Place · Level 3
How Pain Is Made · a decision, not a readout
伤害感受器只报告有事 · 脊髓背角是一道闸门 · 脑干能调音量 · 增益会被调高 · 所以损伤好了痛还在 · 红旗照样要就医
Story path
Chapter 1
Pain is a decision
Pain is a decision
The pain you feel is a conclusion your nervous system computes — not a reading sent up from the wound.
Start with a scene you have probably lived. You take a knock during a ball game, feel almost nothing at the time, and finish the match. You get home, pull off your trousers, and find a large bruise down your shin — and only now does it start to hurt, and it hurts more the longer you sit.
The damage happened at the moment of impact. The pain arrived two hours late.
If pain were a readout of damage, this makes no sense. Tissue tears worst in the second of impact, so that second should hurt most. Yet the same injury delivers completely different pain on the court and on the sofa. Nothing changed in between — except how your nervous system handled it.
So pain isn't a phone line running straight from wound to brain. It's a road with several checkpoints, each able to let a signal through, hold it back, amplify it, or damp it. What reaches your awareness is the sum of that whole run.
This is not a fringe view. The International Association for the Study of Pain revised its definition of pain, and the whole weight of the revision sits in a few words: pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (Raja 2020). That clause or resembling is a formal admission — pain can exist without damage, and it is still real pain.
This story walks the full route, from the outermost detecting fibre to your awareness. By the end you'll see why pain can outlast the injury that started it.
Start with a scene you have probably lived. You take a knock during a ball game, feel almost nothing at the time, and finish the match. You get home, pull off your trousers, and find a large bruise down your shin — and only now does it start to hurt, and it hurts more the longer you sit.
The damage happened at the moment of impact. The pain arrived two hours late.
If pain were a readout of damage, this makes no sense. Tissue tears worst in the second of impact, so that second should hurt most. Yet the same injury delivers completely different pain on the court and on the sofa. Nothing changed in between — except how your nervous system handled it.
So pain isn't a phone line running straight from wound to brain. It's a road with several checkpoints, each able to let a signal through, hold it back, amplify it, or damp it. What reaches your awareness is the sum of that whole run.
This is not a fringe view. The International Association for the Study of Pain revised its definition of pain, and the whole weight of the revision sits in a few words: pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (Raja 2020). That clause or resembling is a formal admission — pain can exist without damage, and it is still real pain.
This story walks the full route, from the outermost detecting fibre to your awareness. By the end you'll see why pain can outlast the injury that started it.
Naming: detection and experience are two things
Two words get treated as one, and separating them removes a lot of confusion.Nociception: a nerve ending detected a noxious stimulus and fired. This is measurable in you.Pain: your experience.
When the IASP revised its definition it attached a note precisely on this: neither implies the other (Raja 2020). Nerves can fire while you feel nothing — that's the two hours on the court. And you can hurt, genuinely, with nothing firing at the periphery.
The distinction matters because it closes off one very damaging sentence. Someone hurts, the work-up finds no matching damage, and the conclusion becomes the pain must be fake. That sentence is physiologically wrong. It treats nociception as pain's entry ticket, and it never was one.
The next four scenes show, checkpoint by checkpoint, exactly how the two come apart.
Chapter 2
The detector only reports trouble
The detector only reports trouble
You have no pain receptors. You have nociceptors, and what they report isn't pain — it's trouble.
Scattered through skin, muscle, joint capsule and the lining of bone is a class of nerve ending that is bare — not wrapped into a corpuscle. They're called free nerve endings. Their membranes carry channels that normally sit shut. Meet a high enough temperature, a hard enough squeeze, or the chemicals released by damaged tissue, and the channels open, positive ions pour into the fibre, the voltage crosses a threshold, and the fibre starts firing (Basbaum 2009).
Notice what it sends up: a train of pulses encoding how strong and where. That's all. Nowhere in that train is the word pain — pain gets computed several stops later.
