How a Migraine Happens

One theory explaining how a migraine happens:

Although there is general agreement among healthcare providers and researchers that a key element in migraines is blood flow changes in the brain, the exact cause of migraine headaches remains unknown.

According to the National Institute of Neurological Disorders and Stroke (NINDS), one theory of how a migraine happens is as follows:

The nervous system responds to a trigger by creating a spasm in the nerve-rich arteries at the base of the brain.
The spasm closes down or constricts several arteries supplying blood to the brain, including the scalp artery and the carotid (neck) arteries.
When the arteries constrict, blood flow to the brain is reduced.
At the same time, blood-clotting particles, called platelets, clump together in a process that is believed to release a chemical called serotonin - which acts as a powerful constrictor of arteries and further reduces the blood supply to the brain.
When reduced blood flow decreases the brain's supply of oxygen, symptoms signaling a headache, such as distorted vision or speech, may result.
Reacting to the reduced oxygen supply, certain arteries within the brain open wider, or dilate, to meet the brain's needs. The dilation spreads, finally affecting the neck and scalp arteries.
Dilation of these arteries triggers the release of pain-producing substances, called prostaglandins, from various tissues and blood cells.
Chemicals that cause inflammation and swelling, and substances that increase sensitivity to pain, are also released.
The circulation of these chemicals and the dilation of the scalp arteries stimulate the pain-sensitive nociceptors.
The result, according to this theory, a throbbing pain in the head.