IRENE TRACEY
Everybody hurts, but why? We explore the science of pain
Everybody hurts, sometimes. Irene Tracey, a neuroscientist at the University of Oxford, guides us through the science of pain.
We may have learnt to tame it with drugs, but pain is one of the certainties of human existence. It can be both physical and emotional, ranging from a searing torment to a mild soreness. But what exactly is it, what function does it serve, and how can we really know how much pain someone is in?
What is pain?
Simple, you’d think. You touch a hot saucepan by mistake and it hurts like hell. You immediately withdraw your hand, rush to the tap and run cold water over it. Phew. No need to rush to A& E. But then it throbs for days, reminding you of the burn and your carelessness until the pain fades away. Lesson learned: you’ll be more careful around cookers in future.
This simple incident can tell us a lot about pain. Mostly, it’s a brilliant warning system. Without it, you would not have withdrawn your hand, and the injury would be much worse. Pain like this – what we call ‘acute pain’ – is a good thing: it’s key to our survival. That’s why the ability to experience pain is shared across species. A few people include plants in this, too, but as plants have no nervous system or brain, it’s hard to know how they’d actually feel pain when injured or cut. Pain is evolutionarily old, an essential warning that something in the environment can cause us injury, harm, or even death.
Without pain, you’re in trouble. We know this, sadly, because there’s a rare genetic condition, which we call ‘congenital insensitivity to pain’ or WORDS: PROF IRENE TRACEY CIP, in which a person doesn’t get the warning ‘hurt’ of pain after severely damaging themselves. Historically, they didn’t survive to adulthood due to the consequences of unfelt injury.
What does pain do?
Pain motivates us to act. Think about that hot pan again. Now imagine you’d picked up the pan before realising it was too hot to handle. Your options are to drop it and make a mess, or bear the pain until a solution is found. In an instant, you detect that the pan is hot (thermal), it’s on your hand ( location), it’s painful (intensity), you don’t like it (unpleasant), it’s engaged your full attention (cognition), and you’re not happy about it (emotional). That’s a lot of things, which is why pain is often called a ‘multidimensional’ experience.
So, what do you do? Well, from past experiences, learnt responses, and potential outcomes (like being told off for dropping the pan) you make a decision and act. Recruiting extraordinary brain-based networks, you are able to block the pain and get the hot pan to safety – then it’s back to that cold tap. Pain drives action, prompting us to run away, avoid it in the first place, or signal to others that we need help and relief.
How do we feel pain?
Just underneath our skin surface, we have an intricate network of ‘pain nerve fibres’ that end with special
receptors called nociceptors. When activated, these receptors send signals along the nerve fibres to the spinal cord and up into the brain, where pain, as a perception, emerges.
The nociceptors can be activated by a variety of triggers: thermal ( heat), mechanical (like a knife cut or hammer blow) and chemical/irritant (for example, acid or chilli pepper). The signals then travel along different V[ RGU QH RCKP PGTXG HKDTG #Ť CNUQ known as A-delta, fibres carry what we call ‘first pain’ – the fast, quick signal that tells you ‘ouch’ when you touch a hot pan. C fibres follow up with the ‘second pain’, which is the slow, constant throbbing that tells you it’s still hurting. Normal touch – feeling something like your clothes or holding a pen – is carried on different peripheral nerves called Aß (A-beta).
The transmission of the pain signal to the spinal cord is helped by other components of the nerve fibres called ion channels, and it’s these that many patients with CIP don’t have, which is why they don’t feel pain. Therefore, targeting and blocking nociceptors and/or the transmission process is a cunning way to block pain. Indeed, that’s what many pharmaceutical companies are currently trying to do.
Interestingly, many nociceptors are ‘polymodal’ – a fancy word meaning that different things can activate the same nociceptor. As an example, let’s look at temperature and food.
How can food be painful?
Different thermal nociceptors in our body are activated by specific temperatures, giving us a painful sensation of intense hot or cold. Amazingly, these same nociceptors are also activated by various natural chemicals, giving rise to the same experience. For instance, when we bite into a chilli pepper, a chemical called capsaicin binds to the same nociceptor that’s activated by painfully hot temperatures of around 42°C and above. That’s why we perceive a curry as hot: the brain can’t distinguish what activated the nociceptor, it just knows that your mouth is burning! Downing a pint of lager won’t help, either, as capsaicin is fat-soluble, not water-soluble – so order a cucumber raita instead.
Some scientists think that plants produce capsaicin to discourage mammals from eating their fruits. Birds don’t seem to react to the chemical, allowing them to eat the chillies and help with seed dispersal, which is what the plant wants!
How does the brain generate pain?
Once the pain signals arrive from