MadSci Network: Neuroscience
Query:

Re: What exactly causes vertigo?

Date: Wed Jan 19 13:53:22 2000
Posted By: Amanda Kahn, Grad student, neuroscience, UCSF
Area of science: Neuroscience
ID: 947273801.Ns
Message:

Hi there!

Vertigo is a sense of dizziness and disorientation, and it often arises due 
to problems with the vestibular system.  Your vestibular organs are 
contained in two structures called the labyrinths, which are often referred 
to as the "inner ear."  There is one labyrinth on each side of your head, 
located near your cochlea (the primary sensory organ for hearing).

The labyrinth is composed of two principal organs: the semicircular canals, 
and the otoliths.  Each semicircular canal consists of three "loops" 
organized at right angles to each other.  If you imagine an X-Y-Z set of 
axes, one loop lies in the XY plane, one in the XZ plane, and one in the YZ 
plane.  The canals are, indeed, filled with fluid.  A specialized expansion 
at the base of each canal contains neurons with little hairs (or cilia) 
sticking into a membrane.  When the head moves in an appropriate fashion, 
the fluid in the canals move accordingly.  This fluid movement causes the 
membrane to bulge in the direction of head turning, which bends the cilia of 
the hair cells.  When the cilia bend, ion channels in the neurons open or 
close, which sends an electrical signal to other neurons.  So, the 
semicircular canals manage to translate head movement into fluid movement 
into electrical (ion) movement.  The semicircular canals are most sensitive 
to head rotations, like looking to your side.

The other labyrinth organ is called the otoliths, and their structure is 
slightly different.  The otolithic organs are located in the same part of 
the body, and also contain hair cells with their cilia embedded in a 
membrane.  Instead of containing fluid, though, the otoliths contain a rocky 
sheet of calcium carbonate crystals (that's where the otolith gets its name: 
oto + lith = ear rock!).  When the head tilts or is subject to acceleration/
deceleration (like in a car or an airplane), the rock moves, and the hair 
cell cilia are bent.  Again, this sends an electrical signal to other 
neurons. 

By letting your brain know how your head is moving, the labyrinths help you 
maintain a sense of orientation.  They also keep your gaze fixed as your 
head moves, so that your eyes can maintain a continuous picture of the world 
around you.  So, when they're not working properly, patients experience 
disorientation, and may also have inappropriate eye movements.

Vertigo is thought to be caused by a disruption of the various components 
that make up the vestibular system.  It is known that certain antibiotics 
can disrupt hair cell cilia, causing vertigo and hearing problems (because 
hair cells in your cochlea are similar to hair cells in your labyrinth).  
Alterations in the fluid composition or pressure could produce vertigo, as 
well.  Another idea is that particles of the otolith could become 
mislocalized, resulting in inappropriate stimulation of hair cells. 

Hope this helps,
Amanda
amandak@phy.ucsf.edu

Most good neuroscience textbooks have information about the vestibular 
organs -- try looking for Kandel's _Principles of Neuroscience_ or Purves' 
_Neuroscience_.


A picture of the labyrinths, as well as information/support options/
treatment plans can be found at the following site: http://www.teleport.com/~veda/

Additional information about various types of vertigo and dizziness can be 
found at a clinic's website: http://
www.earaces.com




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