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The inner Ear
- changes mechanical energy to bioelectric code that can be read by the brain
- -two main functions are filtering and transduction
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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
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structural parts of the inner ear (3)
Cochlea, semicircular canal and vestibule
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bony and membranous labyrinths
both cochlea and the vestibular system have a membranous labyrinth inside a bony labyrinth
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Cochlea
- resembles a tube of decreasing diameter, size of a pea coiled up
- -gets smaller as you move from base to apex
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Cochlea has three ducts
- 1. Scala vestibuli
- 2. Scala media
- 3. Scala tympani
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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shearing of hair cells
cells respond to tip deflection. Causes an action potential to be sent to the brain
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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
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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
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