(Disponível em Português)
The expression “sixth sense” is often used to refer to some sort of special intuition that gives people the ability to know and perceive things without using the five traditional senses. . It is also the title of the 1999 film by M. Night Shyamalan, starring Haley Joel Osment and Bruce Willis. But could pain be a real sixth sense? And would our lives be better without it, or would we be doomed?
The five senses refer to the five traditionally recognized methods of perception, or sense: taste, sight, touch, smell, and audition. The nervous system has specific sensory systems and organs dedicated to each sense. For sight, for example, the sensory organs are the eyes, and the sensory system the visual system. However, what constitutes a sense is a matter of some debate, particularly when it comes to defining boundaries between similar but distinct sensory stimuli.
Let’s think about touch, for example. The information regarding touch is initially conveyed to our brain by specific sensory neurons, and is then perceived in specific areas of the brain. If someone touches your arm, one or multiple specific types of sensory neurons (generally, mechanoreceptors) will be activated, and will convert that ‘touch’ into an electrical signal, which is carried to the somatosensory cortex of the brain, where it is ultimately processed and perceived.
Unlike what most people believe, “pain” isn’t felt due to a stronger activation of mechanoreceptors. Instead, there are specific sensory neurons, called nociceptors, that are only activated once a stimulus reaches a certain intensity.
This means that, for example, a slight touch with a needle will activate mechanoreceptors and be perceived as touch, but if the pressure of the needle on the skin becomes more intense it now begin to activate nociceptors, whose nerve endings are generally located in deeper layers of the skin, and the touch will be perceived as painful.
There are different types of nociceptors, which convert and transmit information to the brain about different types of “pain”, following different types of noxious (“painful”) stimuli. There are multiple pathways this information can take, some of them being distinct from those that carry information about touch. Once this information reaches the brain, it is processed and perceived by specific areas – some of them are the same as the ones that perceive and process touch, but others aren’t.
Additionally, pain, unlike touch, isn’t a purely sensory experience. In fact, the International Association for the Study of Pain defines it as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”. In the scientific community, a distinction is often made between the purely sensory aspect of pain, which is called nociception, and the experience of pain as a whole, which includes the emotional aspect of it. In this article, the term “pain” will be used in both situations for simplicity.
Like the other senses, pain is a highly subjective experience, which can be modulated by dozens of factors including, for example, expectation and attention. This is why children (an adults) are often distracted by nurses when drawing blood – if you’re not paying attention and expecting to feel pain, you’re not likely to feel as much pain, even though the purely sensory aspect of it is pretty much the same (the same nociceptors are activated with the same intensity whether you are looking at the needle or not).
In short, it looks like pain should be considered a different (sixth) sense altogether from touch, since the neurons, pathways, and brain areas responsible for perceiving each of these sensations are different.
If we look at the description of pain, and if we think about our own experience with it, a keyword stands out: unpleasant. Pain really is, in the very least, unpleasant, but often disabling. If we think about the other senses, they all have a very clear purpose and, together, they contribute to making our life in this world (as animals but also as humans) easier. Pain, on the other hand, can be a bit of a pain… Sometimes it can stop us from doing necessary day-to-day activities. People who experience different types of chronic pain often struggle to live normal lives, because of how disabling it can be. So wouldn’t we be better off without it?
In reality, pain serves a critical survival purpose, acting as a defence mechanism at different levels. Firstly, pain has a very clear immediate protective function: it prompts us to withdraw from or escape dangerous or damaging situations. For example, if we put our hand on a hot stove, a potentially damaging situation, we will remove it immediately, which minimises the damage caused by it.
Another function of pain is to protect the damaged body part as it heals. In this example, the burn on our hand will make using that hand painful, and so we would avoid using it, which speeds up healing. The final function of pain is similar, in all aspects, to that of a teacher: pain teaches us to avoid potentially damaging situations in the future. Here, we might be more careful next time we come near a stove, and check whether it is safe to touch it, or just avoid it altogether.
What would actually happen if we were unable to feel pain?
Is it possible to live without it? The answer is yes. And no. There are in fact a few people who are born without the ability to feel pain. They can still feel touch, but not pain, which again argues for making a distinction distinction between pain and touch, when it comes to the senses. This condition is called congenital insensitivity to pain (CIP), and it is mostly caused by a mutation (alteration in the DNA sequence) in the gene that codes for a specific ion channel (a structure located at the cell surface that allows the exchange between the inside of the cell and the outside).
This channel is present in nociceptors and is critical for the transmission of the electrical signal to the brain. If this channel isn’t working properly, the signal will never reach the brain and therefore will never be perceived – and therefore pain will not be felt. These people are often healthy in every other sense, and so should technically be able to live a long, normal life. But, generally speaking, they aren’t. In fact, most of them die during childhood, as they often don’t realise they have injuries such as burns or broken bones, and struggle to learn which behaviours to avoid. Additionally, they may not respond to problems, being unable to detect severe diseases, such as infections, early on.
CIP is an extremely rare disorder, partly due to its relatively specific genetic origin, but mostly because it isn’t necessarily viable from an evolutionary point of view, since so few individuals reach adulthood. There are several reports of male CIP patients that end up killing themselves in early adulthood by doing ridiculously dangerous things, as a result of not having been “taught”, and thus not being restrained by pain. There is also the case of a young Pakistani boy who came to the attention of scientists through his reputation in his community as a street performer who walked on hot coals, and stuck knives in his arms without displaying any signs of pain. He later died in his early teens, after jumping from the roof of a house. A personal account of life without pain can be found in a BBC News article (warning: sensitive readers might find the article disturbing).
Even though congenital insensitivity to pain has a very specific, clear origin, our understanding of pain, both at the molecular level, and at the level of neural circuits, is still quite limited. There is currently extensive research being carried out all around the world, in an attempt to increase our knowledge of this mysterious sixth sense. In the same way that comprehending the mechanisms behind vision or audition was required to design glasses and hearing aid devices, a better understanding of the mechanisms involved in generating and processing pain is crucial for the development of therapies that can improve the quality of life of millions of people worldwide.
One thing, however, appears certain: we can’t seem to live without this sixth sense of ours.
BIBLIOGRAPHY:
1. Hellier, J.L. ed., 2016. The Five Senses and Beyond: The Encyclopedia of Perception: The Encyclopedia of Perception. ABC-CLIO.
2. Kandel, E.R., Schwartz, J.H. and Jessell, T.M. eds., 2000. Principles of neural science (Vol. 4, pp. 1227-1246). New York: McGraw-Hill.
3. McMahon, S.B., Koltzenburg, M., Tracey, I. and Turk, D.C., 2013. Wall & Melzack’s Textbook of Pain, Expert Consult-Online and Print, 6: Wall & Melzack’s Textbook of Pain. Elsevier Health Sciences.
4. http://www.iasp-pain.org/Education/Content.aspx?ItemNumber=1698#Pain
5.http://www.bbc.com/future/story/20170426-the-people-who-never-feel-any-pain
6. https://www.bbc.co.uk/news/magazine-18713585
Elisa Clemente is a PhD student at University College London
Edited by: Ana Gerschenfeld(Science Communication office). Photo credit: scoundrelfighter (Creative Commons CC BY-NC-ND 2.0)
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