Intrinsic spinal cord circuits compare sensory inputs and efference copies to correct for perturbations to ongoing movement

Successful movement requires continuous adjustments in response to changes in internal and external environments. To do so, neural circuits continuously compare efference copies of motor commands with sensory feedback to respond to sensory prediction errors. Some responses need to be very fast, indicating that they likely occur within spinal cord circuits. Here, we describe spinal cord circuits involving dI3 neurons, showing that they receive multimodal sensory inputs and direct efferent copies from both Renshaw cells and motor neurons. We further show that they form connections to motor pools. Reducing dI3 neuronal activity diminished stumbling responses, as did disrupting Renshaw cell circuits, providing evidence for a comparator role of dI3 neurons for online corrections. Together, our findings reveal a pivotal role for dI3 neurons functioning as comparators of efference copies and external sensory feedback to mediate rapid corrections of ongoing movements.