So, if some die through illness or damage, the nervous system can permanently lose some of its abilities. The symptoms of disorders of the nervous system depend on which part is attacked - Alzheimer's disease destroys cells in the memory area of the brain, for example.
Many forms of brain and spinal cord (CNS) damage cut axons. Where axons can regenerate, as in peripheral nerves, they can bring back function. However in the CNS axon regeneration fails. This is the main reason why paralysis and loss of sensation is permanent in conditions such as spinal cord injury.