Axon reflexes in axolotl limbs: Evidence that branched motor axons reinnervate muscles selectively

Abstract

Numerous axon reflexes were found in grafted forelimbs, regenerated forelimbs, and forelimbs with regenerated nerves in the axolotl Ambystoma mexicanum. They were not seen in normal limbs. Each reflex was due to a single motor axon which branched, generally at a point central to the limb. In most cases several consecutive electrical stimuli were necessary to cause a visible contraction or a muscle action potential in the motor units concerned. I observed a total of 13 axon reflexes between the supernumerary limb and the ipsilateral host limb. All but one could be stimulated in either direction. In other animals I demonstrated 13 axon reflexes by cutting a small nerve in the distal limb and stimulating its proximal stump. In at least five, and possibly as many as 14 of these 26 axon reflexes, the two muscles involved were synonymous. In 20 of the 26, the muscles involved were situated in the same limb region and were synergistic or synonymous. Only in five axon reflexes were the muscles in widely different parts of the limb and clearly unrelated in function. Random innervation or mechanical guidance alone cannot account for these muscle-specific axon reflexes. Axon branches innervated muscles selectively, although the mechanism remains unclear. The selectivity was not perfect, though, because axon reflexes between dissimilar muscles did occur occasionally. Although it has been proposed that an axon may remain in an inappropriate muscle but be functionally “repressed,” no evidence of such repressed synapses was found.