The Complexity of Pain: Understanding the Anatomy and Influences

What is Pain?

Pain is deeply personal–everyone's pain experience is different. Although no one wants pain, it is vital to our existence and acts as a protective mechanism by signaling a potential threat. Our own experience with pain allows us to empathize with others and can therefore help foster relationships. 

By definition, pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [1]. Basically, pain is created when our body perceives a threat. 

Pain can be classified as acute or chronic. Acute pain comes on suddenly such as with an injury and typically goes away within 3 months. Chronic pain persists longer than 3 months, or longer than what is considered normal for tissue healing. Acute pain can sometimes become chronic pain for various reasons, including if managed improperly.

The traditional pain model dates back to Descartes in the 16th and 17th centuries. Descartes suggested that pain is a sensory experience resulting from stimulation of specific nerve receptors in our body, usually due to injury or disease. For decades, pain was considered a marker for tissue damage or illness. These older concept models of pain have been criticized as overly simplistic, and do not take into consideration just how complex the human body is. We now know that pain can occur without tissue damage, tissue damage can occur without pain, and attempts to fix damaged tissue does not guarantee pain elimination.

The concepts of pain have begun to shift as health care professionals have failed to treat persisting pain conditions with expensive surgeries, risky medication and traditional therapies. We now consider pain as a warning signal designed to alert us to potential damage and to protect us from injury. Pain is not solely created from damaged tissue. The whole person has to be considered, including the impacts of psychological and social influences.

The Anatomy of Pain

Normally, acute pain goes away once the injury is healed, usually within 3 months (but can sometimes be longer depending on the person). When pain persists after 3 months, it is considered chronic pain, and specific tissue damage is the less likely cause of pain as our tissue completes its typical cycle of healing in this time frame. When our body perceives a continued threat, our brain continues to create pain as an output for protection. But if the stimulus for injury is gone, what is causing pain at this point?

We have nerves and nerve endings located all over our body. Nerve cells are also called neurons. There are specific neurons that normally respond to harmful stimuli, including mechanical (touch, pressure, vibration), thermal (temperature) and chemical stimuli. When these neurons are stimulated, they send a signal to the spinal cord which can in turn send a signal to the brain. This process is called nociception, and it sometimes results in pain. 

Most of the time, instead of creating pain, our brain reacts in other ways to protect us, such as activating our body to move away from the stimuli. Imagine if you accidentally reach for an unknowingly hot burner on your stove–your body reacts by moving your hand away from the hot burner in order to protect it. Once the warning signal reaches the brain, the brain makes sense of the signal based on the information arriving as well as other information our brain stores, such as past experiences with pain or our emotional state. If the brain thinks that your body needs protecting, it will create pain. 

When our body experiences an injury, such as stepping on a tack, the nerves from the foot send signals to our brain which kickstarts a protective process by creating pain. This process begins with pain and tissue inflammation, then the affected tissue heals. As stated earlier, typical tissue healing times range up to 3 months.

When pain persists beyond 3 months, what is happening? Interestingly, we now know that people can experience pain without physical stimuli. There are factors beyond physical stimuli or tissue damage that affect our pain perception. Sometimes our thoughts and certain places can activate these warning signals, and our brain or spinal cord creates pain. When the nervous system is persistently in a high activity state due to these various factors, our threshold for experiencing pain can decrease. This is called central sensitization. With central sensitization, although the nerves from our body provide limited input to the spinal cord and brain, the brain and spinal cord respond as if there have been many painful stimuli [2]. So, we perceive a threat in the absence of tissue damage or intense nerve signals to the brain. Given the intricacies of this topic, it merits a dedicated blog post in the future.

What Else Influences Pain?

Our nervous system which consists of our brain, the spinal cord and all the nerves into our limbs and organs, is an extremely complex system. And beyond physical injury, pain can be influenced by many other factors.

• Sleep: We know that oftentimes an injury or pain can make it difficult to sleep or get comfortable at night. But poor sleep can influence how we experience pain as well. There is strong evidence that having short or disturbed sleep can cause hyperalgesia (i.e., an increased sensitivity to painful stimulation) and the development or exacerbation of spontaneous pain symptoms (such as muscle pain or headache) [3]. 
• Memory: Our memory can impact pain! Areas of our brain that process pain also process memory. If you’ve felt pain in the past, that memory can impact how you experience pain now. A positive memory or association with something may reduce the perception of pain, while negative memories can amplify it. You may worry about pain before it even happens. Overtime, our brain learns how to react to pain based on our past experiences.
• Beliefs: Our beliefs on pain and injury can greatly impact our pain experience. If we believe pain means there is tissue damage, we are less likely to move the extremity that might be painful, which in turn may impact mobility and intensify our pain. This will occasionally lead to fear-avoidance, which can spiral into avoiding movements and activities that our bodies should be able to do.
• Mood: Brain regions involved with pain pathways are similar to those involved in mood disorders. Depression and anxiety have the potential to magnify pain due to these pathways, and often due to behavioral changes associated with them [4]. People with depression and anxiety also have a higher risk of experiencing chronic pain.
• Stress: When we're stressed, our body activates the "fight or flight" response. This is another natural survival mechanism that prepares the body to respond to a perceived threat. Certain hormones are released, which can impact how we perceive and respond to pain. Stress can make the nervous system more sensitive by amplifying the signals related to pain and cause reduced threshold for experiencing pain.
• Social support: When dealing with pain, having people around us who provide emotional, practical and even empathetic support can make our perception of pain less intense. Lack of social support and feelings of isolation or loneliness can amplify our perception of pain, making it more difficult to cope and heal. 

Musculoskeletal Pain

At Aware Health, we treat patients who primarily experience musculoskeletal pain, which is pain in the muscles bones, joints, ligaments and tendons. We can also help people who have nerve-related pain. Below are just some examples of what our providers can help you with!

Final Food For Thought

• Pain is not an indicator of tissue health.
• Pain does not equal tissue damage.
• Pain is a protector.
• Your body learns pain.
• You can retrain your pain system to be less protective! [5]

References

1. Raja, S. N., et al. (2020). The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises. Pain, 161(9), 1976–1982. https://doi.org/10.1097/j.pain.0000000000001939

2. Dydyk AM, Givler A. (2023). Central Pain Syndrome. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK553027/

3. Haack, M., et al. (2020). Sleep deficiency and chronic pain: potential underlying mechanisms and clinical implications. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 45(1), 205–216. https://doi.org/10.1038/s41386-019-0439-z

4. Yang, S., & Chang, M. C. (2019). Chronic Pain: Structural and Functional Changes in Brain Structures and Associated Negative Affective States. International journal of molecular sciences, 20(13), 3130. https://doi.org/10.3390/ijms20133130

5. Baumgartner, J. N., Haupt, M. R., & Case, L. K. (2023). Chronic pain patients low in social connectedness report higher pain and need deeper pressure for pain relief. Emotion, 23(8), 2156–2168. https://pubmed.ncbi.nlm.nih.gov/36996174/

6 Tame the Beast. (2022). Tame the Beast. A collaboration between a pain scientist (LM), a pain physiotherapist (DM), and a professional communicator (SC). Copyright 2022, Tame the Beast. https://www.tamethebeast.org/