Lecture 26: The Auditory System Part 1

Question 1

What are the two types of hearing loss?

Answer 1

i. Conductive

ii. Sensorineural

Question 2

What is conductive hearing loss?

Answer 2

-blockage of sound-conducting path from source to cochlea
Example: otitis media, middle ear infection
Treatable and Reversible

Question 3

What is sensorineural hearing loss?

Answer 3

Damage to inner ear or central-auditory pathway
Hair-cell loss
Auditory nerve loss (loud noise)
Incurable, permanent hearing loss

Question 4

What are causes of sensorineural hearing loss?

Answer 4

a. bacterial, viral, vascular damage
b. genetic origins (presbycusis)
c. ototoxic drug exposure (gentamycin)
d. overstimulation (recreational)

Question 5

What do you feel with pain in ear?

Answer 5

It is due to nerve endings in tympanic membrane (ear drum) that become active when the tympanic membrane moves great extents

Question 6

What is sound?

Answer 6

Higher the amplitude, the louder the sound

-it is the combination of a bunch of different frequency components (beat frequency)

Question 7

What is frequency?

Answer 7

The pitch of the sound

Question 8

What forms the middle ear?

Answer 8

Malleus (lateral)
Incus
Stapes (medial)

Question 9

How does sound flow?

Answer 9

Pinna (outer ear) to middle ear to inner ear

Question 10

What is function of middle ear?

Answer 10

Function: to amplify sound to compensate for the fact that energy of sound is dissipated when goes from air to water (which is the fluid that cochlea is surrounded with)

- so if you lose 20% of energy as air hits water (because air is less dense, it gets bounced off when it hits the more dense water)
- stapes is used to bang the inner ear to make sure energy is transferred

Question 11

What is the function of the cochlea?

Answer 11

Transforms energy of sound to chemical signals that the brain can use
It is breaking down sound into individual frequencies

Question 12

What are the three chambers within cochlea?

Answer 12

i. scala vestibule (upper)
ii. scalia media (middle)
iii. scala tympani (lower)

Question 13

What is filled with upper and lower ear filled with? What is middle ear filled with?

Answer 13

Perilymph and endolymph respectively

Question 14

What is the organ of Corti?

Answer 14

Part of cochlea and contains hair cells

-located in scala media

Question 15

What is endolymph filled with?

Answer 15

Endolymph is the fluid in lumen of cochlea (scala media)

-filled with potassium

Question 16

What is paralymph filled with?

Answer 16

Sodium

Question 17

What are the two types of sensory cells?

Answer 17

i. single row of inner hair cells

ii. three rows of outer hair cells

Question 18

What is the tectorial membrane?

Answer 18

Acellular membrane overlying the hair cells of the cochlea
-it is connected to the tallest row of sensory hairs on hair cells
-up and down movements of organ of corti will cause deflection of sensory hairs
Where do afferent axons go? What are these known as?
Inner hair cells
Shit that we hear from

Question 19

What is transduction?

Answer 19

The means by which the acoustic stimulus energy gets transformed into a type of energy that the brain/CNS can understand

Question 20

How is signal transduction carried out in auditory system?

Answer 20

Hairs in cochlea are connected by TIPLINKS…channels that connect the hairs with one another
-when hair moves in direction of longest hair, the channels STRETCH and the channels OPEN (depolarization)…depolarization is induced by the influx of K+
-when the hair moves in the direction of the shortest hair, the channels close, and you get hyperpolarization
One hair cell contains many stereocilia (which is what moves and is connected by tiplinks..)

Question 21

What is the Basilar membrane?

Answer 21

The membrane on which the hair cells sit

-will vibrate (and vibrate in different places) when subjected to different frequencies

Question 22

Where will high frequency vibrate? Where will low frequency sound vibrate?

Answer 22

the base of cochlea (proximal to middle ear)

apex of the cochlea (most distal to the middle ear)

Question 23

What is mechanism of middle ear/basilar membrane in transducing sound?

Answer 23

Middle ear induces basilar membrane to vibrate (move up and down)
-basilar membrane moving up and down will cause the hair cells to move side to side because hair cells are connected to the tectorium membrane up top (going up will cause longest hair to tilt towards its direction…and going down will cause hair cells to go away from direction of long cell)

Question 24

How is frequency represented in CN 8?

Answer 24

Low frequency: follows periodic motion (vibratory motion) of the sensory hairs
-membrane depolarization follows pattern
High frequency: membrane depolarization is constant

Question 25

What is the stiffness gradient?

Answer 25

Characteristic of how stiff the basilar membrane is
Stiffer and more narrow basilar membrane vibrates at high frequencies (proximal cochlea)
Floppier and less narrow basilar membrane vibrates at low frequencies (distal cochlea
Referred to as “tonotopic organization”
-range of hearing has nothing to do with hair cells and it has everything to do with basilar membrane

Question 26

Timbre

Answer 26

the quality of a note or sound/tone that distinguishes different types of sound production (same frequencies but you notice it is being emitted from different instruments…)

Question 27

What is the concept of a labeled line?

Answer 27

• principle of frequency coding; it does not matter the PATTERN of frequency, but rather the position of basilar membrane that is vibrated
• this is especially significant in high frequency sound because the vibration is constant…thus you can’t tell the frequency of the motherfucker…therefore instead, the axon just by virtue of its position will tell CNS the type of frequency carried

Question 28

What is representation of intensity in CN 8 vs. basilar membrane?

Answer 28

The tuning properties of the hair cell is transferred directly to hair cell which is transferred directly to 8th nerve
The amount of intensity needed to drive a response are the same between basilar membrane and auditory nerve..

Question 29

What is process of hearing sound?

Answer 29

Frequency components of a sound activate specific regions of the basilar membrane which causes the associated hair cells to depolarize which generates action potentials in CN 8

• frequency components are then transformed into a representation of action potentials
• frequency components of a sound are transformed into a series of patterns of AP on different VIII fibers

Question 30

What is the function of outer hair cells?

Answer 30

fine tunes the membrane to allow you to focus your attention on something (e.g. the hook-up at a party analogy Cohen talked about in calss)

Question 31

How does outer hair cell carry out its function?

Answer 31

Contracts due to prestin
Outer hair cells are needed to allow you to finely tune high frequency sound
-that is the first thing to be damaged due to loud noise
-that’s the first type of hearing loss

Question 32

Prestin

Answer 32

motor protein of outer hair cell…so makes OHC contract

Question 33

So when you lesion outer hair cell

Answer 33

• you won’t be able to distinguish sounds
• you won’t be able to focus your hearing
• you will need someone to be shouting right at you to hear