It is generally assumed that the main function of the corticospinal tract (CST) is to convey motor commands to bulbar or spinal motoneurons. Yet the CST has also been shown to modulate sensory signals at their entry point in the spinal cord through primary afferent depolarization (PAD). By sequentially investigating different routes of corticofugal pathways through electrophysiological recordings and an intersectional viral strategy, we here demonstrate that motor and sensory modulation commands in mice belong to segregated paths within the CST. Sensory modulation is executed exclusively by the CST via a population of lumbar interneurons located in the deep dorsal horn. In contrast, the cortex conveys the motor command via a relay in the upper spinal cord or supraspinal motor centers. At lumbar level, the main role of the CST is thus the modulation of sensory inputs, which is an essential component of the selective tuning of sensory feedback used to ensure well-coordinated and skilled movement.

States 14 15Throughout mammals evolution, the contribution of the corticospinal tract (CST) to motor control 16 progressively increases, culminating in primates with tight control of skilled movements such as 17 independent finger dexterity 1 . Although cortico-motoneuronal connections have been shown to be 18 absent or very rare in rodents 2-5 , it is generally assumed by analogy with primates that the main 19 function of the CST is to convey motor commands. Yet the CST has been shown to have other 20 functions 1 , such as sensory gating. By differentially filtering out sensory signals at their entry point 21 in the spinal cord 6,7 , sensory gating is thought to have evolved to increase the gain of relevant 22 feedback information during voluntary movement to improve execution of skilled movements 8-10 . 23Here we show that, unexpectedly, sensory gating is the essential function of the lumbar CST in 24 mice while lumbar motor command mainly involves non-CST pathways. By sequentially 25 investigating each stage of the corticofugal pathways for motor command and sensory gating, we 26 found that they both originate in the same cortical area but follow segregated paths. The hindlimb 27 motor command is mainly relayed through subcortical supraspinal motor centers, and is only 28 anecdotally encoded by the CST, requiring a propiospinal relay in the upper cord. In contrast, 29 corticospinal neurons are essential for sensory gating, and act directly through a population of 30 lumbar interneurons. Our results reveal that the original function of the CST in mammals, still 31 prominent in rodents, is sensory gating, not motor control. This pathway appears to have been 32 later selected through evolution to serve other functions, such as refined motor commands, 33 eventually yielding direct CST-to-motoneuron connection in primates. The rodent lumbar CST thus 34 serves as a prototypical paradigm to study, in isolation, the primary role of CST in sensory gating. 35 36The rodent sensorimotor cortex has a highly somatotopic organization 11 . Mainly studied in a motor 37 command perspective, it has been repeatedly mapped for its ability to provoke motor 38 contraction 11,12 , but never yet with regard to its ability to induce sensory gating. This cortically-39 evoked inhibition of sensory information transfer between primary afferents and the central nervous 40

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