The Science of Pain

What is pain?

Pain is a stimulus experienced in the body that is integrated to the brain. It is the brain’s job to interpret this signal as dangerous and painful. A common misconception about understanding pain is assuming it is a simple cause-and-effect relationship. Some injuries do not match the intensity of pain experienced; an example is a paper cut. There are situations where the body can ignore the threat of pain, such as athletes or emergent situations. There are also times that pain occurs even after the injury is fully healed. These are many of the reasons pain can be such a confusing topic to so many people. I hope this will help clarify and understand this topic further².

Basic Physiology

Three types of pain

1. Nociceptive pain
   • Pain that is caused by actual or threatened damage to non-neural tissue
2. Neuropathic pain
   • Pain that is caused by a lesion or disease affecting the somatosensory system
3. Pain that unknown origin
   • This describes chronic pain. This many times begins as nociceptive pain but changes for various reasons causing an increase in intensity or spread of pain.

First, a nerve fiber receptor is stimulated with the signal heat, cold, or chemical; then, the nerve sends a signal to the spinal cord. It is in the spinal cord that where the signal is sorted and relayed from the spinal cord up to the brain for interpretation of the situation, place, and reaction².

1. Nerve fibers/receptors

Nociceptive fibers are a better term to refer to all these different pain fibers collectively. Nerve fibers are found all over the body outside the brain and spine. There are many types of nerve sensory fibers that these respond to dangerous stimulation the body is experiencing, heat, cold or chemical. These fibers are directly influenced by changes that happen to the body. Changes include an increased amount of Delta A nerve fiber that conducts pain and increases ion channels. Ion channels are the doorways that open and close to nerve fiber cells. They allow ions(sodium, potassium, and calcium to travel in and out of the cell. This exchange of ions in and out of the neuron cell is needed to excite the nerve fiber to send messages. More ion channels lead to more pain signals being received and sent. ².

2. Spinal cord

The signal for nociception(pain) is then sent to the spinal cord, the dorsal horn, and the posterior column. Here, a cell type called an interneuron is a middleman in directing the information to and from the brain. The interneuron is responsible for sending and receiving messages to the brain. Changes in the spinal cord, such as overstimulation, can affect the interneuron, causing it to die ².

3. The Brain-Pain Neuromatrix

Pain depends on multiple factorials; however, it is ultimately the brain that decides the threat level in addition to the intensity of pain. This explains why in emergent situations, injured individuals can overcome serious injuries that would otherwise hinder an individual. This also demonstrates how environment and trauma associated with injury have been shown to influence the patient pain experience².

Over the years, researchers have mapped out the brain’s role in experiencing chronic pain to help understand the enormous impact and dysfunction. This does not mean that pain is “all in the head” and, therefore, not real; in fact, quite the opposite. Below is a short list of all the brain areas and associated functions that change due to pain².

Brain -Pain Neuromatrix	Functional areas that can be impaired by pain
Amygdala	a. Emotional reactions( fear and addictive behaviors
Primary somatosensory cortex	a.     Movement planning b.     Visual stimulus c.     Somatic sensory
Anterior cingulate	a.     Autonomic function:( Blood pressure, heart rate) b.   Cognitive function: (Decision making, Reward anticipation, Concentration, Focus, Empathy, emotion)
Primary cortex	a.     Planning movement b.     Execution of movement
Hypothalamus and Thalamus	a.    Autonomic function: ) Cody temperature, hunger/thirst, fatigue/ sleep) b.     Relaying sensory and motor signal to the cerebral cortex: ( alertness, sleep, consciousness)
Prefrontal cortex	a.     Planning complex cognitive behavior b.     Personality expression c.     Decision-making d.     Moderate social behavior
Hippocampus	a.     Spatial navigation b.     Short-term and long-term memory
Cerebellum	a.     movement b.     proprioception, balance c.     coordination

A quick that is helpful video to summarize the big concepts described above.

What causes the cycle of chronic pain?

Previously patients experiencing chronic pain were blamed, chastised, or accused of lying about experiencing pain. This was often due to a lack of medical evidence to support a “physical” cause for a source of pain. However, just like training the body to develop bigger and stronger muscles, you can also train the body to send more pain signals and perceive a higher level of threat. This is often not intentional but through a series of accumulating factors that create the perfect storm for pain. Research has now been able to show that through a series of changes that upregulate pain².

