Sleep and Run—How do they do that?

When it comes to human muscles, the adage “use it or lose it” universally applies. Whether they belong to a senior citizen who breaks a bone or an astronaut in space, human muscles atrophy, or weaken, due to lack of use. A couple weeks of bed rest makes it difficult for humans to walk, so rehabilitation from injury or surgery, especially for older people, tends to be a long and expensive process.

But oh, to be a grizzly bear! Waking from a four- to six- month hibernation marked by virtually no activity, grizzlies are not only able to start foraging immediately, they’re also ready to run.

David Lin, assistant professor in Bioengineering and Neuroscience at Washington State University, would like to know what mechanisms prevent muscle atrophy in bears, in hopes that some day the research may help humans. In particular, he is interested in muscle plasticity, or the way that muscles change their properties to meet demands. When human muscles atrophy, not only do they lose strength, they also convert muscle fibers from slow-twitch to fast-twitch. Slow-twitch muscles are the postural muscles that allow us to hold ourselves upright when we stand, whereas fast-twitch are those that enable us to move rapidly. By the end of hibernation, grizzly bears experience just a 20-percent loss in muscle strength and minimal conversion of slow to fast muscles.

Among the factors that determine muscle plasticity are neural input, hormonal levels, and how much the muscle is worked. In order to better understand what sorts of neural inputs are sent to the muscles of grizzly bears, Lin implanted tiny transmitters into the leg muscle of a bear during hibernation. The transmitters relayed the electrical activity of the muscle, a measurement of neural input, to a computer in another room. In addition, neuroscience graduate student Jack Hershey will be looking at the microstructure of muscle biopsies from summer-active and hibernating bears.

Lin has been working with Charles Robbins, professor, Zoology and Natural Resource Sciences, and director of the Bear Research, Education, and Conservation Program, and Lynne Nelson, assistant professor, Veterinary Clinical Science, who is studying the heart muscle function of bears. Working with the Bear Research Center, the only facility in the world to house adult grizzlies for research, enables the researchers to have access to the bears throughout the year, including winter hibernation.