Origin of spinal projections to the anterior and posterior lobes of the rat cerebellum.

The present study was carried out to analyze the topography of spinal projections to the anterior and posterior lobes of the cerebellum and to investigate whether projections to the two lobes come from different spinocerebellar neurons or from branching axons of the same cells. We used orthograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) to identify the cerebellar areas where spinocerebellar axons terminate and retrograde double-labeling techniques to estimate the incidence of spinocerebellar neurons projecting to both anterior and posterior lobes via axon collaterals. Orthograde labeling confirmed that the rat, like other mammalian species, has spinocerebellar projections to two different regions of cerebellar cortex, i.e., lobules I-V of the anterior lobe and lobule VIII of the posterior lobe, with the highest incidence in lobules II, III, and VIII. We did not observe a clear difference in the distribution of afferents coming from different spinal segments to either of the two lobes. The double-labeled cells were located primarily in the lower thoracic and upper lumbar segments, almost exclusively in Clarke's column and in the dorso-lateral part of lamina 7 (in the region of the spinal border cells). It is likely that most or all of the spinocerebellar neurons in these structures project to both anterior and posterior lobes. Therefore, the two lobes of the cerebellum are likely to receive common information from these cells, but different information from the separate populations of spinocerebellar neurons that project only to one lobe or the other.