An Overview of Pain Perception
Pain is a commonality among humans as well as among other animal groups. The sensation of pain is very much a subjective experience; when a person experiences pain, only they can feel it. Others, however, can sympathize and even feel pain of their own as a response to our expression of pain, or in response to witnessing the situation that has brought on our pain. We can often recognize when another person is experiencing pain; other times we cannot. Experience has taught us what kinds of situations cause pain. Experience may have also taught us that some people may find a specific situation very painful, such as running a 5K, while others find that same situation to be much less painful. Why is pain felt differently by two people exposed to the same situation? What is the purpose of pain and why does running a 5K hurt? Why do our own perceptions of pain change over time? Why does running a 5K hurt less the more that we train for it?
The first of these questions can be answered by distinguishing between the amount of pain that a stimulus causes and the amount of pain that a person can handle. The former is fairly similar among different people, while the latter can be quite different. Fields & Levine (1) put it succinctly when they write:
“the sensory level (intensity) required for detection (pain threshold) is rather constant, whereas the intensity level required to elicit spontaneous complaints (tolerance “threshold”) is highly variable.” (352)
To further elucidate this idea, they draw on an example from a comparison between the two recognized types of pain, acute versus chronic pain. Acute pain is the unpleasant sensation that is experienced when the tissues of the body are or might be harmed (2). Thus, acute pain serves to protect the body by letting it know when there is a problem. (Perhaps running a 5K is painful because the body recognizes the potential for tissue damage as a result of all of the pounding on your ligaments and joints and the stress on your muscles.) Chronic pain, on the other hand, often originates from a prolonged experience of acute pain (3), but does not warn of tissue destruction (2). Its name hints at the most devastating part of its nature: its long duration (3). Fields & Levine suggest that in acute pain, the intensity of pain is greater than the tolerance threshold, while chronic pain may not start out in this way (1). Overtime, the intensity of chronic pains can exceed the tolerance threshold of the individual. This suggests that there is a dynamic component to pain perception with respect to the tolerance threshold. Before we explore this idea, let us attempt to better understand the mechanism behind the detection of and interpretation of pain stimulus (pain threshold and tolerance threshold, respectively).
The pain threshold aspect of pain perception involves neurons called nociceptors that respond to stimuli that are or might be causing harm to tissue. Some stimuli that are detected are temperature (the neurons that detect unsafe temperatures are called themoreceptive nociceptors), pressure (mechanoreceptive nociceptors), and chemicals (polymodal nociceptors) (4). Nociceptors sense stimuli at their receptor end where an action potential is generated that communicates the presence of the potentially damaging stimulus to other parts of the nervous system (4). The amount of stimulus required for an action potential from these receptor ends of nociceptors is the sensory threshold of pain. Since the components that detect the stimulus are simple nerve cells, it is easy to understand why pain thresholds are “rather constant.” Amazingly, however, these neurons inform the nervous system as to what kind of pain and how strong it is, as well as where in the body the neuron receptors were stimulated (4).
While the first part of pain perception involves the sensing of potential danger, the second part of pain perception involves informing the brain of this potential. It is in this stage that the tolerance threshold for a given stimulus is determined. Once the nociceptors tell the spine (via neurotransmitters) that possible tissue damage is occurring (4), this message is relayed to the brain through neurons called projection cells (1, 5). What these projection cells actually communicate to the brain, however, is affected by the input of interneurons. Generally, these interneurons can be separated into two groups. One group lets the signal get through, allowing the brain to perceive the pain, and the other inhibits the pain signal, keeping the brain from perceiving the pain (1). People have different tolerance thresholds for the same painful event because interneurons in the nervous system are filtering which signals can get through according to what other stimuli are being received from the nervous system (that is, interneurons adapt the signals sent through the projection cells to different situations) (5).
Now we understand why two people may have different pain tolerances, but what about when the pain tolerance of an individual changes? Why does running a 5K become less painful the more you train for it, even though you are still subjecting your body to the same amount of pounding and stress as before? The answer to these questions lies in the concept of neuronal plasticity, the idea that neurons, and the way in which they are wired in the nervous system, can change over time (6, 7). Neuronal plasticity can increase the pain threshold by either increasing the significance of those interneurons that propagate signals along the pathway, or by decreasing the effects of those interneurons involved in inhibition of the pain signal (6). This process by which the body responds to lower threshold levels of pain resulting in an increase in pain perception as a whole is called sensitization (8). When neuronal plasticity works in the opposite direction, increasing threshold levels, the process is called habituation (9). It is sensitization that can change a persistent acute pain into a chronic pain, while habituation may be the reason for a less painful 5K run when the runner has trained more.
The perception of pain is clearly a complex process. It usually acts to tells us when we are potentially damaging our bodies (acute), but sometimes it serves no purpose (chronic). We can share one another’s pain, but we cannot quantify for sure the degree to which another is in pain, making pain a very difficult phenomenon to research and understand. The experience of acute pain is often easier to understand than that of chronic pain because we can imagine our own sensation of pain associated with the given pain-inducing situation. Running a 5k? Most people can imagine at least in some detail what kind of pain such an activity might entail. It becomes much more difficult to imagine the kind of pain one endures daily from the chronic knee, foot, or hip pain caused by running too many 5Ks. Hopefully our understanding of chronic pain will one day rival our current knowledge of acute pain. In the mean time, we should use our body’s perception of acute pain to recognize and respond promptly to problems in our bodies so that chronic pain is less likely to occur. After all, when it comes to pain, our bodies are only trying to tell us something.
(1) Fields, H. L. & Levine, J. D. 1984. Pain- Mechanisms and Management. Medical Progress, 141(3): 347-357.
(2) Yezierski, R. P., Radson, E., Vanderah, T. W. 2004. Understanding Chronic Pain. Nursing, 34(4): 22-23. <http://findarticles.com/p/articles/mi_qa3689/is_200404/ai_n9402167>. 2008.
(3) National Institute of Neurological Disorders and Stroke. “NINDS Chronic Pain Information Page.” Office of Communications and Public Liaison. <http://www.ninds.nih.gov/disorders/chronic_pain/chronic_pain.htm>. 2008
(4) Barrett, Julia. “Pain.” Gale Encyclopedia of Medicine. December 2002. <www.healthatoz.com/healthatoz/Atoz/common/standard/transform.jsp?requestURI=/healthatoz/Atoz/ency/pain.jsp>. August 14, 2006.
(5) Freudenrich, Craig. Howstuffworks. “How Pain Works.” <http://health.howstuffworks.com/pain.htm/printable>. 2008.
(6) Stucky, C. L., Gold, M. S., Zhang, X. 2001. Mechanisms of Pain. PNAS, 98(21): 11845-11846.
(7) The Neurosciences Institute. Neuronal Plasticity. “Learning, Memory and Cognition- Neuronal Plasticity.” < http://www.nsi.edu/index.php?page=iv_neuronal_plasticity>. 2007.
(8) Cellular Mechanisms of Learning and Biological Basis of Individuality. “Sensitization.” <http://www.geocities.com/cell_learning/Sensitization.htm>.
(9) Cellular Mechanisms of Learning and Biological Basis of Individuality. “Habituation.” <http://www.geocities.com/cell_learning/Habituation.htm>.