Lecture 19; Sound 3 Flashcards
Describe the transfer of sound to the inner ear:
The external ear resonates the sound (makes it louder) this displaces the tympanic membrane, displacing the ossicular tree, which articulates and the stapes oscillates the round window
This causes displacement of the fluid which causes the sensory cells to connect with the tectorali membrane and transduction to occur
The round window is a membranous layer that can move to allow fluid displacement.
Describe what happens when the stapes articulates;
Movement in the stapes in the oval window sets up a traveling wave on the organ of corti and basilar membrane
As the travelling wave moves along the organ of corti what happens?
The wave increases in amplitude, decreasing wavelength but maintaining frequency until the point of resonance.
What does the point of resonance signify on the organ of corti?
The point of resonance occurs at a single frequency.
The point of resonance will be defined by the mass and stiffness of the structures (organ of corti)
Additional mechanisms enables this to form a peak of activity and around this area threshold/ sensory cells will be stimulated.
Different frequencies will change the point of resonance.
Describe HZ organisation of the organ of corti;
Base = High frequencies (near oval window)
Apex = low frequencies
Position of peak of travelling wave varies spatially with frequency
How is complex sound processed by the organ of corti?
- Multiple resonate frequencies occur
How does the organ of corti act as a mechanical filter?
Gradient in mechanical properties along length generates mechanical filter
Describe the mechanical properties along the basilar membrane;
Gradual gradient;
Basal end: Thick, narrow and stiff
Apical end: Thin wide and compliant
how is the cochlea mapped?
Tonotopically mapped
- Hz distribution (ability to seperate freqeuncies) thus at those resonating points, those sensory cells are activated and sound is transduced.
Does tonotopicity extend beyond the basilar membrane?
Yes it extends into the cortices where some neurons only respond to specific frequencies..
What sort of motion is the vertical motion of the organ of corti translated to in the sensory cells?
Vertical motion of the organ of corti is converted into radial motion at the stereocilia. (as they rub against the tectorali membrane, which leads to sensory transduction)
Describe radial movement and sensory cell depolarisation;
Movement in one radial direction generates depolarisation, while movement in the other causes hyperpolarisation (but only in the one plane)
What part of the IHC and OHC are the sensory transducers?
The stereocilia are the transduction elements
Describe the stereocilia:
There are an array of different length stereocilia. B/w the longer and shorter one there are little bridges and this is where the transduction channels are.
What are the bridges between stereocilia called?
tip links (transduction channels)
What are tip links?
- Dont know what constitutes the channels
- Mechanical, electrical transduction channels (linking mechanical motion to electrical transduction)
- Made up a single cadhedrin molecule with a proteocadhedrin molecule at either end.
Are transduction channels found in tip links?
No, they movement of the stereocilia causes the tip links to pull, opening the transduction channel son the stereocilia, enabling ions to flow into the cell and activate it.
What is essential of tip links for stereocilia function?
There is always a good amount of tension on tip links to ensure stereocilia are at optimal state… that is the actin filaments can actually increase or decrease the tension if make sure it stays within the optimal range.
Ensuring sound is heard.
How does the tip links system function as an adaptive mechanism?
- It will tone itself down to prevent background noise from being heard
What happens to tip links with loud noise exposure?
They break, this is where initial sound damage occurs, they do repair but lose this ability after being broken a number of times.
What does opening of the transduction channels by tip links lead to?
A channel opening between the endolymph and the sensory cells
K flows into the cells, depolarising it.
Describe receptor potential with a tone being player for a period of time.
Displacement leads to PERIODIC changes in membrane voltage which at low Hz follows HZ.(AC COMPONENT)
(i.e the basilar membrane keeps displacing in the wave nature, so the sensory cells keep depolarising in a wave pattern)
At high HZ leads to net depolarisation during duration of sound. (DC COMPONENT)
- Think about how this will correlate with neurotransmitter release
What is the DC component due to?
The DC component is due to high capacitance of cell membrane and inability to follow Hz
What is the AC component due to?
AC due to Hz dependant changes in receptor potential
What is depolarisation and repolarisation of the hair cells driven by?
Mediated by K from endolymph driven by the endocochlear potential
Whats the RMP of the inner hair cells?
-70mV
(endolymph is ~+80mV)
= endocochlear potential, the very large potential across the surface of the hair cell and is essential for transduction
Why is there such a large potential across the inner hair cell?
B/c there is no K gradient as the fluids both have strong K concentrations, but there is an electrical gradient, it must be higher fro transduction to occur
For every mV this potential drops whats the hearing loss?
1db for 1mV
Why do stereocilia hyperpolarise when moved in the off direction?
B/c this closes the transduction channels, but background channels remain open thus hyperpolarisation occurs.
What is the first key characteristic that tells us the function of IHC?
The innervation pattern
What does the innervation pattern tell us?
Predominant afferent innervation of IHC (thus main sensory cell input)
Predominant efferent innervation of OHC
How many rows of hair cells are there?
1 IHC
3OHC
What do OHC do?
Amplify sound and tuning at low levels (of the mechanical response)
How do OHC function?
The travelling wave is actively amplified by action of OHC to overcome the viscous forces of the fluid and resistance of the tissues.
They oscillate and pulls itself up pushing on the tectorali membrane
How do the OHC achieve their function?
- OHC are electromotile
- Motor proteins (prestin) in the cell membranes
- The affects cochlear mechanical motion known as the cochlear amplifier.
What are the types of afferent fibres?
Spiral ganglion fibres
Type one: innervates IHC
Type Two: innervates OHC
What are special proteins in the IHC for controlled neurotransmitter release?
Hair Cell Ribbon synapses
How do ribbon synapses function?
They have lots of neurotransmitter vesicles docked onto them, allowing for controlled neurotransmitter release
Acts as a converyor belt, ensures locking between stimulus and neurotransmitter release.
Describe the tuning of the auditory nerve fibres;
Auditory nerve fibres are sharply tuned to different Hz
They tend to respond to some extent below their maximum Hz.
Synchronized firing at low Hz but at higher ones they turn on and adapt, continuously firing.
What are the two mechanisms for frequency coding of the auditory nerves?
Place principle
Volley principle
Whats the place principle?
Cochlear is a filter and is tonotopically organised so that Hz is detected by spatial representation from base to apex
What is volley principle?
Low Hz are detected by temporal firing (synch) of nerve fibres in time to the Hz of the stimulus (phase locking)
Only useful at low Hz 1-2hz
How is intensity encoded for?
Rate of firing of an individual auditory nerve fibres increases with sound intensity
