MS ExplainedWhat happens in MS?
It is thought that MS is an autoimmune condition. For some reason - possibly following exposure to an as yet unidentified infection - the body's immune system starts to attack cells within the central nervous system. T-cells manage to pass through the blood-brain barrier where they mistake the myelin sheath for a foreign body and start to destroy it.
The process of damaging or stripping away myelin from an axon is called demyelination. Messages that pass along a demyelinated nerve become delayed or blocked. As the central nervous system controls processes throughout the body, a wide range of symptoms can occur, depending on where the damage has happened.
For reasons that are not yet understood, the attack by the immune system tends to stop after an indefinite period and scar tissue develops on the damaged nerve. The forming of scar tissue over an area of damaged myelin is what forms the plaques, lesions or scarring ('sclerosis') that show up as white blotches on MRI scans. The name multiple sclerosis comes from the fact that it causes areas of sclerosis at different places in the central nervous system.
Once the inflammation caused by the immune attack is over, it is possible for damaged myelin to be replaced, a process known as remyelination. Whether this happens or not depends on damage to the surrounding oligodendrocytes, the supporting cells that produce myelin and which are also attacked by this process. Although the new myelin can work effectively, it tends to be thinner than unaffected myelin and so messages through the affected nerves may not be as fast as before the attack. If there are several periods of damage, the amount of remyelination is reduced.
Remyelination tends to occur in the earlier stages of MS. Over time, with repeated attacks, oligodendrocytes are damaged and destroyed and myelin is not as easily replaced. If an axon is left without the nourishment and protection of myelin it will be more vulnerable to damage.
It has been found that the central nervous system is able to overcome small areas of axon loss by finding ways to reroute messages around an area of damage through undamaged nerve cells. This ability to adapt to areas of damage is called plasticity. Should the area of damage become too large, this rerouting process is no longer able to compensate and messages to or from that part of the central nervous system are permanently blocked, resulting in symptoms that do not improve.
Remyelination and rerouting occur in the stage of MS where an attack of worsening symptoms, or relapse, is followed by a period of remission when some or all of the function returns.
Remyelination, rerouting and the loss of axons can happen at the same time in different parts of the central nervous system.
Next page - The cause of symptoms