What does your pain feel like? Is it: flickering, quivering, pulsing, throbbing, beating or pounding? Is it punishing, grueling, cruel, vicious, killing, tender, taut, rasping, splitting, burning, searing, suffocating, tearing, penetrating, agonizing, annoying, wrenching, gnawing, cramping? Is it exhausting? -- Some qualities of pain as described in the McGill Pain Assessment Scale, developed by pain specialist Dr. Ronald Melzack

Alleviating pain is one of mankind's oldest goals. Possibly its oldest expression was the primitive witch doctor who stripped bark from a willow tree to brew the first salycilic tea. And almost as old, the happenstance -- at least 2,000 years ago -- when some healer serendipitously discovered the pain-killing powers of the juice of the poppy.

Today's pharmacological ancients -- aspirin from the willow bark brew and opium, morphine, dilaudid, codeine (and a slew of synthetics) from the poppy -- are still among the chief aids in the struggle against pain; until very recently, they were the only ones. Indeed, the 17th-century British physician Sir Thomas Sydenham once raved, "of all the remedies which the almighty God has seen fit to bestow upon mankind, none is so universal or so efficacious a remedy as opium."

The explosion in brain research over the past decade has led scientists to begin unraveling the mysteries of how the human organism literally can become its own poppy -- producing its own opium-like chemicals. They are beginning to tease out the intricate mechanisms by which someone hurts in one circumstance but not another, or why one person hurts with a given disorder and someone else -- with an identical physicial problem -- is barely discomforted.

Uncounted millions of Americans are in varying degrees of pain -- ranging from acute pain caused by something as common as hammering a thumb to intractable pain from something as lethal as the final stages of ravaging cancer.

Some 40 million Americans, say the experts, hurt all the time. These are the so-called chronic pain patients whose suffering is characterized by an inability to lead functional, even relatively normal, lives. They continue to hurt after any structural or mechanical injuries are healed, as far as the doctor can determine. Dr. Ronald Dubner, chief of the the National Institutes of Health's principal pain research unit -- the Neurobiology and Anesthesiology Branch of the National Institute on Dental Research -- says that for these people, the natural warning signal of pain "has become like a burglar alarm that nobody can shut off."

The experience of pain differs from individual to individual and from circumstance to circumstance. In addition to the 40 million with chronic pain syndrome, countless others -- some have estimated possibly 40 or 50 million more -- suffer intermittant chronic pain: backs "going out," so-called "pinched nerves" (which may or may not be really pinched), headaches, arthritis. These people suffer pain that may put them out of action periodically or temporarily, but does not necessarily rule their lives. The cost of chronic pain is estimated at between $40 billion and $70 billion a year -- in lost work, medical bills, pills, sometimes legal bills.

The plight of the chronic pain patient, once dismissed by even the most sympathetic doctors as imaginary or a collection of psychoneuroses or even deliberate malingering, is -- as a result of new discoveries -- getting more attention, improved treatment approaches and some marked successes. Scientists now realize that just because they can't find a mechanical cause for the pain there is no reason to deny that the pain and a cause for it exists.

And moreover, even though a lot of pain really may be in the head, it is as much the purview of the neurochemist as of the psychiatrist or psychologist. The professional appelations are changing rapidly to reflect the new approaches to the problem: psychobiologist, psychoneurologist, psychophysiologist, neuropsychiatrist. This cross-pollination of specialties reflects the new awareness of the complex intertwinings of mind, brain, biology, chemistry, physiology. Pain goes from the molecular level to the transient thought, from the nocicepter, the cell that recognizes that the hammer has hit the thumb, to complicated behaviors following a minor automobile accident generated because the brain's pain-producing and pain-relieving chemicals may act on the emotions as well as on the hurt.

The study of pain today is so vast, so complicated, encompassing so much of modern medicine, psychiatry and behavior, that some of its practitioners have given themselves a new name: algologists, from algo, Greek for pain.

The major pain researchers at NIH are housed in the dental institute -- and anyone who has had a toothache can understand why. The program at NIH's "Pain Central" also involves other institutes, encompassing those specializing in cancer, heart, lung, blood or neurological diseases, arthritis and neurology.

