Have you ever wondered why it hurts when you get a cut, get bruised, or break a bone? Everyday our bodies are exposed to thousands of external stimuli that cause pain. Pain is “a distressing sensation, as well as an emotional experience that is linked to actual or potential tissue damage, with the sole purpose of notifying the body’s defense mechanism to react towards a stimulus in order to avoid further tissue damages” (Yam et al, 2018). The pain you feel when you get kicked in the leg or scratched by your dog is a good thing—it immediately signals the brain that something is wrong and that your body needs to react. For example, if you grab the handle of a hot pot, it will burn you. Right away, you feel pain and something tells you to let go. As a result of the pain signal, you let go of the pot before it can cause serious tissue damage. In this way, pain plays an important role in maintaining our body’s health and ensuring that it is in optimal condition to carry out life’s necessary functions.
So what is pain? And how is your body able to let go of the burning pot so quickly? The sensation of pain occurs through a neurological pathway that consists of transduction, transmission, and modification. Transduction occurs when the stimuli comes into contact with a sensory receptor located in the dermis or subcutaneous layer of the skin—this is called the afferent neuron. The signal travels to the spinal cord or brain stem via the first order neuron. From there it is passed on to the second order neuron by neurotransmitters. Second order neurons cross the midline of the body and transmit the signal to the thalamus. The thalamus is located in the center of the brain and acts as a relay point for sensory transduction. Finally, third order neurons transmit the information to the somatosensory cortex to elicit a response. Sending signals through axons (part of neuron) is done through action potentials, which is caused by the crossing of ions across the cellular membrane.
However, when you experience a sudden strong pain, such as grabbing a hot pot, your body will involuntarily react to reduce injury, by a reflex response that occurs in the spinal cord. This response activates neurons in your hand which tell your muscles to relax and let go of the pot. This happens in less than a second, before the full signal has even had time to make it all the way to your brain.
Our daily lives determine the number of pain signals our brains receive each day. When we did the two touch test in class to experiment with the different size of receptive fields in our body, I began thinking about why some people feel more pain than others. I love playing soccer and one of my favorite National team players is Mallory Pugh. I remembered that I once read an article about how she doesn’t have the ability to feel pain, and how one day, when she was 12, she hurt her arm. “When she showed her father the injury—her wrist looked like a bent twig—he was horrified and told her she couldn’t play in her club team’s tournament that weekend. Pugh looked straight at her father and popped the bone back into place” (Kims, 2019). This story really caught my attention and I decided I wanted to learn more about why some people never experience the sensation of pain.
The condition is called congenital insensitivity to pain (CIP), and “is considered a form of peripheral neuropathy because it affects the peripheral nervous system, which connects the brain and spinal cord to muscles and to cells that detect sensations such as touch, smell, and pain” (NIH, 2020). People with CIP have a mutation in their SCN9A gene, which codes for a protein that makes up part of sodium channels. Sodium channels play a critical role in the formation of action potentials and without them, signals cannot be transmitted from neurons. As a result, when someone with CIP experiences a “painful” stimuli, the pain signal cannot be transmitted to the brain. The signal gets lost and the person does not perceive pain.
Although having this condition might seem like a superpower, it is actually quite dangerous to live with—especially for children. Young children often bite their own hands to the point of severe injury because they do not recognize that it hurts. Teenagers may stick their hand in boiling water to get something out or put their hand down on a hot burner for a long period of time. Without the ability to detect pain, people with CIP have a hard time determining what actions are harmful to their body and should be avoided.
One study published in Brain: A Journal of Neurology, examined the ability of people with CIP to perceive and feel empathy for others experiencing pain. Twelve patients with CIP were given two Situational Pain Questionnaires, one for the level of pain they think a stimulus would cause to themselves, and the second for the level of pain they think a stimulus would cause for someone else. To assess the level of pain others felt from various stimuli, short video clips without facial expressions or sound were shown. The same questionnaires and videos were given to a control group of normal pain sensing university students. The results showed that patients with CIP had significantly lower self-pain scores than the university students, proving they had a much harder time differentiating between painful and non-painful stimuli. However overall, there was no significant difference between the two groups in scores for rating the pain of others, meaning that CIP patients have the same level of empathy as normal people (Danziger, Prkachin, Willer, 2006).
“Congenital Insensitivity to Pain – Genetics Home Reference – NIH.” U.S. National Library of Medicine, National Institutes of Health, 2020, ghr.nlm.nih.gov/condition/congenital-insensitivity-to-pain.
Kimes, Mina. “Get Ready to Hear a Lot about Mal Pugh in the Women’s World Cup.” ESPN, ESPN Internet Ventures, 17 May 2019, www.espn.com/espnw/story/_/id/26718450/get-ready-hear-lot-mal-pugh-women-world-cup.
Nicolas Danziger, Kenneth M. Prkachin, Jean-Claude Willer, Is pain the price of empathy? The perception of others’ pain in patients with congenital insensitivity to pain, Brain, Volume 129, Issue 9, September 2006, Pages 2494–2507, https://doi.org/10.1093/brain/awl155
Yam, Mun Fei et al. “General Pathways of Pain Sensation and the Major Neurotransmitters Involved in Pain Regulation.” International journal of molecular sciences vol. 19,8 2164. 24 Jul. 2018, doi:10.3390/ijms19082164
Knee picture: from me
Pain signal pathway picture: https://commons.wikimedia.org/wiki/File:1417_Ascending_Pathways_of_Spinal_Cord.jpg
Mallory Pugh picture: https://www.flickr.com/photos/49182023@N05/47966456047
Graph image: Danziger, Prkachin, Willer study (cited above)