What causes a migraine headache?
Understanding where migraine pain comes from does not explain why someone gets migraines in the first place. It also does not explain why they get a migraine on any particular day. There is evidence that there is a portion of the brain where migraine headaches generate or begin. This concept is based upon some patients developing "migraine like" headaches after implantation of neural stimulators for chronic pain and a patient who had a spontaneous migraine that was observed while she was being imaged in a special type of scanner.
Why do people get migraines on any given day?
A practical way to think about why someone has a migraine on any particular day is the concept of a migraine threshold. Each day when one wakes up without a migraine, they are below their migraine threshold. As the person goes through the day, if they experience certain triggers such as missed meals, certain foods, different smells, etc. they may move closer to their threshold. If they cross over their threshold, a migraine is triggered and the events leading to neurogenic inflammation occur. In this model, migraine preventatives act to move the patient away from their threshold.