Somatotopic organization of motoneurons innervating the pronators, carpal and digital flexors and forepaw muscles in the dog: a retrograde horseradish peroxidase study.

Central distribution of motor neurons innervating the individual forearm and forepaw muscle through the median and ulnar nerves was studied in the dog using the retrograde horseradish peroxidase (HRP) method. All HRP-labeled cells were seen ipsilaterally in restricted parts within the dorsolateral (DL) and retrodorsolateral nucleus (rDL) of the ventral horn at levels from the cranial tip of C7 to the cranial third of T2. The results showed that motoneurons situated more cranially supply the more proximal muscles and those situated more caudally supply the more distal ones, and that the pronator motoneurons occupy longitudinally the central part of the DL; the carpal flexor motoneurons, the medial part of the DL; the digital flexor motoneurons, the dorsal central part of the DL and the lateral fourth of the rDL; and the forepaw muscle motoneurons, the medial three-fourths of the rDL. These somatotopic arrangements of each motor pool could be correlated with the location and action of the muscles.