Investigating High-Frequency Vestibular Function: A Potential Flight Diagnostic (First Award Fellowship)
Jocelyn E. Songer, Ph.D.
Harvard-Massachusetts Eye and Ear Infirmary
When a person suffers from inner ear trouble, they may experience dizziness, balance problems and motion sickness. For astronauts these problems can be especially difficult since their inner ears have to react to the special conditions in space, during re-entry and after landing. These special conditions often lead to space motion sickness, and can make it difficult for the astronauts to correctly interpret motion. On Earth, if a patient has inner ear trouble they can go to the doctor and a wide range of tests can be performed to figure out what their inner ear trouble is, but in space, astronauts are largely on their own and have limited resources for medical tests.
NSBRI Postdoctoral Fellow Dr. Jocelyn Songer’s research project is looking at new ways to evaluate inner ear trouble. The inner ear contains both the organs that sense sound and the organs that sense balance. Songer’s research will focus on some of the ways that sound can be used to test balance. Songer’s long-term goal is to develop fast reliable tests of inner ear function that require minimal training and equipment and can be used by astronauts in space, and by doctors on Earth.
NASA Taskbook Entry
The otolith organs, the utricle and saccule, play a role in a number of clinical vestibular disorders and have been hypothesized to play a role in some of the vestibular dysfunction experienced by astronauts during spaceflight and after re-entry. The mammalian saccule, which senses gravity, accelerations and loud acoustic stimuli, is one of the least studied organs of the inner ear. Although there are morphological changes in the saccular epithelium (to both type I and type II hair cells) as a result of spaceflight, little is understood about how these changes might affect saccular physiology. Improving our understanding of the saccule and its frequency characteristics is of particular interest because measurements of saccular sensitivity in response to loud sounds are starting to be used in the clinic.
The goal of this project is to improve our understanding of the mammalian saccule by focusing on the frequency characteristics of the saccular epithelium. The original project had two aims:
- Characterize the frequency tuning of saccular reflexes in response to tones using both vestibular-evoked myogenic potentials (VEMP) and vestibulo-ocular reflexes in adult rats.
- Evaluate the frequency characteristics of saccular hair cells by receptor potentials and transduction currents from the central (striolar) and peripheral (extrastriolar) zones of the saccular epithelium.
Based on preliminary results from both projects, we decided to focus our efforts on Aim 2. We hypothesized that within the saccular epithelium hair cell, frequency sensitivity will vary with hair cell type and zone, with some cells sensitive to higher frequency stimuli, including acoustic frequencies and others sensitive to lower frequencies, including frequencies of voluntary head motions. To test this hypothesis, we compared voltage-gated currents, receptor potentials and transduction currents from type I and type II hair cells from both striolar and extrastriolar zones of the saccular epithelium using whole-cell patch recording methods in the semi-intact rat saccule.
Preliminary results comparing striolar type I hair cells and extrastriolar type II hair cells suggest differences. Extrastriolar type II hair cells have a midpoint voltage for sodium channel inactivation (-742mV, n=6) that is significantly more positive for type II extrastriolar hair cells than striolar type I hair cells (-931mV, n=19). Additionally, striolar type I hair cells have a faster fast-time constant of adaptation (7.82ms, n=7) in response to step bundle deflections when fit with a double exponential than the extrastriolar type II hair cells (153ms, n=5), a smaller operating range (1.10.2μm, n=8 compared to 1.70.1μm, n=5,) and a higher upper cutoff frequency (6010Hz, n=9 compared to 293Hz, n=4) when stimulated with sinusoidal bundle deflections from 2Hz to 100Hz and fit with a double exponential. This preliminary data is consistent with our hypothesis that frequency filtering may differ between hair cell types and zones, especially when looking at the fast component of adaptation. Evaluating responses to higher frequencies (up to 500 Hz) and additional cell types and zones, we will be able to more fully characterize these differences.
Characterization of the frequency tuning of hair cells in the saccular epithelium will give us insight into what drives vestibular afferent firing patterns, improve our understanding of peripheral contributions to vestibular reflexes, and may improve and expand our understanding of the frequency characteristics of VEMPs. In addition, it may provide insight into how morphological changes in the saccular epithelium resulting from spaceflight contribute to the vestibular dysfunction experienced by astronauts.