The distribution of motoneurons supplying hind limb muscles in the clawed toad, Xenopus laevis

The distribution of motoneurons in the lumbar spinal cord (spinal segments 8-10) of the clawed toad, Xenopus laevis, was studied with the horseradish peroxidase technique. In a total of 13 different hind limb muscles this tracer was applied in a slow-release gel. Motoneurons innervating a particular hind limb muscle were clustered in longitudinally arranged motor pools. Motor pools of different muscles did show considerable overlap both in the rostrocaudal and transverse plane. But, the various motor pools clearly show a somatotopic organization of motoneurons even in such a condensed lumbar spinal cord as in Xenopus laevis. Motoneurons innervating more distally positioned muscles are generally found in more caudal segments, while proximal muscles (with the exception of the m. adductor magnus) are supplied by motoneurons more or less throughout the lumbar enlargement. Flexor muscles usually are innervated by motoneurons situated ventrolaterally in the ventral horn, extensor muscles by dorsomedially found motoneurons. This pattern is particularly apparent for proximal (thigh) muscles, less so for more distal (shank and foot) muscles. The present data are in keeping with those obtained with the retrograde cell degeneration technique in ranid frogs and are consistent with observations in other tetrapods, although a more clear separation of motor pools is evident in "higher" vertebrates such as birds and mammals.