Somatotopy of climbing fiber branching to the cerebellar cortex in cat.

The study was aimed at determining the distribution and incidence of the inferior olive axonal branching as related to the somatotopic organization of the climbing fiber system in cat. Multiple fluorescent tracing was used. In a first set of experiments, spectrally different fluorescent tracers were injected into somatotopically defined areas of the anterior lobe-pars intermedia and of the paramedian lobule. Retrogradely labeled cells were found to be localized in well-segregated face-forelimb or hindlimb olivary domains, within the various subdivisions. A large and consistent number of the olive cells were found to send axonal branches to face-forelimb or hindlimb regions of the anterior lobe-pars intermedia and of the paramedian lobule. It was also found that inferior olive branching is restricted to somatotopically corresponding areas of the two lobes. In a second set of experiments the same technique was used to check whether the inferior olive neurons which project to the forelimb areas of the anterior lobe-pars intermedia and the paramedian lobule also send collaterals to the face-forelimb areas of crus II (medial crus II). The data show that different sets of neurons projecting either to the face-forelimb areas of crus II and anterior lobe-pars intermedia or of crus II and paramedian lobule coexist within the face-forelimb related olivary domains with the set of neurons projecting to the face-forelimb areas of the anterior lobe-pars intermedia and paramedian lobule. Neurons branching to the 3 cerebellar regions were observed only occasionally. Therefore, at least for the face-forelimb-related cerebellar areas, the climbing fiber branching can be said to originate in sets of inferior olive neurons that connect different pairs of somatotopically homologous cerebellar cortical areas. The branching pattern is present in all the cortical zones studied, namely C1-C3, C2, D1 and D2, and appears to be a general feature of the olivocerebellar system.