Spinocerebellar neurons and propriospinal neurons in the cervical spinal cord: a fluorescent double-labeling study in the rat and the cat.

In the cervical spinal cord of the rat and the cat, the distributions of spinocerebellar and of descending propriospinal neurons were investigated using the retrograde fluorescent double-labeling technique. Moreover, a search was made for the presence of neurons with both ascending spinocerebellar and descending propriospinal axoncollaterals. Diamidino Yellow Dihydrochloride (DY) was injected at T2, while True Blue (TB) (in rats) or Fast Blue (FB) (in cats) was injected in the cerebellum. The distributions of labeled neurons were very similar in the rat and the cat. DY-labeled propriospinal neurons, projecting to T2 or below, were most numerous in lamina I and laminae IV to VIII. In the rat, such neurons were also present in the lateral spinal nucleus (LSN). TB- or FB-labeled spinocerebellar neurons were concentrated in the central cervical nucleus (CCN) at C1-C4, in the central part of lamina VII at C5-T1, in the medial part of lamina VI and the adjoining dorsomedial part of lamina VII at C2/C3-T1, and in Clarke's column. They were also found in lamina V at C1 and C7-T1, and in lamina VIII at all levels. In both species only very few DY-TB/FB double-labeled neurons, representing neurons with branching axons, were observed; in C1-T1, only about 0.5% of all TB/FB-labeled spinocerebellar neurons and about 0.05% of all DY-labeled descending propriospinal neurons were double-labeled. The double-labeled neurons were all located centrally in lamina VII at C5-T1, but even in that area they constituted not more than 1.5% (rat) and 4% (cat) of the labeled spinocerebellar neurons. These findings indicate that, in the cervical cord of the rat and the cat, descending propriospinal neurons and spinocerebellar neurons are to a large extent separate populations.