Multiple sclerosis is a disease that is most directly associated with the degeneration of:

Multiple Sclerosis works on a cellular to destroy the Myelin Sheath of nerve cells. MS is ultimately very destructive and progressive, with the Myelin Sheath of nerve cells being lost, and lesions and inflammation forming as a result. Because the nerve cells are sensitive and don't have the sheath that normally protects, them they are constantly destroyed which eventually will lead up on the symptom chain, and eventually Seizures, Coma, Death.