A headache is not pain in your brain. Your brain has no pain receptors at all. The pain you feel during a headache comes from the tissues surrounding the brain: the membranes wrapping it, the blood vessels running through and around it, the muscles and nerves of the head and neck. When these structures are irritated, stretched, compressed, or inflamed, they send pain signals through a major nerve network to your brainstem, which your conscious mind interprets as a headache. The specific combination of structures involved and the way they’re activated determines whether you experience a dull, pressing ache or a throbbing, debilitating migraine.
Why the Brain Itself Can’t Feel Pain
The brain processes pain from every other part of the body, but it contains no pain-sensing nerve endings of its own. Neurosurgeons can operate on exposed brain tissue with patients awake and feel no pain response. The structures that do contain pain receptors are the ones just outside or around the brain: the dura mater (the tough outer membrane covering the brain), large blood vessels inside and outside the skull, the muscles of the scalp and neck, and the nerves that thread through all of these tissues.
Pain can be triggered in these structures by electrical, mechanical, thermal, or chemical stimulation. Stretching a blood vessel in the membrane lining the skull produces pain. So does inflammation around the large arteries feeding the brain. The sensation travels along nerve fibers that are densely packed throughout the dura mater, most of them branching from the trigeminal nerve, the major sensory nerve of the face and head.
The Trigeminal System: Your Head’s Pain Highway
Nearly all headache science converges on a network called the trigeminovascular system. This is the connection between trigeminal nerve fibers and the blood vessels they wrap around inside your skull. The trigeminal nerve has three main branches covering the forehead, cheek, and jaw, but it’s the uppermost branch (covering the forehead and eye area) that plays the biggest role in headaches. Its fibers form a web of mostly unmyelinated (slow-conducting) nerve endings around the brain’s blood vessels, the large venous channels, and the dura mater.
When something activates these nerve endings, they do two things simultaneously. They send pain signals inward to a relay station in the brainstem, which passes the message up to the parts of the brain that register conscious pain. And they release signaling molecules at their outer tips, directly into the tissue around the blood vessels. This two-way function is what makes headaches self-reinforcing: the nerve endings both report pain and worsen the conditions causing it.
How Inflammation Builds Without Infection
One of the key discoveries in headache science is that a headache can involve real inflammation in the membranes around your brain, even though there’s no infection or injury. This is called neurogenic inflammation, meaning the nervous system itself triggers the inflammatory response.
Here’s the sequence. When trigeminal nerve endings fire, they release signaling molecules, particularly one called CGRP (calcitonin gene-related peptide) and another called substance P. CGRP binds to the smooth muscle cells in blood vessel walls, causing them to relax and widen. This increases blood flow through the membranes. Substance P, meanwhile, acts on tiny blood vessels to make their walls leaky, allowing fluid and proteins to seep into the surrounding tissue. Both molecules also trigger mast cells (immune cells stationed in the tissue) to dump their contents: histamine, serotonin, and inflammatory signaling proteins like TNF-alpha and interleukins.
The result is swelling, increased blood flow, and a chemical soup that further sensitizes the nerve endings that started the process. This is why a headache can start mild and escalate: the initial nerve activation creates local inflammation, which activates more nerve endings, which release more inflammatory molecules.
What Happens During a Migraine
Migraine involves all of the above mechanisms but adds a distinctive upstream trigger. Scientists have debated for decades whether migraine is fundamentally a blood vessel problem or a brain cell problem. The current consensus is that it’s both, integrated through the trigeminovascular system. Vascular and neuronal pathways both feed into the same nerve network, and separating them is no longer considered useful.
Many migraines, particularly those with aura (visual disturbances, tingling, or other sensory changes before the pain hits), are preceded by an event called cortical spreading depression. This is a slow wave of intense electrical activity that rolls across the surface of the brain, followed by a period where the affected neurons go quiet. During the wave, brain cells depolarize all at once, flooding the space around them with potassium ions. The pumps that normally restore the balance get overwhelmed, and the excess potassium causes neighboring cells to depolarize in turn, propagating the wave outward at about 3 to 5 millimeters per minute.
