Presynaptic plasticity at cerebellar parallel fiber terminals.

The cerebellum plays a role in the control of sensorimotor functions and possibly also of higher cognitive processing. The granule cells, which are abundant and unique in their characteristic dendritic morphology, allow the cerebellum to combine the advantages of sparse coding with a high sensitivity for individual afferents at the input stage. Plastic changes in the granular layer circuitry may thus control instant transformation of inputs as well as long-term modifications so as to support procedural memory formation. Over recent decades, substantial research has been done to explore the mechanisms of postsynaptic changes that may sustain learning processes in the cerebellum, especially bidirectional plasticity at the parallel fiber to Purkinje cell synapse. In contrast, the presynaptic occurrence of synaptic plasticity has been relatively neglected. Here we review the current models of granular layer processing in the framework of cerebellar functioning with special emphasis on the presynaptic modulations of operations at the parallel fiber to Purkinje cell synapse. We argue that the wide range of possible mechanisms that can strengthen the parallel fiber to Purkinje cell synapse at the presynaptic level endows the cerebellar cortex with optimal computational capacities to potentiate both spatial and temporal cues that are relevant for fine-regulating memory formation.