Distribution of neurons in the main cuneate nucleus projecting to the inferior olive in the cat. Evidence that they differ from those directly projecting to the cerebellum.

The distribution in the main cuneate nucleus of cells projecting to the inferior olive and the intermediate zone of the cerebellar anterior lobe were compared by means of double retrograde labeling methods in the cat. The tracer combinations were either Fast Blue and Diamidino Yellow Dihydrochloride; or horseradish peroxidase conjugated to wheat germ agglutinin and Diamidino Yellow Dihydrochloride. Neurons in the caudal, middle and rostral subdivisions of the main cuneate nucleus project to the inferior olive. Differences exist, however, in its number and location along the rostrocaudal extent of the nucleus. Cells projecting to the inferior olive predominate in the caudal and middle subdivisions, where they concentrate ventrally. No cells in the "clusters region" project to the inferior olive. Main cuneate nucleus neurons projecting to the cerebellum concentrate rostral to the obex, bordering the external cuneate nucleus and partially intermixing with the rostrally located cells projecting to the inferior olive. However, no double-labeled cells were found. The results indicate that the main cuneate nucleus projections to the inferior olive and cerebellar anterior lobe originate from different populations of neurons with high specific locations within the nucleus. This finding is in agreement with previous studies suggesting that each of the main cuneate nucleus targets receives its input from a distinct population of neurons within the nucleus.