Rats, cats and many other mammals have mustaches that they usually use to sense their surrounding environment, similar to the feel. But scientists have yet to determine exactly how mustaches communicate the sense of touch to the brain. Now an interdisciplinary team at Northwestern University has devised a new model to help predict how a rat’s mustache activates different sensory cells to do so, according to a new article published in the journal PLOS Computational Biology. Such work could one day enable scientists to build artificial mustaches as supply sensors in robotics, as well as to further illuminate human touch.
“The sense of touch is incredibly important to almost everything we do in the world, yet it’s very difficult to study touch using hands,” said co-author Mitra Hartmann, a biomedical engineer at Northwestern’s Center for Robotics and Biosystems , said. “Mustache provides a simplified model for understanding the complex, mysterious nature of touch.”
That is why there is such a long history of studying mustaches (vibrissae) in mammals: rats, cats, tree hornbills, hippos, seals, sea otters, pole cats, shrews, tammar wallabies, sea lions and naked mole rats, according to various previous studies, strikingly similar basic mustache anatomy. The current study focused on rats. Rats have about 30 large whiskers and dozens of smaller ones, which are part of a complex “scanning sensor motor system” that enables the rat to perform such diverse tasks as texture analysis, active contact for finding the path, pattern recognition and location of objects, just by scanning. the terrain with its mustaches.
Technically, the mustache is just hair, a collection of dead keratin cells, much like human hair. It is what they are attached to that makes them as sensitive as human fingertips. Each rat beard is placed in a follicle that connects it to a ‘barrel’ made up of as many as 4,000 densely packed neurons. Together they form a grid or array that serves as a topographical ‘map’, which tells the rat’s brain exactly what objects are present and what movements are taking place in their immediate vicinity. All the vessels are in turn connected in a kind of neural network, so that the rat gets multidimensional clues about its environment.
Frequency card
In 2003, Hartmann and several associates discovered that the mustache of a rat resounded at certain frequencies. This is the same principle that applies to the strings of a harp or a piano: longer mustache resonates at lower frequencies, while shorter mustache resonates at higher frequencies (the strings on many musical instruments also reach different pitches by varying the thickness). Rats have shorter mustaches near the nose, with longer ones at the back, which allow them to create a kind of “frequency map” by crossing their noses everywhere. A single mustache works much like a single fork. Put them all together, and a rat can sense size, position, the edges of objects, even slight variations in texture, relative to its small rodent body. For example, a very fine texture will produce a stronger vibration in a high frequency mustache than in a low frequency string.
As it moves across the site, a rat constantly scans its surroundings with its whiskers (‘whisking’) and sweeps them back and forth between five and 12 times per second. When a mustache hits an object, it bends in the follicle and this causes an electrical impulse to the brain that enables the rat to determine the direction and how far each mustache moves. Certain neurons in the gear cortex pulse at very precise frequencies, and these pulses are sent continuously to the thalamus, comparing it to incoming mustache signals. In this way the animal forms an ‘image’ of the world around it.
North-West University / Nadina Zweifel
Hartmann and her colleagues wanted to know more about how this complex observation system responds to various external stimuli, especially during active beating. However, it is not yet possible to measure this interaction experimentally. in vivo, “the authors wrote. Therefore, they decided to create a mechanical model of the follicle sinus complex to simulate the deformation within a follicle.
“The part of the mustache that causes touch sensors is hidden inside the follicle, so it’s incredibly difficult to study,” Hartmann said. “You can not measure this process experimentally, because if you cut the follicle open, then the damage will change in the way the mustache is held. By developing new simulations, we can gain insight into biological processes that cannot be measured directly experimentally. will not be. “
“Active beat”
To build their model, Hartmann et al. relied in part on data from a 2015 ex vivo study of rotten beards, measures tissue displacement in response to the deflection of mustache beards in a dissected row housed in a petri dish. While this earlier experiment focused only on a small region of the entire mustache follicle-sinus complex, the results of the Northwest team nevertheless provided a useful starting point.
The team ended up with something similar to a beam-and-spring model for displacing mustaches in the follicle-sinus complex. The mustache and follicle walls serve as beams, with the distribution of tissue within the follicle wall representing four internal feathers at different locations. The connective tissue and muscle just outside the follicle serve as two external feathers at the top and bottom of the follicle, with distant facial tissue and adjacent follicles serving as rigid ground in the model.
Hartmann et al. found that rat beards are likely to bend in an “S” shape inside the follicle when they touch an object. This flexion then presses or attracts the sensor cells, causing them to send touch signals to the brain. The same bending profile arises regardless of whether the mustache is brushed against an object or touched externally. And both intrinsic contractions and an increase in blood pressure can improve the tangible sensitivity of the system.
The authors admit that this is a simplified model that focuses on the deflection of one follicle at a time, but they hope to simulate the simultaneous deflection of several mustaches in the future. Even the simplified model has interesting implications for future research.
“Our model shows the consistency in the mustache distortion profile between passive touch and active beating,” said co-author Yifu Luo, a graduate student in Hartmann’s laboratory. “In other words, the same group of sensory cells will respond when the mustache is bent in the same direction under both conditions. This result suggests that some types of experiments to study an active mustache can be done in an anesthetized animal.”
DOI: PLOS Computational Biology, 2021. 10.1371 / journal.pcbi.1007887 (On DOIs).