Damage and disease in the brain, spinal cord and other parts of the nervous system are very hard to treat. The first problem is that nervous tissue has a limited ability to repair itself. In the brain and spinal cord, damaged neurones are rarely replaced, so the damage is often permanent. Some functions may be partly regained as other neurones take over, but this is slow and limited.
The second problem is irreversible damage to surrounding tissues. Treatments such as surgery to remove a tumour, or radiotherapy, can harm the healthy tissue next to the target. Because that tissue also repairs poorly, any harm done by the treatment may be permanent too, and doctors must weigh up the risk before treating.
The third problem is difficulties with accessing parts of the nervous system. The brain is enclosed in the skull and the spinal cord in the backbone, so surgery is a major operation with a risk of infection and bleeding. Some regions lie deep inside the brain and are hard to reach without damaging tissue on the way. Drugs carried in the blood also find it difficult to pass into the brain, so they may not reach the damaged area.