Pain is among the most powerful of sensations, and is the body's way of drawing attention to disease. It originates in the pain fibers, which are the thinnest of nerves and are present in an intricate network all over the body. Pain fibers are thin, long projections from the nerve cells for pain.These cells are present in bodies called ganglia lying along the side of the spine. Each point in the body sends its pain information to more than one nerve cell. The nerve fibers from the cells branch and overlap. The pain information reaches the nerve cell in the ganglion, and then goes via another fiber, from the same nerve cell, into the spinal cord. In the spinal cord nerve fibers take the information to the brain, while other fibers send pain information to nearby spinal cells. The fibers that go to the brain are the ones that inform the brain about the site of origin of pain. The fibers that go to nearby spinal cells begin a "reflex arc" that can result in withdrawal of the part of the body that feels the pain. Thus touching a very hot object causes immediate withdrawal of the hand even before the brain registers the information about the object.

The nerve supply to the different areas of the body is fairly constant. This means that if a particular site in the body is insensitive to pain, it is usually easy to determine where in the nervous system the actual disorder lies. The other advantage is that to produce anesthesia at a given region, one only has to block the nerve that supplies the region.

The stimulus that produces pain on the skin is usually one which may cause tissue damage, such as cutting, pricking, crushing, or excessive heat or cold. Ischemia, or lack of blood supply, can produce an extremely severe pain. Inflammation produces moderate to severe pain, which is often throbbing or burning in character. Cancer, unfortunately, produces pain only when advanced.

Mediators of pain

While direct damage to a pain nerve fiber can cause a sensation of pain, the sensation is often caused by disease, which releases chemicals that stimulate the pain nerves. The chemicals involved in the production of pain include acetylcholine, 5-hydroxytryptamine, kinins, prostaglandins and several others. Many of the clinically useful painkillers are prostaglandin inhibitors.

Superficial and deep pain

Pain may be superficial or deep in origin. Superficial pain is felt in the skin, muscles and bones. The site is easy to localize, as years of viewing establish localization of a stimulus to its origin to within a square centimeter or so. Pain is usually more severe than deep pain, sharper and better localized. The skin can identify cutting, pricking, burning etc sensations. Lack of blood supply to muscles causes marked pain. Deep pain, or visceral pain, arises from the deeper organs such as the heart, lungs, and intestines. Pain is poorly localized, because the eye does not see these organs and correlate with the lesion. Cutting and crushing often do not cause pain in most of the visceral organs. Some diseases release chemical mediators, and the result is perceived as a sensation of mild to moderate deep pain. Visceral organs are, however, sensitive to lack of blood supply. Ischemia produces extremely severe pain. A heart attack, for example, is caused by ischemia of the heart muscle. Angina is a term used to denote ischemic pain, and while it is commonly used for the heart, angina may occur in the intestines and in superficial muscles as well.

Referred pain

In many cases the same part of the spinal cord supplies both the skin as well as a deeply situated organ. As an example, the same nerve root (4th cervical) provides nerve fibers to the diaphragm and to the shoulder. Irritation of the diaphragm is thus often perceived as a pain in the shoulder. This form of pain is called referred pain. Pain from the heart may be referred to the inner arm, and pain from the ureter may be referred to the inner side of the thigh and the genitalia.

Radiating pain

A disease at the origin of a nerve may be perceived as pain along the entire pathway of the nerve. A typical example is sciatica. The disease lies in the bones of the spine, which press upon nerve fibers that later enter the sciatic nerve. The pain is perceived as pain along the entire path of the sciatic nerve, ie pain along the back of the leg. This sort of pain is said to be radiating.

Making a diagnosis from pain

Doctors make most of their diagnoses from a careful evaluation of pain.

The site of pain often indicates the organ involved. Thus a headache implies a disease in the brain or its associated layers (meninges) or the skull. Pain in the chest occurs from diseases in the heart or lungs. Pain in the upper abdomen is caused by diseases of the stomach, the duodenum, liver and gall bladder. Pain in the middle abdomen is caused by diseases of the intestines and pancreas, and pain in the lower abdomen from the large intestines or (in females) from the reproductive organs in the pelvis.

External causes of pain are easy to diagnose, and injuries, abscesses, ulcers and other lesions are visible.

In the head, pain arises from the meninges (coverings of the brain), since brain tissue itself has no pain fibers!

In the chest, the main diseases are heart attacks and lung diseases. Pain from the lungs occurs on the sides rather than in the center, while pain from the heart is more central and tends to be felt a crushing force in the chest.

Abdominal pain is more difficult to diagnose.

Treatment of pain

Pain should be treated promptly and adequately, provided the diagnosis has been made. However if relief of pain is likely to mask the symptoms and increase the likelihood of overlooking a serious condition, treatment of pain assumes secondary importance.

Painkillers (analgesics) are commonly available drugs, and in general fairly effective. They may be narcotic analgesics or non-narcotic analgesics. The former are more effective, but can cause respiratory depression and addiction, and are therefore reserved for severe pain.

For ordinary purposes the non-narcotic analgesics suffice. These drugs, also called NSAIDS (nonsteroidal anti-inflammatory drugs) usually interfere with prostaglandins in the brain, and at the site of disease, and decrease inflammation as well as pain. Examples of such drugs are aspirin, paracetamol, ibuprofen, ketoprofen, naproxen, diclofenac,ketorolac, piroxicam and nimesulide. The main side effects of these drugs is gastrointestinal upset and bleeding. Aspirin interferes with clotting of blood, and causes worse bleeding than others. Most drugs can, rarely, harm the kidneys. Some of them worsen asthma.

Narcotic analgesics are usually (but not always) given by injection. The most powerful narcotic analgesic is morphine. Others are pethidine, pentazocine, tramadol, propoxyphene and codeine. They are effective in almost all forms of pain, but can cause respiratory depresssion and addiction. Morphine and its analogs produce constriction of the pupil of the eye.

Combinations of NSAIDS and milder narcotic drugs are often effective. Combinations of NSAIDS with propoxyphene are commonly used.