Multiple sclerosis. Coloured magnetic resonance imaging (MRI) scan of a sagittal (vertical) secti- on through a brain in multiple sclerosis (MS). The front of the brain is at left. In MS the myelin sheaths around the axon nerve fibres of the brain (purple) are destroyed. This produces demyelinated lesions such as the small yellow area at lower right, and abnormally enlarged fluid-filled ventr- icles (one at centre, yellow). Affected axons can no longer conduct nerve impulses, causing highly individual symptoms ranging from tingling to para- lysis. MS is thought to be an autoimmune disorder in which the immune system attacks myelin.