Why study whiskers in mice? Humans don’t have whiskers

Sometimes an exciting research finding is quickly attacked by the internet because it was done #InMice. But some of the most exciting discoveries have resulted from studies  #InMice. And some things #InMice, like their lungs, hearts, livers, and bladders, are surprisingly more similar to humans than we may assume. It’s also just fun to learn about how other animals have adapted to their unique environments. It may just even open our eyes to understanding the world from their point of view. 

Let’s consider whiskers, for example. You may have noticed the whiskers on your dog, cat, or even your pet mouse. Those long and thick hairs on their snouts give them greater ability to touch and feel things close to their face. Imagine you are trying to walk through your home with a blind-fold on. If you aren’t blind, you’ll immediately put up your hands to feel your way to the bathroom or refrigerator. You’re using your hands as whiskers. 

But how is that touch information being processed in the brain? Does it work the same way in humans as it does in mice? The short answer is, yes. In every mammal, you can look at the outer portion of their brain and find an area specific to the sense of touch. It was discovered in humans in the 1920s when Wilder Penfield, a Canadian neurosurgeon, electrically stimulated his patients’ brains to locate the areas causing seizures. Here, Penfield kept his patients awake and they would report back to him what they felt or if their arm moved, for example. When Penfield stimulated one area of the brain, called the somatosensory cortex, it was as if someone was touching their hands. 

But it wasn’t just the hands, soon scientists discovered an entire map of the body in the somatosensory cortex. Interestingly some areas for the lips and hands were larger than for the legs and feet. This larger size also coincided with people being able to more easily discriminate two points on their skin. You can try it yourself. Take two pens and place the tips on your leg a few inches apart. Then close your eyes and move the pen tips closer to one another. Soon it will feel like there is only one pen tip touching you, but when you open your eyes you’ll see that they are pretty far apart. Now try the same thing with your upper lip in the mirror. You’ll quickly learn that you can get the pen tips far closer to one another before they feel like one when you touch your upper lip than when you touch your leg. That’s because a larger portion of your somatosensory cortex is dedicated to your lips than your legs, which is depicted in the homunculus (the image below).

Okay, but how does this all relate to whiskers? Well, the sensory nerve that connects your lips to your brain is actually the same nerve that connects whiskers to a mouse’s brain (i.e., the infraorbital nerve). And, the area of the somatosensory cortex dedicated to the mouse’s whiskers is HUGE compared to the rest of its body. Just like our upper lip. Thus, even though the whiskers don’t look like lips, studying them can reveal how your own somatosensory cortex develops and learns. 

But, yeah, whiskers don’t look like lips, and that’s the cool part. Every one of those whiskers actually has its own specific area in the somatosensory area called a barrel, and each barrel maps back to the whiskers in the same identical area. The map in the brain is the same as the map on the face. This allows scientists to study how unique changes in whisker A1 can alter barrel A1, and how it may alter barrels A2 and B1. 

What’s even cooler is that even if all rodents have whiskers, they use their somatosensory areas in different ways depending on how they have adapted to their environment. Star-nosed moles, for example, have star-arms in their somatosensory cortex instead of barrels. The somatosensory cortex of naked mole-rats is largely occupied by their teeth. Sea lions have whiskers that look like drills, helping them to chase down fish in the water. And every hair on a manatee is actually a whisker that maps to their somatosensory cortex. Could you imagine having a star-nose that can feel like your fingers, or having sensory hairs all over your body like a manatee? Not all research needs to benefit human health, sometimes it can open our eyes to new solutions to problems. Just look at how sea lion whiskers inspired new marine technology.

If you would like to know more about whiskers and the fascinating sensory worlds of other animals, check out Ed Yong’s recent book, “An Immense World” or Professor Kenneth Catania’s book on “Great Adaptations”.

Justin Varholick  

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