Mechanism
Intermediate
9 min read
Pain
Forty-six pages of this corpus use the word as a boundary, none made it a subject. The essential distinction is between the signal sent and the experience lived: they are not the same thing.
A word used everywhere as a border
Forty-six pages of Body Lab mention pain, and almost always for the same reason: to mark where an explanatory page stops and a health professional starts. That border does not move — it is even the subject of the end of this page.
But between the two there is a mechanism, and it is part of the basics of how the nervous system works. Not describing it leaves room for explanations that circulate and are wrong.
What the international definition says
The body that holds authority on the subject revised its definition in 2020, for the first time in forty years.
Certainty level · Established
Pain is defined as an unpleasant sensory and emotional experience associated with — or resembling that associated with — actual or potential tissue damage.
The revision came from an international, multidisciplinary task force, with consultation of the people concerned. It comes with explicit notes, two of which matter here: pain is always a personal experience, and a person's report of an experience as pain should be respected.
Raja SN, Carr DB, Cohen M, Finnerup NB, Flor H, Gibson S, Keefe FJ, Mogil JS, Ringkamp M, Sluka KA, Song XJ, Stevens B, Sullivan MD, Tutelman PR, Ushida T, Vader K (2020)
Two parts of that definition do all the work. Experience: pain is lived, it is not a physical quantity. Or potential: no lesion needs to exist for a pain to exist.
The signal and the experience are not the same thing
Nociception
The detection and transmission, by specialised receptors and fibres, of potentially damaging events — excessive heat, strong pressure, substances released by injured tissue. It is a signal, not a sensation.
damage signalling
The distinction underpins the whole field. Nociception is what fibres carry; pain is what a person undergoes. Passing from one to the other is neither automatic nor proportional.
Certainty level · Established
The nociceptive signal is modulated all along its route: it can be amplified or damped before resulting in a painful experience.
The mechanisms are described at several levels — on entering the spinal cord, through descending pathways from the brainstem, and at the level of brain networks. This is what explains why the same stimulation does not produce the same pain depending on context, attention or the person's state.
Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021) · Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018)
Both dissociations exist and are documented. A serious injury may not hurt at the time — a wound discovered afterwards, with no memory of pain. And intense pain can exist with no lesion identifiable on examination.
What this above all does not mean
This is the most important passage on the page, and the most often misread.
Caution
Three false, and dangerous, readings
"So pain is in your head." No. Every experience is produced by the nervous system — that is true of sight, of taste and of the position of a limb. Saying that a pain is produced by the nervous system makes it neither imaginary, nor voluntary, nor less real.
"So you can decide not to hurt." No. The modulation described here is not voluntary. It can no more be commanded than heart rate can.
"So pain without a visible lesion is not to be taken seriously." The opposite. The international definition states explicitly that a person's report of an experience as pain should be respected, and the absence of a cause found on examination invalidates nothing.
What this illuminates elsewhere in the corpus
The muscle soreness page describes a delayed pain whose causal chain is not established. The distinction set out here explains why an account in terms of damage alone does not suffice: between a tissue stress and a sensation, several stages of modulation come in between.
The what limits range of motion page observes that a movement often stops on a sensation rather than a mechanical stop. Proprioception supplies part of that sensation, nociception another.
Why no scene accompanies this page
Limit
Pathways absent from the models
The receptors, fibres and pathways described here belong to the nervous system, which the anatomical sources Body Lab uses do not contain. Modulation, for its part, cannot be depicted: it is not a structure but a way of working.
What this page does not do — and where Body Lab stops
Caution
No assessment, no treatment
This page describes a general mechanism. It assesses no pain, identifies no cause, and proposes no treatment, no technique and no exercise. Persistent pain is an entire clinical field, with mechanisms, investigations and care of its own, none of which is addressed here.
Pain that persists, that wakes you at night, that follows an impact, or that comes with loss of strength, swelling, a fever or a loss of sensation belongs to a health professional. Chest pain, pain radiating to the arm or jaw, or pain of sudden and severe onset warrants calling emergency services.
No page — this one included — allows you to decide whether a pain can wait.
Sources
Main sources
- Raja SN, Carr DB, Cohen M, Finnerup NB, Flor H, Gibson S, Keefe FJ, Mogil JS, Ringkamp M, Sluka KA, Song XJ, Stevens B, Sullivan MD, Tutelman PR, Ushida T, Vader K (2020). The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises. Pain.
- Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021). Principles of Neural Science, 6th edition. McGraw Hill.
- Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018). Neuroscience, 6th edition. Oxford University Press.
Put it into practice in Shapier
Sparing the nervous system day to day
Building recovery habits that spare the nervous system is done in Shapier; Body Lab only explains how the pain signal is modulated.
Sparing the nervous system day to dayBody Lab explains; Shapier lets you act and track.
Check my understanding
What does actual or potential tissue damage mean in the international definition of pain?
A pain can exist without any lesion being present
A pain is only real if an examination finds a lesion
A potential lesion will inevitably appear later
Why does the same stimulation not always produce the same pain?
Because the nociceptive signal is amplified or damped all along its route
Because everyone decides their own pain
Because the receptors of some tissues malfunction
Saying a pain is produced by the nervous system — what does that mean?
The same as for sight or taste: it makes the pain neither imaginary nor less real
That it is in your head and therefore imaginary
That you can decide not to hurt
Choose an answer
Read next
- The nervous systemThe network that gathers, decides and commands. It works in two directions at once, and a whole part of it escapes the will — otherwise you would have to think about every heartbeat.Described without a scene
- ProprioceptionThe sense that continuously reports the position and tension of the body without your having to look at it. It is what makes walking in the dark possible — and it is not equally reliable for everything it measures.Described without a scene
- Delayed muscle sorenessThe ache that arrives the next day is not lactic acid. What is known about where it comes from, how it runs its course, and why it measures neither the quality nor the value of a session.With a 3D scene
Where to go next
Available offline
Offline download is available in the mobile app.
Trust and method
Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
- No scene accompanies this page: the pathways described belong to the nervous system, absent from every anatomical source Body Lab uses.
- This page describes a general mechanism. It assesses no pain, identifies no cause and proposes no treatment.
- Persistent pain is an entire clinical field, with mechanisms, investigations and care of its own. None of that is addressed here.
- Any pain that persists, that wakes you at night, that follows an impact, or that comes with loss of strength or a fever belongs to a health professional.
Sources
- Raja SN, Carr DB, Cohen M, Finnerup NB, Flor H, Gibson S, Keefe FJ, Mogil JS, Ringkamp M, Sluka KA, Song XJ, Stevens B, Sullivan MD, Tutelman PR, Ushida T, Vader K (2020). The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises. Pain.
- Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021). Principles of Neural Science, 6th edition. McGraw Hill.
- Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018). Neuroscience, 6th edition. Oxford University Press.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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