A migraine begins when cells in the central part of the brain become oversensitive to stimulation. The reason this happens is not known. A migraine trigger causes these cells to suddenly begin firing electrical impulses. This causes a chain reaction inside the brain. The impulses travel through the brain in a wave, similar to the way ripples spread across the water when a stone is thrown into a pond. This spreading electrical impulse causes the release of serotonin and other brain chemicals. Serotonin causes changes in blood flow to the brain. When an impulse reaches the outermost part of the brain, it triggers the release of Substance P and other chemicals. These chemicals contribute to the pain, inflammation, and increased sensitivity. The result is the throbbing pain you feel.