The secret of a rat’s touch? It’s all in the way the mustaches bend

Rats, cats, and many other mammals have whiskers, which they typically use to sense their surrounding environment, similar to the sense of touch. But scientists have yet to determine precisely the means by which mustaches communicate this sense of touch to the brain. Now, an interdisciplinary team from Northwestern University has developed a new model to help predict how a rat’s whiskers activate different sensory cells to do just that, according to a new paper published in the journal PLOS Computational Biology. Such work could one day allow scientists to construct artificial mustaches as tactile sensors in robotics and shed more light on human touch.

“The sense of touch is extremely important to almost everything we do in the world, but it is very difficult to study touch with the hands,” said co-author Mitra Hartmann, biomedical engineer at the Center for Robotics and Northwestern biosystems. “Whiskers provide a simplified model for understanding the complex and mysterious nature of touch.”

This is why there is such a long history of studying mustaches (vibrissae) in mammals: rats, cats, tree squirrels, manatees, harbor seals, sea otters, pole cats, shrews, tammar wallabies, sea lions and hairless mole rats all share surprisingly similar basic mustache anatomies, according to various previous studies. The current study focused on rats. Rats have about 30 large whiskers and dozens of smaller ones, part of a complex “scanning sensorimotor system” that allows the rat to perform tasks as diverse as texture analysis, active touch for research path, pattern recognition and object localization, simply by scanning the terrain with your whiskers.

Technically, whiskers are just hairs, a collection of dead keratin cells, much like human hair. It’s what they’re attached to that makes them as sensitive as human fingertips. Each rat’s mustache is inserted into a follicle that connects it to a “barrel” made up of no less than 4,000 densely packed neurons. Together, they form a grid or table that serves as a topographical “map”, telling the rat’s brain exactly what objects are present and what movements are taking place in their immediate environment. All of these barrels are in turn wired together in some sort of neural network, so the rat receives multidimensional clues about its surroundings.

Frequency map

In 2003, Hartmann and several collaborators discovered that a rat’s whiskers resonate at certain frequencies. The same principle applies to the strings of a harp or piano: longer whiskers resonate at lower frequencies, while shorter whiskers resonate at higher frequencies (the strings of many musical instruments also reach different heights by varying thickness). Rats have shorter whiskers near the nose, with longer ones farther back, which allows them to create a sort of “frequency map” by pushing their noses all over the place. A single mustache acts a bit like a single-tooth tuning fork. Put them all together, and a rat can feel the size, position, edges of objects, even slight variations in texture, compared to its small rodent body. For example, a very fine texture would create a stronger vibration in a high frequency mustache than it would in a low frequency mustache.

While moving across the field, a rat constantly scans its surroundings with its whiskers (called a “whip”), sweeping back and forth between five and 12 times per second. When a mustache hits an object, it bends in its follicle, triggering an electrical impulse to the brain that allows the rat to determine both the direction and distance each mustache is moving. Certain neurons in the rat cortex pulse at very precise frequencies, and these pulses are sent continuously to the thalamus, which compares them to incoming whiskers signals. This is how the animal forms an “image” of the world around it.

Hartmann and his colleagues wanted to learn more about how this complex sensing system responds to different external stimuli, especially during active whipping. However, “it is not yet possible to experimentally measure this interaction in vivo», Write the authors. So they decided to create a mechanical model of the sinus follicular complex to simulate the deformation inside a follicle.

“The part of the mustache that triggers the touch sensors is hidden inside the follicle, so it’s incredibly difficult to study,” Hartmann said. “You cannot measure this process experimentally, because if you open the follicle, the damage would change the way the whiskers are held. By developing new simulations, we can get information about biological processes that cannot be measured directly experimentally. “

“Active whipping”

To build their model, Hartmann et al. was based in part on data from a 2015 ex vivo study of rat whiskers, measurement of tissue displacement in response to deflection of whiskers in a dissected row housed in a Petri dish. While this previous experience only focused on a small region of the entire follicular follicle sinus complex, the data obtained nonetheless gave the Northwest team a useful starting point.

The team ended up with something akin to a beam-and-spring model for moving whiskers in the sinus follicular complex. The whiskers and follicle walls serve as bundles, with the distribution of tissues within the follicle wall representing four internal springs in different places. Connective tissue and muscle just outside the follicle serve as two external springs at the top and bottom of the follicle, with the distant facial tissue and adjacent follicles serving as a rigid floor in the model.

Hartmann et al. found that rat whiskers are more likely to bend into an “S” shape in the follicle when touching an object. This flexion then pushes or pulls on the sensor cells, causing them to send tactile signals to the brain. The same curvature profile occurs regardless of whether the mustache is touching an object or being touched from the outside. And intrinsic muscle contraction and increased blood pressure can improve the tactile sensitivity of the system.

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

“Our model demonstrates the consistency of the mustache deformation profile between passive touch and active whipping,” said co-author Yifu Luo, a graduate student at Hartmann’s lab. “In other words, the same group of sensory cells will respond when the mustache is deflected in the same direction under both conditions. This result suggests that certain types of experiments to study the active whip can be performed in an anesthetized animal. “

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