Effective motor execution needs to accurately integrate proprioceptive sensory feedback to update the motor command centers about the outcome of the movements. The motor system can also generate an internal prediction of the planned actions to reduce delay. Previous studies have suggested that several cerebellar and cortical sites act as integration centers, where internal motor predictions can be made by converging sensory feedback and cortical corollary pathways. On page 1233 of this issue, Hantman and Jessell find that the convergence of the cortical command pathway and the proprioceptive sensory feedback pathway occurs even earlier, at Clarke's column in the spinal cord.

Clarke's column comprises dorsal spinocerebellar (dSC) tract neurons, which form a nucleus spanning thoracic and lumbar spinal cord and that relay proprioceptive sensory information from the hindlimb. Although dSC tract neurons are known to be active upon electrical stimulation of descending corticospinal tracts, the exact nature of corticospinal input and the interaction between corticospinal efferent activity and spinocerebellar afferent activity were unclear. Hantman and Jessell used genetic and anatomical tracers to map out dSC neurons and their inputs and outputs in the mouse and found that dSC tract neurons in Clarke's column receive both proprioceptive axonal projections from the dorsal root ganglion and descending corticospinal projections. To do this, they identified Clarke's column dSC neurons by their expression of glial cell-derived neurotrophic factor (Gdnf); this expression pattern distinguishes them from other spinocerebellar projections neurons. The authors then used the Gdnf promoter to create inducible mice that selectively expressed the fluorescent protein mGFP in their dSC neurons. Using this elegant genetic technique, the authors found that dSC spinocerebellar projections reach cerebellar lobules I, II, III and VIII (see image; mGFP-positive dSC projections terminating at the cerebellum are shown in green, vGlut1 immunostaining is shown in red and Neurotrace Nissl staining is shown in blue). The authors also measured the electrophysiological responses of dSC neurons on corticospinal or dorsal root ganglion stimulation and found that these neurons receive excitatory inputs from proprioceptive dorsal root ganglion projections and direct excitatory inputs from corticospinal axons and/or indirect cortically-evoked inhibitory inputs.

These findings suggest that dSC neurons in Clarke's column represent a spinal cord–level convergence site where descending motor corollary signals and ascending sensory feedback may be integrated, perhaps serving to fine-tune ascending proprioceptive feedback to the locomotor command center.