How does hearing loss initially manifest?
- Difficult to hear in noisy environment
- May require to lip read
- Gets worse as you get older
What is sound?
-
Vibrations of objects --> which set up pressure waves in the surrounding air
-
Elastic property of air --> allows these pressure waves to propagate (spread)
What is the shape of sound?
What are the 2 parts of a sound wave?
- Sine Wave
- Compression & Rarefaction
During pure tones, what is frequency related to?
- Frequency of the tone is directly related to the pitch of the sound
-
Amplitude is related to perceived loudness
If a sound has a particular frequency. It is a ....
Pure Tone Sound
What is the relationship between frequency & pitch + amplitude & loudness?
- Logarithmic Relationships
-
Double in Frequency --> Increases Pitch by 1 Octave
-
Double in Amplitude --> Increase Loudness by 6dB
What is a property of sounds with a clear pitch?
-
Frequencies are related
- They are all multiples of a fundamental frequency
What are sounds that have all integer multiples of a 'fundamental frequency' called?
What does the sound sound like?
- Harmonically Related
- Sounds like a sound with a clear pitch
(e.g. lowest common denominator)
What makes up complex sounds?
- Frequency components (i.e. different sine waves)
What dies a Fourier analysis help us do?
- Plot the amplitude (& phase) for each frequency component of a sound
- This gives us a sound spectrum
- Giving us a compact representation for signals that contain oscillations
How do you do a fourier analysis?
What is the graph you get called?
- Convert PRESSURE as a function of TIME
- to AMPLITUDE as a function of DIFFERENT FREQUENCIES
- Sound Spectrum
What is the advantage of using a Fourier Analysis?
Gives you a compact respresentation of signals that contains oscillations
(graphs contain lots of complicated mini-waves --> thus it helps you to not have to track sound every second --> you can present sound as different frequencies with the amount of each frequency)
What is meant by narrowband?
- Sound contains energy at a narrow range of frequencies
(Pure tone is an extreme example of a narrowband)
These sounds are periodic and may evoke an identifiable pitch
What is meant by broadband sounds?
Give examples
- Contain energy at a lot of different frequencies
- e.g. noises & clicks (natural broadband sounds)
What does a spectrogram do?
-
Divides sounds up into short-time segments --> then calculates a spectra for each time segment in turn
- This helps you track changes of sounds over time (as a sound spectrum only gives you it in one split moment)
Ear effectively converts sound into a spectrogram
What are the 3 dimensions of a spectrogram and how are they represented?
- Time (x-axis)
- Frequency (y-axis)
- Energy (colour)
-
What does the outer ear consist of?
- Pinna (odd bumps and folds)
- Concha
- Ear Canal
Which part of the ear is considered to be unique to each person (like a finger print)?
- Pinna (bumps and folds on it)
Which part of the external ear changes the nature of sound in a subtle way?
- Pinna
What is the function of a pinna?
-
Collect all the sound that hits it
-
Directs the sound into the ear canal (thus acts to amplify sound)
How is the pinna involved in sound localisation?
- Sound hits the ear --> bounces around in a complex way
- Depending on where it came from --> sound bounces in a different way
(Causing subtle changes to nature of the sound)
How is the outer ear involved in localisation?
- Incoming sounds --> are filtered by outer ear --> in a way that depends upon location of the sound source in the vertical plane (elevation)
- It is also important in horizontal plane (but only if you have hearing loss in one ear)
What happens to the frequency if a sound is presented to you at a higher inclinication level?
- Higher Frequency
What happens to the frequency if a sound is presented to you at a lower inclinication level?
- Lower Frequency
What would happen if you had no pinna?
- Would not be able to localise sound above, below, infront or behind you
Name the 2 main functions of the outer ear.
-
Sound Amplification (transforms acoustic energy at the tympanic membrane - pinna transfer function)
-
Sound Localisation (pinna transfers function depending upon sound source position in vertical plane)
Collects & funnels sounds to eardrum but does so differently depending on sound source location
Name the 3 bones in the middle ear.
- Malleus
- Incus
- Stapes
Why is 99.9% of sound reflected from cochlea?
What is the conseuqence of this?
- Due to impedence of fluid in the cochlea
-
30dB sound loss due to impedence mismatch between air & fluid
By how much do the middle ear bones overcome the loss of sound?
- +34dB
What are the 3 methods used to undergo impedence matching?
- Reduce Area
- Increase Force
- Buclking of Ear Drum
How does the middle ear reduce area?
By how much does this increase sound pressure?
- Area ratio of the ear drum : stapes footplate (20:1)
- +26dB
(tympanic membrane is much bigger than the footplate of the stapes --> so focused onto a smaller area)
How does the middle ear increase force?
By how much does this increase sound pressure?
- Lever action of the ossicles (1.3:1)
- +2dB
What is the third method that causes the increase in sound pressure?
By how much does this increase sound pressure?
