The labeling of nociceptors as pain fibers was not an admirable simplification, but an unfortunate trivialization under the guise of simplification.
From Patrick Wall, eminent pain scientist and author of the gate theory of pain.
How we think about pain has been an evolving process. In ancient times, it was often thought to be punishment from gods or demons, something initiated by outside, external forces that invaded the body. During the Middle Ages, pain was seen by some as a spiritual test of faith. In 1664, when he wrote the Treatise of Man, Rene Descartes challenged the idea of pain coming from an outside source. He introduced the idea that pain was a sensation coming from the body itself. He saw the body as a machine and that pain occurred when a noxious stimulus, such as fire causing a burn to the skin, created a disturbance to that machine. He illustrated his idea with a now famous drawing of a man with his foot too close to a fire. He visualized particles of heat activating a thread running to the brain (the nerves), where a valve in the brain would open and cause a response. This model of how pain works, that tissue damage sends pain signals to the brain, causing us to jump with an "Ouch!" stood uncontested for about 350 years.
Rene Descartes was very, very smart, but as it turned out, he was wrong . . . at least about how pain works.
There have been hints all along that the relationship between tissue damage and pain is not always directly related. We see examples of this in our daily lives. Many people have heard of phantom limb pain, a phenomenon where a person who has had an amputation experiences pain in a limb that no longer exists. And who hasn’t found a large bruise on their body without any memory of an injury to the area. Our bruise provides evidence that tissue damage occurred yet, apparently, without any significant pain at the time. History is full of reports of soldiers who were unaware of they were shot until they were off the battlefield. Basketball player Kevin Ware sustained a visibly broken leg during a game, yet said later that he felt no pain and had no idea what had happened until he saw his leg with his own eyes.
Another amazing pain story was reported in the British Medical Journal. A carpenter was brought into an emergency room after he stepped on a nail that penetrated all the way through his shoe. The man was in such pain that he had to be sedated before the nail could be pulled out. When his shoe was removed, it was discovered that, in fact, it had not injured his foot but had gone between his toes. [You can read about the case in this paper; find the section Getting Beyond the Black Box of Symptom Reporting.]
Clearly, as Paul Ingraham of PainScience.org wrote:
Pain is Weird
These may be some unusual cases, yet every day people with no discernable tissue damage suffer from chronic low back pain, or neck pain, or shoulder pain. What is going on? Some people with serious tissue damage have no pain, other people with no tissue damage have serious pain, and some people have pain in tissue that doesn’t exist. How can we make sense of this?
Each of us has a brain, a spinal cord, and sensory nerves that go out to various parts of the body. Sensory nerves going to internal organs, joints, etc., monitor the environment within our body while nerves going to the skin monitor our external environment, responding to temperature, vibration, pressure, etc. Some nerves are “multimodal” and respond to more than one type of stimulus while others are very specific. It was recently discovered, for instance, that some nerve fibers in the skin respond specifically to moderate pressure stroking done at a moderate speed. Apparently we are biologically wired to respond to massage!
Some nerve fibers respond to potential threat. Called nociceptors, (“noci” = noxious; “ceptor” = receptor), these nerve fibers respond to potentially damaging levels of heat, cold, pressure, or chemicals. When these high-threshold (i.e. they will not respond to lower, non-threatening levels of stimulation) nociceptors are activated, they send impulses to the brain. This process is called nociception. Nociception is NOT a “pain signal” nor is it in and of itself pain. It could more rightly be called a “danger signal.” It alerts the brain that something potentially threatening has just occurred.
When these signals travel up the spinal cord and reach the brain, the brain processes the information and assesses whether the event that initiated the impulses is dangerous. Modern brain imaging has taught us that there is no specific “pain center” in the brain, as was once thought. Instead, a complex array of reactions takes place in several parts of the brain that correspond with emotions, thought processes, and the primitive "critter" part of our brain. It is as if one could say that the various parts of the brain are having a conversation about how dangerous is this potential threat. Past experience, our emotional state, the context in which the stimulus occurred, all may play a role in whether the brain perceives the event as dangerous and to what extent. If the brain perceives serious threat, it will create the sensation of pain and send impulses out to the periphery that might make you pull away, fall to the ground, or writhe and yell out in distress. The sensation of pain is a protective mechanism. It gets our attention and makes us take appropriate action, such as pulling away from a sharp object or immobilizing a body part that has just gotten injured. If the brain perceives a very low level of threat, it might make us reposition ourselves without us paying much attention. If it perceives no threat at all, we might not notice that anything happened.
