The most common type of vascular headache involving (it is thought). Abnormal sensitivity of blood vessels (arteries) in the brain to various triggers resulting in rapid changes in the artery size due to spasm (constriction). Other arteries in the brain and scalp then open (dilate), and throbbing pain is perceived in the head. The tendency to migraine is inherited and appears to involve serotonin, a chemical in the brain involved in the transmission of nerve impulses that trigger the release of substances in the blood vessels that in turn cause the pain of the migraine. These nerve impulses cause the flashing lights and other sensory phenomena known as an aura that may accompany a migraine. Not all severe headaches are migraines and not all migraines are severe.