Multiple sclerosis. Coloured magnetic resonance image (MRI) scan of a vertical section through the brain of a patient with multiple sclerosis (MS). The two cerebral hemispheres, containing the fluid-filled ventricles (white), occupy the top part of the brain. Below these is the cerebellum (yellow). Lesions caused by MS appear red in the cerebral hemispheres (upper right and centre left of brain) and white in the cerebellum (lower centre). MS is caused by deterioration of the myelin sheaths around nerve fibres, and leads to such symptoms as unsteady gait, tremor, speech defects, paralysis and incontinence. No cure is yet known.