Figure 1.

Organization of MNs innervating the different muscle types. A, Schematic showing the location of the injection sites in fast muscles. B, Fast MNs are located in the dorsal part of the motor column and comprise pMNs and dorsal sMNs. Middle pMN (MiP) innervating the epaxial and caudal pMN (CaP) innervating the ventral aspect of the hypaxial muscle are labeled in these experiments. C, Site of injection of fluorescent dye in the slow and intermediate muscles at the level of the myoseptum. D, Slow and intermediate MNs are located in the ventral part of the motor column. E, Simultaneous dye injections into fast and slow/intermediate muscles. F, Fast MNs (green) are located in the dorsal part of the motor column and can be distinguished from the ventrally located slow/intermediate MNs (red). The dashed lines in B, D, and F delineate the dorsal and ventral aspects of the spinal cord.