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The secret of mouse touch?How the whiskers bend



Photo of cute face of laboratory rat.
enlarge / Scientists at Northwestern University have developed the first mechanical simulation of rat whiskers inside their follicles to better understand how this sensory input is transmitted to the brain.

Rats, cats, and many other mammals have whiskers, which are commonly used to perceive their surroundings, similar to the sense of touch. But scientists have yet to determine exactly how the whiskers convey this tactile sensation to the brain. According to a new paper published in the journal PLOS Computational Biology, now an interdisciplinary team at Northwestern University has proposed a new model that can help predict how the whiskers of rats activate different sensory cells to do To this point. This work may have an angel scientist able to construct artificial whiskers as tactile sensors in robotics, and further reveal human touch.

Mitra Hartmann, a biomedical engineer at the Northwest Center for Robotics and Biosystems and co-author Mitra Hartmann, said: “The sense of touch is very important for almost everything we do in the world. Learning to touch is very difficult. “Whisker provides a simplified model to understand the complex, mysterious nature of touch. “

This is why the study of whiskers has a long history (Tentacles) In mammals: According to previous studies, mice, cats, squirrels, manatees, seals, sea otters, arctic cats, freshwater mice, sea lions and naked naked mice all have surprisingly similar basic whisker anatomical structures. The current research focuses on rats. Rats have about 30 large whiskers and dozens of smaller whiskers, which are part of a complex “scanning sensorimotor system”

; that allows rats to perform a variety of tasks, such as texture analysis, simply by scanning , Active touch path finding, pattern recognition and object positioning on the topography of whiskers.

Technically speaking, beard is just hair, a collection of dead keratin cells, much like human hair. Their adhesion makes them as sensitive as human fingertips. Each rat whisker is inserted into a hair follicle that connects it to a “bucket” of up to 4,000 densely packed neurons. Together, they form a grid or array, which serves as a “map” of the terrain, telling the mouse’s brain exactly what objects are present in its surrounding environment and what movements are taking place. In turn, all these barrels are connected together to form a kind of neural network, so the mouse can obtain multi-dimensional clues about its environment.

Frequency diagram

In 2003, Hartman and several collaborators discovered that the whiskers of mice resonate at certain frequencies. This is the same principle that applies to the strings of a harp or piano: longer whiskers resonate at lower frequencies, while shorter whiskers resonate at higher frequencies (the strings on many instruments can also Achieve different pitches by changing the thickness). Rats have shorter beards near their noses and longer beards at the back, allowing them to poke their noses all over the place to create a kind of “frequency map.” A single whisker behaves much like a single-fork tuning fork. Putting them together, compared to its small rodent, the mouse can feel the slight changes in size, position, edges of objects, and even texture. For example, a very fine texture will produce greater vibration on high frequency whiskers than on low frequency whiskers.

As it moves over the terrain, the mouse constantly scans the surrounding environment with its whiskers (called “whiskers”), sweeping back 5 to 12 times per second. When a whisker hits an object, its follicle bends, which generates electrical impulses to the brain, allowing the rat to determine the direction and distance of the whisker. Certain neurons in the rat cortex pulse at very precise frequencies. These pulses are sent continuously to the thalamus, which is then compared with the incoming whisker signals. This is how animals form “images” of the world around them.

Mechanics is the key to the feel of the whiskers. When the whisker deflects, its deformed contour in the hair follicle determines the activity of different groups of mechanoreceptors.  ”Src =” https://cdn.arstechnica.net/wp-content/uploads/2021/03/whisker1-640x427.jpg“ width =” 640 “height =” 427“ srcset =” https://cdn.arstechnica .net/wp-content/uploads/2021/03/whisker1.jpg 2x
enlarge / Mechanics is the key to the feel of the whiskers. When the whisker deflects, its deformed contour in the follicle determines the activity of different groups of mechanoreceptors.

Northwestern University/Nadina Doubt

Hartmann and her colleagues wanted to learn more about how this complex sensing system responds to different external stimuli, especially during active agitation.However, “this interaction cannot be measured experimentally in vivo,” the authors wrote. Therefore, they decided to create a mechanical model of the follicular sinus complex to simulate the deformation inside the follicle.

Hartman said: “The part of the whisker that triggers the touch sensor is hidden inside the follicle, so it is difficult to study.” “You can’t measure this process experimentally, because if you cut the follicle, the damage will change the fixation of the whisker. Way. By developing new simulations, we can gain insights into biological processes that cannot be directly measured experimentally.”

“Active Stirring”

To build their model, Hartman Wait. Partly dependent on 2015 data In vitro To study rat whiskers, measure the response of whisker deformation to tissue displacement in a row dissected in a petri dish. Although this earlier experiment only focused on a small part of the entire whisker-follicular antral complex, the data obtained provided a useful starting point for the Northwest team.

The research team finally came up with a principle similar to the beam and spring model used to eliminate whiskers in the follicular sinus complex. The whisker wall and the follicle wall act as beams, and the distribution of tissue in the follicle wall represents four internal springs located at different positions. The connective tissue and muscles outside the hair follicle act as two external springs at the top and bottom of the hair follicle, and the distant facial tissue and adjacent hair follicles play a hard role in the model.

Hartman Wait. It was found that when the mouse’s beard hits an object, it is most likely to bend into an “S” shape in the hair follicle. This bending then pushes or pulls the sensor units, triggering them to send touch signals to the brain. Whether the whiskers are brushed on the object or in contact with the outside, the same curved profile will be produced. Both inherent muscle contraction and increased blood pressure can improve the tactile sensitivity of the system.

The authors admit that this is a simplified model that focuses on the deflection of a single follicle, but they hope to be able to simulate the simultaneous deflection of multiple whiskers in the future. Even the simplified model has interesting implications for future research.

“Our model proves that the whisker deformation curves between passive contact and active stirring are consistent,” said co-author Luo Yifu, a graduate student in Hartmann’s lab. “In other words, when the whiskers deflect in the same direction in both situations, the same set of sensory cells will respond. This result suggests that certain types of research on active whiskers can be performed in anesthetized animals. experiment.”

DOI: PLOS Computational Biology, 2021. 10.1371/journal.pcbi.1007887 (about DOI).


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