Figure 6. Cholera toxin b (CTb) labeling in the spinal cord after intramuscular injections and CST-interneuron topographic relationship.

(A1–3) Examples of labeling with CTb subunit in one 40 µm section at level C7/C8, achieved after injection into either deltoids, biceps and the wrist extensor compartments. Matlab-generated heat maps of CTb-labeled spinal gray matter were produced from all sections for each muscle group at levels C5/C6 (B) and at levels C7/C8 (C). Heat maps show transganglionic labeling of the proprioceptive afferents in the dorsal and intermediate region of spinal cord (afferent; i.e. laminae 2–6) and retrograde labeling of the motoneurons in the ventral regions of the spinal cord grey matter (motor pools; i.e. laminae 9). Spinal overlap of motor cortex CST joint zones and proprioceptive afferent terminations are also shown in B and C. As in Figure 3, CST termination contours were set at 60% threshold (black contour) and 10% threshold (gray contour). The color bar demonstrates density of labeling as pixels per mm² for proprioceptive afferents (left axis) and cells per mm² for motor pools (right axis). Scale bar is 500 µm.