Multiple sclerosis. Magnetic resonance imaging (MRI) scan of a coronal (vertical) section through a brain in multiple sclerosis (MS). In MS there is destruction of the myelin sheaths around the axon nerve fibres of the brain and spinal cord. This produces demyelinated lesions (dark ovals, upper centre) and abnormally enlarged fluid-filled ventricles (black, centre) in the brain (pale/mid grey). Axons in the affected area can no longer conduct nerve impulses, resulting in highly indiv- idual symptoms ranging from tingling to paralysis. MS is believed to be an autoimmune disorder in which the immune system attacks myelin.