Selective vulnerability of the centrifugal and centripetal axons of primary sensory neurons.

The primary sensory neurons are unique in possessing one axon that lies within the peripheral nervous system and another that extends into the central nervous system. The two, therefore, have different environments and are associated with different supporting cells. Most of the distal ("dying-back") axonopathies that have so far been studied display simultaneous distal degeneration of both the peripherally and centrally directed axons of the dorsal root ganglion cells ("central-peripheral distal axonopathy"); others may display selective distal degeneration of the peripheral axons. Recent electrophysiological (sensory nerve conduction and somatosensory evoked potentials) and neuropathological studies have demonstrated that a further pattern in distal axonopathies is a selective or predominant degeneration of the centrally directed axons. This has been suggested for hereditary spastic paraplegia and clioquinol neurotoxicity. Persistent symptoms in central and central-peripheral distal axonopathies may result from imperfect regeneration of the centrally directed axons.