Mapping of dI3 neuron sensorimotor circuits across the cervical and lumbar spinal cord

Abstract

From the fine control of hand movements to the dynamic corrective adjustments during locomotion, spinal circuits integrate descending supraspinal and sensory inputs to modulate diverse motor functions. The integration of such a wide range of signals across the spinal cord is primarily mediated by propriospinal interneurons. In this study, we investigate the connectivity of a population of propriospinal interneurons marked by the expression of Isl1, called dI3 neurons. These dI3s integrate supraspinal and sensory signals to facilitate many important functions, such as hand grasp, locomotion, and motor recovery after spinal cord injury; however, we have a limited understanding of how subpopulations of dI3s modulate network activity across the spinal cord to contribute to these behaviors. Their functional connectivity to motor circuits across the cervical and lumbar spinal cord was assessed through optogenetic activation of dI3s localized in different spinal segments. Our data demonstrates that cervical and lumbar dI3 subpopulations can form local, commissural, intersegmental, and long propriospinal pathways. Furthermore, dI3 subpopulations can be tonically stimulated to elicit locomotor activity. These extensive projection patterns of dI3s across the cervical and lumbar spinal cord suggest that dI3 subpopulations can modulate the activity of multiple motor networks within their respective spinal cord segments or across distant forelimb and hindlimb segments to facilitate a wide variety of motor functions.