Mesencephalic projections to the first cervical segment in the cat.

Mesencephalic neurons projecting to the upper cervical spinal cord were examined by mapping the distributions of labeled cells after injecting fluorescent tracers or wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the C1 segment. Injections into the central or deep regions of the ventral horn produced retrograde labeling in cells of several mesencephalic regions. The majority of cells were found contralaterally in the superior colliculus and red nucleus, and ipsilaterally in and around the interstitial nucleus of Cajal (INC), in the cuneiform region, and in the fields of Forel. Smaller numbers of cells were located in the periaqueductal gray matter, nucleus annularis, and magnocellular nucleus of the posterior commissure. Dorsomedial injections in the ventral horn near the ventral commissure labeled only a subset of these projections, including cells in the mesencephalic reticular formation adjacent to the INC and in the nucleus annularis. Dorsolateral injections labeled some cells in the superior colliculus and were particularly effective at labeling cells in the red nucleus. These results suggest that at least ten different cell groups project to the ventral horn of the first cervical segment. Most, but not all, groups originate from regions implicated previously in the control of eye or head movements.