There is a 2021 study done by King's College that suggests its actually linked to the immune system and not in the brain as originally thought. They noticed that mice that were injected with antibodies of Fibro patients vs healthy people showed the mice had a sensitivity to hot and cold and other factors. Once the antibodies cleared from the mice a few weeks later they were back to normal. This likely indicates that our body is in a constant state of developing antibodies similar to MS patients.