Multiple sclerosis. Coloured magnetic resonance image (MRI) scan of an axial section through the brain of a patient with multiple sclerosis (MS). The cerebral hemispheres are seen here. Grey matter is on the cerebral outer edges; white matter is found deeper in the brain. Lesions caused by MS are seen at left and lower left (white). They are due to destruction of myelin sheaths around nerve fibres. Progressive destruction of myelin in the brain and spinal cord produces such symptoms as unsteady gait, tremor, speech defects, paralysis and incontinence. MS is thought to be an autoimmune disorder.