Axon regeneration: what needs to be overcome?

Axon regeneration is crucial for recovery of function after nervous system injury. Over many years, research has uncovered numerous factors which prevent damaged axons from regrowing and reforming functional connections after damage. These factors are both extrinsic, relating to the central nervous system environment, and intrinsic, relating to the growth capacity of the neurons themselves. In this short review, I summarize these elements with a view to illustrating how they may be overcome to promote nervous system repair.