Two kinds of fibre run this route, differing in whether they're wrapped in myelin:
Aδ fibres: wrapped in a thin myelin sheath. Myelin insulates, letting the signal jump along, so it's fast. This gives you the fast, sharp, well-localised hit.C fibres: no myelin, so the signal crawls segment by segment, more than ten times slower. This gives you the slow, dull, hard-to-place ache.
This explains something you verify constantly. Catch your toe on a table leg and there's a sharp yelp of pain, then about a second later a dull, swelling ache floods in. One impact, two waves of pain — because the fast line and the slow line don't arrive together (Basbaum 2009).
Hold onto this: the outermost layer only shouts. How loud the shout ends up being is decided by the checkpoints downstream.
Scattered through skin, muscle, joint capsule and the lining of bone is a class of nerve ending that is bare — not wrapped into a corpuscle. They're called free nerve endings. Their membranes carry channels that normally sit shut. Meet a high enough temperature, a hard enough squeeze, or the chemicals released by damaged tissue, and the channels open, positive ions pour into the fibre, the voltage crosses a threshold, and the fibre starts firing (Basbaum 2009).
Notice what it sends up: a train of pulses encoding how strong and where. That's all. Nowhere in that train is the word pain — pain gets computed several stops later.
Two kinds of fibre run this route, differing in whether they're wrapped in myelin:
Aδ fibres: wrapped in a thin myelin sheath. Myelin insulates, letting the signal jump along, so it's fast. This gives you the fast, sharp, well-localised hit.C fibres: no myelin, so the signal crawls segment by segment, more than ten times slower. This gives you the slow, dull, hard-to-place ache.
This explains something you verify constantly. Catch your toe on a table leg and there's a sharp yelp of pain, then about a second later a dull, swelling ache floods in. One impact, two waves of pain — because the fast line and the slow line don't arrive together (Basbaum 2009).
Hold onto this: the outermost layer only shouts. How loud the shout ends up being is decided by the checkpoints downstream.
Why this layer isn't a pain nerve
Calling these fibres pain nerves sounds harmless and is actually the root of the whole misunderstanding.The name smuggles in an assumption: this line fires, therefore pain. From which it follows neatly — the worse it hurts, the harder the line is firing, the worse the damage.
But the line does exactly one job: detect a mechanical, thermal or chemical stimulus above threshold, and fire. It doesn't know whether you're on the court or the sofa, whether the injury matters, or whether you hurt. It's an alarm contact, not a pain meter.
Worse, the threshold isn't fixed. Chemicals released by damaged tissue lower the threshold of nearby nociceptors, so stimuli that wouldn't normally qualify can set them off (Basbaum 2009). That's why warm water stings on sunburn — the water didn't get hotter, that patch's threshold got turned down. This is peripheral sensitisation.
Remember the shape of the word: sensitisation = threshold turned down. Scene five shows the same thing happening in the spinal cord, with far heavier consequences.
Chapter 3
The dorsal horn is a gate
The dorsal horn is a gate
When you knock yourself, you rub it without thinking. That isn't psychological comfort — you are physically closing a real gate.
The Aδ and C fibres coming from skin don't connect straight to the brain. They enter the spinal cord first and change connections in a block of grey matter at the back, the dorsal horn. Wherever a signal changes hands, it can be tampered with.
The key point: that same dorsal horn also receives another input, the Aβ fibres. These are the thick, heavily myelinated, fastest-conducting class, and they carry touch, pressure and vibration — rubbing, stroking, pressing and holding all activate them.
Melzack and Wall proposed in 1965 that the two inputs compete in the dorsal horn. Activity in the thick Aβ fibres, working through a set of interneurons, presses down on the thin fibres' route upward; activity in the thin fibres lifts that suppression. Whichever input prevails sets how far the gate opens, and so how much signal travels on (Melzack 1965).
So rubbing genuinely works: you are flooding the gate with Aβ input and crowding the thin-fibre route out. Anyone can verify it; no trust required.
