Tuesday, 18 June 2013

Searchlight on Otoliths!

That the vestibular organ consists of the semicircular canals and the otoliths organs is a well known fact. That most of the conventional  battery of tests of vestibular function assess the semicircular canals mainly the lateral SCC is another facts known to few. 
The otolith organs - the utricle and saccule sense linear acceleration, head tilt and gravity (yeah! that kind of lift going up/coming down feeling). 
What is also interesting is that in lower vertebrates, especially amphibians and fish, the saccule is also sensitive to sound, and is the organ of hearing! In our species, the cochlea has evolved to replace the saccule as the primary organ of hearing. 
Until recently, clinical tests to measure the otolith function were not available, largely due to cumbersome nature of such gadgets (picture a mini railway track with vestibular chair strapped and all the cables to your VNG! Or picture a lab fitted with a vertical lift fitted with all these!) as well as difficulty, even with such gadgets, to selectively stimulate unilateral otolith function. 
That was before VEMP and Subjective Visual Vertical (SVV) were introduced as tests of saccule and utricular functions respectively. 
Assymetry between the left and right otolith organs contributes to an astronaut's individual susceptibility to space motion sickness (SMS) - a phenomenon seen in 50-70% of astronauts during first days of spaceflight. 

1. J Vestib Res. 2011;21(3):141-51. doi: 10.3233/VES-2011-0400.

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