 pain.
The spreading of pain to uninjured areas is common in those with chronic pain. Also it can be experienced in acute injuries. For example, when your thumb is hit with a hammer and your whole hand throbs with pain.
To determine why, the researchers studied pain-sensing nerves in the skin of mouse ears.
The trigeminal V3’s tinier branches extend into the skin of the lower ear. However, an entirely different set of nerves is responsible for the skin of the upper ear. The difference enabled the researchers to compare the responses of two unrelated groups of nerves that are close to each other.
To surmount the difficulty of monitoring nerve responses, Dong’s team placed a gene into the DNA of mice which made the primary sensory nerve cells glow green when activated. The pain-sensing nerves of the face are a subset of these.
Hypersensitivity in Injured and Uninjured Nerve Terminals
Next, patches of skin were bathed in capsaicin. Capsaicin is the active ingredient in hot peppers.
The result was the pain-sensing nerves lit up in both regions of the ear. But V3 nerves in the lower ear were much brighter than those of the upper ear.
The researchers’ conclusion was that pinching the connected though separate V2 branch of the trigeminal nerve had in some way sensitized the V3 nerves to “overreact” to the same amount of stimulus. You can see this in the video clip above.
Applying capsaicin to different areas resulted in more nerve branches coming from a pinched V2 nerve lighting up than those coming from an uninjured one.
This suggests that adding to the pain signals being sent to the brain, nerves that don’t normally respond to pain are able to modify themselves during prolonged injury.
Says Dong: “Chronic pain seems to cause serotonin to be released by the brain into the spinal cord. There, it acts on the trigeminal nerve at large, making TRPV1 hyperactive throughout its branches, even causing some non-pain-sensing nerve cells to start responding to pain. Hyperactive TRPV1 causes the nerves to fire more frequently, sending additional pain signals to the brain.”
Central Terminal Sensitization of TRPV1 by Descending Serotonergic Facilitation Modulates Chronic Pain
Yu Shin Kim, Yuxia Chu, Liang Han, Man Li, Zhe Li, Pamela Colleen LaVinka, Shuohao Sun, Zongxiang Tang, Kyoungsook Park, Michael J. Caterina, Ke Ren, Ronald Dubner, Feng Wei, Xinzhong Dong
Neuron, 23 January 2014 10.1016/j.neuron.2013.12.011
Image credit: Medical Art Service, I. Christensen, Wellcome Images
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