The metaphor stops here: the gate isn't a door, with no hinge and no switch. It's a set of interneurons doing arithmetic, and the arithmetic decides how much goes up. This is also what transcutaneous electrical nerve stimulation stands on — using current to rub for you.
The specific 1965 wiring diagram was substantially revised later, and that should be said plainly. But the core has held: the dorsal horn is an adjustable relay, not a through-wire.
The Aδ and C fibres coming from skin don't connect straight to the brain. They enter the spinal cord first and change connections in a block of grey matter at the back, the dorsal horn. Wherever a signal changes hands, it can be tampered with.
The key point: that same dorsal horn also receives another input, the Aβ fibres. These are the thick, heavily myelinated, fastest-conducting class, and they carry touch, pressure and vibration — rubbing, stroking, pressing and holding all activate them.
Melzack and Wall proposed in 1965 that the two inputs compete in the dorsal horn. Activity in the thick Aβ fibres, working through a set of interneurons, presses down on the thin fibres' route upward; activity in the thin fibres lifts that suppression. Whichever input prevails sets how far the gate opens, and so how much signal travels on (Melzack 1965).
So rubbing genuinely works: you are flooding the gate with Aβ input and crowding the thin-fibre route out. Anyone can verify it; no trust required.
The metaphor stops here: the gate isn't a door, with no hinge and no switch. It's a set of interneurons doing arithmetic, and the arithmetic decides how much goes up. This is also what transcutaneous electrical nerve stimulation stands on — using current to rub for you.
The specific 1965 wiring diagram was substantially revised later, and that should be said plainly. But the core has held: the dorsal horn is an adjustable relay, not a through-wire.
Why rubbing only holds for a moment
Rubbing helps — and you'll have noticed it stops helping the moment your hand stops.That is exactly what the mechanism predicts. The gate is held shut by ongoing Aβ input, not flipped to a setting and locked. Stop your hand, Aβ stops firing, the suppression lifts, and the thin-fibre route takes the lead again.
This yields a transferable judgement: anything that works by pressing the gate shut with external input can only work while the input continues. Rubbing, vibration devices, electrical stimulation pads, massage guns — all of them are this category. They aren't frauds; the gate is real and the suppression is real. But if someone tells you such a device cures something, or opens or reconnects anything, that claim has run past what the mechanism can support.
What they can do is clear and genuinely valuable: buy you a window that hurts less. What the window is for is scene six.
Chapter 4
The brainstem sets the volume
The brainstem sets the volume
Your brainstem runs a line downward that plugs straight into that gate. Those two painless hours on the court were this line's doing.
The route: in the midbrain sits a cuff of grey matter wrapped around the cerebrospinal fluid channel, the periaqueductal grey. It projects down to a region of the medulla, which sends fibres the rest of the way to the dorsal horn — landing exactly on the gate from the last scene (Ossipov 2010).
So information doesn't only travel up. The top reaches down and rewrites the checkpoint's settings directly.
More importantly, this line isn't a knob that only turns down. That medullary relay holds two cell types with opposite jobs: when one fires, pain relayed by the dorsal horn is suppressed; when the other fires, pain is amplified (Ossipov 2010). One pathway, both analgesic and pro-nociceptive. Which type prevails depends on the situation, attention, expectation and mood of the moment.
Among the messengers this line uses are the body's own opioids, and the periaqueductal grey is dense with the receptors that catch them. Morphine relieves pain largely by hitching a ride on this existing system.
Which sets up a beautiful experiment. Give people who've just had wisdom teeth out a placebo, and some of them genuinely hurt less. Now give those responders naloxone — a drug that blocks opioid receptors — and their pain comes back; give it to the people who didn't respond to placebo in the first place, and it changes nothing (Levine 1978).
The conclusion is hard: placebo analgesia is not people pretending. It runs on the body's own opioid pathway — a real anatomical circuit that a drug can block. Anything reversible by naloxone cannot be living only in the imagination.
