Pain from a Scientific View-- Nociceptive Pain

We've discussed pain from a personal perspective over the last two weeks.  For these next two blogs, we'll explore what scientists have learned about pain.  This week, the focus will be on nociceptive pain, and next week it will be about neuropathic pain.

In looking at the definition of pain, I find articles that define it as physical or mental discomfort caused by stimulus to nerve endings that signal harm or possible harm being done to the body.   More simply stated, some thing (i.e. heat, pressure, sharp object, infection) triggers your nerves to send an "It hurts!"  message to the brain.

The Mayo Clinic explains the pain process clearly.  We have a central nervous system (CNS) which is made up of our brain and spinal cord.  The rest of the nerves in our bodies are called the peripheral nerves and are part of the peripheral nervous system (PNS).  On the ends of the peripheral nerves are receptors called 'nociceptors'.  Nociceptors are found throughout our bodies -- in joints,  skin, bones, soft tissue (muscles, cartilage, ligaments, etc), and around internal organs.  They sense stimuli that may cause harm and send electrical impulses through the peripheral nerves to the spinal cord.  If you develop an infection in your appendix you feel visceral nociceptive pain.  Visceral refers to pain in the abdomen or organs.  If you stub your toe or twist your ankle, you feel somatic nociceptive pain,  Somatic refers to pain everywhere else. 

From the spinal cord, the 'messages' are prioritized (or triaged) before being sent to the brain.  The signal(s) sent in the case of a minor bump may be sent to the brain more slowly than signal(s) from a smashed finger.  In the case of a minor bump, the brain's response may only focus on sending messages to your immune system to start the healing process. (Did you ever see a bruise on your leg and have no idea how it got there? You may not have noticed the bump, but your brain signaled your immune system to repair the minor damage.) If your finger is smashed in a door, your brain will immediately signal your body to remove your finger from the door and to start the healing process.  The signals are sent so fast that your reaction is considered to be a reflex -- your brain uses pain to tell you to remove the damaging condition.  In all of these situation, nociceptors initiate chemical and electrical messengers which travel through the peripheral nerves to your spinal cord, and your spinal cord relays those messages to your brain.

This seems relatively straightforward, but there's more to pain.  Scientists are finally trying to learn exactly how pain signaling works in our bodies, and they have made some interesting discoveries and hypotheses.   First, pain physically changes brain and spinal cord cells and creates 'pain memory'.  It's believed that this pain memory may play a role in chronic pain that occurs even after an injury has healed.  Second, genetics may affect how a person responds to pain stimuli - whether that person is very sensitive to pain or has a high tolerance for it.  When someone is very sensitive to pain, it's because the signaling is faster, and possibly stronger, in their PNS and CNS.  Third, there is a gene called COMT which has different activity rates in different people.  Those with a very active COMT may tolerate much higher levels of pain.  

So what does this mean for those with chronic nociceptive pain?  One belief is that those with higher sensitivity to acute pain have a higher likelihood of eventually experiencing chronic pain. 
I'm not convinced that this is the case because I know many people who have a very high pain tolerance who live with severe chronic pain.  I expect more studies need to be done in this area. 

Pain is considered chronic when it lasts beyond the time when an injury has healed.  It's possible that the initial injury caused the nerves to become extra sensitive so that pain is felt more easily in that area.  

Again, I am left with questions.  Given that it takes about 42 weeks (that's nearly a full year) for a damaged ligament to heal, is it really 'pain memory' causing us to imagine that there is still pain?  Is it possible that chronic nociceptive pain is caused by micro damage that takes much longer (i.e. years) to heal?  Scientists don't have answers to these questions yet.  Perhaps one day they will.

Sources:

1.  Mayo Clinic, Understanding Pain, June 26, 2016 
https://www.mayoclinic.org/understanding-pain/art-20208632
Accessed June 10 - 18, 2019

2.  Felman, Adam, What is pain and how do you treat it?, Medical News Today, Last updated July 27, 2017,  https://www.medicalnewstoday.com/articles/145750.php,  Accessed June 10 - 18, 2019

3.  Weir, Kirsten, Owww!  The science of pain,  Science News for Students, 
June 25, 2014 
https://www.sciencenewsforstudents.org/article/owww-science-pain,
Accessed June 14 - 18, 2019

4.  Stace, Richmond, What is Pain?  A Modern Scientific Perspective, Ezine Articles.
April 9, 2010, 
https://ezinearticles.com/?What-is-Pain?-A-Modern-Scientific-Perspective&id=4074205​, Accessed June 14 - 18, 2019