Neuroscience Vol. 10, No. 3, pp. 621-638, 1983 0306-4522/83 $3.00 + 0.00 Printed in Great Britain Pergamon Press Ltd 0 1983IBRO zyxwvu PRINCIPLES OF ORGANIZATION OF THE CORTICOPONTOCEREBELLAR PROJECTION TO CRUS II IN THE CAT WITH PARTICULAR REFERENCE TO THE PARIETAL CORTICAL A REA S P. BRODAL Anatomical Institute, University of Oslo, Oslo, Norway Ah&me--In 13 cats injections of horseradish peroxidase-wheat germ agglutinin in various parts of the cerebra1 cortex were combined with injections in the cerebellar crus II in the same animal in order to study the cortical regions that may influence the crus II via the pontine nuclei. In 2 cats lesions in the cerebral cortex were combined with horseradish peroxidase injectionsin the crus II. In the pons terminal regions (anterogradely labelled from the cerebral cortex or containing terminal degeneration) and cell groups retrogradely labelled from crus II were carefully plotted. The pontocerebellar projection to crus II is mainly crossed, on the average 26% of the labelled cells were found in the ipsilateral pons. Some overlap between sites of ending of cortical fibres and sites of origin of fibres to crus II was present in all cases, but the degree of overlap varied considerably, depending on which cortical region was injected. Typically, partial overlap between terminal patches and groups of labelled cells occurred at multiple sites in the pontine nuclei. A major input to crus II appears to come from the parietal region. Experiments with bilateral cortical injections showed that the pontine projection from the parietal region is topographically organized in a precise mosaic pattern of adjacent but apparently non-overlapping patches of termination. Area 6 also has strong connections with crus II, while only very few of the corticopontine fibres from the sensorimotor region overlap with cell groups labelled from crus II. The second somatosensory area and the visual cortex both seem able to influence a small but significant proportion of cells projecting to crus II. In contrast to other cortical regions, the auditory cortex appears to send fibres mainly to cell groups projecting to the ipsilateral crus II. It is concluded that the input to crus II originates in wide areas of the cerebral cortex. Small subgroups of neurons projecting to crus II can be. differentiated on the basis of their cortical afferents. It appears likely that each subgroup receives fibres mainly or in some instances only from one cortical site. The corticopontocerebellar projection to crus II probably exhibits a high degree of spatial order providing a specific pattern of convergence and divergence in the cerebellar cortex, in agreement with recent physiological evidence from micromapping studies. The corticopontocerebellar pathway is a major afferent route to the cerebellum. Little is understood, however, of its basic principles of organization and the kind of information it carries. The two links of this pathway, the corticopontine and pontocerebellar projections, both exhibit complex patterns of div- ergence and convergence (see ref. 2, 14,34 and 38 for recent reviews). Thus, corticopontine fibres from even a very small part of the cortex diverge to terminate in numerous discrete “patches” in the pontine nuclei. Pontocerebellar fibres to a small part of the cerebellar cortex converge from numerous more or less discrete cell groups in the pontine nuclei. In order to elucidate further the principles of organization underlying these patterns, one needs, among other things, to visualize in the same individual the termination of corticopontine fibres and the location of pontine neurons projecting to the cerebellum. One approach, Abbreuiations: DAB, 3,3-diaminobenzidine; HRP, horse- radish peroxidase; HRF-WGA, horseradish peroxidase-wheat germ agglutinin conjugate; TMB, tetramethylbenzidine. tried in the first experiments of the present study, is to combine anterograde degeneration and retrograde transport of horseradish peroxidase’ (HRP). A sim- pler and more sensitive method is to use anterograde and retrograde transport of HRP or HRP-WGA after injections in the cerebra1 cortex and the cere- bellum in the same animal. This procedure has been used in the monkey” and in a study of the cortical projections to crus I in the cat.15 It appeared that crus I receives minor projections from wide areas of the cerebral cortex, a quantitatively dominant input com- ing from the parietal region. Fibres from one cortical region as a rule contact only a small fraction of the total population of neurons projecting to crus I. However, some overlap between anterograde and retrograde labelling is usually found at numerous discrete sites in the pontine nuclei. The results seem to be compatible with a mosaic pattern in the con- nections between the cerebra1 cortex and the cere- bellum, as indicated by physiological studies in the rat with micromapping methods4 The present study was undertaken to determine in comparable detail the cerebral connections via the 621