The route: in the midbrain sits a cuff of grey matter wrapped around the cerebrospinal fluid channel, the periaqueductal grey. It projects down to a region of the medulla, which sends fibres the rest of the way to the dorsal horn — landing exactly on the gate from the last scene (Ossipov 2010).
So information doesn't only travel up. The top reaches down and rewrites the checkpoint's settings directly.
More importantly, this line isn't a knob that only turns down. That medullary relay holds two cell types with opposite jobs: when one fires, pain relayed by the dorsal horn is suppressed; when the other fires, pain is amplified (Ossipov 2010). One pathway, both analgesic and pro-nociceptive. Which type prevails depends on the situation, attention, expectation and mood of the moment.
Among the messengers this line uses are the body's own opioids, and the periaqueductal grey is dense with the receptors that catch them. Morphine relieves pain largely by hitching a ride on this existing system.
Which sets up a beautiful experiment. Give people who've just had wisdom teeth out a placebo, and some of them genuinely hurt less. Now give those responders naloxone — a drug that blocks opioid receptors — and their pain comes back; give it to the people who didn't respond to placebo in the first place, and it changes nothing (Levine 1978).
The conclusion is hard: placebo analgesia is not people pretending. It runs on the body's own opioid pathway — a real anatomical circuit that a drug can block. Anything reversible by naloxone cannot be living only in the imagination.
Why the same injury swings
The same injury hurts more after a bad night, less when you're distracted, and worse the more you fear it. These get filed under psychological and then dismissed.Mechanistically they aren't in the psychological column at all. They're in the anatomical one. Attention, expectation, mood and stress state all feed into that descending line from the periaqueductal grey, and that line ends physically on the dorsal horn (Ossipov 2010).
So relax and it'll hurt less and the pain is psychological are entirely different sentences, and the difference is worth nailing down:
The first says: there is a real neural pathway through which context can change the gate's setting.The second says: your pain doesn't exist.
The first has anatomy behind it. The second has nothing behind it — scene one already showed that pain needs no nociceptive entry ticket; it is an experience that can stand on its own.
This also puts sleep and stress back where they belong. They aren't soft advice. They are adjustments to this line's operating point.
Chapter 5
The gain gets turned up
The gain gets turned up
After being fired at repeatedly, the dorsal horn becomes easier to fire. This is the story's payload: it explains why pain outlasts the injury.
Scene two showed damaged tissue turning thresholds down at the periphery. The same thing happens inside the spinal cord, and far more consequentially.
When C fibres deliver input to the dorsal horn over and over, the junction gets rebuilt on the dorsal-horn neuron's side: synaptic transmission is turned up, and the neuron's excitability is turned up. This process is central sensitisation (Woolf 2011). Three consequences follow the rebuild:
The threshold drops: inputs that wouldn't have qualified now make it fire.The receptive field widens: a neuron that used to watch one small patch of skin can now be activated from the patch next door. So pain spreads into uninjured territory.It amplifies: the same input strength sends a stronger signal onward.
At the level you can actually feel, that becomes two named things:
Hyperalgesia: what hurt already now hurts more.Allodynia: what never hurt at all — clothing brushing past, warm water, the weight of a duvet — now hurts (Woolf 2011).
Put the three together and the counterintuitive thing follows: when the wound heals, the gain does not automatically come back down. The tissue has finished repairing while the amplifier is still set where it was. Your pain then isn't because something is still breaking there; it's because the gain along this route was changed.
One thing must be nailed shut here, or the whole story gets read backwards. Central sensitisation is not the pain being psychological, and not the pain being imagined. A change in synaptic transmission is a physical change, measurable in an experiment (Woolf 2011). Saying someone's pain is amplified by central sensitisation and saying they're faking are sentences pointing in opposite directions — the first says their nervous system genuinely changed.
Scene two showed damaged tissue turning thresholds down at the periphery. The same thing happens inside the spinal cord, and far more consequentially.
