A team of researchers at Harvard Medical School has shed new light on how different sensations — from the soft touch of an electric toothbrush to the agony of stepping on a Lego — travel through our bodies to our brains.
Previously, the brain was thought to be where all such complex sensations were processed. But in a study published Nov. 23 in Nature, the researchers discovered that the spinal cord, formerly thought to simply relay information to the brain, actually plays a role in differentiating one type of touch from another.
This discovery may help scientists understand and address sensory processing disorders, including neuropathy.
“In neuropathic pain, it’s often the case that light touch of the skin becomes painful,” said David Ginty, a neurobiology professor at Harvard Medical School and the senior author of the paper. “Now that we know more about how light touch information is processed, we’re beginning to understand how that processing goes awry to give rise to the perception of pain in response to what would normally be an innocuous touch.”
By studying mice, Ginty and his colleagues found that two distinct pathways carry information about touch from the skin to the brain. One, a direct pathway, relays light, frequent touches, like vibrations; whereas more intense sensations follow an indirect route through specialized neurons in the spinal cord before reaching the brain.
Vibrations from a nearby train, for example, would travel straight from the skin to the brainstem, the part of the brain that connects to the spinal cord. But the sensation of stepping on a Lego would first be relayed to specific neurons in the spinal cord before reaching the brain.
Ginty believes that vibrations, because they come in waves that each need to be processed immediately, are sent directly to the brainstem to avoid any information being filtered out or lost in the spinal cord. However, the signals for more firm or intense forms of touch are sent to the spinal cord, where specialized neurons can distinguish between the intensity of pressures and pass that information to the brain.
“Think about a mechanical stimulus acting on your skin and vibrating 10 or 100 times per second, which gives the feeling of flutter,” he said. The system of neurons, which sends an impulse to the brain with every vibration, would be overwhelmed if each signal had to first stop in the spinal cord.
To discover the purposes of these pathways, the research team stimulated the paws of anesthetized mice and silenced one pathway at a time to understand the two processes. But, further research is required to fully understand how this system works.
“What’s happening in the spinal cord may be quite like how features of the visual world are processed by the retina to create parallel streams,” such as color and motion, said Alexander Chesler, a senior investigator at the National Institutes of Health and one of the reviewers of the paper.
Ginty plans to repeat the experiment in waking mice to understand how the internal state of an animal, such as its emotional state or hunger levels, can affect how the body perceives touch. He also hopes to investigate how more complex features of touch, such as texture and pliability, factor into the system.
This study opens the door for further research into how different types of touch are perceived by the body, including how the normal touch information process is altered in people with different conditions, including
hypersensitivity to light touch.
Understanding the role neurons in the spinal cord play in processing touch helps scientists understand where the misperception could be happening and, potentially, how to correct it.
“That could potentially have implications for treatments or ways to decrease the perception of intensity,” said Jerry Chen, assistant professor of biology at Boston University, who was not involved in the study. “We can work on targeting one pathway to dial down the intensity while still preserving the quality of what you’re touching.”
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