Projections to the inferior olive of the cat. I. Comparisons of input from the dorsal column nuclei, the lateral cervical nucleus, the spino-olivary pathways, the cerebral cortex and the cerebellum.

The present experiments compare the projections from the dorsal column nuclei (DCN), the lateral cervical nucleus (LCN), the spino-olivary pathways (SO), the motor cortex (Ms1) and the cerebellum (CB) to the inferior olive of the cat. A differential labeling strategy was used for these comparisons. It was found that projections from the contralateral DCN and LCN and the ipsilateral SO overlap extensively with each other in the dorsal accessory division of the inferior olive and the caudal half of the medial accessory olive. Projections from the contralateral motor cortex partially overlap these somato-sensory projections but they also extend into the principal division and into the rostral half of the medial accessory olive where other authors have found input from other "motor" sources such as the red n., the globus pallidus and the caudate n. The contralateral cerebellum projects heavily to most of the inferior olive except for a few regions in the caudal portions of the two accessory nuclei. These results show that there is a partial segregation between somatosensory and motor inputs within the inferior olive and that the cerebellar feedback fibers appear to avoid at least parts of those regions where the somatosensory input is heavy. Some of this segregation, particularly within the medial accessory olive and the principal n., correlates with differences in both the olivo-cerebellar connectivity and the cytoarchitecture of these regions. Although the somatosensory and motor inputs are partially segregated, there are also several regions where these inputs overlap. These regions are: (1) an area in the medial half of the rostral portions of the dorsal accessory olive, and (2) a small oval zone in the middle of the caudal half of the medial accessory olive. This overlap provides an anatomical basis for electrophysiological evidence which demonstrates the existence of cells in some of these regions that respond to activation of both the cerebral cortex and the spinal cord (e.g., Crill and Kennedy, '67).