Beyond parallel fiber LTD: the diversity of synaptic and nonsynaptic plasticity in the cerebellum

In recent years, it has become dear that motor learning, as revealed by ass ociative eyelid conditioning and adaptation of the vestibule-ocular reflex, contributes to the well-established cerebellar functions of sensorimotor i ntegration and control. Long-term depression of the parallel fiber-Purkinje cell synapse (which is often called 'cerebellar LTD') is a cellular phenom enon that has been suggested to underlie these forms of learning. However, it is clear that parallel fiber LTD, by itself, cannot account for all the properties of cerebellar motor learning. Here we review recent electrophysi ological experiments that have described a rich variety of use-dependent pl asticity in cerebellum, including long-term potentiation (LTP) and LTD of e xcitatory and inhibitory synapses, and persistent modulation of intrinsic n euronal excitability. Finally, using associative eyelid conditioning as an example, we propose some ideas about how these cellular phenomena might fun ction and interact to endow the cerebellar circuit with particular computat ional and mnemonic properties.