Cerebellar Cortical and Corticonuclear Circuitry (Continued)

The two major neurotransmitters in the cerebellum are glutamate and γ-aminobutyric acid (GABA). Glutamate is excitatory and is found in the MFs, PFs, CFs, UBCs, and deep cerebellar nuclear neurons that project to thalamus, brainstem, and cerebellar cortex. GABA is inhibitory and is utilized by the PCs, all remaining cerebellar interneurons (stellate, basket, Golgi, Lugaro), and the DCN neurons that project to the inferior olive. Glycine is present in inhibitory interneurons in the DCN. A number of other peptide neurotransmitters are present also in the afferent fibers and neurons of the cerebellar cortex.

The PC generates two different classes of action potentials in response to its principal afferents. The input of hundreds of MFs produces a brief burst of repetitive simple spikes, 50 to 150 per second. The inhibitory basket and stellate cell interneurons produce inhibition of PCs locally and for some distance lateral to the longitudinal strip of active parallel fibers. Therefore MF-induced PC activity consists of a brief burst of action potentials along the course of active parallel fibers, surrounded by a band of inhibited cells. PC excitation is further restricted by Golgi cells, which receive excitatory PF synapses on their apical dendrites and provide a mostly tonic inhibitory input to the glomerulus, decreasing the excitability of granule cells to MF afferents. PF input is thought to provide information about incoming signals, such as direction and speed of limb movement. In the cognitive domain, the PF may provide the PC with the context in which behaviors occur.

Climbing fiber input to the PC induces a complex spike with the very low frequency of 0.5 to 2 spikes per second, the same rate of firing as the olivary neurons from which the CF originates. The CF input to the PC is thought to signal the occurrence of errors. The MF-CF inputs to the PC are relevant to synaptic plasticity involved in learning and memory. Long-term depression (LTD) is characterized by the persistent depression of synaptic transmission from PFs to the PC that occurs when parallel and climbing fiber activation are concurrent. Long-term potentiation (LTP) has also been described. The balance of LTD and LTP enables the cerebellar cortex to adapt to errors by regulating cortical output either down or up.

These corticonuclear circuits and physiology are the basis of theories that the cerebellum functions as an adaptive filter, utilizing internal models to maintain behaviors around a homeostatic baseline, and optimizes cerebellar influence upon motor, cognitive, or limbic behaviors appropriate to the prevailing context. The paracrystalline structure of cerebellar cortical architecture and organization has led to the idea that it has a general signal-transforming ability, a universal cerebellar transform, which is applied to multiple domains of neurologic function. The role of the cerebellum in the nervous system is a result, then, of the combination of the uniform cerebellar structure and function and the complex and varied connections of the cerebellar microcircuits, with extracerebellar areas conveyed by the mossy and climbing fiber inputs and the corticonuclear outputs.

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