The anatomical organization of hindlimb motoneurons in the lumbar spinal cord of the frog, Rana catesbiana

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Abstract
The precise location of lumbar spinal motoneurons was studied in the bullfrog by observing the retrograde reaction following injury to nerves which innervate different hindlimb muscles. The results were confirmed and their usefulness for physiological studies was demonstrated by iontophoretically injecting dye through an intracellular recording micropipette into motoneurons which were identified by antidromic spike invasion. The effects of plexus variation upon the location of motoneuron pools was considered in order to standardize the results and make possible comparisons between different frogs.New determinations were made of frog hindlimb muscle function with reference to jumping in order to assess the possible functional significance of motoneuron organization. In general, motoneurons acting upon proximal joints are located rostrally to motoneurons acting upon distal joints and flexor motoneurons are located rostrally to extensors.The somatotopic significance of motoneuron organization was also assessed and it was found that within the lateral motoneuron column there are three groups of cells, each of which innervates different, but somatotopically related groups of muscles. This organization is quite similar to that found in cats. The developmental and functional implications of this similarity are considered.