The inner Ear

  1. The inner Ear
    • changes mechanical energy to bioelectric code that can be read by the brain
    • -two main functions are filtering and transduction
  2. Inner ear chain of function
    • stapes motion moves the fluid in the cochlea and vibrates other structures in the inner ear
    • -this causes the BM hair cells to be displaced depending on Hz of input creating an auditory filter and elicits neural discharge in the auditory nerve.
    • -auditory nerve sends info about freq, temporal content and intensity to the nervous system
  3. structural parts of the inner ear (3)
    Cochlea, semicircular canal and vestibule
  4. bony and membranous labyrinths
    both cochlea and the vestibular system have a membranous labyrinth inside a bony labyrinth
  5. Cochlea
    • resembles a tube of decreasing diameter, size of a pea coiled up
    • -gets smaller as you move from base to apex
  6. Cochlea has three ducts
    • 1. Scala vestibuli
    • 2. Scala media
    • 3. Scala tympani
  7. Basilar membrane:structure
    • separates scala media from scala vestibuli
    • -BM gets wider as you move from base to apex of cochlea
    • -BM is stiffer at base  and floppier at apex
    • -high HZ resonant closer to base and low freq resonant closer to apex
  8. On top of Basilar membrane
    • -outer hair cells and inner hair cells
    • -auditory nerve endings are located on top of the BM, and connect to the hair cells
    • -tectorial membrane sits on top of BM
  9. Inner hair cells (IHC)
    • 1 row approx 3000
    • -shaped like jugs, approx 50 stereocilia on top
    • -tips of stereocilia not embedded in tectorial membrane
    • -arranged in straight line
  10. OHC-outer hair cells' structure
    • are shaped like test tubes with 100 stereocilia on top
    • -tips are embedded in the tectorial membrane
    • -arranged in v or w pattern
    • -connected to each other by tip links
  11. oval window
    • moves in opposite direction as the stapes
    • -allows flow of cochlear fluids as a result of stapes movement. 
    • -this causes the BM (cilia) to be displaced
  12. Selective resonance of the BM
    • not all parts of it vibrate equally well to a sine wave of a particular frequency
    • -different locations along the BM resonant best to different freq
    • -Frequency-to-place coding, independent of the source (bone or air)
  13. Traveling wave and envelope
    • the envelope is the most important wave form in the cochlea
    • -traveling wave must be above the first envelope to be heard with it.
    • -if it is lower intensity than it won't be heard
  14. 4 types of filters
    • low pass
    • high pass
    • band-pass= a region that is let through
    • band-reject= cuts out a region but lets everything else pass through
  15. BM vibration
    • -band pass filter= the stuff in the envelope passes through but the stuff outside does not
    • -each point on BM corresponds to a band pass filter with a different centre Freq.
    • -as one moves from base to apex the centre freq desceases
    • -BM vibration is non linear: the amp response of the BM depends on the level of the input
  16. Function on band pass filters
    • our ability to use this filter affects our ability to filter out background noise
    • -you need narrow filters
    • -this functions is dependent on stereo-cilia function
  17. Outer hair cells' role (2)
    • 1) amplify sounds that are below 50dB
    •  - amplify and sharpen the filter action of the cochlea
    • 2)sharpen the traveling wave peak
  18. Inner hair cell functions
    • afferent fibres send info to the brain
    • -innervated by the VIII CN
    • -without these we have no hearing
    • -can not pick up sound below 50dB
  19. OHC function
    • Innervated by the olivocochlear bundle
    • they receive messages from the brain and modify BM response depending on stimulus
    • -pulls down tectorial membrane to touch the IHC so they can be sheared.
    • -they stretch and shrink in response to sound
  20. shearing of hair cells
    cells respond to tip deflection. Causes an action potential to be sent to the brain
  21. Active mechanism (or the OHCs)
    • low intensity sounds amplified by OHC
    • above 20dB the amplification declines progressively until about 90 where there is no additional amplification. 
    • -as sound increases in freq the OHC's do less work 
    • -OHC still provides sharpness throughout
  22. Non-linear means
    • 1. BM displacement is more Hz specific
    • 2.BM displacement more sensitive to low level sounds, sounds need to be atleast 50dB if no OHCs are present
    • -these non linearities allow for more accurate transduction of info from IHC to VIII CN to brain
Card Set
The inner Ear
The inner Ear