Multiple sclerosis. Coloured rotating axial magnetic resonance imaging (MRI) scan through the upper regions of the brain of a patient with multiple sclerosis. This scan shows multiple focal areas of disease (scars) in the white matter. 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.