Hearing #2 Flashcards
Why do we have a middle ear at all?
- Air conduction of Sound: by displacement of the eardrum and ossicular chain
- Transformer to match low air impedance to high impendance of the inner ear fluid
What would happen if we didn’t have a middle ear (straight from outer → inner)?
The sound pressure in the ear canal would result in a very small movement of the ear drum to do the high vibration resistance of inner ear fluid.
Majority of energy would be deflected → significant hearing loss
With middle ear E-transfer is ~97%
without 0.3%
**Only lose a small amount of sensitivity 3dB
How exactly does the middle ear structures act as a transformer to overcome inner ear resistance?
- Greater area of eardrum transfers more pressure on smaller stapes footplate
- Malleus arm longer than incus generates greater force at stapes (think of a high heeled shoe!)
if you didn’t have this, the pressure wouldn’t be amplified → 50dB hearing loss)
Why is an inner ear infection so dangerous?
Because it is so close to the Brain! Any infection in the inner ear can be easily spread via the internal auditory meatus/canal (that contains the CN VIII and ear vessels) → brain absyss
What are the differing types of cochlear fluid and what cavities do they reside in?
- *Perilymph:** Fluid in the upper/lower cavities, similar to most ECF, with low K+ and high Na+. Bathes the nerve fibres anfd the organ of corti
- scala vestibule
- scala tympani
- *Endolymph**: Middle cavity. Contains little Na+ and high K+.
- scala media
Why is maintaining the potassium within the endolymph so important, and is this a high or low energy process?
The high levels of K+ maintain the transduction pathway and keeping these high levels against the low levels of the perilymph is a very high energy, precise process
What produces endolymph?
Stria Vascularis: pumps high K+ level fluid
What is the Organ of Corti
- Sits on the ‘basilar membrane’ and is around 2.5cm (same as iner ear)
- Contains 20,000 sensory hair cells (2types)
- -5000 IHC and 15000 OHC*
- Above these/projecting into is the **tectorial membrane
*the hair cells are surrounded by a matrix of supporting cells → integral structural support**
Describe the difference between the 2 hair cells, and where exactly ‘do we hear’?
Specialised micro villi with an actin core
- *Outer Hair Cell**: v-shaped, differing heights, 3-5 rows
- *Inner Hair cell**: linear, one row
Stereocilia: where we hear! Only 100 per cell.
What are the steps of sound being transferred into/down the inner ear?
- Stapes moves against the oval window → sets up a wave of activity that travels along the tube as a “Travelling Wave” along the organ of Corti and Basilar Membrane
- Depending on the frequency the wave will come to a “point of resonance”
- As it gets close to its point of resonance the wave will increase in height
- Once it reaches it’s point of resonance the wave and all its energy crashes into the sensory cells at that point and nothing flows beyond
What is the round window, and what is so special about its design?
Round window sits below the oval window in the inner ear and acts as a pressure relief system moving out into the middle ear as the oval window moves in.
RW sits at 90 degrees to the eardrum as to not be affected by the pressure waves of that and is shaped ‘like a pringle chip’ to avoid crushing. Designed to take pressure and not change shape, and is unaffected by the middle ear.
How is it that the inner ear acts as a frequency filter and how do the hair cells in these regions respond?
High Frequency: wave travels only a short distance; tiny stubby cells
Low Frequency: Wave travels further down; huge cells w big stereocilia
Therefore the organ of Corti has “tonotopicity” and the sound is organised spatially. Therefore every sound gets seperated into it’s component frequency
______ motion at the organ of corti is converted into _____ motion at the stereocilia hairs
Vertical motion at the organ of corti is converted into Radical motion at the stereocilia hairs
What significance do Tip Links have and what do they do in terms of sending a signal?
Between sterocilia, connecting them are Tip Links that connect with a mechanical transduction ion channel.
When bent towards the longest stereocilia the tip links and therefore channels open, which allows K+ from the endolymph to flood in, causing influx of Ca2+ from voltage gated Ca2+ channels depolarising the sensory cell → NT release.
When the hairs go the other way the channels close, and you have frequency dependent depolarisation!
How are the top links held into place and what can affect this?
Stereocilia are stiff due to an actin core (this can be altered via efferent fibres → reducing/inc sound sensitivity by changing the tension of the linkage)
Tip Links ‘cohedrin 23 molecule’ are attached by one end to a electricaltransduction channel, and to a myosin molecule on the other end.
When the stereocilia are at rest to myosin is trying to ‘drive up’ the actin, but the tension of the actin linkage will stop this at some point. This means it’s always about to open.
Depolarization and repolarization of hair cells is mediated by ____. How is this system maintained?
Potassium
The natural gradient is for K+ to flow into the hair cell, and then down its conc gradient to the low K+ perilymph.
These cells don’t require energy to pump K+ out, and the energy required to move that gradient is moved to stria vascularis, so it can pump K+into the endolymph . As the hair cells don’t have a blood supply and get all their nutrients via the fluid.
In generating that potassium in the endolymph you create a high voltage, which is another amplifier to greate a massive voltage gradient to open the cells.
What do the OHC do?
Outer Hair Cells (75%): Local amplifyer tuning circuit, allows us to hear precise frquency and boosts in order for us to hear precise sound (makes us for energy loss in outer/middle ear).
Doesn’t really stimulate nerve endings, but causes the shape change of a protein Prestin that lines the hair cell wall, causing it to mechanically oscillate in time to the frequency = “Cochlear Amplification”.
As the OHC are attached to the basilar membrane they’ll pull the membrane up when stimulated, adding energy to the wave, boosting the wave movement further, sending the energy into the IHC.
Noise is a major and preventable cause of hearing loss, and the sensory cells don’t regenerate.
What levels of sound cause damage?
>85dB starts to get damaging
