According to the dictionary, demyelination means a "loss of myelin from the nerve sheaths, as in multiple sclerosis." So what is myelin? And what are nerve sheaths?
The myelin sheath insulates the the nerve fibers or axons. Nerve impulses travel along the nervous system to and from the brain. The myelin helps the nerve impulses travel smoothly and quickly from one nerve to the next.
When damage to the myelin occurs, the nerve impulses can no longer travel smoothly throughout the nervous system. In multiple sclerosis, it occurs in the CNS. The billions of nerves in the brain and spinal cord as well as the optic nerve are affected.
In MS, lesions form along the myelin, which causes symptoms to occur throughout the body. The symptoms can occur anywhere along the central nervous system. The symptoms can be intermittent - meaning they come and go; or are not continuous.
Here's a short video animation that helps to explain what happens.
Ideally the oligodendrocytes produce and maintain myelin. They also repair damaged myelin. Total repair or remyelination can happen after an attack or exacerbation. In this case the symptoms would go away and the MS patient would return to the point they were before the attack.
Each oligodendrocyte produces myelin for several axons. Each axon has several oligodendrocytes producing myelin for it. If the oligodendrocytes are damaged only partial remyelination may occur. If this were to happen, some disability may remain.
The destruction of the myelin is shown by scars or lesions along the axons. It has been found that sometimes even the axons can become damaged or destroyed as well.
There are glial cells called astrocytes. These are responsible for the scar tissue which forms in place of the myelin. The scars or lesions along the axon cannot perform the way the myelin does. So there will almost always be some type of disability.
There are so many variables in MS, that no one can say for sure:
What will happen.When it will happen.How it will happen.And who it will happen to.
Multiple sclerosis is such a complicated disease.