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Sensory Processing by Single Cerebellar Granule Cells
Michael Häusser
Wolfson Institute for Biomedical Research & Dept. of Physiology, UCL, London, UK
Granule cells constitute the input layer of the cerebellar cortex, translating mossy fibre signals into parallel fibre input to Purkinje cells. I will discuss work which examines the input-output transformations performed by single granule cells during sensory processing using in vivo patch-clamp recordings. Despite receiving ongoing excitatory mossy fibre synaptic input, resting granule cell firing rates are kept low . This is due in part to a tonic GABAergic conductance, which dampens granule cell excitability and prevents the transformation of spontaneous mossy fibre inputs into granule cell action potentials. In contrast, sensory stimulation activates bursts of mossy fibre EPSCs that in turn evoke bursts of granule cell spikes. Our results lend support to the idea that the cerebellar granule cell layer implements an efficient sparse coding scheme by maximizing the signal-to-noise ratio for transfer of salient inputs. Bursts in granule cell axons are also optimal stimuli for activation and plasticity of synapses onto Purkinje cells, providing a link between stimulus encoding and memory storage in the cerebellum.