Evolution provides different myelin sheathing cells (Oligodendrocyte and Schwann cells) in mammalian CNS and PNS. Damaged neurons in the CNS have little chance of recovery whereas damaged neurons in the PNS have an excellent chance of recovery. When ruptured, olidodendrocytes release Nogo that signals the damaged neuron cell body to stop any attempt at regrowing its stump. Ruptured Schwann cells in the PNS do not release Nogo. Why should nature behave like this? Is it because the wiring density of the CNS is orders of magnitude greater than that of the PNS?