Multiple sclerosis. Coloured sagittal magnetic resonance imaging (MRI) scan through the brain of a 53-year-old woman with multiple sclerosis. This scan shows extensive signal abnormality (orange) in the white matter, indicating brain atrophy consistent with severe long-standing multiple sclerosis. The corpus callosum (the structure connecting the two hemispheres of the brain) is much thinner than normal. Multiple sclerosis is a condition affecting the myelin sheaths around the nerve fibres of the brain and spinal cord. Axons in the affected area can no longer conduct nerve impulses, resulting in symptoms ranging from tingling to paralysis. Multiple sclerosis is believed to be an autoimmune disorder in which the immune system attacks myelin.