Cerebellar afferents from the paramedian reticular nucleus studied with retrograde transport of horseradish peroxidase.

Details in the cerebellar projections from the paramedian reticular nucleus (PRN) were studied in cats and monkeys by means of retrograde axonal transport of horseradish peroxidase (HRP). In the cat the majority of the fibres projects to the anterior lobe and to the vermis of the posterior lobe (with the exception of lobules VIIB and VIIIA). A less conspicuous projection was found to the lobulus simplex, the crura and the flocculus. The cerebellar nuclei, the paramedian lobule and the paraflocculus appear to be weakly connected with the PRN. A similar distribution of the cerebellar afferent fibres was found in the monkey material. The three subgroups of the PRN in the cat are not equal in their projection. The dorsal group appears to be connected with the greater part of the cerebellar cortex and with all nuclei. The ventral group lacks a connection with lobulus IX, the flocculus and the paraflocculus, and the accessory group appears to have its strongest connection with lobulus I (lingula), the flocculus and the vermal lobules VII-X. The findings are discussed in relation to other studies on the efferent and afferent connections of the nucleus.