The Curious Neuroscience of Pain
Why the phenomenon of pain continues to baffle scientists.
Posted April 13, 2022 | By: Aditi Subramaniam, Ph.D.
KEY POINTS
- Pain evolved as a mechanism to signal tissue injury.
- But pain is a complex phenomenon, and many different factors influence pain perception, including mood.
- Understanding the mechanisms by which pain turns chronic is essential for the development of more effective treatments.
Try and think back, if you will, to the last time you were in intense pain. Do you remember anything else about the scene? In all likelihood, you don’t. Was it a nice day out? Was there a lovely scent of cherry blossoms in the air?
My guess is that your sensation of pain trumped all other experiences at that moment. This is hardly surprising, given how important a signal acute pain is. Drop everything this moment and attend to me, says pain. I’m a signal that something is wrong and needs your attention.
But all is not so straightforward when it comes to pain. A story that neuroscientists like to tell to illustrate the strangeness of pain is that of a construction worker who fell on top of a nail, which ended up piercing right through his shoe. The man, quite understandably, was in sheer agony and had to be given strong painkillers on the way to the hospital, where it was discovered that the nail had not penetrated his flesh at all but had just gone through a gap between his toes!
This is not to say that the man’s pain was not real; he was most definitely in agony. It’s just that the agony was not in response to a real event but an imaginary one. What essentially happened was this: The brain had constructed a story of the most likely scenario based on cues from the environment (seeing the nail go through the shoe, for instance) and decided that a pain signal was appropriate to protect the man from further harm.
The subjectivity of pain ratings
Of course, this is a great anecdote not just to illustrate how complex a phenomenon pain is but also to shine a light on a very real problem that pain scientists encounter every day: How do you measure pain? Pain, by definition, is subjective; I know you’re in pain because you tell me so. To this day, when doctors want to measure or quantify pain, they rely on a startlingly simple question: Could you rate the intensity of your pain on a scale from 0 to 10, with 10 being the worst possible pain?
The issues with such a subjective scale are obvious, not least of which is that there is so much variability in our tolerance levels and how we perceive pain that one person’s 9 is almost certainly another’s 4.
Factors that influence pain perception
In fact, some of the illustrations of how people perceive pain differently and how the same person perceives pain differently under various circumstances seem downright astonishing. Take, for instance, the fact that while their baseline levels of pain might be similar, people who are religious report feeling less pain than their atheist counterparts when shown a picture of the deity they believe in. Or the fact that negative mood not only increases pain but also reduces overall pain tolerance in patients with chronic conditions.
I know firsthand the effect of mood on pain and vice versa. I recently ended up with a pinched nerve in my back (a condition called sciatica), and the path to recovery has been slow and agonizing, but at the same time, an excellent case study on the psychology of pain. There were days in the month (anyone who has ever menstruated knows what I mean) when my pain tolerance was alarmingly low. The same intensity of pain that I had been able to bear (somewhat) bravely in the days previously would easily bring me to tears.
And what of the effect of pain on mood? Unfortunately, I was a decent case study on this aspect as well. Apart from losing my patience with the two other members of my houselhold more frequently than usual (sigh), I also noticed I was—and I know this sounds dramatic—taking a bleaker view of nearly everything in life.
In the television series House, Hugh Laurie does an excellent job of conveying how Dr. House’s mood largely depends on how much pain he is in on any particular day. There are moments in the series when House is in little to no pain at all, and those are the moments that the viewer gets to see, well, not a sweet-natured Dr. House as such, but at least a moderately pleasant one.
The neuroscience of pain
As with any other sensation, pain is processed in the brain. When we touch something that causes pain, the pain signals travel from the hands to the spinal cord and then up to the brain, where they are processed, and the appropriate response is decided upon. This works differently for situations in which immediate action is warranted—if you touch a hot iron, for instance, the information to the motor neurons in the hand is sent by a neuron in the spinal cord. Why waste time involving the brain when a simple reflex arc can do the trick several times faster? Of course, the information about a potential injury and pain is eventually sent to the brain, and more complex decisions about the injury can be made.
The neuroscience of pain is complicated further by the fact that there isn’t one particular brain region or network for pain. There is also a considerable difference in the way different people’s brains seem to process pain—we all know that person who crumbles at the smallest sign of pain and others who seem to have a much higher tolerance. But one part of the brain that seems to be involved in most kinds of pain is a tiny region deep within the brain called the insula. Functional MRI studies have also shown correlations between blood flow to the dorsal posterior insula and pain intensity ratings.
When pain morphs from signal to disease
When pain is a signal for injury, of course, it’s great and potentially life-saving. But what happens when the pain ceases to be useful? My experience with a pinched nerve that took several weeks to heal gave me a little taste of what it must feel like to be in chronic pain. Chronic pain is the opposite of the acute pain example I started this article with. If acute pain is a symptom of an underlying condition or a tissue injury, chronic pain is a disease in and of itself. The pain outlasts any injury and even modifies the way in which the brain responds to innocuous stimuli such as touch. In some patients with chronic pain, a simple feather touch to the skin feels like intense burning.
Right now, treatment for chronic pain involves temporary symptomatic relief using drugs that can have serious side effects when used long-term. Managing pain becomes the focus, and a combination of approaches, including nontraditional ones like acupuncture and meditation, seems to provide at least some relief.
As scientists gain a deeper understanding of how pain works and the molecular mechanisms by which it becomes chronic, treatment approaches are bound to get more specific and personalized. For anyone who has dealt with pain that interferes with their daily lives, that day can’t come soon enough.