- Buckling of the Ear drum
- 2x increase in pressure
- +6dB
When the ear drum vibrates it also buckles (complex) --> this is equivalent to a secondary lever effect --> causing an increase in pressure
How does the middle ear amplify different frequencies?
- It does not amplify all frequencies to the same level
At what frequency does it amplify well?
-
1 kilohertz (around level of human speech)
- However higher & lower frequencies --> it is not as good
Which muscle in which reflex is activated to protect hearing?
- Stapedius Muscle
- Contracts in the Middle Ear Reflex
-
Contract --> oscilles unable to move (amplification not allowed)
-
Relax --> oscilles able to move (amplification allowed)
The reflex controls the amount of amplification
How does the middle ear enhance sound?
- Enhances sound transmitted to inner ear
- Done in a controlled & frequency-dependent way
What are the dimensions of the cochlea?
- 1cm wide
- 5mm from base to apex
- Snail-shaped (coiled)
How does the cochlea connect to the middle ear?
- Via the footplate of the stapes --> which rests against the oval window
After the fluid in the cochlea moves --> what moves?
- Round window
Name the 3 layers in the cochlea. Where are they in relation to each other?
- Scala Vestibuli (top)
- Scala Media (middle)
- Scala Tympani (bottom)
Where do hair cells sit?
Down what do they send signals?
What do they respond to?
- Sit on the basilar membrane
- Respond to movements of fluid (created by sound vibration)
- Send signals down the auditory nerve (cranial nerve VIII)
What is found in the 3 layers of the cochlea?
-
Scala Vestibuli & Scala Tympani --> contains PERILYMPH
-
Scala Media --> contains ENDOLYMPH
What is the composition of endolymph?
- High K+
What is the composition of perilymph?
- Low K+
What maintains the composition of the endolymph?
- Very High K+ Concentration
- Unusual in the body
- Due to stria vascularis
-
Sits in the lateral wall
- Actively increases K+ concentration of endolymph
What would happen if K+ dropped in endolymph?
- Complete Deafness
- Needs to be maintained at a high level by stria vascularis (lateral wall of scala media)
What is the membranous labrinyth made up of?
What does it comprise of?
- Semi-Circular Canals + Otolithic Organs
- Contains endolymph
Why do syndromes/diseases affecting balance and vestibular apparatus affect hearing?
- Fluid environment of the vestibular apparatus is continuous with that of the cochlea
- Problems usually come together (e.g. mania syndrome)
What do vibrations do to the basilar membrane?
- Causes complex deflections
- Complicated waveform
What determines where the peak/centre of the fixed point on the basilar membrane will be?
-
Depends on the frequency (stapes will vibrate at a certain frequency causing fluid to vibrate at that frequency causing the peak at a certain point)
What is the wave from the ovale window (stapes) called?
- Travelling Waveform
What deteremines where the wave peaks are on the basilar membrane?
- Sound Frequency
Where is the wave peak during low frequency sounds?
-
Close to Apex (i.e. centre)
Where is the wave peak during high frequency sounds?
-
Close to Base (i.e. near ovale window)
What happens if there is a complex sound? (in terms of basilar membrane)
- More than 1 frequency
- Therefore different energies
- Movements of the basilar membrane will be at multiple different points
What is the equation that links stiffness (k), mass (m) & frequency (w)?
w2 = k/m
Where is the basilar membrane most stiff?
-
Base (near ovale window)
What happens in terms of stiffness from base to apex?
-
Stiffness decreases as you go from the base to the apex
How does mass change as you go from the base to the apex?
-
Base --> Little Mass
-
Apex --> Lots of Mass
- The mass increases as you go towards the apex
Where does low frequency sound produce movement on the basilar membrane?
-
Apex of the Basilar Membrane
Where does high frequency sound produce movement on the basilar membrane?
-
Base of the Basilar Membrane
What two properties of an object determine the frequency they are likely to vibrate at?
- Stiffness (rigidity)
- Mass
What is the mass & rigidity like at the apex?
- Apex
- Wide & Floppy
-
Large Mass & Low Rigidity
What is the mass & rigidity like at the base?
- Base
- Narrow & Rigid
-
Small Mass & High Rigidity
What frequency does the base 'like'?
High Frequency
What determines the frequency of the Basilar Membrane vibration?
- Mass
- Stiffness
What is the stiffness & mass like at the base?
- Stiff
- Less Mass
What is the stiffness & mass like at the apex?
- Less Stiff
- More Mass
What is the basilar membrane like?
- Acoustic Prism
- Takes in light --> splits it up into lots of different frequencies
What is the component of the basilar membrane that responds to sound called?
- Organ of Corti
What does the Organ of Corti do?
- Converts movement of the basilar membrane into electrical activity
What does the up-down movement of the basilar membrane cause?
-
Tectorial Membrane --> to slide sideways over the membrane
- Causes sideways displacement of the hair cell bundles in the cochlear hair cells
(sliding of tectorial membrane to one side relative to basilar membrane pushes hairs in a particular direction (towards depolarisation or hyperpolarisation)
How are the adjacent stereocilia joined up?
