The distribution of motoneurons innervating the flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU) muscles was studied utilizing retrograde labeling with horseradish peroxidase (HRP). The motor nuclei showed considerable differences in their longitudinal extents. The FCR nucleus occupied spinal segments C6–T1 while FCU ranged from the C7–C8 junction to T1. Results with localized i.m. injection of HRP suggested a somatotopic distribution of these motoneurons.
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