This wave of activity is what produces the aura: as it crosses the visual processing area, you see zigzag lines or blind spots. As it crosses sensory areas, you feel tingling. More importantly for the headache itself, the wave appears to activate the trigeminal nerve endings in the overlying membranes, launching the cascade of neurogenic inflammation described above. Genetic studies have found that mutations in the sodium-potassium pumps responsible for maintaining normal ion balance are linked to migraine with spreading depression, reinforcing how central this mechanism is.
CGRP plays such a pivotal role in migraine that infusing it into migraine-prone individuals can provoke an attack. This finding led directly to a class of migraine treatments that work by blocking CGRP receptors, preventing the peptide from triggering blood vessel dilation and inflammation.
Why Tension Headaches Feel Different
Tension-type headache is the most common headache, affecting roughly 25% of the global population in any given year compared to about 14% for migraine. It typically produces a dull, pressing, band-like pain on both sides of the head rather than the one-sided, throbbing pain of migraine.
The most consistent finding in tension headache research is increased tenderness in the muscles and connective tissue of the head and neck, detected by pressing on specific points. This tenderness correlates directly with how often and how intensely someone experiences these headaches. For occasional tension headaches, the pain likely originates from these peripheral tissues: tight, irritated muscles sending pain signals through local nerves.
But when tension headaches become frequent or chronic, something changes in the central nervous system. People with chronic tension headaches show lower pain thresholds not just in their head muscles but across their entire body, responding to pressure, heat, and electrical stimulation at levels that don’t bother people without chronic headaches. This pattern points to central sensitization, a state where the brainstem and spinal cord amplify incoming pain signals. Prolonged input from sore muscles and connective tissue essentially retrains the pain-processing system to be more reactive, turning what started as a peripheral muscle problem into a brain-level pain disorder. The pain response even changes in character: instead of the normal diminishing response to repeated pressure, people with chronic tension headache show a more linear, undiminishing response, a hallmark of central sensitization.
Secondary Headaches: Pain With an Underlying Cause
Scientists classify headaches into two broad categories. Primary headaches (migraine, tension-type, cluster) are the disorder itself, with no separate underlying disease. Secondary headaches are symptoms of something else: dehydration, sinus infection, medication overuse, head injury, or in rare cases, serious conditions like bleeding in the brain or tumors.
The International Classification of Headache Disorders defines a secondary headache by requiring evidence that the headache appeared in close timing with the other condition, worsened as it worsened, and improved as it improved. A headache that looks exactly like a migraine but started right after a head injury would be classified as a secondary headache attributed to the injury, not as migraine.
The mechanisms vary by cause. Dehydration headaches, for example, occur because fluid loss causes brain tissue to shrink slightly, pulling away from the skull and tugging on the pain-sensitive membranes and nerves surrounding it. A sinus headache involves direct inflammation pressing on nerve-rich tissue in the sinus walls. Each secondary headache activates the same pain-sensitive structures (membranes, blood vessels, nerves) through a different route.
Why Headaches Escalate and Persist
One of the most practical insights from headache science is why headaches often worsen if left untreated. The neurogenic inflammation loop, where activated nerves cause inflammation that activates more nerves, creates a feedback cycle. The longer it runs, the more sensitized the nerve endings become, and the harder the headache is to interrupt. This is why treating a migraine early tends to work better than waiting: you’re breaking the cycle before it fully establishes.
In people who experience very frequent headaches, the repeated activation of these pathways can cause long-term changes in how the brain processes pain. The trigeminal relay station in the brainstem becomes more excitable, lowering the threshold for future headaches. This central sensitization helps explain why chronic headache sufferers often develop sensitivity to stimuli that wouldn’t bother most people, like normal room lighting or mild pressure on the scalp.