This entry’s intended to clear up the common misconception that the relationship between injury and pain is causal. Injury doesn’t cause pain. Well, not directly anyway. In my view, it’s best to think of the relationship as functional; injury is related to pain by the function of pain.
The relationship between injury and pain is not directly causal: I think most people have a confused understanding of the relationship between pain and injury. Most would, I think, say that injury causes pain. Meaning that pain is directly caused by damaged tissue. But if this were so, we should expect to feel pain constantly for the whole period of an injury; i.e. constantly (and I mean constantly) for weeks and months with the intensity of pain easing as the injured tissue heals. Yet most would, I think, see this as abnormal. Instead, they expect to feel pain at the moment when tissue becomes damaged and, perhaps, for a short time afterwards; and they expect pain to disappear when the injured body part is rested. Most also have an intuitive understanding that it’s normal to feel pain in the absence of injury; when you grab a hot pan-handle or your hair is pulled, for example. So almost everyone has an intuitive understanding inconsistent with the commonly stated belief that injury causes pain.
Surprisingly, scientific discourse is littered with much the same inconsistency. For example, the International Association for the Study of Pain (the ‘IASP’) state, “Many people report pain in the absence of tissue damage or any likely pathophysiological cause” and “if they report [pain] in the same way as pain caused by tissue damage…” But scientists also causally link pain with a noxious stimulus: “A stimulus that is damaging or threatens damage to normal tissues.” In this context, the word ‘stimulus’ refers to the mechanical and thermal energy detected by the sensory receptors involved in pain. And injury’s a tissue state (the state of being damaged); it’s not a type of energy.
While there’s no direct causal relationship between injury and pain. There are two ways that injury is an indirect cause of pain. First, a startling variety of biochemicals are released by and in the vicinity of damaged tissue. These biochemicals make sensory nerves more sensitive; the upshot being that the sensory nerves involved in pain react to lower intensities of mechanical and thermal energy. This is why you feel pain in response to lower intensities of energy when you have an injury; for example, when a bruise is quite gently pressed or you walk on a sprained ankle. To emphasise the point, this is indirect causation; the direct cause is the mechanical or thermal energy (that results from what you’re doing) at the site of sensitive nerves. The evidential reasons for rejecting direct causation are mentioned above: if injury were a direct cause of pain, then it’d be normal to feel pain constantly whenever you’re injured and it’d be abnormal to experience pain intermittently; and the sensory nerves involved in pain are directly stimulated by energy not damaged tissue.
The relationship between injury and pain is functional: There’s another (very) indirect way it might be stated that injury is causally related to pain. Injury compromises our ability to survive. This fact has lead to us evolving the capacity to feel pain. However, it isn’t helpful to think of this evolutionary relationship in causal terms; it’s far more informative to think of it functionally.
Scientists don’t often discuss the function of pain. Generally, the consensus is that pain functions as both a perceptual experience (as the conscious awareness of noxious stimuli) and a motivational experience (it motivates behaviour that prevents or minimises tissue damage). There are other schools of thought. Perhaps the most influential of these is the idea that pain has a homeostatic function. Specifically, pain functions to maintain bodily integrity. I’ve reservations about both these views, but these concerns are for another blog. My point here, has been to dispel the erroneous belief that injury is a direct cause of pain. The relationship between injury and pain is functional; it is not causal.
(A little more on the function of pain: The germ of the idea that pain functions to maintain bodily integrity can be found in Patrick Wall’s 1979 article “On the relation of injury to pain”, which inspired the title of this entry. In this paper, he argues that pain functions to promote healing not prevent injury. The more explicit notion that pain is a homeostatic emotion is to be found in A.D. Craig’s 2002 paper, “How do you feel? Interoception, the sense of the physiological condition of the body”. Unfortunately neither paper is open access. If you have institutional access you can find these papers here: Wall and Craig. Other accounts, by scientists, that are specific to pain’s function are difficult to find. Most references to function are to be found in scientific literature with another focus, but I’ll put my neck on the line and claim that, largely, these accounts conceive pain as a sensory (i.e. perceptual) experience. For example, Woolf and Ma state that pain functions as the conscious awareness of a noxious stimulus – this article is not open access, either, but here’s the link: Woolf and Ma. Although this may appear inconsistent with the view that pain motivates behaviour that prevents or minimises injury, it is not. Presumably the idea is that conscious awareness of a noxious stimulus fulfils a critical role in such motivation. By contrast, philosophers have plenty to say about pain’s function. The majority, under a variety of closely related but subtly distinct theories, tow the scientifically endorsed view that pain has a perceptual function. A recently published paper by Laurenz Casser challenges this view. I mention this paper specifically because it is very recent, well argued and provides strong reasons to reject the position that pain has a perceptual function. It’s available here, if you have institutional access: Casser.)
The Pain Jam 