The brainstem is a key centre in the control of body movements. Although the precise nature of brainstem cell types and circuits that are central to full-body locomotion are becoming known^1,2,3,4,5, efforts to understand the neuronal underpinnings of skilled forelimb movements have focused predominantly on supra-brainstem centres and the spinal cord^{6,7,8,9,10,11,12}. Here we define the logic of a functional map for skilled forelimb movements within the lateral rostral medulla (latRM) of the brainstem. Using in vivo electrophysiology in freely moving mice, we reveal a neuronal code with tuning of latRM populations to distinct forelimb actions. These include reaching and food handling, both of which are impaired by perturbation of excitatory latRM neurons. Through the combinatorial use of genetics and viral tracing, we demonstrate that excitatory latRM neurons segregate into distinct populations by axonal target, and act through the differential recruitment of intra-brainstem and spinal circuits. Investigating the behavioural potential of projection-stratified latRM populations, we find that the optogenetic stimulation of these populations can elicit diverse forelimb movements, with each behaviour stably expressed by individual mice. In summary, projection-stratified brainstem populations encode action phases and together serve as putative building blocks for regulating key features of complex forelimb movements, identifying substrates of the brainstem for skilled forelimb behaviours.

脑干是控制身体运动的关键中心。尽管对全身运动至关重要的脑干细胞类型和回路的确切性质越来越为人所知^1,2,3,4,5，但了解熟练前肢运动的神经元基础的努力主要集中在脑干上中心和脊髓^{6,7,8,9,10,11,12}. 在这里，我们定义了脑干外侧延髓 (latRM) 内熟练的前肢运动的功能图的逻辑。在自由移动的小鼠中使用体内电生理学，我们揭示了一个神经元代码，它可以将 latRM 种群调整为不同的前肢动作。这些包括伸手可及和食物处理，这两者都因兴奋性 latRM 神经元的扰动而受损。通过组合使用遗传学和病毒追踪，我们证明了兴奋性 latRM 神经元通过轴突靶标分离成不同的群体，并通过脑干内和脊髓回路的差异募集起作用。调查投影分层 latRM 种群的行为潜力，我们发现这些种群的光遗传学刺激可以引起不同的前肢运动，每种行为都由个体小鼠稳定表达。总之，投射分层的脑干群体对动作阶段进行编码，并共同作为调节复杂前肢运动的关键特征的假定构件，识别脑干基质以进行熟练的前肢行为。