- Tip-Links
How does transduction by the hair cells take place?
-
Stereocilia of the hair cell bundles connected via tip links
-
Movement of the bundle changes the tension on the tip links --> causing opening/closing stretch sensitive K+ channels
-
K+ Channel Opening --> causes influx of K+ from endolymph --> causing depolarisation of the hair cell membrane
- This causes opening of voltage-gated Ca2+ channels --> increasing probability of transmitter release
How does the hair work?
- Hairs vary in length
- Connected to a stretch-sensitive K+ channel
- If they move in one way --> distance between hair either increases/decreases --> causing an increase/decrease in tension in the tip-links --> thus opening/closing K+ channels
What happens if the distance between hair cells decrease, therefore tension in tip-link decreases?
- Hyperpolarisation
What happens if the distance between hair cells increases, therefore tension in tip-link increases?
- Increase in tension
- Open K+ channels
- Depolarisation
What happens when K+ channels open?
-
K+ flood in from endolymph
- This causes voltage-gated Ca2+ channel to open --> causing an increase in intracellular Ca2+
- This causes depolarisation
- Causing NT release (glutamate)
What are the 2 types of hair cells?
What are they both connected to?
- Inner Hair Cells (IHCs)
- Outer Hair Cells (OHCs)
Both connected to Auditory Nerve Fibres
What is the function of the Inner Hair Cells (IHCs)?
- Passive
- Respond to vibration of the basilar membrane
- Perform a process of mechano-electrical transduction (convert mechanical energy to electrical)
- They release NT onto Auditory Nerve Fibres (electro-chemical transduction) --> transmitting information along auditory nerve
What happens at low frequencies in terms of membrane potential?
-
Membrane potential of the hair cell --> follows every cycle of the stimulus (AC response)
-
Fluctuate rapidly (but can keep up)
What happens at high frequencies in terms of membrane potential?
- Membrane potential is unable to follow individual cycles
- Therefore remains depolarised throughout duration of the stimulus (DC response)
Cannot keep up anymore with the rapid fluctuations (around 4000 hertz) --> thus cell will depolarise and not change (frequency too high)
What happens at intermediate frequencies?
- Membrane potential exhibits a mixed AC + DC response
What is the function of Outer Hair Cells (OHCs)?
- Active
- Electric-mechanical transduction
- They contract --> feed energy back into the basilar membrane
Electric signals feedback into it to cause mechanical mvoement so that it moves the basilar membrane --> causing positive feedback (IHCs and OHCs propogate each other)
- Takes small signals/vibrations and amplifies them to make movement of BM even bigger
What are the important implication of OHC function?
- Process is responsible for sensitivity of hearing
- Vulnerable to damage by NOISE or OTOTOXIC DRUGS
What do OHCs contain which allows them to change their shape/length when they are depolarised?
- Prestin
What are the OHCs connected to and why is it important?
-
Connected to Basilar Membrane
- Therefore causes movement of the BM
- This amplifies vibrations of the basilar membrane
Generally, what are the active cochlea's 2 main functions?
- Increase Sensitivity
- Increase Tuning
How do OHCs increase sensitivity & tuning?
-
Increase gain in the cochlea by boosting vibrations --> this also results in narrow tuning
What happens if OHCs are not working (i.e. just passive sound)?
-
Most of the Adjacent Basilar Membrane will move (displace) too at a similar level
- Therefore will not be tightly tuned
-
Not much difference between different points
- Reduced sensitivity
- Reduced tuning
What happens if OHCs are not working? (generally)
- Remove the Tuning
- Less Sensitive
- Need louder sounds for auditory nerve fibres to respond (cause there is no 'help')
OHC FUNCTION
Why does a healthy cochlea 'generate sound'?
- Due to active mechanical feedback (electro-mechanical transduction) of the OHCs
How is the theory about OHCs incorporated in baby hearing tests?
-
OHC amplification system --> technically produces sounds that were not put in at all (produces own sounds)
- Therefore if you feed 2 frequencies --> you should get a third if the OHC is working fine
- If it is not there --> problem with hearing --> need further checks
What are the effects of antibiotics on hearing?
-
Antibiotics can affect Organ of Corti
- Cause wiping out of the hairs on the hair cells
- Need to do hearing test after to ensure they are okay
What can cause damage to Outer Hair Cells?
- Drugs
- Noise
Specifically what antibiotic can cause hair damage (on OHCs)?
Why is this AB usually given?
- Kanamycin
- Often given to children for juandice & other conditions --> but too much causes hair cell destruction
What are the outer & middle ear important for?
-
Transmitting sound to the inner ear
- In a form compatible with the fluid environment of the inner ear
What is special about the basilar membrane?
- Tuned to sound frequency
-
Splits complex sounds into constituent components (e.g. acoustic prism)
What are inner hair cells?
- Mechano-electrical transducers
- Convert mechanical deflections into electrical potentials
What are OHCs?