Hearing: Lecture 13-14 Flashcards
How does sound travel?
Transmitted through the air as a sinusoidal pressure wave. Period of cycle = frequency (Hz) , loudness = amplitude (dB)
How sensitive is human hearing?
Child: 20,000 Hz- 20Hz Adult: 16,000- 20Hz 4dB = acoustic threshold, >120dB = extreme pain
How can pressure levels be calculated
Phons= subjective sound level 130 = pain, 120 = pneumatic drill 50-75 = speech
Describe the anatomy of the human ear

How does the ear respond to sound?
Eardrum moves in and out depending on frequency and amplitude.
What is the cochlea
3 bones which conduct the pressure waves Converts pressure waves to sound, and amplifies dound Normally coiled up Contains the basilar membrane
Label the uncoiled cochlea
Insert diagram

What are the three fluid filled compartments which are part of the cochlea?
Scala vestibuli which becomes scala tympani - Filled with Extracellular Fluid Scala media (the middle) which has an increased K+ concentration.
Describe the anatomy of the Cochlea
Encased in bone Scala vestibuli and scala tympani seperated by scala media, tectorial membrane , tunnel of corti.
What is the organ of corti?

How does the organ of corti work?
1 row of internal hair cells 3 rows of outer hair cells which push against tectorial membrane and cause it to bend.
What are the cochlear hair cells?
Kinocilium is big hair at one end, sterocilia are the hair cells which decrease in size after. Stereocilia are linked by small proteins (tip links) Tips of hairs are embedded in tectorial membrane - cochlear hair cells move in shear manner Insert diagram

How do these hair cells move?
Small V’s of hairs with kinocilium in the middle
90% of sensory fibres go to innner row of hair cells
Bends with the vibrations of the sound
What happens if the hair cells become damaged?
Tip links broken so sterocilia are no longer organised / not held together
Large gaps in rows
What is the basilar membrane?
Wide near the heliotrema, narrow near the oval and round window
Resonates at different regions depending on the frequencies.
- Basal end (near oval window) high frequencies (stiff)
- Apex (wide end) low frequencies (flexible)
- Amount of vibration increases with distance from base

What is the role of the tectorial membrane?
At rest the hairs are perpendicular to tectorial membrane.
Change of angle (shearing) in either direction causes an opposing response due to being di-directionally sensitive

How does membrane potential change with hair cell displacement?
Bend to the left –> hyperpolarisation
Bend to the right –> depolarisation (glutamate release)

What are the properties of hair cells?
Depolarised at rest (-60mV)
Directionally sensitive
- right –> depolarisation –> increased firing
- elft –> hyperpolarisation –> decreased firing
Mechanically tuned- hair buundle length varies
Electrically tuned (sinusoidal Em response)
How are the hair cells tuned?
Location in the cochlea
Length of the sterocilia
Electrical tuning
How does the movement of the hair cells produce membrane potential change
Shearing movement –> opening of K+ channels due to stretch or mechanically mediated channels
Triggers an influx of potassium from scalomedia into the cell –> opening of VG calcium channel –> Ca+ influx
Ca2+ is taken up by mitochondria and allows the exit of K+ through Ca2+ mediated K+ channels.
Ca2+ leaves through ion pump extrustion
This allows the hair cells to straighten.
How is the repeatable wave produced?
K+ in starts upstroke, which is continued by Ca2+ influx.
Downstroke caused by efflux of positive ions.
Only a tiny movement in hair cells is required to produce a large change in the membrane potential.
Em may oscillate due to the continous movement of hair cells back and forth.