On the diffusion of horseradish peroxidase into muscles and the "spurious" labeling of motoneurons.

There is some evidence to suggest that the fascial sheaths of muscles are important in preventing the labeling of motoneurons that occurs, apparently, as a result of the diffusion of horseradish peroxidase (HRP) into the muscles. To test this possibility, Gelfoam soaked in HRP was implanted over flexor and extensor muscles in the proximal forelimb of the rat. When the fascial sheaths were damaged, labeled neurons were found in the motoneuronal pools of the exposed muscles; if intact, virtually no labeling of motoneurons was observed. These results suggest that, if intramuscular injections are to be used as a method for identifying motoneuronal pools, care should be taken when exposing the muscle to be injected, to ensure that surrounding muscles and their fascial sheaths are not damaged.