These researchers are trying to understand the different types of pain and looking at the body's own painkilling opiates. They are developing drugs that mimic those opiates as well as ways to make the body produce more of them. Pain Pathways

There are, the scientists have learned, several different pathways over which the brain receives and acts upon pain messages. Take a sprained ankle, for example. On one pathway the message of sharp pain moves swiftly -- at the rate of about 30 meters (about 98 feet) a second -- communicating to the thalamus in the midbrain the type of pain, where it is and how bad the hurt. Then, back at the ankle, the injured tissue secretes a chemical called arachidonic acid, which is changed into hormonal substances called prostaglandins.

One way to understand the process is to imagine these prostoglandins as tiny demons, hopping up and down, busily making the ankle swollen and inflamed and dispatching slower messages (taking a whole second to traverse a single meter) along a different set of nerve fibers. These messages presumably tell the person to get off the ankle until it gets better. The slower pain messages get into all sorts of places in the brain where moods and emotions, as well as pain perceptions, may be involved.

Researchers now know that aspirin (and aspirin-like drugs such as ibuprofin and other so-called non-steroidal anti-inflammatories) work by keeping the arachidonic acid from turning into prostaglandins -- cutting them off at the pass, as it were -- and thereby interrupting the nagging pain messages, the inflammation and the swelling. (Acetaminophin -- Tylenol, for example -- lessens pain but not inflammation, leading scientists to speculate that it does not work at the site of the injury.)

Both fast and slow messages are carried along their respective nerve fibers to a part of the spinal cord called the dorsal horns. This is the body's principal pain switching station, where pain reports are dispatched to the brain via neurotransmitters -- chemical brain messengers that are fired from neurons and sizzle from brain cell to brain cell, carrying their special message of pain. The dorsal horn "station" is also where return messages are mediated, messages sent from the brain using different neurotransmitters, ones that may relieve pain.

These chemical messengers -- neurotransmitters -- may work in several ways. They may simply keep any more pain messages from getting past the dorsal horns into the brain, or they may block the pain message at the cell level. Or they may function in ways not yet understood.

The first of these pain-fighting chemicals in the brain was identified in the mid-1970s and named endorphin (from endogenous morphine). Other neurotransmitters, including enkephalins and serotonin, have been linked to pain relief as well as mood states, because the pathways along which these messages travel go up into the brain's limbic system, the center of emotion and mood, as well as down to the pain station. Endorphins and enkephalins act on the same receptor sites in the brain as the drugs made from, or based on, the opium poppy juice. Others act on different receptors in different parts of the brain.

There are other chemicals the brain and other parts of the body produce when the organism is under stress, with some unexpected and still not wholly explained results.

Say, for example, that you're a primitive man, with that same sprained ankle. You haven't the conscious sense to stay off your foot, so your body and brain work together to make it too sore to use. But then, while you're sitting under a giant fern, along comes a saber-toothed tiger. Something in your brain clicks in, the stress hormones and neurotransmitters flow and in the wink of an eye you're hotfooting it away as fast as you can -- pain in the ankle superseded by fear of the tiger.

The modern protoypes of this are legion -- the wounded soldier who carries his disabled buddy behind the lines, unaware of his own injuries; the quarterback who finishes the game with a pulled hamstring; the marathon runner who finishes the race with a broken toe.

Stress clearly can block pain, and much current research is trying to explain how -- with, of course, an eye toward harnessing it. At a recent conference sponsored by the New York Academy of Sciences, researchers from all over the world compared notes on current findings. The potential is provocative, but the research is only just beginning to hint at answers. Pain Control

It's all very well -- and undeniably fascinating -- to understand some of the ways our brains run our lives. But one of the main reasons the scientists are trying to learn how the brain works is to help it do its job better. Once science learns how, for example, to augment the brain chemicals that make us feel better, it's a short step to making us feel best.

This might be through new drugs -- perhaps some that affect only the pain, not the moods that may be changed when the drug gets into other parts of the brain as well, or perhaps by learning how to turn on our own chemicals at will, especially when pain messages seem to refuse to turn off as is often the case in chronic pain.

A recent discovery may add a new class of drugs to relieve certain pain situations. Tricyclic antidepressants, drugs used to elevate moods, have been found in some cases to relieve pain as well. Research at the NIH clinic has demonstrated that one of these drugs called amytryptoline has an effect on relieving pain separate from its effect on depression. The research also showed that the expectations of the patients had an effect on how well the drug worked.

Scientists believe these drugs work by keeping the brain's feel-good chemical, serotonin, from being reabsorbed by the nerve cells after its message has been transmitted, so that it keeps on sending its messages of pain relief.

