If you have ever been rejected by someone you will know how it feels. As if they have punched you hard in the guts. Last week researchers found out why - the brain registers mental and physical pain in much the same way. A team from the University of California measured blood flow to the brains of volunteers who were put inside a large imaging machine, given goggles and told to play Cyberball, the video game. The volunteers were told that the electronic ball they saw on the screen was being passed around by real players. In fact, it was controlled by a computer that deliberately didn't pass the volunteers the ball. The result? They felt hurt and left out. Blood rushed into the part of their brains that feels pain, lighting up the imaging screen.

Pain, both mental and physical, is something we all experience. From the extremes of childbirth and broken hearts, to childhood fractures and stubbed toes, we all hurt. Many of us hurt all the time. According to statistics released yesterday, one in seven people in Britain are in constant pain. Of those, one in 10, will have developed their symptoms in their teens, and one in five will be diagnosed with depression as a result. There are countless different kinds of pain and countless ways to feel it, yet we have a limited understanding of it and many continue to suffer because of inadequate treatment.

"There's a tendency in our culture to differentiate between physical and mental pain," says Matthew Lieberman, assistant professor of psychology at the University of California. "When people have pain from a broken arm we don't hold them accountable for getting over it, but we pay lip service to the mental pain of depression and anxiety - we tend to think people have some control over them. But the pain of a broken arm and the pain of someone leaving you both stimulate the brain in a physical way - so you feel hurt."

Lieberman says that the way the brain feels pain is likely to be more complicated than we can imagine. The traditional theory is that pain is a linear alarm system. Your hand touches something hot or sharp and triggers the pain nerves in your skin to send electrical signals through your spinal cord up to the brain to warn that you are in danger.

But in the 1960s, researchers found that the brain didn't always get the warning. Signals from other nerves that feel sensations such as light touch could, if triggered, block the pain nerves. This is why, if you bump your elbow and rub it, it doesn't hurt as much. Modern imaging techniques that measure blood flow to the brain have further challenged our understanding of pain by showing that all sorts of brain areas light up in response to pain.

The traditional view is that it is caused by tissue damage. How much pain you feel should, therefore, be related to how bad your injury is. But again, this often isn't the case. "The degree to which you feel pain is influenced by many impulses reaching the brain," says Dr Cathy Stannard, a consultant in pain medicine at Frenchay hospital, Bristol. "In fact, the onward transmission of pain from the spinal cord is modified by impulses coming down from the brain as well as activity in other nerves that don't carry pain messages. Your culture, gender, beliefs about pain and the social context all modify how much pain you feel."

Although pain is meant to make you remove yourself from the source of pain and to get help, it can be overruled in the heat of the moment. In May this year, Aron Ralston, from Aspen, Colorado, was climbing alone in a remote canyon when he dislodged a 800lb boulder that fell on his right forearm, trapping him against the rock face. After five days and no sign of rescue, Ralston, 27, decided he had to free himself, so, using a small penknife, he hacked off his arm, allowing him to escape.

"Everyone knows of stories of sportsmen who have broken bones and carried on playing," says Stannard. "On the battlefield soldiers will come into field hospitals with a foot blown off but use innocuous language to describe how it happened - they'll say they felt a thump on their foot. But once they are out of the battle and you stick a needle in their arm to give them blood, they'll say the needle hurt in much the same way as anyone else would say it hurt."

People who have had limbs amputated may complain of terrible pain where their leg used to be. "We know pain can happen without any tissue damage, " says Stannard. "The pain processing system is very complicated. People who have had their legs cut off can get phantom limb pain. The nerves don't have any input, they don't feel anything, but people get pain. It may be that if nerves don't get any normal sensations reaching them they get hyperexcitable and start firing off pain signals spontaneously."

Different parts of the brain process the two main components of pain. One bit processes the emotion - how unpleasant it is - while the other registers what it feels like and where it is. If the emotional part, called the anterior cingulate cortex is damaged (for example by a stroke), people can feel how bad a pain is and where it hurts but it doesn't upset them.

So complicated is the pain processing system that when it goes wrong it can have long-term effects. One of the great debates in pain medicine is why people develop chronic pain. Campaigners on behalf of chronic sufferers argue that it is a disease in its own right. Yet it is still a mystery to many of those treating it.

