Brainstem nucleus MdV mediates skilled forelimb motor tasks

Translating the behavioural output of the nervous system into movement involves interaction between brain and spinal cord. The brainstem provides an essential bridge between the two structures, but circuit-level organization and function of this intermediary system remain poorly understood. Here we use intersectional virus tracing and genetic strategies in mice to reveal a selective synaptic connectivity matrix between brainstem substructures and functionally distinct spinal motor neurons that regulate limb movement. The brainstem nucleus medullary reticular formation ventral part (MdV) stands out as specifically targeting subpopulations of forelimb-innervating motor neurons. Its glutamatergic premotor neurons receive synaptic input from key upper motor centres and are recruited during motor tasks. Selective neuronal ablation or silencing experiments reveal that MdV is critically important specifically for skilled motor behaviour, including accelerating rotarod and single-food-pellet reaching tasks. Our results indicate that distinct premotor brainstem nuclei access spinal subcircuits to mediate task-specific aspects of motor programs. The authors use a combination of viral tracing and genetics to characterize the diversity of neurons projecting from mouse brainstem to motor neurons that control limb movements; in particular they discover that the medullary reticular formation ventral part (MdV) is functionally specialized for skilled forelimb motor control. Brainstem circuits controlling precision movement Two papers published in this issue of Nature address a long-standing puzzle in mammalian motor control — the organization and function of circuits between the brain and spinal cord that control motor movements such as reaching. Thomas Jessell and colleagues investigate a class of mouse spinal interneurons known in other species to be involved in fine forelimb movements. They show that in mouse, these neurons have appropriate anatomical innervation to carry both motor commands and an internal copy signal, and ablation of these neurons impairs reaching movements. Furthermore, optogenetic activation of the ascending branch recruits a cerebellar circuit, and also disrupts reaching movements. These findings implicate these neurons as part of an internal copy pathway for rapid updating of motor output during reaching. Silvia Arber and colleagues use a combination of viral tracing and genetics to characterize the diversity of neurons projecting from mouse brai