Motor circuits in action: specification, connectivity, and function.

Mammalian motor behavior is enabled by a hierarchy of interleaved circuit modules constructed by interneurons in the spinal cord, sensory feedback loops, and bilateral communication with supraspinal centers. Neuronal subpopulations are specified through a process of precisely timed neurogenesis, acquisition of transcriptional programs, and migration to spatially confined domains. Developmental and genetic programs instruct stereotyped and highly specific connectivity patterns, binding functionally distinct neuronal subpopulations into motor circuit modules at all hierarchical levels. Recent work demonstrates that spatial organization of motor circuits relates to precise connectivity patterns and that these patterns frequently correlate with specific behavioral functions of motor output. This Review highlights key examples of how developmental specification dictates organization of motor circuit connectivity and thereby controls movement.