"Acute pain has served people well by warning them of danger," says Stannard. "But why have we evolved into getting chronic pain - it's like a glitch in an otherwise robust system. We don't know why persistent pain develops. Doctors can search for a cause but if they don't find it they are left to try to treat the pain rather than a condition."

If there is no external cause for pain and nothing is revealed on x-rays or scans, people are sometimes told their pain is psychological. Stannard warns that this misses the complexity of the pain process. "Damage to any part of the system can make you experience pain," she says. "Scans show the structure and anatomy of the body but not how things work."

If their backs were to be x-rayed, most people in their 70s would show signs of wear and tear. But while some with perfect spines complain of terrible back pain, others, with severe degeneration, don't seem to suffer at all. "How some people can spend their lives in pain without any obvious physical cause is a mystery - it shows our failure to understand what pain is," says Dr Simon Farmer a consultant neurologist at Queens' Square neurological hospital in London. "Some people's pain systems are clearly set differently. It may be that there is a strong genetic predisposition to how we feel pain."

There are also behavioural influences at work. "Fear and poor understanding can determine how disabled someone gets with back pain," says Stannard.

"If you have pain you avoid things that cause pain, so you sit in bed and rest. This makes back pain worse. If you can reassure someone that their back pain is normal, that it won't get worse, that their spine won't crumble, that what they have isn't life threatening, you can get them active again. We need to educate people out of the biomedical idea that pain is always a symptom of something wrong in the body, that once it has been investigated it can be treated and will go away. There is not a straight-line relationship with pain."

"People will say they've got a high pain threshold but research shows that everyone's pain fibres fire off at the same amount of painful stimuli. It depends how well they tolerate it."

Among the factors that influence tolerance is how people around you respond. "If people are solicitous, tell you to sit down and say, 'how awful for you', it reinforces your behaviour," says Standard. "If they ignore it then your pain behaviour tends to be extinguished."

This ability to influence the emotional part of how we feel pain can be extended to psychological pain. Lieberman believes that just as rubbing a hurt elbow can relieve the pain, so distraction techniques can reduce emotional pain. "There is evidence that thinking about negative experiences and emotions dampens them down," he says.

"This only works if you think about them using language - if you talk about them or write poetry - and not if you re-run events in a visual way. Processing negative emotional experiences through language seems to stop the hurt. That's why people tend to write diaries and poetry when they're sad - not when they're happy."

The painful facts

· Research shows that women feel pain more intensely than men. But we all know they make less fuss about it.

· Babies who have been circumcised without anaesthetic seem to get sensitised to pain. Studies show they find vaccinations at three months more painful than other infants.

· In China 10% of adults have anaesthetic for tooth-drilling compared with 99% of patients in the US.

· Research suggests that cultural patterns of behaviour shape our response to pain. There are differences in how two-month-old Chinese children behave in response to pain compared with Canadians of the same age.

· In hospitals people from ethnic minorities are given less pain relief for broken bones than white people. There's no evidence that people from ethnic minorities have less pain.

· People with chronic pain often feel angry and resentful. A new study shows that using "loving-kindness meditation", a Buddhist approach to developing love and forgiveness, can reduce pain. When people with chronic back pain were taught to understand and feel compassionate towards people they felt angry with, their pain improved and they were less likely to get depression.

· We are not very good at consciously remembering pain. "If we didn't forget the pain of childbirth then we wouldn't go on and have other children so clearly there is a reason linked with survival," says Dr Cathy Stannard.

Pain killers

Paracetamol: For muscle aches and mild to moderate pain. Doesn't irritate the stomach so often tried before aspirin. Don't overdose - it damages the liver and kidneys.

Aspirin: For mild to moderate pain. Don't use if you have asthma or kidney or liver disease. Never give to children under 16 as it can cause Reye's disease. Can cause allergic reactions and gut irritation.

Ibuprofen: Used for arthritis and headaches. Can cause stomach ulcers and sickness and diarrhoea, as well as allergic reactions.

Codeine: For mild to moderate pain. From the same family as morphine but much weaker. Causes constipation.

Morphine: Best for moderate to severe pain, especially abdominal rather than muscular aches. Prescription only. Side effects include sickness and sleepiness.

Source: The British National Formulary (the medicines bible for doctors)