Cerebello-thalamic synapses and motor adaptation.

The cerebellum's influence on voluntary movement is mediated, in large part, through the cerebello-thalamo-cortical (CTC) pathway. Of particular relevance here are those neurons in the cerebellar nuclei that project, via thalamus, to pyramidal tract neurons in primary motor cortex. Several lines of evidence implicate cerebello-thalamic (CT) synaptic plasticity as a neural substrate underlying movement adaptation in adult animals. CT synapses exhibit a number of structural characteristics suggestive of a capacity for both formation of new synapses, and alterations in efficacy of transmission across existing synapses. Long-term potentiation can be evoked across CT synapses in vitro by high frequency stimulation, albeit in young animals. Evidence regarding the contribution of CT synaptic plasticity to two different types of movement adaptation in adult animals is conflicting. Adaptation involving a strengthening and re-coordination of voluntary movement is associated with an increase in density of CT synaptic boutons and an increase in number of synaptic vesicles available for immediate neurotransmitter release within each bouton. On the other hand, adaptation involving associative conditioning of a reduced sensorimotor neural circuit is associated with plasticity at thalamo-cortical but not CT synapses. These conflicting findings may reflect differences in the extent of synaptic re-organization that occurs at thalamic versus cortical levels, differences in the neural circuitry mediating each behavior, and/or differences in the spatio-temporal convergence of activity in the thalamus during the adaptive processes. It is concluded that CT synaptic plasticity can underlie movement adaptation if the adaptation requires reorganization of the cerebellum's influence on cerebral cortex.