The location of spinal projection neurons in the cerebellar nuclei (cerebellospinal tract neurons) of the cat. A study with the horseradish peroxidase technique.

The distribution of spinal projection neurons was studied in the cerebellar nuclei of the cat following injections of horseradish peroxidase (HRP) into the cervical, thoracic and lumbar cord. HRP-positive (labeled) neurons were found in the medial (fastigial) and the posterior interpositus nuclei on the side contralateral to the cervical injection, being most numberous in cases with injections between the C2 and the C3 segments. In the medial nucleus (M) labeled neurons were distributed in the central to the caudal portions, and there was a conspicuous group of labeled small neurons extending from the ventrolateral part to the intermediate zone between the M and the anterior interpositus nucleus. With an increasing number of medium-sized neurons, this neuronal group persisted caudally in a similar position, ventromedial to the posterior interpositus nucleus (IP). Labeled large neurons were seen in the medial third of the IP. In the two cases labeled neurons of medium and small sizes were equal in number, and the neurons of the IP constituted about 10% of the total number of the spinal projection neurons. The present study suggests that the neurons of the M and the IP, including those of the intermediate group located between the two, project the bulk of the crossed descending fibers as far caudally as the C2 and the C3 segments.