Preview: https://www.cell.com/cell-reports/fulltext/S2211-1247(19)30664-3

Our daily actions are precise and well-coordinated. This aspect of movement is often considered to be mediated by the cerebellum. In humans and non-human primates, the cerebellum affects movements indirectly, through its impact on the motor cortex. However, the way in which cerebellar signals regulate motor cortical activity and subsequently motor fluency is still unclear. In this study we addressed this question by specifically and reversibly blocking the cerebellar outflow of information. This manipulation replicated the symptoms of cerebellar ataxia: it produced slow and uncoordinated movements, and decomposed multi-joint movements into fractionated actions. As soon as the stimulation ceased, the motor behavior returned to normal. The behavioral impairments were preceded by altered brain activity. Our findings suggest that the cerebellum controls the temporal but not the spatial properties of motor cortical neurons, leading to inefficient and uncoordinated movements observed in ataxia.