10. Central nervous system regeneration

•The body responds very differently to CNS injuries as compared to PNS injuries.

•Just like in the PNS, Wallerian degeneration and regeneration follow an injury in CNS, but the cells involved in it and the final outcomes are very different.

•The major difference is the fact that CNS injuries involve neuron bodies along with their axons.

•Ultimately, even a minor injury to the CNS is very difficult to repair and obtain a complete functional recovery.

•There are number of factors that stop or hinder CNS regeneration:

•CNS injuries involve neurons’ cell bodies long with axons and the loss of neurons is irreversible.

•Oligodendrocytes are the cells that create myelin sheath in CNS. But they die when the axon is degenerated and so, the sheath is lost. Without the sheath, the axons form a non-functional messy ball of nerve fibres when they try to regrow.

•Astrocytes, the main supporting cells of the CNS start to form a scar at the injury site. This astroglial scar initially helps strengthen the injured tissue but later it physically bars the axons from growing across the injury site.

•When the axons degenerate after injury, they leave debris behind, which is not cleared efficiently in CNS. This left over debris also slows down re-growing axons.

•In spite of all this, some functional recovery may still occur.

•This usually happens because, the remaining neurons in brain and spinal cord try to compensate for the lost neurons by taking over some of their workload.

•In some cases, the central nervous system assigns other neurons to the task that the damaged neurons used to do. These other neurons are usually taken away from non-essential functions and assigned with more important duties. This phenomenon is called ‘neuronal plasticity’.