Networking brainstem and basal ganglia circuits for movement

The execution and learning of diverse movements involve neuronal networks distributed throughout the nervous system. The brainstem and basal ganglia are key for processing motor information. Both harbour functionally specialized populations stratified on the basis of axonal projections, synaptic inputs and gene expression, revealing a correspondence between circuit anatomy and function at a high level of granularity. Neuronal populations within both structures form multistep processing chains dedicated to the execution of specific movements; however, the connectivity and communication between these two structures is only just beginning to be revealed. The brainstem and basal ganglia are also embedded into wider networks and into systems-level loops. Important networking components include broadcasting neurons in the cortex, cerebellar output neurons and midbrain dopaminergic neurons. Action-specific circuits can be enhanced, vetoed, work in synergy or competition with others, or undergo plasticity to allow adaptive behaviour. We propose that this highly specific organization of circuits in the motor system is a core ingredient for supporting behavioural specificity, and at the same time for providing an adequate substrate for behavioural flexibility. In this Review, Arber and Costa discuss the anatomical and functional specificity of circuitry essential for executing diverse body movements. They focus on specific neuronal populations in the brainstem and the basal ganglia, and the integration of these circuits into systems-level networks that afford flexibility and learning.