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So when you touch a hot stove, the nerve endings in your fingers react instantly. But the ouch comes a split-second later, when that information finally reaches your brain. Well, now, scientists have reconstructed one of the sensory pathways that convey these pain signals all in a dish. NPR's Jon Hamilton reports.
JON HAMILTON, BYLINE: Pain signals often start on the body's surface. Then, says Dr. Sergiu Pasca of Stanford University, they make a long journey.
SERGIU PASCA: Nerve terminals in the skin send that information all the way to the spinal cord. And then the spinal cord will relay it up to the thalamus and then all the way to the outer layer of the brain, which is the cortex.
HAMILTON: Where the signals register as pain - Pasca wanted to recreate this pathway in the lab. So his team created four different brain organoids, spherical clumps of human nerve cells that grow in a dish. Pasca says the team coaxed each organoid to resemble one specific type of brain or spinal tissue.
PASCA: And then we put them together, really put them in close proximity, and watched them as they connected with each other.
HAMILTON: Much the way nerves in the skin connect with the spinal cord, which connects with the brain - the result, which took more than six months to build, created a pathway linking four organoids. Pasca calls it an assembloid. He says it began communicating spontaneously.
PASCA: The cells are just working in a coordinated fashion across the four parts of this assembloid.
HAMILTON: To test their creation, Pasca's team expose the nerve endings on one organoid to the chemical that makes chili peppers painfully hot.
PASCA: Then you start seeing that information traveling. The neurons that sense the signals get activated, and they transmit that information to the next station and the next station, all the way to the cortex.
HAMILTON: Pasca says the model is designed to detect a painful stimulus but doesn't link to the brain areas that cause an emotional response to discomfort.
PASCA: So we don't believe that this pathway that we've built is in any way, like, feeling pain.
HAMILTON: The pathway in a dish is described in the journal Nature. Pasca says it offers a way to study how pain signals travel through the body and perhaps how to block them. Dr. Stephen Waxman of Yale University, who was not connected to the research, says the model could give researchers a new way to test potential pain drugs.
STEPHEN WAXMAN: We generally study them in single cells and then in whole animals. But here we have a miniature nervous system that might be a very useful platform.
HAMILTON: Waxman says the model also might help researchers understand a rare genetic condition he studies. It's called man on fire syndrome.
WAXMAN: These individuals feel searing, burning, scalding pain in response to mild warmth - putting on a sweater, wearing shoes, mild exercise or going outside when it's 72 degrees Fahrenheit.
HAMILTON: The condition is caused by a gene mutation. When Pasca's team tried including this mutation in their pain pathway, it became much more sensitive to stimuli. Dr. Guo-li Ming of the University of Pennsylvania says the pain pathway model is useful but has limitations. For example, in a dish, signals travel only a fraction of an inch. Ming says that's very different than what happens in a human body.
GUO-LI MING: It can be a meter long - right? - from your foot to your spinal cord. So definitely there's a lack of structural organization.
HAMILTON: Even so, Ming says organoids are giving scientists a new way to study parts of the nervous system. Her own lab, for example, has created a model of a human neural tube, the structure in an embryo that becomes the brain and spinal cord. Jon Hamilton, NPR News.
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