Itching is weird. Our itch sense may have evolved to the exquisite sensitivity we have today to warn us about insects that carry disease. Scientists used to believe that the annoying sensation arose from a mild activation of the pain receptors, but now we know itching has dedicated neuron pathways, explains Atul Gawande in a disturbing New Yorker story. (It includes a woman with a scalp itch so persistent that she scratched through her skull and into her brain while she slept.)
Our itch sense depends on a complex system of cells, molecules and nerve circuits that researchers are still untangling. But why we scratch is less of a mystery, now. Scratching actually causes pain, which over-rides the sensation of itch for a moment, writes Tanya Lewis for LiveScience.com. But when that wears off, as it does, the itch can comes back with vengeance.
Zhou-Feng Chen of the Washington University School of Medicine in St. Louis, Mo., explained to the New Scientist how his team researchers figured out why this cycle of itch-pain-itch happens:
We know that the neurotransmitter serotonin helps control pain, and that pain – from the heavy scratching – helps soothe an itch, so Chen's team set out to explore whether serotonin is also involved in the itching process. They began by genetically engineering mice to produce no serotonin. Normally, mice injected with a chemical that irritates their skin will scratch up a storm, but the engineered mice seemed to have almost no urge to scratch. Genetically normal mice given a treatment to prevent serotonin leaving the brain also avoided scratching after being injected with the chemical, indicating that the urge to scratch begins when serotonin from the brain reaches the irritated spot.
The team also showed that also the extra serotonin released when we scratch can activate not just the pain-modulating neurons but also ones that ramp up the itch sensation. This crossing of the wires can trap people in a loop discomfort: itch, scratch, pain, serotonin, itch… etc. The researchers reported their findings in the journal Neuron.
Chronic itch plagues millions of Americans, reports the NIH. But researchers are making strides to understand the complexity of itches. For example, we know there are special nerve fibers that are tuned to itches, Gawande writes:
Unlike, say, the nerve fibres for pain, each of which covers a millimetre-size territory, a single itch fibre can pick up an itchy sensation more than three inches away. The fibres also turned out to have extraordinarily low conduction speeds, which explained why itchiness is so slow to build and so slow to subside.
Researchers even discovered a neurotransmitter that passes the "itch" message along the nerve fibers and to the brain, reports Joseph Stromberg for Smithsonian.com. Still, many things about itches remain unknown — just not the extreme, sometimes dangerous, annoyance they create.