Regeneration and functional reconnection of an identified vertebrate central neuron.

I have examined the axonal regeneration of a pair of identified central neurons, the Mauthner (M) neurons, in Xenopus laevis tadpoles. Lucifer Yellow injections reveal regenerative sprouts arising from the proximal stumps of the M axons within a few days after axotomy; some of these can cross the lesion within 1 week. Many specimens examined at later times (up to 21 weeks) have processes that extend more than 2 mm (equivalent to 5 to 10 spinal segments) beyond the lesion. M axons which have regenerated caudal to the lesion can re-establish functional synaptic contacts with their normal targets, spinal motor neurons. Functional reconnection has been demonstrated as early as 9 days after axotomy and as far as 10 segments caudal to the lesion. In most of the specimens tested, the regenerating M axons appear to exhibit the same degree of specificity for appropriate postsynaptic targets as normal, untransected M axons. M axons retain the ability to generate throughout the range of stages included in this study. The results provide evidence that a return of normal function in the transected vertebrate spinal cord can be mediated by the reconnection of a regenerating neuron with its normal targets.