Noxious Stimulus. Tis but a flesh wound.

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IASP Definition of Noxious Stimulus

A stimulus that is damaging or threatens damage to normal tissues.

Definitions You Might Have Seen

Noxious Stimulus or ...

Danger (in me)



Thoughts, beliefs


People, places, and things

Normal tissues?

My assumption is that though nerve tissue may be damaged in injury or pathology, the definition relates to noxious stimuli of non-neural tissue that then activates nociceptors and nociceptive neurons in the periphery.

Thermal (noxious heat/cold), Mechanical (noxious pressure, movement), Chemical (chemical irritants).

Now, I've always included inflammation as a noxious stimulus but I'm not sure that fits the bill. Classic or acute inflammation accompanied by swelling, warmth, redness, and pain is a reaction to damage or injury that involves immune components that interact with nociceptors and nociceptive neurons. But that's not the only kind of inflammation that stimulates nociception. All inflammation is related to immune or homeostatic process, and only some inflammation may detectable with known biomarkers, like C-reactive proteins (CRP). Inflammation that is not a response to tissue injury or infection may not be noxious (involved with homeostasis). So inflammation doesn't fit the current definition because the interaction is between systems (immune and nervous, for instance). It's more process than application.

Now some of you may know this already, but it's been a sticking point for me and articulating it helps to make sense of the boundaries of meaning related to these terms.

Consider, if you will

We cannot define pain in terms of its stimulus.

How do you think of activation of nociception with pain? What is being activated, how?

I know that for myself it's long been associated with having to have a noxious stimulus of some sort. But if there are processes that activate nociception that don't fit the bill of damage or threat of damage to normal tissues, what then? The IASP also defines Nociceptive Stimulus, which is "an actually or potentially tissue-damaging event transduced and encoded by nociceptors." Both definitions engender immediacy of an event or stimulus, include damage or potential damage, are peripherally related, and don't lend themselves to longer-term thinking about nociceptive activation by interactive processes.

Example, knee OA. This condition can develop seemingly silently. Bone and tissue are changed, damaged, and eroded often without notable symptoms, until it gets to a point where the tissue is changed or damaged enough to result in inflammation. The process of osteoarthritic change does not fit the definition of noxious stimulus and yet ... it is damaging to normal tissue. The OA process may include activation of nociception, even if not always sufficient to result in a pain sensation, but it's not until pain is present that we think of what might have caused it.

My thought processes around this terminology are still to be fully explored and I'm happy to have your input.

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