Organization of corticopontocerebellar connections to the paramedian lobule in the cat.

To reveal the organization and relative magnitude of connections from various parts of the cerebral cortex to the cerebellar paramedian lobule via the pontine nuclei, horseradish peroxidase conjugated to wheat germ agglutinin was injected in the paramedian lobule in conjunction with injection of the same tracer in various parts of the cerebral cortex in 14 cats. Termination areas of cortical fibres (anterogradely labelled) and pontine neurons projecting to the paramedian lobule (retrogradely labelled) were carefully plotted in serial sections through the pons. On the average 89% of all labelled cells were found in the pontine nuclei contralateral to the cerebellar injection, 11% in the ipsilateral pontine nuclei. The highest degree of overlap between anterograde and retrograde labelling was found after injections in the posterior sigmoid gyrus (SmI), while less overlap was found after injections of the anterior sigmoid gyrus (MsI). Injections of the second somatosensory area (SmII) and the parietal association cortex (areas 5 and 7) gave moderate degrees of overlap. Very little or no overlap was found after injections of the premotor cortex (area 6), the visual areas 17, 18 and 19 and the auditory cortex (AI and AII). It is concluded that a major cortical input to the paramedian lobule arises in the posterior sigmoid gyrus (SmI), but that additional contributions arise in the anterior sigmoid gyrus (MsI), the parietal areas 5 and 7 and the second somatosensory cortex (SmII). Among the latter regions probably the parietal areas contribute most. Overlap between terminal regions of cortical fibres and cells projecting to the paramedian lobule takes place at numerous discrete sites at virtually all rostrocaudal levels of the pons. Cerebrocortical afferents via the pontine nuclei to the intermediate zone of the posterior lobe are organized according to the same principles as described previously for cortical afferents to the hemispheral parts of the posterior lobe (crus I and II).