Uncrossed and crossed projections from the upper cervical spinal cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing.

In the upper cervical spinal segments, neurons in the medial part of lamina VI give rise to uncrossed spinocerebellar axons, whereas the central cervical nucleus (CCN) and neurons in laminae VII and VIII give rise to crossed spinocerebellar axons. Using anterograde labeling with biotinylated dextran in the rat, we examined the projections of these neuronal groups to the cerebellar nuclei. Uncrossed and crossed projections were distinguished by cerebellar lesions placed on the side contralateral or ipsilateral to the tracer injections confined to the second and third cervical spinal segments (C2 and C3, respectively). Labeled terminals of uncrossed projections were seen in the middle, dorsal, and ventrolateral parts of the middle subdivision and in the ventral part of the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, terminals were seen in the middle of the mediolateral extent, whereas, in the posterior interpositus nucleus, they were seen in lateral and caudal parts. The terminals of crossed projections from the CCN were distributed ventrally in medial to ventrolateral parts of the middle subdivision of the medial nucleus. Some terminals were seen in the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, labeled terminals were seen mainly in rostromedial parts, whereas, in the posterior interpositus nucleus, they were seen in caudal and dorsal parts of the medial half. The present study suggests that the medial lamina VI group and the CCN in the upper cervical segments project to the different areas of the cerebellar nuclei and are concerned with different functions. Copyright 2001 Wiley-Liss, Inc.