How is it that a basketball player could have a shin bone broken so badly that it protrudes from his leg yet not feel pain? Well, consider this as a possible explanation: he was doing something very familiar, playing basketball. It was an activity in which he’d spent probably thousands of hours. He was intently focused on the activity. No doubt that in the course of playing many games over a period of many years, he had jumped many times, fallen many times, run into other players many times, without any serious harm. It’s all part of the game. Apparently his brain accepted all this as a familiar and non-threatening pursuit and ignored the nociceptive impulses that surely must have occurred at the time of injury.
On the other end of the experiential spectrum – how is it that a person can have no tissue damage and yet suffer from debilitating pain? Think back to the carpenter with the nail in his shoe. Construction is a hazardous occupation. A construction worker needs to be ever vigilant about safety. Construction workers face potential threats on a daily basis and all of them know of other workers who have been seriously injured on the job. When the carpenter’s brain saw the nail protruding through his shoe, the situation appeared to be a serious threat. Creating an intense sensation of pain would force the carpenter to stop everything and have this taken care of immediately. The pain was NOT imaginary, it was very real. Pain is not all in our head, but the sensation of pain is created by the brain.
Our brains are sometimes easily fooled. We’ve all experienced optical illusions where we know something is not what it seems yet our brains are tricked anyway. Our brain creates a subjective experience that does not always match objective reality.
While the basketball player and the carpenter are unusual examples, they illustrate how the relationship between tissue damage and pain is not directly linked in a one-to-one fashion. One unit of tissue damage or nociception does not produce one unit of pain. In fact, one can have nociception without pain and pain without nociception. Nociception is the input to the brain from the tissues. It is the process of transmitting information. Pain is an output of the brain, the brain's response after it has processed the information and determined that protective action needs to be taken. Neuroscientist V. S. Ramachandran has called pain an opinion of the brain about the state of health of the organism.
This concept, the distinction between nociception and pain, may be difficult to grasp at first, but it is important for both those who work with individuals with pain and those who suffer pain, especially chronic pain.
Why does it matter?
When we understand that pain does not equal tissue damage, but that, instead, it is a warning signal, it can help to remove some of the worry that often accompanies pain. When we have an injury, the normal course of healing is that there is usually a spike of pain in the beginning and then, as damaged tissue heals, pain decreases until it is completely gone and everything returns to normal. With chronic pain, something goes awry and the pain continues long after the tissue has healed. Even the worst injury has usually healed within six months, but chronic pain can continue for years. We don’t understand exactly why this happens in some people and not in others. Rather than turning off the alarm after the danger has passed, the body continues to perceive threat. It becomes like an oversensitive smoke detector that goes off when we turn on the shower or a faulty engine light in the car that comes on even when the engine is running fine.
We should never ignore pain, just as we don’t ignore the smoke detector when it goes off because there is always the possibility that there really is a fire. However, we can put pain into its proper context and realize that, just as the smoke detector may “interpret” steam from the shower as smoke when there is no fire, our brain may “interpret” low levels of nociception as something more dangerous than it really is.
Understanding how pain works is not a magic bullet to make it go away. However, when the way we think about pain shifts, it begins to shift our experience of pain. When we live with chronic pain, it can dominate our lives and dramatically alter the way we live, like an obnoxious cab driver who insists on taking us where HE wants to go rather than where WE want to go. When we understand more accurately how pain works, it may still be unpleasant, but putting it into its proper context can help us cope. If pain is alerting us to a condition that needs medical attention, we should attend to it promptly. However, if we’ve been assured that, in fact, we are essentially okay, we can mentally reassign it to the "unpleasant but not threatening" corner of our mind. Over time, as we find ways to treat it and overcome it, the pain may not disappear completely but it may become more like an annoying back seat passenger rather than the obnoxious driver who is in control of the vehicle. Understanding the difference between nociception and pain can help put us back in the driver’s seat and help us find ways to rehabilitate our pain.
If the distinction between nociception and pain seems difficult to grasp, don’t worry. We have a history of 350 years of Cartesian thinking about pain behind us and a whole world around us that still accepts that paradigm. By updating our thinking about how pain works, we increase our chances of finding more effective ways of responding to and diminishing pain. Don’t be surprised if your colleagues, friends, and acquaintances – even your doctors, physical therapists, and other health care providers – are still operating under the old paradigm. While researchers have been aware of these changes in our understanding of pain for about 30 years, many clinicians have not yet updated their thinking. Fortunately, you don’t have to wait for them to catch up with the pain science to put this knowledge to use. Understanding how pain works can help those living with chronic pain as well as health care providers, including massage therapists, who work with individuals with chronic pain.
If you’d like to learn more, I’ve listed some resources about pain science and how to put it to use in your life or in your practice at the end of another article.
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