1. During a migraine headache, arteries in the head (especially in the temporal area) dilate.
2. The widened arteries stretch nerve fibers that encircle the arteries, causing them to send impulses to the brain. In turn, these nerve impulses cause pain and activate the sympathetic nervous system (SNS), which originates in the spinal cord and extends to organs throughout the body, including the stomach and intestines. The SNS controls the body's "fight or flight" response, mobilizing the body for action by speeding up the heart rate, raising blood pressure, and slowing digestion.
3. To slow digestion, the SNS closes the pyloric sphincter (the ring of smooth muscle that separates the stomach from the upper part of the intestines). As a result, the stomach dilates and any leftover food stays in the stomach, which can cause the nausea and vomiting that often accompany migraine headaches. This phenomenon also explains why migraine medications taken by mouth aren't very effective once the headache is intense—they aren't well absorbed into the bloodstream.