Walking heel-to-toe in a straight line is not much of a test if our balance faculties are intact. However, it is a tough task for astronauts just returning to Earth. Many astronauts say they feel unsteady when they come back to gravity after being in the weightlessness environment of space. How does balance depend on gravity, and what happens physiologically when the downward pull isn’t there?

Two structures in the inner ear sense gravity to balance the body: the utricle and the saccule. Together, they provide information to the brain about linear motions—up, down, backward, forward—and the head’s position in relation to gravity—whether the body is sitting up, leaning backward or lying down. To do so, the utricle and saccule have fine hairs covered by a gel with small crystals called “otoliths” suspended in it. When the head changes position—for instance, when a person bends over—gravity pulls the otoliths down and bends the hairs, causing signals to be sent out to the brain.

In space, the otoliths are weightless and don’t bend the hairs, so the brain cannot sense up and down. Studies conducted on lab animals in space report that the utricle becomes more sensitive to motion to compensate for this absence of information. Structural changes in the nerves that relay information from the ears to brain and the areas in the brain that process this information also occur. These data demonstrate what the astronauts experience firsthand: The balance system when returning to Earth is not operating the same way as when leaving.

Astronaut-scientists from NASA STS-90 space mission who investigated how the nervous system works in space will speak at APS’s annual meeting on April 1. The presentation coincides with the National Research Council’s Space Science Week, March 31 to April 2. Looking ahead, NASA continues to advance the understanding of human physiology in space with its Twins Study, launched March 27.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.

Reviewed by Jay C. Buckey, Jr, MD