Gravity-Sensing System In Inner Ear Studied On Neurolab Space Shuttle Flight

Stephen M. Highstein, M.D., Ph.D., professor of otolaryngology and anatomy & neurobiology, Allen Mensinger, Ph.D., research instructor in otolaryngology and other members of Highstein's laboratory are studying the response of these saltwater fish to the near-zero gravity conditions in a shuttle orbiting above Earth's atmosphere. By doing so, they hope to reveal clues to help understand why astronauts suffer from something akin to motion sickness the first few days in space.

"The inner ear of these fish, which helps sense motion, is highly similar to that of humans and other mammals," Highstein says. "By studying toadfish, we can gain a very good sense of how astronauts respond to the absence of gravity." Highstein says these studies also may help explain the mechanism of motion sickness back on Earth. The toadfish study is one of 26 experiments carried out during the 16-day Neurolab mission, which was planned to begin on April 16 but was rescheduled to launch the next day due to problems with the communication system in the crew cabin. The mission is studying the effects of microgravity upon aspects of neurobiology ranging from development to perception. Besides studies of the predatory, slow-moving toadfish found off the Atlantic coast, other research will address related questions using rats, crickets and other animals. Volunteers in the seven-member crew also will serve as study subjects.

The Washington University researchers are studying responses to the altered gravitational state by analyzing signals sent to the fishes' brains from gravity-sensing cells in otolith organs of the inner ear. Humans and other vertebrates require a sense of up and down to live in Earth's gravitational field. Lack of gravity is thought to alter signals coming from the inner ear, leading to nausea in what is known as space adaptation syndrome.

Highstein notes its resemblance to motion sickness. "You can't think, you can't move, you can't eat, you can't do anything but lie still." Before the flight, Highstein and Mensinger will cut the toadfish nerve that detects gravity signals and place a detector called a wafer electrode assembly in the nerve's path. The nerve will grow through pores in the assembly and begin carrying signals again. Special electrodes encircling nine of the pores will pick up nerve signal information before, during and after the Shuttle flight.

Highstein says the electrodes, developed by David J. Anderson, Ph.D., professor of otorhinolaryngology and electrical engineering & computer science at the University of Michigan, are an exciting aspect of the experiments. "They have the potential to be the first truly permanent interface with the nervous system allowing signals to be picked up from brain circuitry," he says.

The electrodes will send information to a wireless transmitter surgically mounted on the head of each fish. These telemetry transmitters were devised by the National Space Development Agency of Japan. The transmitters, which gain power from a magnetic field located outside the fish tanks, will then send information through a series of devices to a recorder that translates nerve activity into marks on a scroll.

The Neurolab crew will monitor the recording device as the fish adapt to microgravity. And by sliding the fish tanks back and forth on special rails, they will expose the toadfish to linear acceleration equal to half the Earth's gravity. These experiments will permit Highstein and colleagues to detect signals sent from the inner ear to the brain as the fish adjust to changing gravity.

"Our work should yield fundamental information about how the gravity-sensing system works in normal and microgravity situations," Highstein says. "This may have an impact on future therapies for motion sickness."

The National Science Foundation and the National Aeronautics and Space Administration provided funding for this research.

To find out more about the Neurolab mission, go to http://shuttle.nasa.gov. For more information on Highstein's research, go to the Aquatic Team site at http://neurolab.jsc.nasa.gov/aquahome.htm. For toadfish photos, go to http://www-pao.ksc.nasa.gov/kscpao/captions/hotpics.htm.