Headaches form when pain-sensitive structures in and around your skull send distress signals to your brain. The brain itself has no pain receptors at all. Instead, the tissues surrounding it, particularly the thin protective membranes called meninges, blood vessels, muscles, and nerves in your head and neck, detect problems and relay pain signals through a major nerve pathway to create the sensation you experience as a headache.
Why Your Brain Can’t Feel Pain
This surprises most people: the organ that processes all your pain sensations is itself completely numb. Brain tissue lacks nociceptors, the specialized nerve endings that detect potentially harmful stimuli. That’s why neurosurgeons can operate on the brain with a patient fully awake.
The pain you feel during a headache originates in the structures that surround and protect the brain. The meninges, three layers of membrane that wrap around the brain and spinal cord, are densely packed with nociceptors. So are the blood vessels running through and around the brain, the muscles and skin covering the skull, and the nerves of the head and face. When any of these tissues are stretched, compressed, inflamed, or irritated, they generate pain signals that your brain interprets as a headache.
How Pain Signals Travel to the Brain
Nearly all headache pain travels through the trigeminal nerve, the largest nerve in your head. It has three branches that cover your forehead, cheeks, and jaw, which is why headaches can radiate across different parts of your face and skull.
When nociceptors in the meninges or blood vessels detect a problem, they fire signals down the trigeminal nerve’s fibers into the brainstem, specifically into a region called the spinal trigeminal nucleus and the upper part of the spinal cord (around the C1 to C3 vertebrae). This is where the initial processing happens. From there, signals climb up to the thalamus, a relay station deep in the brain that sorts incoming sensory information and passes it along to the cortex, where you consciously feel the pain.
This pathway also explains why headaches so often come with neck stiffness. The trigeminal neurons in the brainstem receive overlapping input from the head and from the muscles and skin of the upper neck, so a headache can make your neck hurt, and neck tension can feed into a headache.
What Happens During a Tension Headache
Tension-type headaches, the most common kind, typically produce a dull, pressing sensation on both sides of the head, like a tight band around your skull. The exact mechanism is still debated, but the prevailing explanation involves the muscles and soft tissues of the head and neck becoming tense or irritated, which activates pain receptors that feed into the trigeminal pathway.
Stress, poor posture, lack of sleep, and eye strain are common triggers. These don’t damage tissue, but they can keep muscles contracted long enough to sensitize the surrounding nerve endings. Once those nerves become more reactive, even normal muscle activity can register as pain. Interestingly, research has found that the sensitivity of specific muscle knots (myofascial trigger points) in the head and neck doesn’t consistently correlate with how frequent or severe someone’s tension headaches are. This suggests the problem isn’t purely muscular. Over time, the central nervous system itself may become more sensitive to incoming signals, amplifying pain that would otherwise be mild.
How Migraines Build
Migraines involve a more complex cascade. They likely begin inside the brain itself, with a malfunction in how the brain regulates its own activity, before the pain-sensitive meninges get involved.
In people who experience migraine with aura (visual disturbances, tingling, or speech changes before the headache hits), the process often starts with something called cortical spreading depression. This is a slow-moving wave of intense electrical activity that rolls across the surface of the brain, followed by a period of suppressed activity. It moves at about 3 to 5 millimeters per minute, which matches the gradual spread of aura symptoms over 20 to 30 minutes.
This electrical wave does more than cause visual disturbances. Animal studies show it triggers inflammation around the meningeal blood vessels, prompting them to release inflammatory molecules and pain-signaling chemicals. One of the most important of these is a protein called CGRP. During a migraine attack, CGRP levels rise measurably in the blood, saliva, and even tear fluid. CGRP dilates blood vessels and promotes inflammation in the meninges, which activates trigeminal nerve fibers and starts the pain signal cascade described above. In people with chronic migraines, CGRP levels remain elevated even between attacks, which may help explain why their threshold for triggering the next headache is lower.
Serotonin also plays a central role. Low serotonin levels allow trigeminal nerve endings to release more CGRP and other inflammatory peptides. Triptan medications, the most widely used acute migraine treatments, work by mimicking serotonin at specific receptor sites on trigeminal nerve endings and blood vessels, which dials down CGRP release and constricts dilated blood vessels.
What Makes Cluster Headaches Different
Cluster headaches are rarer but extraordinarily intense, often described as a stabbing or burning pain behind one eye. They strike in cyclical patterns (clusters), sometimes at the same time each day for weeks or months, then disappear for long stretches.
This clocklike regularity points to the hypothalamus, the brain region that governs your circadian rhythm, body temperature, and hormonal cycles. Neuroimaging studies consistently show the hypothalamus activating during cluster headache attacks. The current understanding is that the hypothalamus ignites the trigeminal pain pathway from above, using top-down signals rather than responding to peripheral triggers. It also activates an autonomic reflex that causes tearing, nasal congestion, and eyelid drooping on the affected side, all hallmarks of cluster headache.
When Pain Amplifies Itself
One reason headaches can worsen and become harder to treat over time is a process called central sensitization. In a normal headache, pain signals travel from the meninges or muscles through the trigeminal nerve to the brainstem and then to the thalamus. Each relay point processes and filters those signals. But when pain signals are sustained or intense enough, the neurons at these relay stations become hypersensitive. They start amplifying incoming signals and responding to stimuli that wouldn’t normally register as painful.
This happens in stages. First, the nerve endings in the meninges become more reactive. Then the neurons in the brainstem that receive their signals become sensitized, which is when you might notice that coughing, bending over, or physical activity makes the headache worse. Finally, neurons in the thalamus can become sensitized. At this point, even light touch on the skin of your face, scalp, or arms can feel painful, a phenomenon called allodynia. If you’ve ever had a bad migraine where brushing your hair or wearing glasses hurt, that’s thalamic sensitization at work.
This escalating sensitization may also be one mechanism behind the transformation from occasional headaches into chronic daily headaches. The pain system essentially learns to stay in a heightened state.
Everyday Triggers and Why They Cause Pain
Understanding the underlying pathways makes it easier to see why common triggers produce headaches. Dehydration, for instance, reduces the volume of fluid surrounding the brain. As fluid drops, the brain can slightly shrink and pull away from the skull, stretching the pain-sensitive meninges and blood vessels. This mechanical traction activates nociceptors directly.
Alcohol works through multiple channels: it promotes dehydration, dilates blood vessels in the meninges, and triggers inflammatory responses. Caffeine withdrawal causes a rebound dilation of blood vessels that had been constricted by regular caffeine intake, stretching vessel walls and activating surrounding nerve endings. Poor sleep disrupts serotonin regulation and lowers the threshold for trigeminal activation. Bright lights and strong smells can trigger headaches in sensitized individuals because sensory processing pathways overlap with pain pathways in the brainstem.
In each case, the final common pathway is the same: something irritates, stretches, or inflames the pain-sensitive structures around the brain, trigeminal nerve fibers carry that signal to the brainstem and thalamus, and you feel a headache. The type and location of headache you experience depends on which structures are involved and how far the sensitization process has progressed.