When C fibres deliver input to the dorsal horn over and over, the junction gets rebuilt on the dorsal-horn neuron's side: synaptic transmission is turned up, and the neuron's excitability is turned up. This process is central sensitisation (Woolf 2011). Three consequences follow the rebuild:
The threshold drops: inputs that wouldn't have qualified now make it fire.The receptive field widens: a neuron that used to watch one small patch of skin can now be activated from the patch next door. So pain spreads into uninjured territory.It amplifies: the same input strength sends a stronger signal onward.
At the level you can actually feel, that becomes two named things:
Hyperalgesia: what hurt already now hurts more.Allodynia: what never hurt at all — clothing brushing past, warm water, the weight of a duvet — now hurts (Woolf 2011).
Put the three together and the counterintuitive thing follows: when the wound heals, the gain does not automatically come back down. The tissue has finished repairing while the amplifier is still set where it was. Your pain then isn't because something is still breaking there; it's because the gain along this route was changed.
One thing must be nailed shut here, or the whole story gets read backwards. Central sensitisation is not the pain being psychological, and not the pain being imagined. A change in synaptic transmission is a physical change, measurable in an experiment (Woolf 2011). Saying someone's pain is amplified by central sensitisation and saying they're faking are sentences pointing in opposite directions — the first says their nervous system genuinely changed.
The good news: the rebuild is reversible
The previous page reads like a verdict. It isn't.Woolf's own definition of central sensitisation is carefully worded: a prolonged but reversible increase in the excitability and synaptic efficacy of neurons in central nociceptive pathways (Woolf 2011). Reversible is built into the definition, not offered as consolation.
What can be built up can be taken down. That's why this mechanism doesn't lead to despair — it leads to the rather plain measures in scene six, which are aimed at exactly these settings.
An honest boundary too, or this scene gets abused. Central sensitisation explains that pain can exist apart from damage. It does not prove that any particular person has no damage. Those are very different claims. Someone can have both a real tissue problem and a turned-up gain; in fact that's common. Working out which applies to one individual is a job for a doctor in the room, not something an explainer can do for you.
This story gives you the why, not the which one are you.
Chapter 6
So how to use this
So how to use this
Put the five scenes together and three things fall out ready to use.
1 · The degeneration on the scan isn't why you hurt
This one needs the anatomy and the population data together — neither half carries it alone. The paragraph below was meant to run on anatomy alone; that version leaves out the fact that matters most.
Pain needs two things: a nociceptive fibre that has been set off, plus a gate willing to let it through.
In a healthy disc, only the outer third of the annulus is innervated; the water-rich nucleus at the centre has essentially no blood vessels and no nerve endings (Hartvigsen 2018 · Freemont 1997). Which sounds like it settles it: no alarm fitted, nothing rings when it breaks.
But a degenerated disc grows nerves into itself. In biopsy samples, 57% at the painful level had nerve ingrowth into the inner annulus or the nucleus — and so did 25% at non-painful levels (Freemont 1997). So the premise fails at exactly the step that needed it: you cannot argue from it never had an alarm to the conclusion that an already degenerated disc can't hurt.
Add the stops downstream: how much you hurt depends on how the gate is set, where the descending line is dialled, and whether the gain has been altered. Not one of those settings is visible on a scan.
So what survives is the weaker — and more useful — claim: degeneration on a scan is neither necessary nor sufficient for your pain. The population data says both halves: degeneration is extremely common in people with no symptoms (Brinjikji 2015), and yet the same authors' other meta-analysis found that in people under fifty, findings like disc bulge and extrusion genuinely are more common in those with back pain (Brinjikji 2015). Both are true. Together they don't mean pain comes from somewhere else — they mean that scan can't answer, on its own, why you hurt.
2 · Pain doesn't mean you're being damaged — which is why you can move
Since pain intensity is a modulated quantity, the rule pain means stop has no physiological basis. This is exactly where progressive loading stands: in chronic musculoskeletal pain, exercise protocols that permit pain do at least as well as protocols demanding pain-free movement, and slightly better in the short term (Smith 2017).
