Mapping corticospinal connections with spinal circuits

The majority of corticospinal projections in primates and all corticospinal projections in other mammals exert their influence by integrating into spinal interneuronal circuits. Interneurons therefore represent an evolutionarily conserved target through which the corticospinal tract achieves motor and sensory control. Here I describe the postnatal maturation of the corticospinal tract. I show work aimed at fractionating cortical projections by combining recently developed retrograde viral tracing strategies with genetic access to distinct spinal interneuron populations. Using a 3-D reconstruction pipeline, I discuss the cortical origins of the corticospinal pathway in mice. I then demonstrate the differential distribution of corticospinal inputs to three interneuron populations with divergent functional roles. These studies suggest that primary motor cortex contacts functionally diverse spinal interneurons whilst premotor and somatosensory cortical regions are more restricted in their postsynaptic targeting.