Distribution of median, ulnar and radial motoneurons in the monkey spinal cord: a retrograde triple-labeling study.

Topographic distribution of motoneurons innervating hand muscles through the median (Mn), ulnar (Ul), or radial (Rd) nerves was examined using a retrograde multiple-labeling technique in the macaque monkey. The Mn and Ul motoneurons, i.e. flexor motoneurons, were distributed from C6 to T2 and from C7 to T2 segments of the spinal cord, respectively, while the Rd motoneurons, i.e. extensor motoneurons, were distributed from C4 to T2. The present study further revealed partial intermingling of the cell bodies and partial overlap of the dendritic fields among the motoneurons projecting through different nerves, indicating that subregions of motoneuronal pool participate in coordination between the flexor and extensor, or among the flexor muscles. It was suggested that there exists a control mechanism for precise hand movements in the spinal cord.