A micro-CT scan of a fin whale nerve, which shows a nerve coiled within an outer layer. This inner coiling allows nerves to stretch twice their length without getting injured or damaged.
Nerves are extremely delicate structures. They don't tend to be very flexible and can get injured if they are stretched even the slightest bit too much. At the same time, nerves are needed in areas of the body that put up with a lot of lengthening and straining. Here’s an extreme example: When it opens its massive mouth to feed, the rorqual whale's nerves stretch to more than double their resting length and back—all while making extremely sharp ‘hairpin’-like turns—without being strained or broken. But how do they get away with treating their delicate nerves like a bunch of bungee cords?
In a recent report in the journal Current Biology, researchers present a possible explanation: Whale nerve cells are coiled and coated in two different layers of waviness. A better understanding of how this works could help researchers find new ways to treat nerve damage.
Previous studies on rorqual whale nerves found that the combination of an inner and an outer layer gave them such extreme stretchiness. The inner layer coils up like a snake when the nerve is untaxed, then unravels as needed. That unraveling allows it to elongate without actually stretching. But Margo Lillie, a biologist at the University of British Columbia who studies biomechanics and lead author of the new study, noticed that the nerves also have to make super sharp, hairpin-like turns as they stretch and relax. Even a coiled nerve shouldn’t be able to handle such turns without taking a beating.
To figure it out, she turned to a micro-CT scan of a rorqual whale nerve. On the outside of the inner coils, she found flexible tissue bundles—fascicles—that stretch when the nerves turn to and fro. The inner coils have a large-scale "waviness"—like a telephone cord—that allows for incredible stretching power. But at a smaller scale, the wavy fascicles continue to provide slack to the nerve as it twists and turns. It's waviness all the way down.
“With this second layer of waviness, it has introduced just enough slack for these sharp turns,” says Lillie. What tipped her off, she says, was what the nerves looked like when they were turning. They looked scrunched at the inner curve and stretched at the outer curve, a mechanical term known as bending strain—similar to what a pool noodle looks like when it’s forced into a curve.
This is all great for a hungry rorqual whale, but how will the research help us humans? Understanding how some animals' nerves are able to withstand extreme bending could help us figure out better ways to treat nerve damage. When nerve damage happens, Lillie says, there’s a little gap between the two damaged nerve endings. “You want to get those two ends back together.” Figuring out what whale nerves are made of, how they work, and how they evolved could help inspire new methods and materials for reattaching damaged nerves.
In the future, Lillie says, she would like to find other animals that might also have a similar mechanism. The bullfrog, she says, would be a good place to start—its big throat stretches a lot when it expands. She also wants to study arteries and see how they, with their hollow centers filled with rushing gushes of blood, are able to move and swerve about. Do they have similar mechanisms to nerve cells, or have they evolved to work in a totally different way?
Answering all these questions, she says, will help her and other researchers better understand human nerve cells, and how to treat them when something goes wrong. “Sometimes you need a really exaggerated case to make something apparent."
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