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The Science Behind Topicals
The theory’s premise is that there are two types of nerve fibers that transmit pain: A fibers and C fibers. The former transmits acute pain such as that from a knife wound or breaking a bone. These nerve signals travel rapidly to the spine because humans must be quickly notified that their injury is serious and has to be addressed immediately. C fibers transmit pain signals more slowly and tend to communicate chronic pain, such as that from arthritis. Melzack and Wall posited that there are a number of cognitive, physical, electrical and chemical factors that can determine how and whether the pain signals reach the spine and brain.
In essence, these factors will open or close “gates,” allowing or preventing pain signals from reaching the brain. It is well-known that patients suffering from depression experience pain more acutely. Other patients have been taught to use mindfulness techniques to reduce awareness of pain. Still others use TENS units to interfere with the electrical impulses that move from the site of pain to the spine. The theory asserts that the above behaviors/technology impact the gates that control transmission of pain to the dorsal horn in the spine and, in turn, to the brain. Gate theory posits that topical analgesics contain active ingredients that interfere with such transmission and modulate the degree of pain experienced by the user.
For a few decades, the above explanation was sufficient for explaining the science behind pain and its relief. Researchers discovered further detail on pain transmission in A and C nerve fibers. Residing on the membranes of nerve cells in these fibers are ion channels called Transient Receptor Potentials, or TRPs. These channels come in several “flavors,” but all of them regulate the flow of ions such as calcium and sodium into and out of the nerve cells, allowing the transmission of sensory signals, including pain.
There is a subset of TRPs that are sensitive to temperature, a fact that is relevant to the ingredients in topical analgesics. TRPM8 is sensitive to the cold caused by menthol. TRPV1 is activated by heat of capsaicin. TRPV3 is activated at physiological temperatures between 22 and 40 degrees centigrade, which results from camphor. TRPA1 is sensitive to citrus oils. Most of these channels sit on C nerve fibers, which means that they regulate perception of chronic pain.
When you administer a topical analgesic containing one or more of the above ingredients, you cause an influx of ions to the TRP channel, which results in an immediate strong perception of pain. As ions flood the channel, it shuts down, stopping the travel of nerve impulses on that nerve fiber. Your patient quickly notices that her arthritis pain is reduced.
Depending on the number of TRP modulating ingredients and the number of channels reached, your patient’s pain may get a little better or a lot better. You will find that a topical containing menthol, camphor, capsaicin and, perhaps, a citrus oil, will be particularly effective for relieving chronic pain because all the TRP channels for these ingredients reside on C nerve fibers, which transmit the slow, persistent, chronic pain signals.
After a few hours, TRPs will reset themselves as the ions leave the channel due to the sloughing of ingredients and the body’s tendency towards equilibrium. Your patient will need to reapply her topical analgesic to return to freedom from pain. For this reason, it is a good idea to sell your patient a topical to take home.
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