5 Changes that Train the Pain

1. Increase in the number of ion channels on nerve fiber, which increase the intensity and sensitivity of pain².
2. A change in receptor protein on nerve fiber. This causes the signals sent to remain open on nerve fibers, which causes a longer duration of pain².
   • These gates are only meant to be open for milliseconds; however, due to upregulation with injury and overstimulation, the gate is replaced. This creates a longer frequency signal of pain being sent ².
3. Death of interneuron in the spinal cord
   • The brain is no longer able to distinguish the different signals being sent. This causes the inability to regulate any signal being received or sent . The brain cannot regulate its natural endogenous chemicals like opioids, which are the body’s natural painkiller. It affects the immune processes and causes the loss of the ability to distinguish other sensory information².
4. The brain is moldable to change and therefore repurposes its daily duties when chronic pain is present.
   1. The brain changes its functional hierocracy in response to pain. The comprehensive Neuromatrix table above shows the major function of the brain. Instead of managing its regulated function, these areas are recruited to help respond to the pain. So things that may have seemed easy, like organizing your fridge or remembering your grocery list, may become challenging. In addition, the brain may preserve an injury even after it has fully healed. This even occurs after amputation after the receptor field to nerve is gone. Amputation patients can experience this type of pain, it is called phantom pain ².
5. Stress is a contributing factor to the intensity of pain. Research supports that stress, how an injury happens, and the environment play key role in the amount of pain experienced.
   1. For car accidents and stressful jobs, they have a great chance are 7- 8 times more likely to experience pain.
   2. Stress or perception of stress (environment, finances, family) is another contributor to the intensity of pain reported.

Want to learn more?

• GOT PAIN?: Introduction
• The Opioid Crisis: The History
• The Science of Pain: Knowledge & Education
• Pain Management in Healthcare: The Team
• Physical Therapist 6 Steps to Evaluating Pain!: The Process
• Know Pain, KNOW GAIN!: 8 Common Initial Treatments for Pain: The Treatment

References

1. Massé J. Wow! science helps understand “Your brain in pain”. Advance Physical Therapy. https://advance-physicaltherapy.com/2015/science-helps-understand/. Published March 25, 2016. Accessed October 12, 2022.
2. Louw A. Pain Neuroscience Education: Teaching People about Pain: A Clinical Guide. Minneapolis, MN: Orthopedic Physical Therapy Products; 2018.
3. Chronic pain: A cycle of stress and pain. Curable Health. https://www.curablehealth.com/blog/brain-infographic. Accessed October 12, 2022.
4. An illustration of the dorsal horn in the spinal cord. afferent fibers … https://www.researchgate.net/figure/An-illustration-of-the-dorsal-horn-in-the-spinal-cord-Afferent-fibers-enter-the_fig1_336331957. Accessed October 13, 2022.
5. Sponsored Content by Tocris BioscienceAug 17 2019. Ion channels involved in pain. News. https://www.news-medical.net/whitepaper/20190817/Ion-Channels-Involved-in-Pain.aspx. Published May 28, 2020. Accessed October 13, 2022.
6. Holmes D. The pain drain. Nature News. https://www.nature.com/articles/535S2a. Published July 13, 2016. Accessed October 13, 2022.
7. Karolinska InstitutetFollow this publisher – current follower count:707. Medical science 2012. Issuu. https://issuu.com/karolinska_institutet/docs/medical_science_2012/13. Published September 13, 2012. Accessed October 13, 2022.
8. Brightbraincentre. Chronic pain in the brain. BRIGHT BRAIN CENTRE – LONDON’S EEG, NEUROFEEDBACK AND BRAIN STIMULATION CENTRE. https://www.brightbraincentre.co.uk/chronic-pain-brain/. Published November 20, 2017. Accessed October 13, 2022.
9. Massé J. Wow! science helps understand “Your brain in pain”. Advance Physical Therapy. https://advance-physicaltherapy.com/2015/science-helps-understand/. Published March 25, 2016. Accessed October 12, 2022.