A Functional Topographic Map for Spinal Sensorimotor Reflexes.

Cutaneous somatosensory modalities play pivotal roles in generating a wide range of sensorimotor behaviors, including protective and corrective reflexes that dynamically adapt ongoing movement and posture. How interneurons (INs) in the dorsal horn encode these modalities and transform them into stimulus-appropriate motor behaviors is not known. Here, we use an intersectional genetic approach to functionally assess the contribution that eight classes of dorsal excitatory INs make to sensorimotor reflex responses. We demonstrate that the dorsal horn is organized into spatially restricted excitatory modules composed of molecularly heterogeneous cell types. Laminae I/II INs drive chemical itch-induced scratching, laminae II/III INs generate paw withdrawal movements, and laminae III/IV INs modulate dynamic corrective reflexes. These data reveal a key principle in spinal somatosensory processing, namely, sensorimotor reflexes are driven by the differential spatial recruitment of excitatory neurons.