The brain and spinal cord are very complex and delicate, and they are hard to treat. Neurones in the central nervous system have very little ability to repair themselves or regenerate once they are damaged, so damage is often permanent.
In a spinal injury, neurones in the spinal cord are cut or crushed. Impulses can no longer pass along the cord past the injury, so the person may have paralysis in the parts of the body below it. Because the neurones cannot regenerate, the paralysis is usually permanent and treatment mainly focuses on managing the effects.
Brain tumours are also hard to treat. Surgery is risky because the brain is delicate and the surgeon may damage nearby healthy tissue, causing loss of function. A tumour deep in the brain, or near a vital region, may not be possible to remove. Tumour cells spread into the surrounding tissue, so some may be left behind.
Drug treatment is limited too. The blood-brain barrier stops many drugs in the blood from entering brain tissue, so they cannot reach the tumour.