Hearing: Lecture 13-14

Question 1

How does sound travel?

Answer 1

Transmitted through the air as a sinusoidal pressure wave. Period of cycle = frequency (Hz) , loudness = amplitude (dB)

Question 2

How sensitive is human hearing?

Answer 2

Child: 20,000 Hz- 20Hz Adult: 16,000- 20Hz 4dB = acoustic threshold, >120dB = extreme pain

Question 3

How can pressure levels be calculated

Answer 3

Phons= subjective sound level 130 = pain, 120 = pneumatic drill 50-75 = speech

Question 4

Describe the anatomy of the human ear

Answer 4

Question 5

How does the ear respond to sound?

Answer 5

Eardrum moves in and out depending on frequency and amplitude.

Question 6

What is the cochlea

Answer 6

3 bones which conduct the pressure waves Converts pressure waves to sound, and amplifies dound Normally coiled up Contains the basilar membrane

Question 7

Label the uncoiled cochlea

Answer 7

Insert diagram

Question 8

What are the three fluid filled compartments which are part of the cochlea?

Answer 8

Scala vestibuli which becomes scala tympani - Filled with Extracellular Fluid Scala media (the middle) which has an increased K+ concentration.

Question 9

Describe the anatomy of the Cochlea

Answer 9

Encased in bone Scala vestibuli and scala tympani seperated by scala media, tectorial membrane , tunnel of corti.

Question 10

What is the organ of corti?

Answer 10

Question 11

How does the organ of corti work?

Answer 11

1 row of internal hair cells 3 rows of outer hair cells which push against tectorial membrane and cause it to bend.

Question 12

What are the cochlear hair cells?

Answer 12

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

Question 13

How do these hair cells move?

Answer 13

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

Question 14

What happens if the hair cells become damaged?

Answer 14

Tip links broken so sterocilia are no longer organised / not held together

Large gaps in rows

Question 15

What is the basilar membrane?

Answer 15

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

Question 16

What is the role of the tectorial membrane?

Answer 16

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

Question 17

How does membrane potential change with hair cell displacement?

Answer 17

Bend to the left –> hyperpolarisation

Bend to the right –> depolarisation (glutamate release)

Question 18

What are the properties of hair cells?

Answer 18

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)

Question 19

How are the hair cells tuned?

Answer 19

Location in the cochlea

Length of the sterocilia

Electrical tuning

Question 20

How does the movement of the hair cells produce membrane potential change

Answer 20

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.

Question 21

How is the repeatable wave produced?

Answer 21

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.