Keeping strong during a long winter nap

Astronauts and the bedridden have one thing in common: they have to be very concerned with their muscles. When muscles don’t work, they shrink. So after a few moths in freefall, astronauts come back to Earth weak. Muscles truly are, “Use it or lose it.”

How do bears spend months not moving and then wake up far from weak?

This new paper by Lin and colleagues is a nice, simple strong inference test of two competing hypotheses of how bars keep their muscle tone over winter.

1. With neurons. That is, bears use their nervous system to periodically activate their muscles and shivering.
2. Without neurons. Intrinsic resistance to atrophy within the bear’s muscles, by activating some signalling pathway.

Conceptually, the experimental test is easy: remove neural signals from bear’s muscles. The hard part is carrying it out. Working with bears is not like working with lab mice. The brown bears (Ursus arctos) here were all bears that had been captured. They knocked the bears out, and measured the size of ankle muscles using ultrasound before cutting the nerve in the left leg. They did sham surgery on the right, so each bear acted as its own control.

They did five of these surgeries at the start of summer, and five at the start of winter, so Lin and colleagues were able to compare seasonal effects.

Muscles shrank after surgery in both seasons, but much less so in winter. The second hypothesis wins out: the nervous system isn’t responsible for the bears maintaining muscle tone. The bears have some other mechanism going on here. Time for the neurobiologists to hand over to the muscle physiologists for the bulk of the research on atrophy resistance.

The neurobiologists might not be entirely out of the picture, however. I would have thought that the muscles’ resistance to atrophy might be constant year round. That muscles atrophy in summer but not winter sows there are some kinds of signals from some source that are regulating seasonal cues. Day / night length is an obvious candidate, or environmental temperatures. The nervous system might be involved in the early regulation of the muscle condition.

Reference

Lin D, Hershey J, Mattoon J, Robbins C. 2012. Skeletal muscles of hibernating brown bears are unusually resistant to effects of denervation. The Journal of Experimental Biology 215(12): 2081-2087. DOI: 10.1242/jeb.066134

Photo by Li'l Wolf on Flickr; used under a Creative Commons license.