Cell type and circuit modules in the spinal cord

•Cell type-specific perturbations in the spinal cord reveal modular units of sensorimotor computation and behavior.•Transcriptional profiling unmasks molecular diversity in spinal cord interneurons across the dorsoventral, mediolateral, and rostrocaudal axes.•Motor neuron subtypes map onto a musculotopic plan, whereas interneuron subtypes are defined more abstractly by inputs, outputs, and physiology. The spinal cord contains an extraordinarily diverse population of interconnected neurons to process somatosensory information and execute movement. Studies of the embryonic spinal cord have elucidated basic principles underlying the specification of spinal cord neurons, while adult and postnatal studies have provided insight into cell type function and circuitry. However, the overarching principles that bridge molecularly defined subtypes with their connectivity, physiology, and function remain unclear. This review consolidates recent work in spinal neuron characterization, examining how molecular and spatial features of individual spinal neuron types relate to the reference points of connectivity and function. This review will focus on how spinal neuron subtypes are organized to control movement in the mouse.