How does this happen?

You have mylin around the neuron, when you get MS, the immune system attacks the mylin, which is classified as an auto-immune disorder. This can lead to a build up of scar tissue, then the neuron is unable to send messages to the brain properly. (The neuron is a peice of your body that sends nerve impulse to the brain EX: if you are hit your brain is sent a message from the neuron)