Cellular plasticity at the climbing fibre-Purkinje cell synapse as a model of plasticity in adulthood and ageing

In the adult brain there is a high degree of plasticity which is documented here for climbing fibre terminal arborization impinging upon the proximal Purkinje cell dendrites. Such an arborization presents a more distal compartment, including 90% of branches and varicosities, whose integrity depends on the target and a more proximal compartment which survives to target degeneration. The same terminal arborization enlarges its territory of innervation through collateral sprouting when nearby Purkinje cells are deprived of their climbing fibres or when embryonic Purkinje cells are placed on the surface of an intact cerebellum. In the latter case axonal elongation occurs in the absence of degeneration products and of macrophage/microglia or astrocyte activation. In addition, the embryonic Purkinje cells are able to invade the intact host molecular layer where they develop dendritic arbours which become also innervated by climbing fibre collaterals.