Anatomical studies on the cuneocerebellar neurons in the gerbil by using HRP method

The projection from the external cuneate nucleus (ECN) to the cerebellum was studied in the gerbil following the retrograde transport of minute injections (0.05-0.1 microliter, 30% solution) or implantations of horseradish peroxidase (HRP) in various folia of the cerebellar cortex and deep nuclei. The morphology of the labelled neurons, as well as their quantitative distribution, was also examined. The projections were in general bilateral but predominantly ipsilateral. A remarkable finding, however, was the predominant contralateral projection from the medial cell column of ECN representing the paw region. Topographically, the nuclear areas receiving the thoracic, shoulder and neck muscle afferents projected bilaterally to vermis lobules I-V and VIII-IX, lobulus simplex, crura Ia and Ib, paraflocculus and flocculus, and contralaterally to crus II and nucleus interpositus; the nuclear regions receiving the forelimb muscle afferents sent fibers bilaterally to vermis lobules V-VIII, paramedian lobule and copula pyramidis, and ipsilaterally to crus II and nucleus interpositus. Based on their somal size, three classes of neurons were distinguished: small, medium and large cells. The small and medium cells constituted 79 and 20% of the population of the labelled cells respectively, whereas the large cells were only occasionally identified. In the cell columns of the caudal ECN representing the forelimb (Lan et al. 1994), most labelled neurons were medium cells which made up 65% of the total labelled medium cells. In contrast, the majority (90%) of the labelled cells in the rostral ECN were of the small-sized variety. The functional significance of the segregation, both in the projection patterns and sizes of somata of the cuneocerebellar neurons is discussed.