Supplementary Figure 4: MSE neurons target functionally related motor pools.

Individual MSE neurons that directly contact two motor pools were identified using RabΔG:GFP that was injected into the medial gastrocnemius and RabΔG:Cherry was injected into the lateral gastrocnemius or quadriceps at P0. Spinal cords were analyzed at P8. The locations (a-c) and high magnification views (d-f) of individual cells that were directly pre-synaptic to the medial gastrocnemius muscle (green) and either the lateral gastrocnemius (red) (a,d) or the quadriceps (red) (b,c,e,f) are shown, as projected confocal images. Double pre-motor MSE are yellow. We found that 13/389 (n=6 cords) pre-medial gastrocnemius neurons were yellow following lateral gastrocnemius or quadriceps injections. We did not observe any yellow cells following injections of the antagonistic pair of medial gastrocnemius and tibialis anterior (0/278), suggesting that yellow double premotor neurons are specifically associated with functionally co-recruited muscles. Scale bars are 250 μm in a-c and 25 μm in d-f.