Cerebellar physiology – links between microcircuitry properties and sensorimotor functions

The cerebellar neuronal circuitry is here summarized as a gigantic associative memory, consisting of multiple functional subunits working in parallel, by which useful contingencies between diverse types of information can be stored. The sources of information are the multiple mossy fiber systems that inform the cerebellum about original motor commands/motor plans and sensorimotor information at different levels of abstraction. The output of each functional subunit of the cerebellum is refining the ongoing movement command by linking learnt appropriate sensorimotor functions for the particular context or state. Because of its size, the memory element can house the appropriate associations, translatable to synaptic weights, relevant for all contexts or states that the individual can be expected to experience during a lifetime. It follows that a key component necessary to further improve our understanding of cerebellar function is a precise knowledge of the types and formats of information that reaches the cerebellum via the vast variety of mossy fiber pathways, a subject area that has long been neglected. In parallel, a large part of the cerebellar circuitry is proposed to be devoted to regulating the activity balance in the circuitry. The set points of the balances are internal to the cerebellar neuronal circuitry but influence the excitability of the extracerebellar motor structures so that the drive on individual muscles, for example, is not exaggerated or fall below operative excitation level. The review explores the consistency of this novel interpretational framework with multiple diverse observations at the synaptic and microcircuitry level within the cerebellum.

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