Project Summary The cerebellum is essential for coordinating motor behavior through rapid adjustments of ongoing movements. To refine movement, the cerebellum processes motor and sensory information, and transmits output that ultimately modulates motor neuron activity to ensure successful execution. The path through which the cerebellum can influence limb movement is through output circuits in the cerebellar nuclei (CN). Yet little is known about how CN circuits are organized and whether discrete pathways are dedicated to specific motor functions for limb control. The recent identification of a molecularly distinct subset of CN neurons that project to the cerebral cortex via the thalamus and can affect forelimb movement begins to reveal a broader neural subtype logic to cerebellar output. Further exploration indicates that there are also CN neurons that directly innervate the cervical spinal cord. These descending projections could provide a more direct route for influencing motor output, but the specific roles they have in movement execution and refinement remain poorly understood. Based on preliminary evidence, the major hypothesis of this proposal is that anatomically and molecularly distinct subsets of CN neurons have discrete contributions to forelimb motor control. Specifically, cerebellar-spinal circuits play a critical role in rapid online correction, and their functional output is needed to prevent forelimb ataxia. Three Aims will explore how spinal-projecting CN circuits differ from the more heavily studied CN circuits that project to the thalamus. Aim 1 defines specific subtypes of neurons within the cerebellar nuclei by delineating their input and output connectivity and molecular identities. The distinctions in efferent and afferent connectivity of spinal- and thalamus-projecting CN neurons will be defined using combinatorial genetic and viral circuit tracing tools in mice. In addition, molecular distinctions between these two classes of cerebellar output neurons will be identified using single nuclei RNA-sequencing and multiplexed in situ hybridization. Aim 2 sets out to establish the functional connectivity between these distinct CN subpopulations and forelimb muscles and examines how spinal- and thalamus-projecting cerebellar output pathways influence goal-directed forelimb movements. Selective optogenetic perturbation, electromyographic (EMG) recording, and high-resolution kinematic analysis will be used to determine how spinal- and thalamus-projecting CN neurons differentially affect muscle activity and online refinement of behavior. Aim 3 explores how the activity of discrete CN output pathways correlates with forelimb online correction and endpoint precision. This goal will be accomplished by recording from spinal- and thalamus- projecting CN populations and forelimb muscles during dexterous reaching behaviors, and by applying generalized linear models to determine if neural activity predicts EMG and kinematic movement features. By defining the organization of two major cerebellar output pathways and identifying the ways in which they influence dexterous movements, this work will provide insight into how diverse circuits differentially participate in motor control, and clarify how injury and disease of cerebellar circuits can lead to motor impairments in humans.

Project Narrative Given the central role that movements of the arms and hands play in everyday life, impairment of these functions by injury or disease is especially debilitating. This proposal aims to dissect the organization and function of neural circuits that coordinate and refine limb movement, providing greater insight into how the nervous system controls dexterous behaviors. A better understanding of the mammalian motor system will help to identify strategies for improved diagnosis and repair of motor circuit damage and dysfunction.