What hope for repair of the brain?

The consequences of axonal injuries in the adult nervous system depend on the location of the axons: complete anatomical and functional repair can occur in the peripheral but not the central nervous system. A quantitative electron microscopical comparison of the reactions to axonal injuries in a peripheral site (the superior cervical sympathetic ganglion) and in a central site (the septal nuclei) in adult rats shows that in both cases the denervated postsynaptic sites are reinnervated by new formation of synapses. However, in the ganglion it is the originally severed axons which form the new synapses (regeneration), whereas in the septum the new synapses arise from local, undamaged axons and the cut axons do not reestablish their original contacts. Failure of regeneration in the central site is probably due to the fact that the growing axon sprouts are incapable of finding a pathway to the denervated tissues. Since the observations indicate that the neuropil of the central nervous system is capable of highly efficient synapse formation after injury, true repair of central nervous system damage might be possible if methods could be devised for dealing with the "pathway" problem.