In practice, read pain as a signal rather than a prohibition:
A bit sore, not steadily worsening, back to baseline the next day → usually fine to continue.Worse every session, clearly worse the following day, each time worse than the last → scale back.
3 · Red flags still need a doctor — and the reason is inside this mechanism
This has to be said out loud: understanding that pain can exist apart from damage does not mean you can skip the doctor.
Quite the opposite. This story argues one thing from beginning to end — pain intensity is adjusted by the gate and the descending line, so it was never a readout of danger. Which means using how much it hurts to decide whether to seek care is the wrong instrument from the start. Danger is screened by the accompanying signals.
But one exception comes first, because it is the mirror image of the judgment above: the way a pain arrives is itself an accompanying signal. Sudden onset, peaking within seconds, and the worst of your life — those three together are an emergency wherever it hurts. The don't judge by how badly it hurts rule is about chronic, recurring pain. A pain that has never happened before and detonates is a different thing entirely.
The mechanism in this story explains away none of the following:
Emergency department now, head: a thunderclap headache peaking within seconds, especially the worst of your life. Could be a subarachnoid haemorrhage.Emergency department now, chest: crushing or pressing, like a stone on you, radiating to jaw, left shoulder or left arm; or a tearing pain. Could be acute coronary syndrome or aortic dissection.Emergency department now, abdomen: sudden severe abdominal pain, especially with a rigid abdomen, fever, vomiting, or signs of shock.Emergency department now, low back: numbness around the perineum, anus or inner thighs (saddle anaesthesia); sudden inability to pass urine or to hold it; bowel incontinence; progressive weakness in both legs at once. This is cauda equina syndrome, and a delay of hours can be irreversible.See a doctor promptly: progressive neurological deficit (steadily weaker, an enlarging area of numbness, foot drop).See a doctor promptly: unexplained weight loss, or a history of cancer with new pain, especially pain that wakes you at night and doesn't ease with rest.See a doctor promptly: fever or chills alongside the pain.See a doctor promptly: severe pain after significant trauma (a fall, a crash, an impact).
This site offers general education and explanation. It does not replace a physician's diagnosis and treatment, and it prescribes no specific treatment. For pain carrying the red flags above, or pain that keeps worsening, see a doctor in person as soon as you can.
1 · The degeneration on the scan isn't why you hurt
This one needs the anatomy and the population data together — neither half carries it alone. The paragraph below was meant to run on anatomy alone; that version leaves out the fact that matters most.
Pain needs two things: a nociceptive fibre that has been set off, plus a gate willing to let it through.
In a healthy disc, only the outer third of the annulus is innervated; the water-rich nucleus at the centre has essentially no blood vessels and no nerve endings (Hartvigsen 2018 · Freemont 1997). Which sounds like it settles it: no alarm fitted, nothing rings when it breaks.
But a degenerated disc grows nerves into itself. In biopsy samples, 57% at the painful level had nerve ingrowth into the inner annulus or the nucleus — and so did 25% at non-painful levels (Freemont 1997). So the premise fails at exactly the step that needed it: you cannot argue from it never had an alarm to the conclusion that an already degenerated disc can't hurt.
Add the stops downstream: how much you hurt depends on how the gate is set, where the descending line is dialled, and whether the gain has been altered. Not one of those settings is visible on a scan.
So what survives is the weaker — and more useful — claim: degeneration on a scan is neither necessary nor sufficient for your pain. The population data says both halves: degeneration is extremely common in people with no symptoms (Brinjikji 2015), and yet the same authors' other meta-analysis found that in people under fifty, findings like disc bulge and extrusion genuinely are more common in those with back pain (Brinjikji 2015). Both are true. Together they don't mean pain comes from somewhere else — they mean that scan can't answer, on its own, why you hurt.
