By John Quintner, Physician in Pain Medicine and Rheumatology (retired) and Milton Cohen, Specialist Pain Medicine Physician and Rheumatologist
Expressed in everyday terms, pain is one of our body-centred sensations, characterised by being unpleasant if not also threatening. This is reflected in its name, which is derived from ancient Greek and Latin words for punishment, penalty, retribution, and vengeance for the crimes of murder and manslaughter.
Through the processes of evolution, sentient beings have been endowed with survival systems, one of which is an apparatus built into their nervous system, which functions to detect actual or impending tissue damage. It is called the “nociceptive apparatus” (“noci-” means harm; nociception means harm detection).
Activation of this apparatus usually occurs in association with tissue damage.
Pain is a response to activation of this apparatus but never a stimulus (although the experience can motivate us to take action to seek relief or avoid dangerous situations). Just as there are no “pain receptors”, “pain messages” or “pain pathways”, there is no such thing as a “pain system”. The more accurate terms are “nociceptors”, “nociceptive impulses”, and “nociceptive pathways” and “nociceptive apparatus”. This sensory aspect is relatively easy to study.
In humans, who have language, another dimension to the experience of pain is perception or interpretation of the sensation, including what is its meaning. This dimension is much more difficult to study and understand.
When we become aware that our nociceptive apparatus has been activated, we usually experience pain. However, in certain circumstances such activation may not reach the level of consciousness. For example, a soldier wounded in the heat of battle may not experience any pain at the time of injury.
But activation can also occur indirectly when we perceive a traumatic situation as being a threat to our very existence. Witnessing a violent robbery is such an example. For some people, a complex stress response is activated and the neurological, endocrine and immune systems can produce an excessive amount of chemicals that sensitise the nerve cells (nociceptors) of the apparatus.
As a result, these cells can fire off, either spontaneously or in response to non-harmful stimuli, such as light touch or cool temperature. In this case, even though the experience of pain is not associated with tissue damage, the sensation is no different.
We can always be confident about our sensations, whereas we can be quite wrong in our perceptions (i.e. interpretations). For example, a clap of thunder is simply appreciated by us as a loud noise that activates the auditory apparatus.
The brain does not have to make a “decision” as to how we should perceive the sound. But whether we interpret that loud sound as thunder, a car backfiring, a cannon firing, or as a bomb exploding is a matter of perception. We can always check and find out whether our interpretation is correct.
Thus, we may be mistaken about the cause of activation of the auditory (“hearing”) apparatus but, to reiterate, we cannot be mistaken about the experience of having heard the sound. This sensation is “given” to us.
The same can be said of pain. We are not mistaken when we experience pain. But we may come up with a number of different interpretations of the experience.
There is a popular but in our opinion misguided school of thought claiming that pain functions as a “protector” of our body and is produced by the brain whenever it – the brain – “decides” that we are in danger and in need of protection. But the case of phantom limb pain refutes this hypothesis. How can a limb that is no longer present be a threat, let alone one from which we require protection?
Furthermore, because the experience of pain is an evolutionary consequence of activation of the nociceptive apparatus, it is not dependent upon higher centre decision-making. People – not their brains – are responsible for making decisions such as interpretations.
Some authorities even claim that we can “unlearn” pain, somewhat along the lines of it being like a “bad habit”. But this cannot be true because it is a component of our survival systems. From an early age we learn, from experience, when it is appropriate to use the word “pain” for that unpleasant experience that is commonly – but not always – associated with tissue damage.
This learning stays with us through life – and it has strong survival value. It can also influence our interpretations of related events that might occur during a lifetime. Those aspects may be amenable to “unlearning” but the experience itself has not been learned: it is a built-in feature of being human.
Furthermore, there is no evidence whatsoever that we can experience pain without prior activation of our nociceptive apparatus. We cannot conjure up the sensation of pain through our memory or our imagination!
Let us not downplay the experience of pain as a function only of perception: there is always an unpleasant sensation involved.