Cells of origin of propriospinal connections to cat lumbosacral gray as determined with horseradish peroxidase

Propriospinal cells projecting to the lumbosacral spinal cord of cat were identified using the technique of retrograde transport of horseradish peroxidase (HRP). An injection technique was used to preferentially label cells via their terminals rather than through damaged axons of passage. Large volumes (1 to 5 μl) of 35% HRP were slowly ejected from a micropipet penetrating the cord dorsum midway between the dorsal root entry zone and the midline, while the pipet was slowly withdrawn from a depth of 1500 to 2000 μm below the cord surface. This technique resulted in diffusion of HRP throughout the gray matter on the side of the injection and, usually, sperad to the contralateral gray matter. HRP-labeled cells were observed from C1 to S3 after injections in segments L3-S3. Significant differences were seen in the rostrocaudal distributions of labeled cells after injections at different lumbosacral levels: specifically, L6-L7 injections resulted in less labeling of cells at cervical and high thoracic levels than injections that included segments L3-L5 or S1-S3.