Migraines are caused by abnormal activity in the brain’s nerve networks, not by blood vessel problems as scientists believed for decades. The process involves a chain reaction: overexcited neurons fire in a spreading wave across the brain, triggering inflammation along the nerves that supply sensation to the head and face. This inflammation produces the throbbing, often debilitating pain that defines a migraine. About 45% of a person’s risk for migraines comes from genetics, with the rest driven by hormonal shifts, environmental triggers, and nutritional factors that make the brain more excitable.
The Old Theory Was Wrong
For most of the 20th century, doctors believed migraines were essentially a blood vessel problem. The idea was straightforward: arteries in and around the brain would first constrict (causing aura symptoms like visual disturbances), then dilate (causing the pounding headache). This “vascular theory” seemed to make sense because drugs that constrict blood vessels, like ergotamine, could relieve migraine pain.
But the theory started falling apart as imaging technology improved. A pivotal study found no significant changes in blood vessel size during spontaneous migraines in humans, leading researchers to conclude that dilation simply isn’t the cause. Other evidence piled up: some potent vasodilators don’t trigger migraines at all, and many migraine symptoms, like food cravings, mood changes, and light sensitivity, can’t be explained by blood vessel activity. The scientific consensus has shifted decisively toward a neurogenic model, where the problem starts in the nervous system itself, and any vascular changes are a downstream side effect rather than the root cause.
How a Migraine Starts in the Brain
The current understanding centers on a phenomenon called cortical spreading depression, or CSD. It begins when a wave of intense electrical activity sweeps across the surface of the brain. During this wave, neurons become flooded with sodium and calcium ions in concentrations that overwhelm the cells’ ability to pump them back out. The result is sustained, uncontrolled firing that propagates slowly across the cortex, leaving silenced neurons in its wake. Potassium builds up in the space between cells, and excitatory signaling molecules like glutamate spill out, driving the wave further.
This electrical wave is what causes migraine aura, the visual distortions, tingling, or speech difficulty that about a quarter of migraine sufferers experience. But CSD also does something more consequential: it releases inflammatory molecules that drift to the meninges, the sensitive membranes surrounding the brain. Once those molecules reach the meninges, they activate the trigeminal nerve, the main sensory nerve for your face and head. That activation is the bridge between the electrical event in the brain and the pain you actually feel.
The Pain Pathway: Your Trigeminal System
When the trigeminal nerve system activates, its nerve endings release a signaling molecule called CGRP (calcitonin gene-related peptide). CGRP is now recognized as a major driver of migraine pain. It triggers what’s called neurogenic inflammation: the nerve endings themselves cause blood vessels in the meninges to leak fluid and swell, which in turn activates more pain-sensing nerve fibers. Those fibers send signals back to the brainstem and up to the brain’s pain-processing centers.
This creates a feedback loop. Inflammation activates more nerve endings, which release more CGRP, which causes more inflammation. Over time, this process can sensitize the entire pain system, which is why a migraine that starts as a dull ache can build into severe, throbbing pain that worsens with any movement or sensory input. It’s also why migraines can last hours or even days. The discovery of CGRP’s central role led to an entirely new class of preventive medications that block this molecule, and they’ve proven effective for many people with frequent migraines.
It Starts Before the Pain
Many people notice subtle warning signs hours or even a day before a migraine strikes: unusual thirst, food cravings, neck stiffness, frequent yawning, or mood changes. This premonitory phase isn’t random. Brain imaging studies have shown that deep brain structures, particularly the hypothalamus, become unusually active during this early stage. The hypothalamus regulates basic functions like sleep, hunger, thirst, and body temperature, which explains why those functions go haywire before the headache itself arrives.
Research has also revealed altered connectivity between the hypothalamus and the brainstem in the lead-up to a migraine. These changes in brain communication begin before any pain starts, suggesting the migraine process is already underway long before you feel it. Recognizing premonitory symptoms can be practically useful, since treating early in this phase tends to be more effective than waiting for full-blown pain.
