Exp Brain Res (1991) 87:141-152 Experimental BrainResearch 9 Springer-Verlag1991 A study of branching in the projection from the inferior olive to the x and lateral cl zones of the cat cerebellum using a combined electrophysiological and retrograde fluorescent double-labelling technique R. Apps 1, J.R. Trott ~, and E. Dietrichs 2 1 Department of Physiology, School of Medical Sciences, University of Bristol, University Walk, Bristol, BS8 ITD, UK 2 Department of Anatomy, Institute for Basic Medical Sciences, University of Oslo, N-0317 Oslo 3, Norway Received January 8, 1991 / Accepted May 23, 1991 Summary. The pattern of transverse branching in the olivocerebellar projection to the x zone in the vermis and the lateral ca zone in the paravermis of the cat anterior lobe was studied using a combined electrophysiological and retrograde double-labelling tracer technique. Fluorochrome-tagged latex microspheres were well suited for this purpose. The results show that the region of olive that supplies climbing fibres to the two zones forms a continuous, rostrocaudally directed column about 2.25 mm in length, in a caudo-lateral to rostro- medial part of the medial accessory olive (MAO), on average between A P levels 12.50-10.50. This column may be divided into caudal and rostral halves that project respectively to the x and lateral cl zones in the apical folia of lobules V/VIa. Partial overlap between these two ter- ritories occurs in an intermediate region (A-P levels 12.00-11.00) in middle MAO where olive cells that sup- ply climbing fibres to either x or lateral ct are intermin- gled with a smaller population of cells whose axons branch to provide climbing fibres to both zones. Quan- titative analysis showed that, when different tracers were injected into each zone in the same animal, double-label- led cells represented only 5-7% of either single-labelled cell population within this area of overlap. It is con- cluded that, although some transverse branching is present within the olivocerebellar projection to the x and lateral cl zones in the apical folia of lobule V, such branching is not extensive. Key words: Inferior olive - Cerebellum - Climbing fibres Branching - Fluorescent tracers Introduction It is now well-established that the cerebellar cortex in the cat is divisible into a number of narrow, longitudinal Offprint requests to : R. Apps zones each of which receives its climbing fibre afferents from cells in a restricted portion of the inferior olivary complex (for a review see Brodal and Kawamura 1980). Overall, eight zones have been identified on either side of the midline in the anterior lobe of the cerebellum: from medial to lateral these have been designated the a, x and b zones in the vermis, the ca, c2 and c3 zones in the paravermis and the dl and d2 zones further laterally (e.g. Groenewegen and Voogd 1977; Groenewegen et al. 1979). Further studies have shown that among these zones the c1 and c3 zones are each subdivisible into a medial and a lateral half (Ekerot and Larson 1982; Campbell and Armstrong 1985; Trott 1989). Anatomical studies have demonstrated that ol- ivocerebellar axons branch in the rostrocaudal plane (e.g. in the cat: Brodal et al. 1980; Rosina and Provini 1983, 1987 and in the rat: Wiklund et al. 1984; Wharton and Payne 1985; Payne et al. 1985; Hrycyshyn et al. 1989). Additional anatomical studies in the rabbit have shown that some branching also occurs in the mediolateral plane (Takeda and Maekawa 1984). However, none of these studies investigated the pattern of branching in the olivocerebellar projection to individual cortical zones. Anatomical studies at a higher level of resolution are required since physiological investigations have shown that olivocerebellar axons frequently branch in the ros- trocaudal plane so that an individual olivary neurone provides climbing fibres to several Purkinje cells located at different points along the length of a single cortical zone. This was initially established by using electrical stimuli to excite in one branch of the parent axon an action potential that propagated antidromically back to the branch point and then orthodromically along anoth- er branch to evoke a complex spike in the Purkinje cell innervated by that climbing fibre (Armstrong et al. 1973). Similarly, the axon-reflex technique has been used to show that some olivocerebellar axons also branch in the mediolateral plane so as to provide climbing fibres to two cortical zones which are thereby 'linked' into pairs. Such linking has been found between the x zone and the lateral