Great question. If we knew the answer tot his one, we would probably already be able to regrow neurons.

In this study the protein EphA4 is named as

a candidate target. EphA4 prevents nerve repair

in this context.

http://medicalxpress.com/news/2013-06-blocking-protein-key-spinal-cord.html

Other articles:

http://www.eurekalert.org/pub_releases/2013-10/ki-scs102913.php

http://www.eurekalert.org/pub_releases/2013-07/si-ssd072513.php

http://drexel.edu/now/news-media/releases/archive/2013/March/Nerve-Axon-Growth/

http://www.eurekalert.org/pub_releases/2012-11/uol-dmh110512.php

http://www.eurekalert.org/pub_releases/2012-09/uoc--nsc091012.php

http://www.eurekalert.org/pub_releases/2012-07/gsoa-zrp070612.php

http://www.eurekalert.org/pub_releases/2011-09/uoc--usd092111.php

http://phys.org/news/2010-11-uncovers-extensive-natural-recovery-spinal.html

http://www.eurekalert.org/pub_releases/2010-06/sumc-ssf061010.php

Regards,

Joachim

Joachim and Mark, thank you very much. That was very useful.

Useful links Joachim. The evolution ideas to which Mark alluded (2004 Nature 427, 685-686) may be supported by the recent outstanding work by Takao Hensch with respect to critical periods and neural plasticity. More 'plastic' and adaptable nervous systems seem to correlate with capacity for regeneration.

See: https://www.mcb.harvard.edu/mcb/faculty/profile/takao-hensch/