The secret of a rat’s sense of touch? It all depends on how the whiskers are bent

Photograph of the adorable face of a laboratory rat.
Enlarge / Scientists at Northwestern University have developed the first mechanical simulation of a rat whisker inside its follicle, to better understand how that sensory information is communicated to the brain.

Rats, cats, and many other mammals have whiskers, which they often use to sense their surroundings, similar to the sense of touch. But scientists have yet to pinpoint the means by which whiskers communicate that 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 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 build artificial whiskers as touch sensors in robotics, as well as shed more light on human touch.

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

That is why there is such a long history of studying whiskers (vibrissae) in mammals: Rats, cats, tree squirrels, manatees, harbor seals, sea otters, pole cats, shrews, Tammar wallabies, sea lions and naked mole rats share strikingly similar basic whisker anatomies, according to several previous studies. The current study focused on rats. Rats have about 30 large whiskers and dozens of smaller ones, part of a complex “sensorimotor scanning system” that enables the rat to perform tasks as diverse as texture analysis, active touch for path finding, pattern recognition and location of objects. objects, just by scanning the terrain with their whiskers.

Technically, whiskers are just hairs, 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 whisker is inserted into a follicle that connects it to a “barrel” made up of up to 4,000 densely packed neurons. Together, they form a grid or matrix that serves as a topographic “map,” telling the rat’s brain exactly what objects are present and what movements are taking place in its immediate environment. All those barrels, in turn, are connected in a kind of neural network, whereby the rat receives multidimensional signals about its environment.

Frequency map

In 2003, Hartmann and several collaborators discovered that a rat’s whiskers 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 of many musical instruments also reach different pitches varying the thickness). Rats have shorter whiskers near the nose and longer whiskers further back, allowing them to create a kind of “frequency map” by poking their noses from all sides. A single mustache acts as 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, relative to its little rodent body. For example, a very fine texture would generate a stronger vibration in a high frequency whisker than in a low frequency whisker.

As it moves across the terrain, a rat is constantly scanning its surroundings with its whiskers (called “flapping”), sweeping from side to side between five and 12 times per second. When a whisker hits an object, it bends in its follicle and this triggers an electrical impulse in the brain that allows the rat to determine the direction and distance each whisker is moving. Certain neurons in the rat’s cortex pulse at very precise frequencies, and these pulses are continuously sent to the thalamus, which compares them to incoming signals from the whiskers. This is how the animal forms an “image” of the world around it.

Mechanics are key to the tactile feel of a mustache.  When a mustache is deflected, its deformation profile within the follicle determines the activity of different groups of mechanoreceptors.
Enlarge / Mechanics are key to the tactile feel of a mustache. When a mustache is deflected, its deformation profile within the follicle determines the activity of different groups of mechanoreceptors.

Northwestern University / Nadina Zweifel

Hartmann and his colleagues wanted to learn more about how this complex sensing system responds to different external stimuli, particularly during active shaking. However, “it is not yet possible to experimentally measure this interaction Live“the authors wrote. So they decided to create a mechanical model of the follicle’s sinus complex to simulate deformation within a follicle.

“The part of the whisker that activates the touch sensors is hidden within the follicle, so it is incredibly difficult to study,” Hartmann said. “You cannot measure this process experimentally because if the follicle is cut, the damage would change the way the mustache is held. By developing new simulations, we can obtain information about biological processes that cannot be directly measured experimentally.”

“Active shake”

To build his model, Hartmann et al. was based in part on data from 2015 ex vivo rat whisker study, measuring tissue displacement in response to whisker deflection in a dissected row housed in a Petri dish. While this earlier experiment only focused on a small region of the entire whisker follicle sinus complex, the resulting data gave the Northwestern team a useful starting point.

The team ended up with something like a beam and spring model for the displacement of whiskers in the sinus complex of the follicle. The walls of the whiskers and follicles serve as beams, and the tissue distribution within the follicle wall represents four internal springs at different locations. Connective tissue and muscle just outside the follicle serve as two external springs at the top and bottom of the follicle, with distant facial tissue and adjacent follicles serving as the rigid base in the model.

Hartmann et al. found that rats’ whiskers are more likely to bend into an “S” shape within the follicle when they touch an object. This flexing then pushes or pulls on the sensor cells, activating them to send touch signals to the brain. The same bending profile is obtained regardless of whether the whisker rubs against an object or is externally touched. And both 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 deviation of one follicle at a time, but they hope to simulate the simultaneous deviation of multiple whiskers in the future. Even the simplified model has interesting implications for future research.

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

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

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