27. ear

Question 1

3 parts of outer ear?

Answer 1

• pinna (ear shaped part)
• auditory canal
• tympanic membrane

Question 2

4 parts of middle ear?

Answer 2

Ossicles (mini bones):
- malleus
- incus
- stapes
–> stapes hits oval window to transmit to cochlea

• eustachian tube

Question 3

2 Parts of inner ear?

Answer 3

• Oval window
• Cochlea

Question 4

Why do we need the bones in the middle ear?

Answer 4

• to amplify the vibrations that come from the auditory canal

Question 5

Why do we need to amplify the vibrations?

Answer 5

• the vibrations must be amplified in the middle ear because there is a density change from air to fluid in the inner ear
• without the amplification (approx 30 dB), everything would be really muffled

Question 6

What other structure helps with the amplification?

Answer 6

The tympanic membrane is 15-20x larger than the oval window, so sound energy is much more concentrated when it gets to cochlea

Question 7

What are the 3 chambers of the cochlea? What 2 membranes separates them?

Answer 7

• Vestibular canal
• Cochlear duct
• Tympanic canal

Reissner’s membrane separates Vestibular and Cochlear

Basilar membrane separates Cochlear and Tympanic

Question 8

What are the 2 types of fluid that fill the cochlea? Which fluids are in which chambers?

Answer 8

• perilymph
  –> vestibular canal
  –> tympanic canal
• endolymph
  –> cochlear duct –> electrochemical properties that facilitate transduction

perilymph –> both the side ones
endolymph –> one important one

Question 9

Basilar membrane def? What does it respond to? What does it do?

Answer 9

• membrane between the walls of cochlea, separating cochlear duct and tympanic canal
• thicker, narrower, and stiffer at BASE than at apex
• each section responds most strongly to different frequencies
• reflects the traveling sound waves in the perilymph
• its displacement moves the hair hairs, which starts the process of transduction

Question 10

Basilar membrane:
- characteristics from base to apex?
- frq most sensitive at each point?
- memory cue?

Answer 10

BASE:
- thicker, narrower, stiffer
- HIGHER frq

APEX:
- thinner, wider, floppier
- LOWER frq

Memory cues?
- uptight at the start, then loosens up a bit
- guitar strings –> the tighter (stiffer) they are, the more high frequency it will be
- the high will fade over time… :)

Question 11

Characteristic frequency (of basilar membrane) def?

Answer 11

• The frequency to which each location on the basilar membrane responds most readily
• each location has a characteristic frequency, the frq it’s most sensitive to

Question 12

Displacement envelope meaning? Which side is wider for high frq? Low frq?

Answer 12

• the shape of the displacement of the basilar membrane from base to apex over time
• higher frq –> wider by base
• lower frq –> wider by apex

Question 13

Organ of Corti:
- where is it located?
- what 3 important things does it consist of?

Answer 13

• within the cochlear duct, resting on the basilar membrane

Consists of:
- inner hair cells
- outer hair cells
- tectorial membrane

Question 14

Inner hair cells def / purpose?

Answer 14

• Neurons in the organ of Corti
• responsible for auditory transduction
• connected to Type I nerve fibers

Question 15

Outer hair cells def / purpose?

Answer 15

• Neurons in the organ of Corti
• serve to amplify and sharpen the responses of inner hair cells
• connected to Type II nerve fibers

Question 16

Tectorial membrane

Answer 16

A membrane that lies above the hair cells in the organ of Corti

Question 17

Auditory nerve def?

Answer 17

• Conveys signals from the hair cells in the organ of Corti to the brain
• made up of Type I and Type II auditory nerve fibers bundled together

Question 18

Tip links def? On what kind of cells? What do they do?

Answer 18

• fibers that connect the top ends of adjacent cilia (hair cells)
• BOTH types of hair cells
• when the hair cells bend, the distance between them increases, and the tip links get really tense
• this opens up membrane channels for potassium and calcium, causing depolarization

Question 19

Depolarization response in inner hair cells vs outer hair cells?

Answer 19

Inner:
- release NTs
- action potentials in Type I nerve fibers
- starts transductions

Outer:
- results in change of shape of proteins
- makes the cell and cilia stretch
- called motile response

Question 20

Motile response def? Relationship to activity on basilar membrane?

