Hearing Flashcards
How does sound travel?
Via longitudinal waves in the air
What are longitudinal waves?
Changes in air pressure- has areas of compression (peaks) and rarefraction (troughs)
What is the eardrum also known as?
Tympanic membrane
Where do sound waves travel in humans?
Ear canal- sound waves vibrate off of ear drum
How sensitive is our hearing and what frequencies can we hear?
Very sensitive- can detect signals that vibrate ear drum by a few picometres- amplifies signals by more than 100 fold
- can hear frequencies of 20Hz - 20000Hz
What are the 3 small ear bones in the inner ear and where are they located?
Malleus
Incus
Stapes
- create sound waves with varying frequencies in the perilymph of cochlea
What is the perilymph?
Type of extracellular fluid with components (e.g. sodium, potassium) similar to normal extracellular fluid
What does the active process enhance and what is this?
Frequency selectivity = capacity to discriminate between similar tones
What is the organ of corti and where is it found?
= strip of sensory epithelium running 33mm along the spiral cochlea
What is the key component of the organ of corti?
Hair cells = sensory receptors of inner ear
How many hair cells does the organ of corti contain?
16000
How are the hair cells in the organ of corti arranged?
one row of inner hair cells (IHCs) + 3 rows of outer hair cells (OHCs)
Describe the structure of the organ of corti
Made up of tectorial + basilar membrane
Hair cells lie within these 2 membranes with the IHCs connected to auditory nerve
What is the tectorial membrane and what is it attached to?
= extracellular matrix that covers the apical surface (highest point) of the organ of corti
- attached to stereocilia of OHCs
Why does the sound wave need to be amplified?
sound waves travel at different speeds in air vs perilymph
Define: amplification
The process of increasing the volume of sound
What chamber is the perilymph located in?
Scala vestibuli
How do sound waves travel through the perilymph?
- Stapes transmit sound waves through to the scala vestibuli
- Sound wave activates hair cells within organ of corti depending on the frequency
What are the 2 different fluids in the organ of corti and where are they found?
Perilymph- scala vestibuli + scala tympani
Endolymph- scala media
What is the main difference between the 2 fluids in organ of corti?
Endolymph has higher potassium content
What is the basilar membrane and what happens when it vibrates?
= extracellular matrix that covers the basal surface (lowest point) of the organ of corti
Vibration =
1. Hair cells deflected = stereocilia move in a back and forth direction
2. = opens mechanically-gated ion channels = hair cell depolarisation releasing action potentials = converting sound wave into electrochemical signal
What are the 3 chambars in the organ of corti?
Scala vestibuli
Scala media = cochlear duct
Scala tympani
What is ear hair cell morphology optimised for?
Mechanosensation
How are the hair cells optimised for mechanosensation?
Stereocilia within hair bundle are organised in rows of decreasing height
- bundle deflection only in direction of longest stereocilia leads to increased probability of mechanoreceptors opening
What are mechanoreceptors?
= sensory receptors that respond to mechanical stimuli (touch, sound)
Where is the kinocilium located?
Produced 1st during developmeny and moves to outer edge of top of hair cell as ear matures and then the stereocilia form
Do stereocilia have a cytoskeleton and why?
They have a very structured actin cytoskeleton as they have to remain upright
How are stereocilia connected?
via tip links that connect top of one stereocilia to another
What are roolets and what is their function?
= actin filaments in stereocilia- anchor stereocilia into specialised actin network = cuticular plate
What happens if certain proteins that make up tip links are removed?
Mechanosensitivity of stereocilia is decreased which could lead to deafness
How can stereocilia be seen in a lab?
- Hair bundles stained with phalloidin
- Can be viewed via light or electron microscope depending on resolution
Name and describe the model that explains how mechanotransduction works
THE TETHER MODEL:
1. Tip link connected to mechanosensitive channel
2. Tension causes channel to open = influx of calcium ions
3. Channels are also opened in other stereocilia
4. Adaption phase = motor protein slios down which relieves tension in tip link = mechanosensitive channel closes quickly
5. Upper tip link climbs up stereocilium to re-establish tension
Name 2 proteins that form tip links
Protocadherin 15
Cadherin 23
Name 2 places where mechanosensitive channels are located in the ear
Stereocilia
apical membrane
What 2 TMCs are candidates for essential functioning of mechanoreceptor channels and whats the evidence for this?
TMC1 + TMC2:
- Both are selectively expressed in developing hair cells at the onset of mechanosensitivity
- Mutations of TMC1 = deafness
- Absense of both = hair cells of mice lack mechanosensory responses to forward deflection of hair bundles
- Transient exogenous expression of either restores mechanosensitivity in hair cells in mice who have had them knocked out
How is the organ of corti adapted for function?
= gradual changes in structure + morphology from base to apex e.g. height of stereocilia + width and thickness of basilar membrane
- stiff and taute at base- picks up high frequencies
- More flaccid at apex- can move freely so picks up lower frequencies
What happens when the cochlea recieves a pure tone acoustic stimulus?
- Pressure differences between its liqiud filled compartments cause basilar membrane to oscilate at frequency of stimulation
- Waves propagate from base of cochlea towards apex
- As wave progresses, it grows in amplitude but drecreases in wavelength and the quickly vanishes when it reaches relevant frequency position on basilar membrane
Describe the active process
OHCs locally activated and enhance basilar membrane vibration
- Depolarisation causes OHCs to shorten as well as enhances vibrations via contractions and hyperpolarisation causes them to elongate
- IHCs detect the vibrations and activate afferent neurons e.g. auditory nerve
Where are piezo2 channels found in the ear?
apical membrane
