Fig. 1.

Locations of commissural interneuron cell bodies in the lumbar and cervical spinal cord. Commissural interneuron (CIN) cell bodies are shown on the left side of transverse sections and all injections were made on the right side. A: CINs labeled with the b subunit of cholera toxin (CTb) in L5 of the adult rat spinal cord. CTb was injected into the contralateral L3 intermediate gray matter. B: distribution of cells labeled as in A made from eight sections from L5. Adapted from Liu et al. (2010) with permission from Elsevier. Note the clusters of cells in lamina VIII, the medial intermediate gray matter and the lateral deep dorsal horn. C: transverse section of a C4 lumbar segment from a young mouse. Last-order premotor interneurons are labeled transsynaptically with a rabies virus that was injected into the right tibialis anterior muscle. Note the concentration of CINs in lamina VIII (demarcated by yellow dots; A. Bannatyne, B. S. Bhumbra, A. J. Todd, D. J. Maxwell, M. Beato, unpublished observations). D: plot of distribution of Fluorogold-labeled CINs in a single C4 section of an adult rat. Adapted from Mitchell et al. (2016). The black area on the right represents the injection site. E: cells in segment C4 labeled with a retrograde virus [Hiret-GFP (Kinoshita et al. 2012)] that was injected unilaterally into C7–C8 segment of an adult macaque. The area demarcated by the box is shown at higher magnification in F; note the labeled CINs in lamina VIII (A. Bannatyne, E. Bagdatlioglu, D. J. Maxwell, D. S. Soteropoulos, unpublished observations).