By Donavyn Coffey 

The brain doesn't have its own pain receptors, so why do headaches hurt so much?

Headaches are extremely common and they can take many forms, ranging from mild to debilitating and lasting minutes to days. When your cranium is in pain, it's easy to think your brain tissue itself must be hurting. But that's not likely.

Ironically, the brain senses pain throughout the body, but doesn't actually have its own pain receptors. So why, then, do headaches hurt?

Headaches can stem from an underlying medical condition, for instance, swollen sinuses, low blood sugar or a head injury. But broadly speaking, most headaches arise due to "referred pain," meaning you feel the pain in a different place than where it's actually occurring, said Dr. Charles Clarke, a neurologist and headache specialist at Vanderbilt Health in Tennessee. It's similar to how a herniated disk in your back can cause sciatica, a pain down your leg. For most headaches, an issue somewhere else in the body — like the jaw, shoulders and neck — causes pain in the muscle and nerves around the brain, he said.

Take tension headaches, which according to the World Health Organization, are the most common type of recurring headaches. Tension headaches often occur as pain in the muscles across the top of your head or forehead, where a sweatband or a headband would sit, Clarke said. The pain is caused by tightened muscles in the face, neck and scalp and can be stress-related, according to the National Institute of Health (NIH). But the head pain and the tightening of cranial muscles can be secondary to another stress response, like tight shoulders or a clenched jaw, Clarke said.

According to the NIH, pain-sensing nerves in the muscles and blood vessels around the head, neck and face can be triggered by different processes, such as enlarged blood vessels, stress or muscle tension. Once activated, these nerves send messages to the brain, but it can feel as if the pain is coming from deep within the brain tissue.

Migraines are another type of headache, although technically, headache is just one symptom of the neurological disorder. Migraine headaches can be felt in a variety of ways and places: deep pain, surface pain; the back, left or right of the head; or behind the eyes. What sets migraines apart, Clarke said, is their severity.

Migraine pain is more intense than other headaches and can be longer lasting. The disorder is often genetic and can cause additional symptoms, such as nausea. The underlying causes of migraines are not completely understood, but one theory is that the pain is linked to the trigeminal nerve, the sensory nerve for the head and face; and the dura, the brain's protective layer where blood vessels expand and contract.

One possible explanation for migraine pain is that an electrical event in the brain stimulates the trigeminal nerve pathways and sets off an inflammatory reaction. The inflammation spreads through the dural blood vessels and the trigeminal nerve fibers send signals back to the brain stem. The inflammation then spreads to pain-sensitive meninges — protective tissue around the brain — triggering a headache.

This cascade of inflamed blood vessels and irritated nerves is "a fire that's burning out of control," Clarke said. It's like a feedback loop that becomes more and more irritated, causing the experience of a migraine to build, he said. This is why many migraine treatments work better if deployed earlier.

While the relationships between pain around the body and head pain are well established, the mechanisms that cause headaches still aren't fully understood, Clarke said. But the good news is that "we are very good at treating these," he said. For instance, lifestyle changes, such as practicing yoga; over the counter medications, such as ibuprofen and aspirin; and prescription medication for more severe headache disorders can go a long way to reduce headache severity and frequency.

"If people need help [with headaches] we can often make them a lot better," Clarke said.