Gracile, Cuneate, and Spinal Trigeminal Projections to Inferior Olive in Rat and Monkey

In the cat, somatosensory nuclei send substantial projections to the inferior olive, where they terminate in a somatotopic fashion. Although the organization of the cat inferior olive has been used to interpret data from other species, published data suggest this organization may not occur universally. The present study investigated whether the inferior olive in albino rats and cynomolgus monkeys receives the same brainstem somatosensory inputs, whether these inputs are organized somatotopically and, if so, how the organization compares with that in the cat. Projections from the gracile, cuneate and spinal trigeminal nuclei were labeled with wheat germ agglutinin conjugated to horseradish peroxidase or with biotinylated dextran. The results were compared with data from cats (Berkley and Hand [1978] J. Comp. Neurol. 180:253-264). In the rat and monkey, the gracile, cuneate and spinal trigeminal nuclei all project to the contralateral inferior olive, where each nucleus has a distinct preferred terminal field. As in the cat, projections to the medial accessory olive and caudal dorsal accessory olive did not terminate in a precisely organized fashion. Projections to the rostral dorsal accessory olive, however, formed a clear somatotopic map. These somatotopic maps differed from those in the cat in that input from the trigeminal nucleus was confined rostrally, so that the caudal end only received input from the gracile and cuneate nuclei. These data indicate that similar organizational principles characterize the somatosensory projections to the inferior olives of the three species. Nevertheless, distinct species differences occur with regard to the details of this organization.