Technically, the whiskers are just hairs, a collection of keratin cells, much like human hair. It’s what they’re 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 as many as 4,000 densely packed neurons. Together, they form a grid or array that serves as a topographic “map,” telling the rat’s brain exactly what objects are present and what movements are taking place in their immediate environment. All those barrels in turn are wired together into a kind of neural network, so the rat gets multidimensional cues about its environment.
In 2003, Hartmann and several collaborators discovered that a rat’s whiskers resonate at certain frequencies. It’s the same principle that applies to the strings of a harp or a piano: longer whiskers resonate at lower frequencies, while shorter whiskers resonate at higher ones (the strings on many musical instruments also achieve different pitches by varying in thickness). Rats have shorter whiskers near the nose, with longer ones further back, enabling them to create a kind of “frequency map” by poking their noses all over the place. A single whisker acts much like a single-pronged tuning fork. Put them all together, and a rat can sense size, position, the edges of objects, even slight variations in texture, relative to its little rodent body. For instance, a very fine texture would set up a stronger vibration in a high-frequency whisker than it would in a low-frequency whisker.
As it moves across the terrain, a rat is constantly scanning its surroundings with its whiskers (called “whisking”), sweeping back and forth between five and 12 times per second. When a whisker hits an object, it bends in its follicle, and this sets off an electrical impulse to the brain that enables the rat to determine both the direction and how far each whisker moves. Certain neurons in the rat cortex pulse at very precise frequencies, and these pulses are sent continuously to the thalamus, which compares them with incoming whisker signals. That’s how the animal forms an “image” of the world around it.
Hartmann and her colleagues wanted to learn more about how this complex sensing system responds to different external stimuli, particularly during active whisking. However, “it is not yet possible to experimentally measure this interaction in vivo,” the authors wrote. So they decided to create a mechanical model of the follicle sinus complex to simulate the deformation inside a follicle.
“The part of the whisker that triggers touch sensors is hidden inside the follicle, so it’s incredibly difficult to study,” said Hartmann. “You can’t measure this process experimentally because if you slice open the follicle, then the damage would change the way the whisker is held. By developing new simulations, we can gain insights into biological processes that cannot be directly measured experimentally.”
To build their model, Hartmann et al. relied in part on data from a 2015 ex vivo study of rat whiskers, measuring tissue displacement in response to the deflection of whiskers 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 nonetheless gave the Northwestern team a useful starting point.
The team ended up with something akin to a beam-and-spring model for the displacement of whiskers in the follicle sinus complex. The whisker and follicle walls serve as beams, with the distribution of tissue inside the follicle wall representing four internal springs at different locations. The 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 rigid ground in the model.
Hartmann et al. found that rat whiskers are most likely to bend in an “S” shape within the follicle when they touch an object. This bending then pushes or pulls on the sensor cells, triggering them to send touch signals to the brain. The same bending profile results regardless of whether the whisker brushes against an object or is externally touched. And both intrinsic muscle contraction and an increase in blood pressure can make the tactile sensitivity of the system better.
The authors admit 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 consistency in the whisker deformation profile between passive touch and active whisking,” 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 is deflected in the same direction under both conditions. This result suggests that some types of experiments to study active whisking can be done in an anesthetized animal.”