The way MS does its damge is by attacking myelin.
Myelin is the insulation that surrounds nerve fibers in the brain and spinal cord.
Think of myelin like the insulation around an electrical wire...and MS just bites into it at various places, causing the electrical/nerve signals to get all messed up.
This is also why people with MS are so sensitive to heat. If the body's electrical system gets too hot, it short circuits causing wacky things to happen! Luckily, once the core body temperature returns to normal, usually the system "reboots" and you are back to normal. :-)