On the outside, the brain is protected by the skull or cranium. The spine protects the spinal cord. On the inside the brain is protected by the blood-brain barrier. This normally keeps things out of the brain that don't belong there.

MS, which is a disease of the CNS, causes the nerves in the brain to come under attack. They become damaged by demyelination. There are so many areas in the body that can possibly be affected by the damage. That is why there are so many different symptoms common to MS.

Here's a short video animation that helps explain what happens in the CNS.