An interdisciplinary team from Northwestern University has created a new model for studying how mammalian whiskers bend, offering insights that could help us better understand the sense of touch in both animals and humans.
In the study, published Thursday in PLOS Computational Biology, the researchers set out to examine the complicated ways that whiskers interact with their environment. Rats and some other mammals can actually move their whiskers back and forth to sense their surroundings; the behavior, appropriately enough, is called "whisking." We know that both whisking and passive touch by the whiskers involve complex interactions between the whisker's shaft and cells called mechanoreceptors, which link incoming sensory stimuli to the wider nervous system. But it is not easy for scientists to study all of those interactions directly.
"The problem is we can't see inside the follicle," Mitra J.Z. Hartmann, a professor of biomedical engineering and a professor of mechanical engineering at Northwestern University and a co-author of the paper, told The Academic Times. In whiskers like the ones this group studied, the follicle is a small structure at the base of the whisker. "But this follicle, if you unwrapped it and looked on the inside, would be packed with sensors. If something comes along and pushes the whisker, we can see with a video camera or by eye, 'Here's how the whisker bends.' But what is it doing inside the follicle, where those sensors are? We made the invisible visible through simulations, by predicting how that whisker will deform."
Hartmann and her colleagues relied on a beam-and-spring model with two beams and six springs. The two beams were the whisker and the follicle wall, respectively, while four of the springs represented tissue distribution in the follicle wall and two other springs represented muscle and connective tissue outside the follicle. The researchers used parameters from a 2015 paper from Samuel J. Whiteley, then a researcher in the physics departments of the University of Chicago and the University of California, San Diego. Whiteley and his coauthors had used tissue samples to directly measure portions of a rat's whisker follicle within a structure called the ring sinus. Hartmann and her colleagues also used their own anatomical measurements of the follicle-sinus complex. Their study marks the very first simulation of the whisker's changing shape as it actively whisks or passively touches its surroundings.
"The springs just push or pull, but it's [about] how that pushing or pulling relates to the mechanoreceptors that then transmit the signal outside the follicle [up to the brain]," explained John W. Rudnicki, a professor of civil and environmental engineering and a professor of mechanical engineering at Northwestern and a coauthor of the paper.
"This is the first step towards modeling how those mechanoreceptors will be deformed, and what our work shows is that the way that they'll be deformed will depend on the blood pressure in the follicle as well as the muscles around the follicle," Hartmann added.
Using their new model, the researchers discovered that whiskers could theoretically deform into one of three broad shape profiles — two types of S-shapes or a C-shape. Yet only the S-shapes fit reasonably well with existing biological data, which were taken from the Whiteley paper.
"The results also show that it is possible future experimental data will show that the deformation is maybe a C-shape," said Yifu Luo, a researcher at Northwestern and lead author of the paper. "This shape, no matter if it's an S-shape or a C-shape, can be modulated by some of the factors — for example, blood pressure or muscle stiffness."
Luo explained that the neural pathways related to touch in rats are similar to those in humans. Their findings on whiskers could have broader ramifications for understanding how human brains interact with the world.
"A big question in neuroscience is, how does an animal change its motor output — how does it change the way it moves — based on the incoming sensory input?" Hartmann explained. "A way to phrase that is 'closing the sensory-motor loop.' That loop is certainly present in humans, and it's also present in rats. Our work, along with a lot of others, starts to help close the sensory-motor loop. We're at the gateway to that loop."
While the model's simplicity makes it useful for studying overall whisker deformation, Rudnicki noted that it could not capture all aspects of real-world whiskers.
"They say a good model is one that's as simple as possible and no simpler, and in a way, that's a good description of this model," Rudnicki said. "You can think of a more complicated model that would have a distribution of springs rather than a few discrete ones. It has no time dependence — that is, everything is proportional to how you push the whisker. We think that time dependence will affect how the mechanoreceptors respond to the whisker deformation. It does not include vibration of the whisker, and it basically focuses on one whisker. I think we know that there's an interaction with adjacent or even farther away whiskers."
"It's also two-dimensional," Hartmann added. "We'd like to expand it to three dimensions."
She expects her team will soon be able to look at whisking on an even more granular level. "These experiments are just beginning — and it's not clear what our resolution will be," Hartmann said. "It might be possible to see inside the follicle at a resolution that would tell us more information."
The study, "Constraints on the deformation of the vibrissa within the follicle," published April 1 in PLOS Computational Biology, was authored by Yifu Luo, Chris S. Bresee, John W. Rudnicki, and Mitra J.Z. Hartmann, Northwestern University.