracers.

The present investigation demonstrates the morphological relationships among the main shoulder motor nuclei within the spinal cord of the cat. The intraspinal position of these nuclei has been revealed by retrograde labelling of spinal motor neurones via their peripheral nerves supplying anatomically identified shoulder muscles. Multiple pressure injection of up to four fluorescent tracers (Bisbenzimide, Fast Blue, Fluoro-Gold, Rhodamine-b-isothiocyanate) in one experiment was used to show the longitudinal distribution and topographical relations of motor neurones projecting to muscles acting on the scapulo-humerus joint. Tracer-positive cells have been found from middle C5 to rostral Th2 in the cervical cord, forming coherent longitudinal cell clusters separated in medial and lateral projection fields in the ventral horn. The present data suggest that the anatomical organization of spinal shoulder motor neurones corresponds to the embryonic origin of their later target muscles. All medial motor nuclei project to muscles deriving from ventral embryonic origins, while those motor nuclei lying in lateral positions innervate muscles originating from dorsal muscle primordia. Therefore, the spinal topography of shoulder motor nuclei seems to be independent of both the position and the function of a given muscle in the adult animal.