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Pain is not a direct reading of damage: the nervous system produces it, weighing signals from the body against danger, attention, memory and context. That is why the same injury can hurt very differently from one person or moment to the next.

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Nerve endings called nociceptors detect heat, pressure or chemicals that could harm tissue and send warning signals to the spinal cord and brain, but it is the brain that decides whether, and how much, it hurts. During the Second World War, the American anaesthetist Henry Beecher found that most of the badly wounded soldiers he treated did not ask for morphine, perhaps because surviving and leaving the battle outweighed the injury. In 1965, Ronald Melzack and Patrick Wall proposed the gate control theory: signals can be turned up or down in the spinal cord, which is partly why rubbing a knock helps. Phantom limb pain, felt in an arm or leg that has been amputated, shows that pain can exist where there is no tissue. In 2020 the International Association for the Study of Pain revised its definition, noting that pain and nociceptor activity are not the same thing.

Türkçe oku
Ağrı, vücuttaki hasarın birebir ölçümü değildir: Onu sinir sistemi üretir ve vücuttan gelen sinyalleri tehlikeyle, dikkatle, hafızayla ve içinde bulunulan durumla birlikte tartar. Bu yüzden aynı yara farklı kişilerde, hatta aynı kişide farklı anlarda çok farklı acıtabilir.

Nosiseptör denen sinir uçları, dokuya zarar verebilecek sıcaklığı, basıncı ya da kimyasalları algılar ve omuriliğe, oradan da beyne uyarı gönderir; ama acının olup olmayacağına ve ne kadar olacağına beyin karar verir. İkinci Dünya Savaşı sırasında Amerikalı anestezi uzmanı Henry Beecher, tedavi ettiği ağır yaralı askerlerin çoğunun morfin istemediğini gördü; belki de hayatta kalıp cepheden uzaklaşmanın rahatlığı yaradan ağır basıyordu. Ronald Melzack ile Patrick Wall 1965'te kapı kontrol kuramını öne sürdü: Ağrı sinyalleri omurilikte güçlenip zayıflayabilir; darbe alan yeri ovuşturmanın acıyı hafifletmesinin bir nedeni de budur. Kesilip alınmış bir kolda ya da bacakta hissedilen hayalet uzuv ağrısı, ortada doku olmadan da ağrı olabileceğini gösterir. Uluslararası Ağrı Araştırmaları Derneği 2020'de ağrı tanımını güncelledi ve ağrının, nosiseptörlerin etkinliğiyle aynı şey olmadığını vurguladı.

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Background reading: Pain
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The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

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The word placebo is Latin for “I shall please”. One of the first clues that the effect is physical came in 1978, when Jon Levine and colleagues studied patients recovering from wisdom tooth removal. They gave them a placebo and naloxone, a drug that blocks the effect of endorphins, the body's own painkilling chemicals. In patients who had responded to the placebo, naloxone increased the pain, which suggested that the placebo relief came from endorphins. In 2004 Tor Wager and colleagues used brain scans to show that a placebo painkiller reduced activity in pain-processing regions during a painful stimulus. Expectation works the other way too: in the nocebo effect, from the Latin for “I shall harm”, expecting harm makes symptoms worse. None of this means the pain was imaginary, or that placebos can replace effective treatment.

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