New research with mouse whiskers links a group of neurons to the sense of touch, a finding that could eventually allow robotic limbs and prosthetics to actively sense and receive tactile input.
We often take the sense of touch, or somatosensation, for granted—that is until our leg falls asleep and we aren’t able to stand, or when we experience acute pain. Scientists say it’s the one sense they know the least about.
“Somatosensation is critical,” says Alison Barth, professor of biological sciences at Carnegie Mellon University. “You can somewhat overcome losing your sense of smell, sight, taste, or hearing.
“But if you lose your sense of touch, you wouldn’t be able to sit up or walk. You wouldn’t be able to feel pain. We know less about the features that make up our rich tactile experience than we do about any other sense, yet it’s such a critical sense.”
Somatosensation occurs in a number of forms, like feeling texture, temperature, pressure, pain, or vibration. It’s responsible for proprioception, which helps us know where we are within our environment.
It tells us if our feet are firmly planted on the floor, or if we’re holding a paper cup tightly enough that it won’t slip out of our hand, but loosely enough that we don’t crush the cup.
Puff of air on whiskers
Scientists know a good deal about the molecular receptors that mediate the different types of somatosensation, but they know little about how touch is represented in the brain.
“When someone gets pricked by a pin, we know how information about that sensation travels from the skin to the spinal cord. But what happens in the brain has been much less clear—it seems like all different sorts of touch information get jumbled together,” Barth says.
In previous studies, researchers discovered that certain groups of neurons in the brain’s neocortex are reliably more active than others.
Using a transgenic mouse model Barth created to study activity in live neurons, she and colleagues set out to see if these neurons were generally more excitable, or if they responded specifically to one tactile stimulus.
They found that they react much more quickly and strongly when a puff of air was directed at the mouse’s whiskers, while other neurons had little or no response.
“This is the first time we’ve been able to visualize neurons in the somatosensory cortex that ‘like’ a specific tactile stimulus,” Barth says. “It shows that neurons are individuals. They have different jobs to do in the cortex. In this case these neurons had a special feature: they responded when all of the mouse’s whiskers moved at once.”
The researchers also discovered that the neurons in question received direct synaptic input from the posteromedial nucleus of the brain’s thalamus. This shows that the neurons that react to the puff-of-air stimulus have a dedicated, unique sub-network of connections that enable them to communicate with one another and amplify the information they are receiving from the stimulus.
“Now that we have isolated the neural underpinnings of a certain feature, we can try to manipulate and change the interactions between cells. Can we train the mouse and strengthen the connections between neurons? What happens to perception if we remove the connections? It’s really the frontier of truly understanding somatosensory function,” Barth says.
The research also could lead to work that will identify how somatosensory information is coded, which could be used to incorporate sensory information into brain-machine interfaces, which would in turn allow robotic limbs and prosthetics to actively sense and receive tactile input.
Doctoral student Nick J. Audette is a coauthor of the study. Other coauthors are from Max Delbrück Center for Molecular Medicine, the Neuroscience Research Center at the Charité-Universitätsmedizin, the Bernstein Center for Computational Neuroscience at Humboldt University, all in Berlin.
The Deutsche Forschungsgemeinschaft, the European Research Council, the Alexander von Humboldt Foundation, the European Union, and the National Institutes of Health funded the study.
Source: Carnegie Mellon University