Causes

Until recently, the general theory on the migraine process rested solely on the idea that abnormalities of blood vessel (vascular) systems in the head were responsible for migraines. Now, however, experts tend to believe that migraine starts with an underlying central nervous system disorder. When triggered by various stimuli, this disorder sets off a chain of neurologic and biochemical events, some of which subsequently affect the brain's vascular system. No experimental model fully explains the migraine process.

There is certainly a strong genetic component in migraine with or without auras. Researchers have located a single genetic mutation responsible for the very rare familial hemiplegic migraine, but several genes are likely to be involved in the great majority of migraine cases. Numerous chemicals, structures, nerve pathways, and other players involved in the process are under investigation.

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General Theories to Explain Migraine

Central Nervous Disorder. One theory that attempts to integrate many of the known events in the migraine process is as follows:

Stress or some unknown factor triggers the release of certain protein fragments called peptides (Substance P, calcitonin gene-related peptide, and others).
These peptides dilate blood vessels and produce an inflammatory response that triggers over-excitation of the nerve cells in the trigeminal pathway. [This nerve pathway runs from the brain stem to the head and face. These nerves spread to the meninges (the membrane covering of the brain).]
While the brain itself is insensitive to pain, the meninges and blood vessels around the brain are sensitive to pain. Some experts suggest that pain occurs when blood drains from the center of the head to the blood vessels around the brain.
Auras are believed to be a response to blood flow changes that cause a rapid reduction in brain activity that reaches the cerebral cortex (the outer layer of the brain), referred to as spreading depression. This effect may be visualized as an electrical wave spreading through the brain just as a wave of water is caused by the dropping of a pebble. Some research suggests that in people with auras, the cortical spreading depression itself activates the inflammation in the trigeminal nerves that triggers pain in the meninges.