2 · Pain doesn't mean you're being damaged — which is why you can move
Since pain intensity is a modulated quantity, the rule pain means stop has no physiological basis. This is exactly where progressive loading stands: in chronic musculoskeletal pain, exercise protocols that permit pain do at least as well as protocols demanding pain-free movement, and slightly better in the short term (Smith 2017).
In practice, read pain as a signal rather than a prohibition:
A bit sore, not steadily worsening, back to baseline the next day → usually fine to continue.Worse every session, clearly worse the following day, each time worse than the last → scale back.
3 · Red flags still need a doctor — and the reason is inside this mechanism
This has to be said out loud: understanding that pain can exist apart from damage does not mean you can skip the doctor.
Quite the opposite. This story argues one thing from beginning to end — pain intensity is adjusted by the gate and the descending line, so it was never a readout of danger. Which means using how much it hurts to decide whether to seek care is the wrong instrument from the start. Danger is screened by the accompanying signals.
But one exception comes first, because it is the mirror image of the judgment above: the way a pain arrives is itself an accompanying signal. Sudden onset, peaking within seconds, and the worst of your life — those three together are an emergency wherever it hurts. The don't judge by how badly it hurts rule is about chronic, recurring pain. A pain that has never happened before and detonates is a different thing entirely.
The mechanism in this story explains away none of the following:
Emergency department now, head: a thunderclap headache peaking within seconds, especially the worst of your life. Could be a subarachnoid haemorrhage.Emergency department now, chest: crushing or pressing, like a stone on you, radiating to jaw, left shoulder or left arm; or a tearing pain. Could be acute coronary syndrome or aortic dissection.Emergency department now, abdomen: sudden severe abdominal pain, especially with a rigid abdomen, fever, vomiting, or signs of shock.Emergency department now, low back: numbness around the perineum, anus or inner thighs (saddle anaesthesia); sudden inability to pass urine or to hold it; bowel incontinence; progressive weakness in both legs at once. This is cauda equina syndrome, and a delay of hours can be irreversible.See a doctor promptly: progressive neurological deficit (steadily weaker, an enlarging area of numbness, foot drop).See a doctor promptly: unexplained weight loss, or a history of cancer with new pain, especially pain that wakes you at night and doesn't ease with rest.See a doctor promptly: fever or chills alongside the pain.See a doctor promptly: severe pain after significant trauma (a fall, a crash, an impact).
This site offers general education and explanation. It does not replace a physician's diagnosis and treatment, and it prescribes no specific treatment. For pain carrying the red flags above, or pain that keeps worsening, see a doctor in person as soon as you can.
Background: pain-free people scan the same
Point 1 above was derived from anatomy. It also has independent corroboration worth a look — but note its position: this is supporting evidence, not the argument. Even if none of the numbers below existed, point 1 would still hold.A systematic review pooling imaging from large populations with no back pain at all found disc degeneration in about a third of asymptomatic 20-year-olds, approaching universal by 80; disc bulges in around 30% at 20 and over 80% by 80 (Brinjikji 2015). None of these people hurt.
In other words, pull a random asymptomatic middle-aged person into a scanner and you'll likely image an abnormality or two. This matches the mechanism's prediction exactly: a scan shows structure changing normally with age, while pain is produced somewhere else.
It's also why mainstream guidelines don't recommend routine imaging for ordinary back pain without red flags (NICE NG59). The Lancet back-pain series puts it more bluntly: the world's response to back pain is frequently too aggressive — over-imaging, over-injecting, over-operating — while the explanation and activity most worth giving are under-supplied (Foster 2018).
This story doesn't teach you what to do about your body part. That's the division of labour: the low back pain, neck pain, knee pain and sedentary-body stories each handle their own region. This one only explains why the line they all share — pain is not damage — is true in the first place.
brinjikji-2015-ajnr-imagingfoster-2018-lancet-lbpfreemont-1997-disc-nerve-ingrowthbrinjikji-2015-degeneration-vs-lbp