Kralj-Hans, Ines; (1999) Cortico-ponto-cerebellar pathway in rats. An anatomical study of the somatosensory cortical input to the cerebellum. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Cortico-ponto-cerebellar pathway is one of the major subcortical pathways linking sensory cortical areas with distant motor regions of the brain. In this thesis I focus on the projections from the primary somatosensory cortex in order to understand the principles in the organisation of this pathway in rats. Primary somatosensory cortex in rats contains aggregates of granule cells in layer IV, named "barrels" by Woolsey and Van der Loos (1970). There are about 32 whiskers and an equal number of large cortical barrels within the posteromedial barrel subfield (PMBSF) in the primary somatosensory cortex. In addition to its cortical representation the peripheral organisation of the whiskers is replicated in other stations within the afferent pathway, the trigeminal nuclei and the thalamus. This organisation is easily identified both anatomically and physiologically. Therefore rat somatosensory system can serve as a model system to study sensory-motor integration. I mapped terminals from small regions of the primary somatosensory cortex of rats (within and surrounding the PMBSF) in the pontine nuclei using the anterograde tracers Phaseolus vulgaris leucoagglutinin, biocytin and biotinilated dextran-amine. The results demonstrate that cells in layer Vb of all the cortical barrels project to the ipsilateral pons. Most barrel columns terminate within the same set of pontine nuclei with slight medio-lateral shifts in their termination zones. The most rostral, smaller barrel columns and the cortical regions representing intervibrissal fur project bilaterally. This dichotomy is not present in corticotectal projections. Both small and large barrel columns as well as the less granular zones of the somatosensory cortex project to the deep layers of the ipsilateral superior colliculus. Retrograde tracing with fluorescent latex beads demonstrated that at least 70% of layer Vb cells in the primary somatosensory cortex in rats send collaterals to the deep layers of the superior colliculus and the pontine nuclei. Pontine nuclei project as mossy fibres onto the cerebellar cortex. These projections are predominantly contralateral but there are also ipsilateral mossy fibre terminals. Some pontine cells in cats are known to send axons that collateralise within the cerebellar white matter and supply the ipsilateral as well as the contralateral cerebellar cortex (Rosina and Provini, 1982). The experiments in the third chapter were designed to analyse whether and to what extent the axons of the pontine cells in rats, which receive somatosensory cortical inputs related to the whiskers, bifurcate. Whisker sensitive patches in lobules VII, IX and Crus I of the cerebellar cortex were injected with differently coloured fluorescent beads. Both sides of pontine nuclei contained large numbers of retrogradely labelled cells, a small percentage of which was double labelled.

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