DISTRIBUTION OF MOTONEURONS SUPPLYING FELINE NECK MUSCLES TAKING ORIGIN FROM THE SHOULDER GIRDLE

A combination of fluorescent retrograde tracers and horseradish peroxi dase (HRP) was used to compare the spinal distributions of motoneurons supplying shoulder muscles with attachments to the skull and cervical spinal cord that suggest a significant role in head movement. Two mus cles, the rhomboideus and the levator scapulae, were innervated by mul tiple segmental nerve bundles that entered the muscles at different ro strocaudal locations. Motoneurons that were labelled retrogradely from rhomboideus nerve bundles formed a single, long column in the ventral horn from C4 to C6, lateral to previously studied motor nuclei supply ing deep neck muscles. When different tracers were used to differentia te motoneurons supplying specific nerve bundles, discrete subnuclei co uld be identified that mere organized in a rostrocaudal sequence corre sponding to the rostrocaudal order of the nerve bundles. Levator scapu lae motoneurons formed a second elongate column immediately lateral to the rhomboideus motor nucleus. Three other muscles, the trapezius, st ernomastoideus, and cleidomastoideus, were supplied by cranial nerve X I. Labelled motoneurons from these muscles formed a single column from the spinomedullary junction to middle C6. Within this column, the thr ee motor nuclei supplying the sternomastoideus, cleidomastoideus, and trapezius were laminated mediolaterally. Sternomastoideus and cleidoma stoideus motoneurons were confined to upper cervical segments, whereas trapezius motoneurons were found from C1 to C6. In C1 and C6, the mot oneuron column was located centrally in the gray matter, but, between C2 and C5, the column lay on the lateral wall of the ventral horn in a position dorsolateral to motor nuclei supplying the rhomboideus and t he deeper neck muscles. The findings in this study suggest that descen ding and propriospinal systems responsible for coordinating head movem ent may have to descend as far caudally as C6 if they are to project o nto muscles controlling the mobility of the lower neck. (C) 1997 Wiley -Liss, Inc.