Headache, the Newsletter of the American Council
for Headache Education. Vol. 6, issue 4, December 1995.
[unbylined article by Carol Hart]. Copyright
©1995 American Council for Headache Education. Reprinted
with permission. For private use only, not to be reprinted.
When we burn a finger or bang a knee, we know what causes the pain. We also understand that the pain, however unpleasant, serves a purpose. It shouts the message, "Stop what you're doing--it's dangerous! And take care of this problem." With a headache, the mechanisms causing the pain, and the reason for the pain, are much less clear.
The brain itself has no sense of pain. The pain-sensitive parts of the head are the scalp and its blood supply, the head and neck muscles, the nasal sinuses, some of the arteries of the brain, the lower portion of the membrane surrounding the brain (the dura mater), and some of the neck and brain nerves. Although the pain is not in the brain, we would not feel pain in the head or any other part of the body without the brain's involvement.
The brain and the nerves communicate by means of a special group of chemicals called neurotransmitters. The neurotransmitters are essential for all nervous system functioning, including muscle contraction, sensory perception, thought, mood, and awareness of pain. Changes in the availability of the neurotransmitter serotonin are believed to have a primary role in the genesis of a migraine, and perhaps of other forms of headache as well. A number of migraine medications, including the antidepressants and sumatriptan, act to stabilize the brain's supply of serotonin.
Nerve signals, moving at speeds up to 100 feet per second, warn of danger, and we yank our finger off the hot stove before we are aware of what we are doing. The reflex of withdrawing the injured finger doesn't need the brain's involvement. It occurs at the level of the spinal cord. The pain signal is first processed by the brain stem at the base of the brain. The brain stem regulates many body processes, such as breathing and heart rate, that are involuntary and unconscious.
From the brain stem the pain signals pass upward to the thalamus, which lies at the very center of the brain. It partially interprets and relays sensory nerve signals to other parts of the brain. The thalamus is closely connected to the hypothalamus, a major regulator of sleep, appetite and other cycles. The hypothalamus is thought to have a role in cluster headaches and migraine. We become "aware" of pain and other sensory perception when the nerve signals reach the cortex, the wrinkled outer "gray matter" of the brain.
Neurotransmitters and other chemicals in the nervous system modulate perception of pain, magnifying or minimizing the pain in response to environmental factors or emotions. The endorphins, the body's natural opium-like chemicals, can reduce perception of pain. Athletes who suffer serious injuries during competition or persons injured while undergoing extreme stress sometimes report feeling very little pain. (At other times or for other people, stress may increase pain or be experienced as pain.) Narcotic drugs mimic these natural pain-relieving chemicals.
Other chemicals, including one known as "substance P," increase pain awareness. Serotonin has a role in promoting the release of substance P. Substance P, histamine, bradykinin, prostaglandins and other irritating chemicals are released by injured tissues. Nerves supplying the irritated tissue relay the "bad news" to the brain, and a throbbing pain is experienced in the tissue itself, or elsewhere along the path of the nerve. Aspirin and other NSAID (nonsteroidal anti-inflammatory) drugs relieve pain by suppressing some of these irritating substances.
The pain-sensing and -responding pathways of the brain and body work in a complex feedback system. The brain stem, which first receives the pain message, also has an important role in regulating pain perception--in effect determining how bad the pain should be. As we learn more about the role of the brain stem and other pain centers in the brain, we hope to develop better nonaddictive drugs that will act at this level to "influence" the brain's "decisions" about the painfulness of the injury, trigger or irritant that launches the pain response.
The pain of occasional headache, like the pain of a hurt finger, can be educational. It can tell us that too much alcohol, too little sleep, or excessive eye strain should be avoided. Or that we need to find better ways of coping with daily stress. Most chronic headache sufferers, in contrast, are believed to have abnormalities in the complex chemistry of the brain and nerves. They may be more sensitive to the effects of some of the pain-triggering chemicals of the body, or have an imbalance in some of the key neurotransmitters involved in pain response, such as serotonin.
brain stem--essential to many
life processes; contains the pathways connecting the brain and
spinal cord; now known to have an important role in migraine [back]
cerebellum--responsible for
coordination of movements; not known to have a role in headache
or pain perception
cortex--responsible for sensory
perception, decision-making, many aspects of behavior and the
integration of these functions
hypothalamus--an internal
"clock" and regulator that coordinates many body
functions and cycles; believed to have a role in migraine and
cluster headache [back]
limbic system--governs instinct
and emotional reactions; believed to have a role in the emotional
response to pain; the cingulate cortex is usually considered part
of the limbic system
thalamus--a central relay
station for processing and forwarding sensory perception,
including pain [back]
spinal cord--carries pain and
other sensory information from the trunk and limbs to the brain;
communicates the brain's commands to the nerves controlling
the body's muscles [back]
Headache, the Newsletter of the American Council for
Headache Education. Vol. 6, issue 4,
December 1995. Copyright 1995 American Council for Headache
Education. Reprinted with permission. For private use only, not
to be reprinted.