How you feel pain

Your experience of pain is part biology, but it's also influenced by psychological and cultural factors. Despite years of research, questions linger about exactly what happens between the moment you stub your toe and the moment you say "ouch."

How pain messages travel

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

• Your peripheral nerves. These nerves extend from your spinal cord to your skin, muscles, bones, joints and internal organs. Some peripheral 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.

  Nociceptors are most concentrated in areas prone to injury, such as your fingers and toes. When nociceptors detect a potentially harmful stimulus — such as the hard surface that stubbed your toe — they relay 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 deep inside your brain. The thalamus forwards the message simultaneously to three specialized regions of the brain: the physical sensation region that identifies and localizes the pain (somatosensory cortex), the emotional feeling region that experiences suffering (limbic system), and the thinking region that assigns meaning to the pain (frontal cortex). Your brain can respond to pain by sending messages to the spinal cord that modulate the incoming pain signals.

How pain messages travel

How you react to pain messages

Some of our current understanding of pain is based on the "gate-control theory," which grew out of observations of World War II veterans and their reactions to different types of injuries. The central concepts of gate-control theory are:

• 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, may be filtered or blocked out by the gate.

  Nerve fibers that transmit touch also affect gatekeeper cells. This explains why rubbing a sore area — such as the site of a 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, increasing the strength of the pain signal that reaches your brain. This is called wind-up or sensitization. At the same time, inflammation at the site of injury may add to your pain.
• Messages 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
• Age
• Sex
• Beliefs and values
• Social and cultural influences
• Attitude
• Expectations

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. Fear and anxiety often increase the intensity of the pain experience.

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.

The location of your pain can affect how you perceive it. Pain that you can escape by assuming a more comfortable position is easier to tolerate than a constant headache that interferes with work or concentration.

How you feel chronic pain

When pain persists longer than expected, it can become a chronic condition — in other words, an illness unto itself. Pain is generally described as chronic when it lasts six months or longer.

Chronic pain may remain constant, or it can come and go, like the pain of migraines. As with acute pain, chronic pain can feel:

• Tingling
• Jolting
• Burning
• Dull
• Aching
• Sharp

Sometimes, chronic pain is due to a chronic condition, such as arthritis, which produces painful inflammation in your joints. Chronic pain may also stem from damage to 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.

Nerve damage can result from:

• Accidents
• Infections
• Surgery

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.

If you have a mood disorder such as depression or anxiety, you're more likely to experience chronic pain — and to feel it more intensely — than do people without a mood disorder. Treating mental health conditions can improve or even eliminate chronic pain.

Sensitization turns up the pain volume
Doctors and researchers think chronic pain may be partly caused by a process called sensitization. In this process, your nervous system amplifies and distorts pain, much like the way your stereo speakers distort the character of recorded music when you crank up the volume. 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.

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• Chronic pain can interfere with sexuality
• Phantom pain
• Complex regional pain syndrome
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