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by Alison McGinnis, PT, DPT, FAFS

Posted in Blog.

August 30th, 2020

The Science of Pain

Pain is a normal, healthy human experience; chronic pain is not. Most tissue injuries (muscles, ligaments, tendons, and joints) heal within the first 3-6 months, but pain may persist for months to years after that. This is due to a sensitive nervous system and the way the brain processes information it receives from the body and the environment.


Your body contains over 45 miles of nerves that work as a network to connect all your body parts to the spinal cord and up to the brain. Your nerves monitor everything going on in the body, and send this information to your brain for processing. There is a resting level of nerve activation throughout the body at any given moment that your brain is interpreting or ignoring. When a danger signal occurs, like when you step on a nail, there is increased neural input to the spinal cord and brain alerting you to take action. This “firing level” is the threshold amount of neural input the brain needs to take action. Once you remove your foot from the nail, the danger signal subsides and the neural input should return to its resting level. Same thing happens when you injure a muscle or other body part. The nerves wake up, alerting the brain of danger in the area, and hopefully gets you to seek out treatment or rest from activity.


For some people, the nerves stay awake or calm down slower, increasing the resting activation level of the nervous system. This means it takes much less input from the body to signal the brain that there is danger and producing pain. Now normal daily movements trigger a pain response that doesn’t correlate with actual dangerous situations in the body since the nerves are extra sensitive. This means normal daily activities you used to do easily and for long periods without pain now cause pain after only a few minutes or a smaller range of motion.



Everyone’s nervous systems are tuned differently. This makes the pain experience highly personal, and means no two people will recover in the same time frame from a similar injury. And your personal alarm system is influenced by many factors, including previous experience, stress, uncertainty, impact on job/family/personal life, failed treatments, and fear. Research has shown that fear of injury or re-injury, and fear of movement or exercise will keep the alarm system on instead of turning it off. This keeps the nerves extra sensitive.


Nerves provide information to the brain from specific sensors throughout the body. There are no “pain” sensors in the body, but rather sensors that respond to temperature, stress chemicals, pressure/movement, blood flow in the tissues, and inflammation. Any changes in these will wake up nerves, and the brain decides what to do with this information. If the resting sensitivity level is already high, normal inputs can now stimulate a pain response from the brain.


When the nerve alarm system goes off in one area, it also tends to stimulate the nerves in surrounding areas as well. Since the nervous system is interconnected, this can result in the sense of pain spreading. When your nerves send an alarm signal, the body responds by sending immune molecules to check out the area and assess for any damage. It will check on the area from the initial signal along with the surrounding areas to see if everything is okay. They will also check on areas of old injuries or surgeries just to be safe. This is a normal response from your body, but this increased stimulation of nerves may increase your sense of pain or discomfort through a much larger region, and may awaken the sensation of old aches and pains. This is due to the sensitivity of the nervous system, not injury to more tissues.


Once your brain becomes aware of an area that is sending danger signals, it will continue to check in on that area for a report. This results in increased information being sent to the brain more often over a longer period, and makes the brain more aware of the painful body part. The brain will also check in and snoop around other body parts to stay on top of what’s going on, which further elevates nerve sensitivity. This is all normal during the acute phase of injury, where the body figures out the problem and produces the correct response to resolve the issue, but can lead to chronic pain if the nerves remain overly sensitive. Everyone’s individual response system is different, making pain a very individualized experience.


There does not even need to be an initial injury to cause pain. Most of us have had the sensation of pain starting out of nowhere one day and not knowing what caused it. Research has shown that tissue injury is not needed for pain, that emotional stress can cause pain, and that prolonged periods of high stress of any kind can lead to pain. Our nervous system is designed to rev up for short periods to escape a threat (fight or flight) then calm back down to our resting level. But in our current society where chronic stress is ever present, a heightened stress response may remain active.


This prolonged stress response also has effects on mood swings, appetite changes, fatigue, weight gain, sleep disturbances, poor posture, low sex drive, stomach irritability, poor concentration and focus, and depression. Your pain experience is no longer localized to a specific area, but has become a global system’s response. This is also a feedback loop where these new problems further drive your pain response.


Treatment for Persistent Pain:

  • Knowledge:  Research shows the more you know about pain and how it works, the more control you gain over your pain. Realizing that much of the pain experience is due to overly sensitive nerves, not tissue damage, will actually decrease nerve sensitivity.
  • Aerobic Exercise: Research shows that this type of exercise increases blood and oxygen pumping through your body and can calm down nerves. A brisk walk for 10-20 minutes is effective for reducing nerve sensitivity. But remember to start small. Doing too much can send your system back into a heightened sensitivity mode.
  • Sleep: Insufficient sleep is linked to increased pain, increased sensitivity/tenderness, obesity, depression, and a slew of other health consequences. Check out the previous blog on sleep for more information and tips on improving sleep habits.
  • Goal Setting: Setting small goals and accomplishing them provides positive reinforcement to reduce pain. Many people in sustained pain either don’t have goals or have set goals that are so big they may seem out of reach. Set simple daily goals that get progressively harder as your pain gets better. By accomplishing the task your brain realizes it’s capable of much more.


The common saying that “pain is in your head” implies that it is not real. This is not the case. Pain is located in the brain not the body, and how you process distress signals from the body determines how you will experience pain, but the pain is real, and measurable, and it can be changed for the better. Once you have an understanding of the role your nervous system plays on your experience of pain, you can take control of your recovery. Remember pain is a normal and healthy experience, but living in pain is not.


References: Why Do I Hurt by Adriaan Louw

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