Pain researchers, notes Dubner, also are interested in techniques like transcutaneous electrical nerve stimulation (TENS), a portable device that offers some relief to many chronic pain patients by electrically stimulating the skin over the area that aches.

Because researchers have been able to identify areas in the brains of rats and other animals where the body's morphine-like chemicals are produced in particularly large quanitities, they have experimented with electrical stimulation of these areas to enhance the production of the natural opiates. Because this seemed to work in animals, neurosurgeons are now implanting electrodes in the brains of some people whose pain has failed to respond to anything else.

Dubner's team is assessing some patients who have had electronic brain implants. Ruth Faulk, a Southern Californian who had electrodes implanted by University of Southern California neurosurgeon Ronald Young last winter, was at NIH this spring for her assessment. Faulk had intractable pain of the hand and the arm that restricted virtually all activity, even driving. At NIH she reported great relief after her operation, so that she was even able to resume some intricate needlework she had had to give up earlier.

But despite the reports of relief from Faulk and others who have received the implants, the NIH researchers have so far been unable to confirm that the relief is caused by stimulation of the natural opiates. Pain Assessment

One way pain specialists determine either what causes a pain or which pathways it is using (and therefore which treatment might help) is by helping an individual who is hurting best describe the sensation.

There are some pain sensitivity tests that can be performed on both humans and animals, but to distinguish between the intensity of the pain and the unpleasantness of the same pain requires the human's "unique ability to symbolize pain in language," says psychologist Richard Gracely, who works with Dubner. Gracely has devised some questionnaires to test for these two pain components.

"It's worked out," says Dubner, "because, for example, we've been able to show that drugs that reduce anxiety, such as Valium, will affect the unpleasantness component but not the intensity, and narcotics will affect the intensity, and usually not the unpleasantness component.

"We were among the first to show that drugs such as opiates narcotics actually do influence pain intensity, when many believed that they only affected how you felt about the pain."

"There is," says pain specialist Dr. Bruce Smoller, orthopedist and psychiatrist, and psychiatric consultant to the NIH pain unit, "some psychological, some physical or structural and some neurochemical components in all pain. Sometimes the psychological is more important, sometimes it is the physical."

Smoller and other psychiatrsts have found personality clusters around certain pain that becomes chronic. The pain, for example, of the upper back, neck and head known as myofascial syndrome often occurs in patients who are "perfectionists, subject to a lot of deadlines," says Smoller, "and a lot of demands. Lower back patients tend to be rigid, angry at not becoming cured, and tend to have unreal expectations. But the stresses of daily living affect the upper back much more. Lots of stresses become transduced easily into pain."

Approaches like biofeedback -- which Smoller says can reduce the amount of pain by 25 to 50 percent -- family therapy and certain physical therapies can be "bridges between the physical and the psychological."

And Gracely, who also works with Smoller in his private practice, adds, "What is so insidious about chronic pain is that only a short time after it begins, it makes not-so-subtle changes in almost every apsect of your life -- your social life, financial, behavioral, communications. It affects your sense of humor, causes personality changes and the distressing sensations become just a small part of the whole syndrome. Proper therapy needs to be directed at all these different parts. One alone isn't that effective.

"People think of the doctor as a mechanic and 'I'm bringing my old Chevy of a body and I'll get a new carburetor and be back on the road.' "

"That's a much too simplistic view," says Gracely, "of a very complex matter." Pain, most specialists agree, can be managed, and managed very well. But although it can often be put in its place, it cannot always be totally banished. Resources

Programs offering a variety of approaches to pain control, with or without drugs, are increasingly available to chronic pain patients. Area resources include: National Chronic Pain Outreach Association Inc. Newsletter ($5 per year) and support groups. For information on chapters in Potomac, Manassas and Alexandria, write 8222 Wycliffe Court, Manassas, Va., or phone (703) 368-7357. Members compare techniques and prices of apparatus (as, for example, TENS units), hear lectures on advances and offer general support. Psychiatric Institute Pain Management Program. A behaviorally oriented inpatient program, several weeks long, for seriously disabled pain patients. Director, Dr. Ronald Harmon. Phone: 828-1800 for information.Washington Pain Assessment Group, Dr. Bruce Smoller, medical director. 4520 East West Hwy., Bethesda, Md. 20814. Phone: 951-3325.Washington Pain Center, Dr. Lorenz K.Y. Ng, medical director. Phone: 387-4735. 2026 R St. NW Washington D.C., 20009.