Spinal neurons projecting to anterior or posterior cerebellum in the pigeon.

Spinal afferent fibers have been shown to project both to lobules III-VI and lobule IX of the cerebellum in the pigeon. In the present investigation the cells of origin of these projections and the course of the axons at spinal levels have been studied by the retrograde transport of fluorescent dyes injected into both parts of the cerebellum. In the upper cervical segments labeled neurons are located predominantly in the ventral horn; the axons cross to the contralateral side. In the cervical enlargement labeled neurons concentrate in the avian cervical Clarke's column (ClC) and in cervical "spinal border cells" (SBC). The axons of ClC neurons project ipsilaterally into the dorsolateral funiculus and SBC project ipsilaterally into the ventrolateral funiculus. In caudal cervical and in thoracic segments dorsal horn neurons (laminae IV/V) are at the origin of an ipsilateral spinocerebellar pathway in the dorsalmost part of the lateral funiculus. In the lumbosacral enlargement there are mainly three spinocerebellar cell groups all of which project contralaterally into the ventral funiculus: ClC, SBC and paragriseal cells. During its ascent this pathway shifts to the lateral funiculus. In addition there is a crossed pathway from ventral horn cells throughout the spinal cord. Whereas approximately equal numbers of dorsal horn cells project to lobules III-VI and to lobule IX, the number of ClC neurons is strongly reduced after lobule IX injections and SBC neurons are nearly absent. Altogether lobule IX has a substantial input from dorsal horn neurons (cutaneous mechanoreception) whereas that to lobules III-VI is dominated by ClC and SBC (proprioreception).