Why nervous tissue does not repair

1. Most tissues heal because their cells divide to replace the ones that were lost, which is how a cut in the skin closes within days.
2. Neurones in the brain and spinal cord do not divide once they are mature, so a neurone that dies is not replaced by a new one.
3. A cut axon inside the central nervous system does not regrow along its old route, so the connection it made is lost even if the cell body survives.
4. The gap fills instead with scar tissue, which physically blocks any regrowth and does not carry impulses.
5. Even if an axon did regrow, it would have to find its original target among billions of others to restore the same function.
6. The effects of the damage are therefore usually permanent, which is what makes this different from an injury to a muscle or a bone.

Irreversible damage to surrounding tissue

1. Nervous tissue is soft and easily torn, so instruments passing through healthy tissue to reach a damaged region destroy neurones on the way.
2. Because those neurones are not replaced, the harm done by the operation itself is permanent as well.
3. Bleeding and swelling after an injury press on nearby tissue, and since the skull cannot expand that pressure kills further neurones.
4. Squashed blood vessels then cut off the oxygen supply, so more cells die of starvation in the hours after the original damage.
5. Neurones lie so close together that different functions are packed within millimetres, so a small area of damage can remove several abilities at once.
6. If the motor pathways in the spinal cord are cut, impulses can no longer reach the muscles below that point and paralysis follows.
7. Sensory pathways usually run in the same region, so feeling is often lost from the same part of the body.

Difficulty in reaching the nervous system

1. The brain is enclosed by the skull and the spinal cord runs inside the vertebrae, so bone has to be cut through before either can be reached.
2. The protection that keeps the CNS safe from everyday knocks is the same protection that keeps a surgeon out.
3. Many important structures lie deep inside the brain, so reaching them means passing through the regions above.
4. A tumour often grows in among healthy neurones rather than as a separate lump, so it cannot be removed without taking healthy tissue too.
5. Drugs are another route in, but the blood vessels of the brain have an unusually tight lining that stops most substances leaving the blood.
6. That barrier protects the brain from poisons and pathogens, but it also blocks many medicines that would otherwise treat the damage.
7. The nerves outside the CNS are spread throughout the body, so damage to them can be equally hard to locate and reach.

What treatment can achieve

1. Most treatment manages the effects rather than repairing the tissue, which is the key limitation to state in an answer.
2. Surgery can prevent further damage by removing a tumour, draining a bleed or relieving pressure, but it cannot restore neurones already lost.
3. Drugs can control symptoms such as pain, seizures or muscle stiffness, and some slow the rate at which further neurones are lost.
4. Physiotherapy helps a patient recover some function because repeated practice strengthens the connections in pathways that survived, so the task is rerouted rather than the damage repaired.
5. Electrical implants can take over a particular job, such as stimulating a muscle directly or steadying the tremor of a movement disorder.
6. Research into replacing lost neurones with stem cells is under way, and the difficulty is not only growing new neurones but getting them to connect correctly.

Why prevention matters so much

1. Since the damage cannot be reversed, avoiding it in the first place is far more effective than any treatment.
2. A helmet spreads the force of an impact over a larger area and over a longer time, so less force reaches the skull and brain.
3. Seatbelts, head restraints and airbags reduce the sudden movement of the head, which is what tears neurones inside the skull during a crash.
4. Rules about head injuries in sport keep a concussed player off the pitch, because a second impact before recovery causes far worse damage.
5. Not moving someone with a suspected spinal injury matters for the same reason, since moving them can turn a partly damaged cord into a completely severed one.

Weighing up a treatment

1. Deciding whether to operate means comparing the harm the damage is already doing with the harm the operation itself could cause.
2. A tumour pressing on the medulla may be worth the risk of surgery, while one sitting deep among the pathways for speech may not be.
3. The patient shares that decision, because only they can judge which abilities they are least willing to lose.