Why Some Brains Are More Vulnerable
Twin studies estimate that about 45% of the variation in migraine risk comes from genetic factors. This doesn’t mean there’s a single “migraine gene.” Instead, dozens of genetic variants each contribute a small amount to how excitable your brain’s nerve networks are, how efficiently your cells clear excess potassium and glutamate, and how sensitive your trigeminal system is to inflammatory signals. If your parents had migraines, your nervous system is likely wired to have a lower threshold for the chain reaction described above.
The remaining 55% of risk comes from environmental and physiological factors, which is why migraines can appear, worsen, or even disappear at different points in a person’s life. Genetics loads the gun, but triggers pull it.
Hormones as a Major Trigger
Estrogen plays a significant role in migraine frequency, which is a key reason migraines are roughly three times more common in women than men. The critical factor isn’t how much estrogen is present but how quickly levels change. The drop in estrogen just before menstruation is one of the most reliable migraine triggers, and many women report their worst attacks in the two days before or during their period.
Pregnancy often brings relief because estrogen rises quickly in the first trimester and stays consistently high throughout. But migraines frequently return after delivery, when estrogen plummets. The years leading up to menopause (perimenopause) tend to be the worst stretch for hormonal migraines, because estrogen levels fluctuate unpredictably. After menopause, when hormone levels finally stabilize at a low baseline, many women find their migraines improve or stop altogether.
Food, Environment, and Other Triggers
Common migraine triggers work by pushing an already-susceptible brain past its excitability threshold. Certain foods contain chemicals that appear to interact with the migraine pathway. Tyramine, an amino acid found in aged cheeses, fermented foods, and some cured meats, seems to be processed differently in people with migraines. Nitrates and nitrites in processed meats like bacon and hot dogs are another well-documented trigger, as are sulfites in red wine. Not everyone with migraines reacts to the same foods, which is why keeping a trigger diary can be more useful than following a generic avoidance list.
Sleep disruption, stress, dehydration, bright or flickering lights, strong smells, and weather changes are other common triggers. These likely work through similar brain mechanisms: they alter the activity of deep brain structures like the hypothalamus and brainstem, lowering the threshold for the cortical spreading depression and trigeminal activation that produce the attack.
The Role of Magnesium
Magnesium deserves special mention because it directly influences several steps of the migraine chain reaction. In nerve cells, magnesium acts as a natural brake on excitatory receptors called NMDA receptors, which respond to glutamate. When magnesium levels are adequate, it physically blocks the calcium channel on these receptors, preventing them from firing too easily. When magnesium is low, those receptors become overactive, making the brain more prone to the spreading wave of electrical activity that initiates a migraine.
Magnesium deficiency has been specifically linked to cortical spreading depression, imbalanced neurotransmitter release, and increased platelet activity. Research shows that adequate magnesium can help dampen neurogenic inflammation by regulating glutamate signaling and modulating pain transmission within the nervous system. This is why magnesium supplementation is one of the few nutritional interventions with meaningful evidence behind it for migraine prevention, particularly for people whose levels run low.
Why Migraines Aren’t Just Bad Headaches
Understanding the biology makes it clear why migraines are fundamentally different from tension headaches or sinus headaches. A migraine involves a cascading malfunction across multiple brain systems: abnormal electrical activity in the cortex, inflammatory signaling in the meninges, trigeminal nerve activation, brainstem sensitization, and disrupted function in the hypothalamus and other deep brain structures. This is why migraines come with nausea, light and sound sensitivity, cognitive fog, and fatigue, not just pain. The entire brain is involved, not just its pain circuits.
It also explains why migraines are so individual. Your specific combination of genetic susceptibility, hormonal patterns, magnesium status, and environmental exposures determines your personal threshold and trigger profile. Two people can have the same diagnosis but very different experiences, because the underlying vulnerability expresses differently depending on which parts of this complex system are most affected.