You have been asked to see D.V. in the neurology clinic. D.V. has been referred by his internist, who thinks his patient is having symptoms of multiple sclerosis (MS). D.V. is a 25-year-old man who has experienced increasing urinary frequency and urgency over the past 2 months. Because his female partner was treated for a sexually transmitted disease (STD), D.V. also underwent treatment, but the symptoms did not resolve. D.V. has also recently had 2 brief episodes of eye "fuzziness" associated with diplopia and brightness. He has noticed ascending numbness and weakness of the right arm with inability to hold objects over the past few days. Now he reports rapid progression of weakness in his legs.

MS is characterized by remissions and exacerbations. What happens to the myelin during each of these phases?