Multiple sclerosis. Coloured magnetic resonance imaging (MRI) scan of an axial (horizontal) sect- ion 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 creates demyelinated lesions (small rounded specks, lower centre) and abnormal- ly enlarged fluid-filled ventricles (blue, centre) in the brain (red). Axons in the affected area can no longer conduct nerve impulses, resulting in symptoms ranging from tingling to paralysis. MS is believed to be an autoimmune disorder in which the immune system attacks myelin.