Fig. 1

Basic connection scheme of the cerebellum. Granule cells (GC) receive afferent and efferent information from the mossy fibers (MF), and convey that information through their parallel fibers (PF). The PFs excite both Purkinje Cells (PC) and molecular layer interneurons (MLI); in turn the MLIs send axons to the PCs. Purkinje cells constitute the only output of the cerebellar cortex, and they send axons that form GABAergic inhibitory connections on the cells of the deep cerebellar nuclei (DCN). Neurons in the inferior olivary nucleus (IO) send axons known as climbing fibers (CF) which form thousands of synapses on PCs. Each PC receives excitation from a single CF. An action potential in a CF reliably causes an action potential in the PCs it innervates; these action potentials are known as complex spikes and are easily distinguishable from simple spikes, which are action potentials tonically generated by the PC, and modulated by the PFs. Not illustrated in the figure is the fact that the ascending axons (AA) of the granule cells can make multiple connections on the dendritic arbor of PCs [4], the orientation of PC dendritic arbors perpendicular to parallel fibers, or the organization in parasagittal modules [5]