Otoliths in Space! The extreme environment of space flight significantly affects the vestibular system. What happens and how does it recover back on Earth? All Ears on Audiology
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All Ears on Audiology  |   October 01, 2017
Otoliths in Space!
Author Notes
  • Jamie M. Bogle, AuD, PhD, CCC-A, is assistant professor of audiology at Mayo Clinic College of Medicine in Scottsdale, Arizona. bogle.jamie@mayo.edu
    Jamie M. Bogle, AuD, PhD, CCC-A, is assistant professor of audiology at Mayo Clinic College of Medicine in Scottsdale, Arizona. bogle.jamie@mayo.edu×
Article Information
Balance & Balance Disorders / All Ears on Audiology
All Ears on Audiology   |   October 01, 2017
Otoliths in Space!
The ASHA Leader, October 2017, Vol. 22, 14-15. doi:10.1044/leader.AEA.22102017.14
The ASHA Leader, October 2017, Vol. 22, 14-15. doi:10.1044/leader.AEA.22102017.14
Humans have always held a fascination with outer space. While beautiful to simply observe from Earth, this environment can be extreme and unforgiving for the scientists who study it in a quest for new scientific insights and knowledge.
Traveling among the stars has required ingenuity, tenacity and significant research involving mathematics, engineering, the cosmos—and effects of space on the human body. From the beginning, vestibular system physiology has limited human exploration in space. Based on the collective experiences of astronauts, the impact of space travel on the human body’s ability to interpret sensory information may have ramifications for future space endeavors.
The vestibular system
The vestibular system is a set of sensory end organs housed in the temporal bone. It encompasses five end organs on each side, including three semicircular canals encoding rotational movements and two otolith organs encoding gravity and linear acceleration.
The otolith organs are especially important for understanding the impact of space travel. These structures include a gelatinous matrix embedded with calcium carbonate crystals. When gravity is applied, the weight of the calcium carbonate crystals pulls on the gelatinous matrix and underlying hair cells. This triggers the otolith organs to relay information regarding gravity and linear acceleration.
The balance system incorporates sensory information from the vestibular, visual and proprioceptive systems to appropriately interpret the environment and allow humans to perceive motion and maintain balance and spatial orientation.

On entry into microgravity, the otolith organs suddenly lose gravity information, an expected sensory input that significantly alters spatial-orientation cues.

What happens during space travel?
All aspects of space travel significantly affect the vestibular system. During liftoff, astronauts experience an increase of three times the gravitational force we feel on Earth as the space shuttle launches into orbit. Astronauts often report false perceptions of pitch and reduced ability to read instrument panels.
Our ability to read during motion is related to the vestibulo-ocular reflex. This reflex uses information from the semicircular canals as an “internal gyroscope” to maintain stable vision during motion. The reduced reading ability that astronauts report could be due to interference with the vestibulo-ocular reflex that’s spurred by the high-amplitude vibration of liftoff (see sources).
One of the most alluring facets of space travel is weightlessness. Once out of the Earth’s atmosphere, astronauts experience a reduced effect of gravity called microgravity or weightlessness. One of the most fundamental forces in the universe, gravity is a measure of acceleration.
On Earth, life has adapted to our 1g environment. When in space, astronauts experience a slight reduction in g-force (0.9g), but because they are in orbit, the corresponding centrifugal force creates a cancelation effect, leaving astronauts to float around their environment in a continuous freefall.
On entry into microgravity, the otolith organs suddenly lose gravity information, an expected sensory input that significantly alters spatial-orientation cues. Astronauts often describe perceptions of spatial disorientation, backward tumbling and the room being upside down. Up to half of astronauts report space motion sickness, known as space adaptation syndrome, as they adapt to microgravity.
Symptoms can begin minutes to hours after the onset of weightlessness and can include vomiting, nausea, disorientation and visual illusions. Space adaptation lasts two to four days and is typically resolved as the vestibular system adapts to lost otolith information (see sources).
During this time, astronauts often avoid challenging tasks until they acclimate to the environment. Spatial disorientation continues, however, as the otolith organs no longer have gravity to use as a reliable reference for movement, and the eyes become the main motion sensor (see sources).
Astronauts report difficulty with diminished coordination and knowledge of limb positions, and rely instead on visual, tactile and internal cues as substitutes for vestibular information (see sources).
The return trip to Earth leads to additional sensory conflict. As the astronauts re-enter the Earth’s atmosphere, they experience a significant increase in g-force, up to 1.3g, which they describe as an increased acceleration sensation (see sources). Back on Earth, the otolith organs re-engage, and astronauts’ bodies must learn to incorporate gravity information into the balance system once again.
Initially, astronauts struggle with controlling their posture and standing upright. Up to 67 percent describe a sense of clumsiness and difficulty walking a straight line, while up to 30 percent describe vertigo or an abnormal perception of motion (see sources). These symptoms typically resolve in a few days, as the astronauts reintegrate back to life in a 1g environment.

Back on Earth, the otolith organs re-engage, and astronauts’ bodies must learn to incorporate gravity information into the balance system once again.

Questions for future space exploration
Given the expenses of space travel and the challenges it poses for the human body, only a handful of astronauts have spent more than a year in orbit. This leaves many questions unanswered about humans’ ability to live in microgravity for long durations.
Is it possible for us to live in a microgravity environment? What about living on the moon (0.181g) or on Mars (0.38g)? As humans develop new technologies that allow for further space exploration, our physiological limitations must be considered. While space motion sickness is a significant inconvenience for the first few days of space flight, the long-term effects of reduced vestibular input should be considered. The vestibular system has known interrelationships with various other systems, including regulation of blood pressure and bone loss.
Understanding and better management of these effects is needed before we can be successful in a microgravity environment for longer durations. Further study on Earth is already underway with tools such as parabolic flight and centrifuge technologies, while our astronauts on the International Space Station provide data for numerous research projects to better understand the effects of microgravity on the human body. The vestibular system, in particular, plays a crucial role in determining how far and long we may travel.
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October 2017
Volume 22, Issue 10