The Path to Chronic Pain: Unraveling the Complex Causes
How does pain, usually a helpful sensation, turn into the unworkable problem we call chronic pain, affecting over a quarter of the world’s population? Chronic pain is a lingering and persistent sensory experience, but it’s far more complex than that. It emerges from interactions within our body’s systems, shaped by a mix of biological, psychological, social, and environmental factors. Over the last 50 years, advancements in evolutionary and complexity sciences have given us new ways to look at this frustrating human problem. In previous posts, we introduced evolution as the science of change, defined the nociceptive apparatus—our body’s internal harm detector—and explored how complexity science can help us better understand health and wellness. Now, we’re ready to explore this different perspective of pain.
The Nociceptive Apparatus: A Constantly Adapting System
Our body’s harm detector, the nociceptive apparatus, has evolved to help us survive by quickly sensing when we’re injured or might get injured. This system can become more sensitive when needed, which is helpful to avoid further harm.
For example, if you step on something sharp, your harm detector might become more sensitive, increasing its responsiveness to detect additional potential injuries. It does this through multiple processes of change, including making changes in the spinal cord (part of the connecting pathways of the harm detector), allowing it to receive more information from nearby nerves. As a result, the harm detector starts picking up signals from nerves not typically involved in sensing harm, like those for light touch, gentle pressure, or blood flow. At the same time, the brain sends signals and chemicals that can speed up or slow down how these messages travel through the spinal cord. These processes are technically called peripheral and central sensitization.
Along with these changes, the harm detector undergoes structural and functional changes That’s why we look at the harm detector from 3 levels: local, long, and global. These levels reflect the changes happening throughout the body in response to actual or potential injury. This process can lead to a phenomenon called allodynia, where things that usually aren’t painful, like a light touch, are suddenly experienced as painful.
While this increased sensitivity helps with survival, it can cause problems when it happens too often and doesn’t return to normal. This is an example of progressive change in a complex system, like the harm detector within the body’s ecosystem. Over time, small changes can accumulate and reach a tipping point, at which one final change causes the system to enter a completely different state. This new state is known as a Complex Maladaptive System (CMS) in complexity sciences.
To understand how this happens, it’s essential to know that the body’s harm detector, like everything else in the body, is constantly changing and adapting to its environment. Let’s take a look at how these changes can develop over time.
How the body’s ecosystem primes the harm detector for change
Increasingly, scientific research highlights the complex interactions between the nervous, immune, and endocrine systems in developing chronic pain and other health conditions. Together, these interrelated systems are seen as the neuroendocrine-immune (NEI) axis, which I often call the ‘holy trinity’ of survival. The nervous system triggers rapid responses such as sensory awareness, movement, and changes in position. For example, when you quickly pull your hand away from a hot object, the harm detector plays a vital role in this process. The immune system fights infections and manages processes like cell breakdown and generation. The endocrine system, together with the autonomic nervous system, controls the body’s stress response—fight, flight, or freeze—by releasing chemicals in the form of hormones and neurotransmitters, such as cortisol and catecholamines (like adrenaline). These chemicals help prepare the body for real or perceived harm, such as in stressful or threatening situations. They do this by raising heart rate, directing blood flow to vital organs, and exciting the nervous system to enhance our ability to see, hear, feel, and move faster. These chemicals and interactions with the nervous system are essential for survival.
While this system evolved to handle immediate threats like encountering a lion, our advanced ability to think, predict, and analyze has introduced internal threats. This complex thinking relies on language, which not only helps us communicate but also enables us to form internal symbols. These symbols allow us to troubleshoot and assess potential dangers—whether they come from past experiences or imagined future scenarios. This ability can trigger the same stress responses as real physical dangers, leading to feelings like worry, fear, anxiety, or anger, all of which are meant to protect us—not just from physical harm, but also from social threats involving strangers or relationships with friends, family, and work.
However, if these stress responses are activated too often or for too long, the body adapts by making them easier to occur, even in less threatening situations. These changes don’t happen in isolation; they involve many systems in the body, including the harm detector.
Local changes to the harm detector
As we previously described, our harm detector is a physical part of the nervous system made up of specialized nerves. These nerves have sensors, called ion channels, that respond to stimulation from their surroundings. Interestingly, these ion channels have a surprisingly short lifespan—some lasting only 3 hours, while most survive up to 3 days—before they are replaced. Our genetic code, which acts like a long-term blueprint we inherit, gives a basic template for this. However, our epigenetic code, influenced by our environment and can change during our lifetime, helps determine what kind of sensor will replace the old one. There are over 300 types of sensors that we have classified scientifically so far (likely more for us to find!). But for simplicity, we can categorize nerve sensors into the following categories:
