Pain is your body’s alarm system. It alerts you when something is wrong and you need to take action to fix the issue. Pain is an early warning system, which causes you to react to avoid something potentially harmful. It is also a continuous alarm, to remind you something is damaged and you need to protect the damaged area. But when pain doesn’t go away, it becomes a serious problem. Sometimes, even after the injury has healed and the suspected cause of pain has been addressed, your body continues to hurt. This “pain with no cause” is part of what makes chronic pain so difficult to treat.
Understanding what pain is, how it works, and how to control it can help you work with a medical provider to determine the pain management strategies that might work best for you.
Types of pain
Pain has 3 different types depending on what triggers the signals to your spinal cord and brain. The first is called nociceptive pain, which is your body’s early warning system. This is a response to a painful stimulus, such as touching something hot, cold, sharp, or getting a chemical burn. The purpose of nociceptive pain is to prevent an injury, which usually causes you to react reflectively. For example, when you touch a hot stove, you pull your hand back quickly, often before you feel the burn. Your body first “recognizes” pain when the A-delta nerve fibers—or “fast pain” nerve fibers—send signals to your spinal cord. This causes a physical reflex, such as pulling your hand back, as your spinal cord relays the pain up to your brain, where you become aware that you felt something.
You might remember a time where you reacted to a burn or cut quickly, but then the pain got worse after the injury first happened. Nociceptive pain is also transferred by type C nerve fibers, also called “slow pain” fibers. This is what causes the throbbing, dull, or achy pain after an injury.
The second type is called inflammatory pain. The inflammation that occurs in the first couple days after the injury activates the A-delta and C nerve fibers (together called nociceptors), which send pain signals to your brain. At this point, the damage has already happened, so the reflex-type nociceptive pain signals have gone away. The inflammation chemicals activate the pain receptors, which cause tenderness and a dull, achy pain.
Inflammatory pain’s purpose is to “remind” you of an injury so you can protect it and prevent more damage. As the inflammation process slows down after the first few days, your pain level should also decrease unless you do something to trigger pain again.
Pathological pain is the last type. Think of it as a malfunctioning alarm continuously sending signals. Within this type are 2 categories. Neuropathic pain is caused by damage to the nervous system, such as a herniated disk that presses on the spinal cord. Pathological pain you feel without damage to the nervous system, recent injury, and little to no inflammation is called dysfunctional pain. Most cases of low back pain would be classified as dysfunctional pain because there is rarely any injury that causes pain to flare up. Pathological pain is difficult to treat because there is no one cause—such as an injury—that needs to be fixed. Treatment of pathological pain has to address physical, mental, social, and even nutritional factors.
Dysfunctional pain can be caused by conditions that affect mental health—such as depression and anxiety— and an overactive stress response. The same areas of the brain that control emotion and motivation also play a role in pain perception. Hormone imbalances that affect these areas can lead to you feel pain when your body is not physically damaged.
Pain is also classified according to how long you feel it. Acute pain is pain that lasts for less than 6 weeks. Sub-acute pain continues without interruption for 6–12 weeks. Chronic pain persists for at least 12 weeks without a reduction in severity or break in symptoms.
Your body has 2 physical ways to reduce pain. The first is called the gate control theory. The nociceptors in your body all go to your spinal cord, where they are initially processed, and then relayed up to your brain. A-beta nerve fibers, which sense touch and vibration sensations, send that information to the same part of your spinal cord as your nociceptors (A-delta and C nerve fibers), and then relay that information to your brain. However, A-beta fibers send signals to the spinal cord faster than nociceptors and activate an “inhibition” nerve cell. When a touch sensation is felt in an area where pain signals are also being sent, your brain will only process the touch sensation. This is because the inhibition nerve cell “closes the gate” to the pain signals being relayed to the brain. That’s why when you bump your shin, you quickly rub the area. You activate the touch receptors and block the pain signal from being processed.
Gate control only works for relatively minor pain or in areas where there are fewer nociceptors. For example, your head has lots of nociceptors. If you bump your head on the corner of a cabinet, rubbing your head will make it hurt a little less, but it still hurts a lot. Gate control also only works while the touch stimulus is applied. Remove the touch and the pain returns almost immediately. This makes a good short-term method for pain control, but it won’t get you through a day unless you consistently apply a stimulus. Your body also tends to filter out touch sensation over time. (That’s why you don’t feel the clothes you wear all day—although now you’re probably feeling them because you’re thinking about how you don’t feel your clothes all day).
So, in order to manage pain over longer periods of time, the intensity of the touch stimulus needs to be increased to wake up the A-beta fibers again. TENS units—which provide electrical stimulation to the area—work by leveraging the gate-control theory. They can be worn throughout the day, and can provide some pain relief to help you move. However, the pain often comes back after the device is removed or if you become desensitized to the constant stimulation.
The other physical way to reduce pain is through the descending inhibition pathway, which can also be triggered mentally and emotionally. Descending pain control starts in the brain and releases endorphins and other hormones. These act in the brainstem and spinal cord to inhibit A-delta and C nociceptor signals. Pain relief through the descending pathway lasts longer because it’s run by the hormones, which stay active from a few minutes to a few hours.
You can activate the descending inhibition pathway a few ways. Counterirritants—such as ice, heat, and creams that contain menthol, camphor, or capsaicin (look on the label for the active ingredients in your favorite muscle rub)—activate your nociceptors which also transmit hot and cold sensations. Activating the nociceptors in a controlled way with a counterirritant causes your brain to release pain-control hormones down your spinal cord, which stops the pain signal to your brain. Although a heating pad can be effective to control pain, it should never be done in the first few days after an injury. If you use heat during this inflammation phase of healing, it can speed up the inflammatory process and increase the swelling too much.
Another way to activate the descending inhibition pathway is with medications. NSAIDS such as ibuprofen, analgesics such as acetaminophen, and opioids such as morphine, oxycodone, and hydrocodone all work to activate the descending pathway in different ways. One type of medication might also work better for you than the others, but it depends on the type of injury, healing phase, and severity of your pain. For example, long-lasting inflammatory conditions can be managed with NSAIDs, while acetaminophen is best for non-inflammatory injuries such as tendinopathies. Opioids are some of the most effective pain relievers, but they are dangerous, have a high risk of addiction, and should only be used for short periods of time under the close supervision of a medical professional.
Finally—just as mental health conditions can cause dysfunctional pain—mental-fitness tactics such as mindfulness and guided imagery can help manage pain conditions. For example, chronic stress is known to contribute to dysfunctional pain. These tactics can reduce an overactive stress response, which reduces the cause of the pain.
“Pain threshold” has two accepted definitions: (1) the smallest amount of pain you can recognize, and (2) the lowest level of stimulation you perceive as painful. For example, if you warm your skin with a heater, you feel a transition from comfortably warm to painfully hot if you stay in front of it for too long. The point where you perceive that heat as pain is your pain threshold. It isn’t always something you can control. It’s determined by a number of factors, including injury type and location, mental-health conditions (such as depression), your sex, your race or ethnicity, and even your hair color—redheads can be more sensitive to pain from heat and cold, and anesthetics can be less effective. If you’re a medical provider, it’s important to consider things such as mental-health history, sex, race, and ethnicity when someone reports pain to you.
How you experience pain is also related to your pain tolerance. Pain tolerance is the amount of pain you are willing to bear, or your ability to “tough out” the pain you experience. Pain tolerance can be affected by anything, including injury type and location, type of stimulus (such as heat, cold, or pressure), genetics, race or ethnicity, sex, socioeconomic background or even the presence of other people. For example, two people can have the same pain threshold and score their pain the same on a 0–10 scale. However, one person might say it’s more unpleasant or be more afraid of that pain coming back than the other person.
A high pain tolerance can be both a blessing and a curse. Because nociceptive and inflammatory pain are signals of injury, pushing through the pain can lead to further injury. However, in cases such as low back pain, where there is often no physical damage to your body, a higher pain tolerance can help you keep moving so you can work through the pain. Especially for Military Service Members, pain tolerance is a delicate balancing act between prioritizing your health and completing the mission. Knowing when to take a knee—even if you can push through the pain—is an important lesson to learn in your military career.
To learn more about pain and pain management, visit the Defense & Veterans Center for Integrative Pain Management and HPRC’s Pain Management section.