Written By Kevin Kerfoot / Reviewed By Ray Spotts
Research recently published in The Journal Of Neuroscience states that rubbing skin activates an anti-itch pathway in the spinal cord. Even though scratching damages skin, especially in sensitive areas like the eyes, stroking can relieve an itch.
The researchers investigated the neural pathway behind this less-damaging form of itch relief. They triggered the urge to scratch in mice by administering an itch-inducing chemical underneath their skin.
Then they recorded the electrical response from dorsal horn neurons in the spinal cord while they stroked the animals' paws. The neurons fired more often as the mice were stroked and less often after the stroking ended.
These neurons respond to both touch and itch, so the increase corresponds to the added touch, not increased itchiness, while the decrease corresponds to itch relief. The same decrease could be seen when the team directly stimulated touch-sensing neurons under the skin.
However, inhibiting both sensory neurons and a subtype of anti-itch interneurons in the spinal cord failed to decrease the response from dorsal horn neurons, while activating sensory neurons stopped the mice from scratching.
The results show that stroking sets off a cascade, activating sensory neurons under the skin that then activate anti-itch interneurons in the spinal cord, resulting in reduced dorsal horn neural activity and itch relief.
The biomechanical effects of skin rubbing
Tohoku University biomechanical engineers have developed a better understanding of the damage that can be caused by something rubbing against the skin. Understanding the skin damage caused by rubbing could lead to better topical skin treatments and help prevent the formation of new routes for viral and bacterial infection.
The study - published in the International Journal of Pharmaceutics - suggests that damaged skin from rubbing face masks and coverings could potentially provide a route for transdermal virus infection, however further research would be needed to investigate how much rubbing is cause for concern.
The uppermost part of the skin - called the stratum corneum - is formed of layers of keratinocyte cells suspended in a lipid matrix. The stratum corneum plays an important protective role, forming a barrier against the invasion of viruses and bacteria, maintaining skin hydration, and managing skin recovery following damage.
The researchers measured the mechanical effects of rubbing on pig skin, which is very similar to human skin. A gear rotated against skin samples at known rates and pressures.
Rubbing face masks
Next they measured the damage this caused by exposing the skin samples to a fluorescent dye. The more damaged the skin, the more the fluorescent dye was able to permeate it.
The mechanical rubbing caused keratinocytes to shrink and wrinkle in the direction of the rubbing. Gaps also formed between the keratinocytes, degrading the skin's barrier function. They then developed a mathematical formula to describe the permeability of rubbed skin, which can be estimated from the amount of strain applied.
"We believe our findings could improve transdermal drug delivery and we plan to investigate the development of novel topical drugs that can be applied to the skin by rubbing," says study participant Kenji Kikuchi.
The findings could apply to the current pandemic if rubbing face masks were found by further research to cause skin damage, potentially allowing another avenue for COVID-19 infection. The team did not specifically investigate this topic and Kikuchi encourages people to continue to wear face coverings that fit comfortably over the nose and mouth.
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