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 th e median (Mn), ulnar (UI), or radial (Rd) nerves was examined using a retro grade multiple-labeling technique in the macaque monkey. The Mn and UI moto neurons, i.e. flexor motoneurons, were distributed from C6 to T2 and from C 7 to T2 segments of the spinal cord, respectively, while the Rd motoneurons , i.e. extensor motoneurons, were distributed from C4 to T2. The present st udy further revealed partial intermingling of the cell bodies and partial o verlap of the dendritic fields among the motoneurons projecting through dif ferent nerves, indicating that subregions of motoneuronal pool participate in coordination between the flexor and extensor, or among the flexor muscle s. It was suggested that there exists a control mechanism for precise hand movements in the spinal cord. (C) 2001 Elsevier Science Ireland Ltd. All ri ghts reserved.