An evaluation of myotypic respecification in axolotls

Nerves supplying forearm flexor muscles were exchanged (crossed) with those supplying forearm extensors. After complete recovery, the movements of the forelimb were the same as those of normal limbs. Dissections showed that the nerves remained crossed, and electrical stimulation of the crossed nerves close to the muscles they supplied showed that the nerves were functionally connected to the foreign muscles. On the basis of these criteria, it appeared that axolotl nerves could respecify themselves. Stimulation of the crossed nerves further from the muscles they innervated, however, yielded contractions in the muscles that the nerves had originally supplied. Therefore, one of the two crossed nerves was severed at the shoulder and allowed to degenerate so that any movements elicited by subsequent electrical stimulation could be ascribed unambiguously to the remaining, non‐degenerated nerve. Electrical stimulation now showed clearly that the nerve which had originally innervated forearm flexors continued to produce flexion responses after it was crossed, and the nerve which had formerly innervated extensors continued to produce extension. Thus the fibers had somehow recrossed to reinnervate their original muscles. It therefore appears from the present investigation that axolotl nerves do not respecify themselves. Instead, a few fibers find their way back to their original muscles, and this is enough to produce normal movement.