A World Without Pain Knowing Neurons Jillian Shaw Ouch

In Northern Pakistan, a ten-year old street performer awed and horrified crowds with his ability to place knives through his arms and walk across burning coals all seemingly without pain.  Closer examination by physicians revealed that he could perceive sensations of touch, differentiate between hot and cold temperatures, distinguish between a tickle and applied pressure, but had no aversion to painful stimuli.  This seemingly superhero power has dangerous and deadly consequences for those afflicted with this mysterious condition.  Just imagine what would happen if touching a scorching hot stove elicited no response by your brain.  Yes, you would undoubtedly impress your friends, but the burn to your hand would require an immediate hospital visit.  Pain is in fact a crucial survival mechanism that instructs us in the ways of injury avoidance and aversion from harmful situations.  If a broken bone did not hurt, it is unlikely that an individual would avoid using the limb until it healed.

Scientists have zeroed in on the cause of this dangerous indifference to pain.  Gain-of-function mutations in the SCN9A gene lead to severe neuropathic pain whereas a loss-of function mutation results in the inability to experience pain (Cox et al., 2006). SCN9A encodes a voltage-gated sodium channel (specifically, Nav1.7 for all of you channel connoisseurs) that regulates sensory neuronThe functional unit of the nervous system, a nerve cell that... More excitability.  Sodium channels enable positively charged sodium ions to be transported in order to play a role in generating electrical signals crucial for communication between neurons.  This sodium channel is preferentially expressed in peripheral neurons of the dorsal root ganglia and in the majority of nociceptors (pain- or damage-sensing neurons) as outlined in this infographic.  In patients who are unable to feel pain, SCN9A produces a protein that cannot function.  (Article continues below)

Pain Infographic Knowing Neurons Nociceptive Inflammatory Neuropathic Dorsal Root Ganglia Spinal Cord

Neuropathic pain results from diseases of the somatosensory system and involves chronic pain that is usually accompanied by tissue injury.  In neuropathic pain, the nerve fibers become damaged leading to incorrect signals being sent to pain centers in the brain.  One intense example of neuropathic pain is phantom limb syndrome. In the aftermath of an injury leading to the loss of a limb, the brain still gets pain messages from the nerves that originally carried signals from the missing limb.  When the nerves misfire, the result is pain.

In our daily lives we see frequent examples of individuals having different thresholds for pain tolerance – one child is kicked during a soccer match and runs off the field screaming while another carries on through an injury to score a goal.  It is possible that mutations that cause sodium channels to open too easily and close too sluggishly could underlie sensitivity to pain.  Since the mutation in people who are pain-free results from inactive sodium channels, it would make logical sense that a pharmacological block in sodium channels should provide pain relief.  The problem with this theory is that if you block all sodium channels, you die.  In fact, the reason puffer fish is so toxic is that tetrodotoxin blocks all voltage-gated sodium channels.  Thus, a drug that selectively blocked a specific type of sodium channel would have to be used to safely alleviate pain.  But before we get too excited about using drugs to mimic the SCN9A mutation that prevents the experience of pain, let’s remember the benefits of those painful lessons that taught us to avoid jumping out of trees and entering boxing matches with bigger siblings.

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References:

Cox J.J., Reimann F., Nicholas A.K., Thornton G., Roberts E., Springell K., Karbani G., Jafri H., Mannan J., Raashid Y. & Al-Gazali L. (2006). An SCN9A channelopathy causes congenital inability to experience pain, Nature, 444 (7121) 894-898. DOI: 

Images made by Jooyeun Lee, Jillian L. Shaw, Kate Jones, and Ryan Jones

Jillian L. Shaw

Jillian decided to dedicate herself to a life of exploring the mysteries of the brain after reading neurological case studies by Oliver Sachs and Ramachandran as a student at Vassar College.After completing a B.A. in Neuroscience with honors in 2009, Jillian headed to USC to pursue a Ph.D. in Neuroscience where she is now in her 5th year.A research stint in Belgium exposed Jillian to the complexities of cell signaling pathways, and her interests shifted from cognitive neuroscience to cellular and molecular neuroscience.Her current research focuses on the link between Down syndrome and Alzheimer’s disease using Drosophila as a genetic model to explore axonal transport, mitochondria dysfunction, synaptic defects, and neurodegeneration.When she is not in the lab, Jillian is forming new synapses by rock climbing throughout Southern California.

Jillian L. Shaw

View posts by Jillian L. Shaw
Jillian decided to dedicate herself to a life of exploring the mysteries of the brain after reading neurological case studies by Oliver Sachs and Ramachandran as a student at Vassar College. After completing a B.A. in Neuroscience with honors in 2009, Jillian headed to USC to pursue a Ph.D. in Neuroscience where she is now in her 5th year. A research stint in Belgium exposed Jillian to the complexities of cell signaling pathways, and her interests shifted from cognitive neuroscience to cellular and molecular neuroscience. Her current research focuses on the link between Down syndrome and Alzheimer’s disease using Drosophila as a genetic model to explore axonal transport, mitochondria dysfunction, synaptic defects, and neurodegeneration. When she is not in the lab, Jillian is forming new synapses by rock climbing throughout Southern California.

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