Nervous Tissue Does Not Repair Itself

1. Most neurones in the central nervous system do not divide once they are mature, so lost neurones are not replaced.
2. A cut axon in the CNS does not regrow, because scar tissue and chemical signals at the injury site block it.
3. Skin and bone heal because the cells there divide by mitosis, and nervous tissue simply cannot do the same.
4. Damage to the CNS is therefore usually permanent, and treatment aims to limit further damage and manage the symptoms.

The Problem of Access

1. The brain is enclosed by the skull and the spinal cord by the vertebral column, so surgery means cutting through bone.
2. Neurones are densely packed and interconnected, so an instrument reaching a deep structure must pass through healthy tissue.
3. Damaging healthy tissue on the way in can cost the patient an ability the operation was never meant to affect.
4. Some structures lie so deep that they are considered inoperable, and the risk of surgery outweighs the benefit.

The Blood Brain Barrier

1. The capillaries supplying the brain have tightly joined walls, forming a blood brain barrier.
2. This barrier protects the brain by preventing many substances in the blood from reaching the neurones.
3. It also stops many drugs from reaching the brain, including some chemotherapy drugs and antibiotics.
4. A drug that works well elsewhere in the body may therefore be useless against a brain tumour or a brain infection.

Treating Brain Tumours

1. A tumour deep inside the brain may be impossible to reach without destroying healthy tissue.
2. A tumour may have no clear boundary, so a surgeon cannot tell exactly where the tumour ends and healthy brain begins.
3. Radiotherapy damages the surrounding healthy neurones as well as the tumour, and those neurones will not be replaced.
4. Chemotherapy is limited by the blood brain barrier, so the drug may not reach the tumour in a high enough concentration.
5. Even a benign brain tumour is dangerous, because the skull cannot expand and a growing mass raises the pressure inside it.

Treating Spinal Injuries

1. The spinal cord carries the neurones connecting the brain to the rest of the body, so damage cuts that connection.
2. The severed axons do not regrow, so impulses can no longer pass between the brain and the body below the injury.
3. The result is a loss of sensation and of voluntary movement below the level of the injury, which is paralysis.
4. The higher up the spine the injury is, the more of the body is affected, and an injury in the neck can affect breathing.
5. Surgery can stabilise the spine and relieve pressure on the cord, but it cannot reconnect the cut neurones.

The Consequences for Patients

1. Damage is usually permanent, so patients live with the effects for the rest of their lives.
2. Physiotherapy helps patients make the most of the function they still have, and other regions of the brain sometimes take over part of a lost role.
3. Patients may need long term care, adaptations to their home, and support for the mental health effects of losing an ability.
4. Stem cell research offers a possible future treatment, and trials are testing whether transplanted cells can replace damaged neurones.