Jabbing, throbbing, burning, stinging, tingling, nagging or aching — pain comes in many forms. Acute pain warns of tissue damage. It's the type of pain that generally accompanies illness, injury or surgery. It may be mild and last just a moment, such as pain from a sting. Or it can be severe and last for weeks or months, as pain from a burn, a pulled muscle or a broken bone sometimes does.

How you feel pain is a complicated phenomenon. Your individual experience of pain is part biology, but it's also influenced by a range of psychological and cultural factors. In fact, despite years of research, questions still remain about exactly what happens between the moment you stub your toe and the moment you utter "ouch" — or some other choice word.

Pain basically results from a series of exchanges among three major components of your nervous system:

• Your peripheral nerves. Your peripheral (pe-RIF-er-ul) nerves extend from your spinal cord to your skin, muscles and internal organs. These nerve fibers vary in their size and the rate at which they conduct messages to the brain. The receptors at the ends of these nerves vary as well. Some types of nerve fibers end with receptors that respond to touch, pressure, vibration, cold and warmth. Other types of nerve fibers end with nociceptors (no-sih-SEP-turs) — which are receptors that detect actual or potential tissue damage.

  You have millions of nociceptors in your skin, bones, joints and muscles and in the protective membrane around your internal organs. But their greatest concentration is in areas prone to injury, such as your fingers and toes. When nociceptors detect a harmful stimulus — such as the hard surface that stubbed your toe — they relay their pain messages in the form of electrical impulses along a peripheral nerve to your spinal cord and brain. Sensations of severe pain are transmitted almost instantaneously.

• Your spinal cord. The nerve fibers that transmit pain messages — such as the throbbing pain from that stubbed toe — enter the spinal cord in an area called the dorsal horn. There, they release chemicals (neurotransmitters) that activate other nerve cells in the spinal cord, which process the information and then transmit it up to the brain.
• Your brain. When news of your stubbed toe travels up the spinal cord, it arrives at the thalamus — a sorting and switching station located deep inside your brain. The thalamus quickly forwards the message simultaneously to three specialized regions of the brain: the physical sensation region (somatosensory cortex), the emotional feeling region (limbic system) and the thinking region (frontal cortex). Your brain responds to pain by sending messages that moderate the pain in the spinal cord.

Severe pain grabs your attention more quickly and generally produces a greater physical response than mild pain. The location of your pain also can affect your perception of pain. A headache that interferes with your ability to work or concentrate may be more bothersome — and therefore receive a stronger response — than arthritic pain in your knee or a cut to your finger.

Before the mid-1900s, doctors generally believed that pain signals traveled from your pain receptors through your pain fibers to the pain center in your brain, much the way a telephone message travels along a telephone line. And they believed that the intensity of pain was directly related to the amount of actual tissue damage.

During World War II, doctors found many veterans whose injuries didn't correlate with the amount of pain they experienced. These veterans were partly responsible for new theories about pain, including the gate control theory. This theory has been criticized and modified over the years, but some of its general concepts are still accepted:

• Pain messages don't travel directly from your pain receptors to your brain. When pain messages reach your spinal cord, they meet up with specialized nerve cells that act as gatekeepers, which filter the pain messages on their way to your brain. For severe pain that's linked to bodily harm, such as when you touch a hot stove, the "gate" is wide open, and the messages take an express route to your brain. Weak pain messages, however, such as from a scratch, may be filtered or blocked out by the gate.

  The gate is also affected by messages on the nerve fibers that transmit touch. This explains why rubbing a sore area — such as the site of your stubbed toe — makes it feel better. The signals of touch from the rubbing actually decrease the transmission of pain signals.

• Messages can change within your peripheral nerves and spinal cord. Nerve cells in your spinal cord may release chemicals that intensify the pain, affecting the strength of the pain signal that reaches your brain. This is called wind-up or sensitization. At the same time, a series of reactions takes place at the site of injury, which causes inflammation. This results in swelling, redness, warmth and increased pain, but inflammation also increases blood flow and promotes healing.
• Messages coming down from your brain also affect the gate. Rather than just reacting to pain, your brain actually sends messages that influence your perception of pain. Your brain may signal nerve cells to release natural painkillers, such as endorphins (en-DOR-fins) or enkephalins (en-KEF-uh-lins), which diminish the pain messages.

This last idea explains how your brain — and its psychological and emotional processes — can affect your experience of pain. In fact, how you interpret pain messages and tolerate pain can be affected by your:

• Emotional and psychological state
• Memories of past pain experiences
• Upbringing
• Attitude
• Expectations
• Beliefs and values
• Age
• Sex
• Social and cultural influences

For example, a minor sensation that would barely register as pain, such as a dentist's probe, can actually produce exaggerated pain for a child who's never been to the dentist and who's heard horror stories about what it's like.

But your emotional state can also work in your favor. Athletes can condition themselves to endure pain that would incapacitate others. And, if you were raised in a home or culture that taught you to "Grin and bear it" or to "Bite the bullet," you may experience less discomfort than do people who focus on their pain or who are more prone to complain.

When pain persists beyond the time expected for an injury to heal or an illness to end, it can become a chronic condition. No longer is the pain viewed as just the symptom of another disease, but as an illness unto itself. Pain is generally described as chronic when it lasts six months or longer.

As with acute pain, chronic pain can feel tingling, jolting, burning, dull, aching or sharp. It may remain constant, or it can come and go, like the pain of migraines.

Sometimes, chronic pain is due to a chronic condition, such as arthritis, which produces painful inflammation in your joints. Occasionally, chronic pain may stem from an accident, infection or surgery that damages a peripheral or spinal nerve. This type of nerve pain is called neuropathic (noor-o-PATH-ik) pain — meaning the damaged nerve, not the original injury, is causing the pain. Neuropathic pain can also result from diseases such as diabetes or in the aftermath of shingles (postherpetic neuralgia).

Occasionally, the cause of chronic pain isn't well understood. There may be no evidence of disease or damage to tissues that doctors can directly link to pain. Or pain may remain after the original injury shows every indication of being healed.

Doctors and researchers think chronic pain may be partly caused by a process called sensitization. Although this process is complex, the basic idea behind it is straightforward. Sensitization can be compared to what happens to the sound when you crank up the volume on your stereo. The pain message gets amplified and distorted, much as music blasted through regular speakers does. The result is a painful condition that is severe and out of proportion to the disease or original injury.

In the peripheral nervous system, sensitization can result from inflammation, which causes your nociceptors to fire with greater intensity, for a longer time, and at a lower threshold than usual. In the spinal cord, sensitization is the result of chemical reactions that increase pain messages being sent to your brain. Sensitization may affect all the pain-processing regions of your nervous system, including the sensing, feeling and thinking centers of your brain. When this occurs, chronic pain may be associated with emotional and psychological suffering.

Pain researchers are focused on identifying the biology that underlies sensitization. They're also investigating other genetic and psychological factors behind how you feel pain, with the goal of developing new and better pain treatments.