Fig. 3.

A schematic overview of the cerebellar circuit. The cerebellar Purkinje cells (PCs) organize the Purkinje cell layer in the cerebellar cortex and their axons form the unique output coming from the cerebellar cortex to the cerebellar nuclei. The PCs receive inhibitory and excitatory impulses through multiple synapses formed on their dendritic tree, which is located in the molecular layer. Climbing fibers (CF, in blue) originate from neurons in the inferior olive, climb to the cerebellum, and form between 250 and 1500 synapses upon a single PC dendritic tree, in a unique 1:1 ratio. Parallel fibers (PF, in pine) originate from granule cells in the granule cell layer of the cerebellar cortex and form a few synapses upon the distal dendritic tree of a single PC. Each PC dendritic tree forms as many PF–PC synapses as 200,000 parallel fibers contacting them. Mossy fibers (MF, in orange) originate from neurons in the spinal cord and brain stem and transport information from the periphery and cerebral cortex to PCs through granular cell axons forming PF. The pine and orange starbursts depict supposed sites of CF-mediated, associative forms of synaptic plasticity required for associative motor learning (LTD of PF inputs and LTP of MF inputs, respectively). These forms of plasticity may be impaired in SCA2. Modified from Smeets and Verbeek [8] and Meera et al. [164]