The organisation of the olivocerebellar projection to lobules VI, VIII, and IX of the posterior lobe of the rat cerebellum was investigated in detail by using the retrograde tracer wheat germ agglutinin‐horseradish peroxidase. Small, well‐defined rostro‐caudally orientated columns of olive cells were found to project to different parasagittal areas in the posterior lobe. A column of olive cells about 2,000 μm in rostro‐caudal length in subnucleus “c” and nucleus β of the caudal medial accessory olive (MAO) provides climbing fibre input to the most medial part of lobules VI and IX, but this projection is displaced laterally in lobule VIII by a projection from a column of cells about 600 μm in rostro‐caudal length in lateral caudal MAO (subnucleus “a”). It is possible that each of these columns of olivary neurones may be further subdivided in the rostro‐caudal axis so that different sections project to different medio‐lateral parts of the cortex. A fine‐grain ‘sublobular’ localisation may also exist: the projection to midline lobule VIc arises at caudal levels of the olive from a band of cells in the transition region between subnucleus “c” and nucleus β, whilst by comparison the projection from caudal levels of the olive to lobules VIa and VIb arises from cells located more ventrally in nucleus β. Evidence is also presented to confirm that the posterior lobe vermis in the rat extends further laterally than in other mammals and that part of it receives a projection from a column of olive cells, 1,000 μm in rostro‐caudal length, in a newly defined region of caudal MAO, termed subnucleus “b1”. Copyright © 1990 Wiley‐Liss, Inc.
