Redefining cerebellar ataxia in degenerative ataxias: lessons from recent research on cerebellar systems

Recent advances in our understanding of neurophysiological functions in the cerebellar system have revealed that each region involved in degenerative ataxias contributes differently. To regulate voluntary movements, the cerebellum forms internal models within its neural circuits that mimic the behaviour of the sensorimotor system and objects in the external environment. The cerebellum forms two different internal models: forward and inverse. The forward model is formed by efference copy signals conveyed by the corticopontocerebellar system, and it derives the estimated consequences for action. The inverse model describes sequences of motor commands to accomplish an aim. During motor learning, we improve internal models by comparing the estimated consequence of an action from the forward model with the actual consequence of the action produced by the inverse model. The functions of the cerebellum encompass the formation, storage and selection of internal models. Considering the neurophysiological properties of the cerebellar system, we have classified degenerative ataxias into four types depending on which system is involved: Purkinje cells, the corticopontocerebellar system, the spinocerebellar system and the cerebellar deep nuclei. With regard to their respective contributions to the internal models, we speculate that loss of Purkinje cells leads to malformation of the internal models, whereas disturbance of the afferent system, corticopontocerebellar system or spinocerebellar system leads to mis-selection of the proper internal model. An understanding of the pathophysiological properties of ataxias in each degenerative ataxia enables the development of new methods to evaluate ataxias.