Answer 20

• A response by outer hair cells that magnifies the movements of the basilar membrane, amplifying sounds and sharpening the response to particular frequencies
• fine-tunes / specifies info from basilar membrane, especially for LOW frq

Question 21

What 2 ways is frequency represented on the basilar membrane?

Answer 21

• place coding
• temporal coding (timing)

Question 22

Place coding def?

Answer 22

• Frequency representation based on the displacement of the basilar membrane at different locations
• knowing where the max. displacement is can tell you what the frq is

Question 23

What did von Bekesy do?

Answer 23

• discovered the physical basis for place coding
  –> ie. that the basilar membrane changes in stiffness, which determines its response to frequencies
• he also determined envelopes of displacement

Question 24

Characteristic frequency (of type I nerve fiber) def? Accounted for by…?

Answer 24

• the frequency to which the auditory nerve fiber is most sensitive
• can be accounted for by the frequency tuning of the basilar membrane

Question 25

Frequency tuning curves for nerve fiber and basilar membrane. What does the graph show? What's the important point? Why is this important?

Answer 25

- shows the sound level (dB SPL) needed to produce a response in that nerve / location for each frequency --> generally higher as its farther away from characteristic frq - the lowest point is the characteristic frequency - the 2 curves directly mirror each other, which suggests that the tuning curve for the nerve fibers can be accounted for by the tuning curve for the basilar membrane

Question 26

Frequency tuning curves shape? Which direction?

Answer 26

- they're ASYMMETRIC - they extend more towards LOWER frequencies - ie. a nerve fiber will respond to a lot of frqs _lower_ than its characteristic frequency, but not that many that are _higher_ than its characteristic frq

Question 27

Frequency tuning curve: what shapes for lower and higher frequencies? What does this tell us?

Answer 27

- lower frqs have much broader curves - higher frqs have much narrower curves - lower frequencies need more information from the outer hair cells to specify frq

Question 28

Relationship between tuning curves and masking effects?

Answer 28

- tuning curves are ASYMMETRIC towards LOW frqs Therefore... - masking effects extend MORE towards HIGHER frequencies - a _low_ frq masks a _high_ frq WELL - a _high_ frq masks a _low_ frq BADLY

Question 29

Temporal coding def? 2 ways?

Answer 29

- frq representation based on firing rates of auditory nerve fibers 1. firing matches frequency for some low frqs (up to a few hundred Hz) 2. Phase synchronization (locking) extends the range (up to 4000-5000 Hz)

Question 30

Volley principle def? For what issue?

Answer 30

- for phase synchronization with temporal coding for frequency - no cell can fire action potentials at every peak, but in a _population_, they can match the rate of the frequency - individual cells fire in phase with peaks of sound wave, but not at every peak Book def: - idea that each nerve fiber in a population of auditory nerve fibers produces action potentials in phase with the peaks in the incoming sound wave, even if not at every peak; explains how a temporal code could represent frequencies much higher than the maximum firing rate of any individual fiber

Question 31

How is amplitude represented / coded?

Answer 31

- individual fibers are _ambiguous_ - info ab amp comes from which nerves and how many are responding - look at frq tuning curves - if you know frq, and you know which ones are responding, you can determine a RANGE of amplitudes it could be

Question 32

How will a cells response to its characteristic frequency change with a change of amplitude?

Answer 32

- for different nerves, their response to the characteristic frq VARIES with amplitude - if you have 2 nerves with the SAME ChFrq, but DIFFERENT thresholds and diff responses... - they'll have a DIFFERENT firing rate at the SAME amplitude - so, you can know a range for the amplitude

Question 33

Dynamic range def? (for a nerve fiber) What is it's general shape? What are the two _ends_ of the dynamic range called?

Answer 33

- the range of amplitudes over which the _firing rate_ of the fiber changes - generally an S shape - threshold at the beginning --> amp. where the nerve starts to fire above baseline - SATURATION LEVEL at the end

Question 34

Saturation level def?

Answer 34

- the amplitude where a fiber reaches its MAXIMUM firing rate - no matter how loud the sound is, the fiber won't fire any faster