In the CNS, neurogenesis, the birth of new neurons from stem cells, is limited to the hippocampus in adults. In other regions of the brain and spinal cord, neurogenesis is almost non-existent due to inhibitory influences from neuroglia, especially oligodendrocytes, and the absence of growth-stimulating cues. The myelin produced by oligodendrocytes in the CNS inhibits neuronal regeneration. Furthermore, astrocytes proliferate rapidly after neuronal damage, forming scar tissue that physically blocks regeneration. It follows that injuries to the brain or spinal cord are typically irreversible.
However, in the PNS, repair is possible if the cell body is intact and Schwann cells remain active. Post-injury, the Nissl bodies in a neuron start to disintegrate, a process known as chromatolysis. Within a few days, the region distal to the damaged axon swells and breaks into fragments, and the myelin sheath deteriorates. This process is called Wallerian degeneration. Despite these changes, the neurolemma remains intact. Macrophages clear the debris, and RNA and protein synthesis increases, promoting the rebuilding or regeneration of the axon. The Schwann cells multiply and may form a regeneration tube across the injured area, guiding the growth of a new axon. However, if the injury gap is too large or filled with collagen fibers, new axons cannot grow.
