Olivary afferents from the raphe nuclei as studied with retrograde transport of horseradish peroxidase.

The afferent projection to the olive from the raphe nuclei in the cat has been studied in animals where microinjections of horseradish peroxidase have been made into the inferior olive from a ventral approach. Retrogradely labelled cells were present in the three caudal most raphe nuclei: the nucleus raphe pallidus, obscurus and magnus. The retrogradely labelled cells are of all sizes, and there appears to be a higher concentration of cells on the side ipsilateral to the injection. The observations demonstrate that all three major subdivisions of the olive--the medial accessory, the dorsal accessory and the principal olive--all receive efferent raphe fibres. No retrogradely labelled cells were present in the other raphe nuclei. The findings are discussed and related to other tracer studies dealing with olivary afferents from the raphe complex.