Re-innervation of axolotl limbs - I. Motor nerves

The motor innervation of the hindlimb by the segmental nerves of the lumbar plexus was determined by observation of muscle contraction and leg movement following nerve stimulation and by mapping the location of junction potentials recorded by an intracellular microelectrode. There is variation between animals in the composition of the plexus and in the muscles they supply in the limb but in any one animal the motor innervation of the two limbs is symmetrical. After severing the two main nerves to one limb and re-directing each nerve towards the muscles normally supplied by the other, the limbs became re-innervated and coordinated movement returned. Mapping the segmental innervation of re-innervated limbs showed that the normal symmetrical pattern, as judged by the unoperated normal limb was restored. Therefore motor nerve axons from each segmental nerve had regained control of their original muscle territories in spite of being directed at operation to foreign muscle. The above is probably sufficient to explain the return of coordinated movement to limbs, on the basis of specific functional connexions made between motor nerve fibres and muscles, rather than on any reorganization of the firing pattern of motoneurones with misdirected axons.