Lecture 18 - Ear Anatomy/ Auditory Transduction

Question 1

equal loudness curve

Answer 1

we don’t hear all frequencies equally, so we don’t have the same threshold for all frequencies

to get an idea of how this relates to perceived magnitude you create an ELC, you start with a standard stimulus and then another stimulus and try to judge whether they’re equal

Question 2

physical signal (pressure waves) collected by

Answer 2

pinna and resonates in the inner ear canal

Question 3

Middle ear muscles can dampen….

Answer 3

…..the ossicles’ vibrations to protect the inner ear from

potentially damaging stimuli.

Question 4

Stapes is pulled away from…..

Answer 4

…..the oval window to

keep from transmitting all of the pressure wave.

Question 5

acoustic reflex of the ossicles

Answer 5

can reduce transmission of loud sounds by 20 dB, protecting the inner ear.

similar to the pupillary light reflex

Question 6

main structure is the cochlea

Answer 6

– Fluid-filled snail-shaped
structure (35 mm long) set into
vibration by the stapes

– Sound waves are transduced (changed from physical, mechanical waves) into neural signals within the cochlea.

– Signals will be transmitted out
of the cochlea via the auditory
nerve (comprised of axons called spiral ganglion cells).

Question 7

transduction takes place in the

Answer 7

cochlea (fluid filled)

Question 8

The __________ pushes against the ___________, transmitting

atmospheric vibrations to the inner ear.

Answer 8

stapes, oval window

Question 9

Structural organization of the

cochlea

Answer 9

– Hard, boney structure

– Divided into chambers: the scala vestibuli (above, where it starts) and scala tympani (below) by the cochlear partition

• the round window bulges out a bit to maintain a balanced pressure as the stapes pushes on the oval window

– Cochlear partition extends from 
the base (stapes end) to the apex (far end - the "top")

– Organ of Corti contained by
the cochlear partition (scala media)

Question 10

organ of corti

Answer 10

contains all of the cells that are crucial for transduction

extends all the way through the cochlear partition

sits on top of the basilar membrane

Question 11

basilar membrane

Answer 11

the floor of the cochlear partition

as the pressure wave goes through, it’s going to make the basilar membrane bounce (vibrate) up and down

Question 12

tectorial membrane

Answer 12

vibrates side to side as the basilar membrane vibrates up and down

makes contact with the inner hair cells

Question 13

hair cells

Answer 13

are doing the transduction

inner and outer

in the organ of corti in the basilar membrane

Question 14

inner hair cells

Answer 14

most responsible to changing (transducing) the physical stimulus to neural signal

Question 15

for each cochlea there are _____ inner hair cells and _____outer hair cells

Answer 15

3,500

12,000

Question 16

cilia

Answer 16

how transduction takes place:
- bend with the combined movement of the basilar and tectorial membrane

- the contact triggers the hair cells to fire: creating an action potential 

on the end of hair cells bending in response to movement of the organ of Corti (as the basilar membrane move up and down) against the tectorial membrane.

Question 17

Basilar membrane vibrates in response to….

Answer 17

….sound and supports the organ of Corti

Question 18

Transduction and the cilia

Answer 18

Movement of inner hair cell cilia in
one direction opens ion channels (influx of positive ions triggers action potential).
• This process causes a release
of neurotransmitters.

• These bind to the auditory nerve fiber, causing depolarization (action potentials)

– Movement in the other direction (of the two membranes) closes the channels (closes the tip links)

this is all INNER HAIR CELL STUFF

Question 19

outer hair cell

Answer 19

are involved in transduction by amplifying transduction,

they themselves do not create the signal

Question 20

Two ways nerve fibers signal frequency:

two theories:

Answer 20

1. Which/where nerve fibers are responding

• Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers.
• Place Theory: what place on the spiral basilar membrane is being activated in associated with which frequencies

2. How/when fibers are firing

• Rate or pattern of firing of nerve impulses.
• Temporal coding

Question 21

Békésy’s place theory of hearing

PLACE THEORY

Answer 21

Frequency of sound is indicated by the place (of hair cells) along the cochlea that has the highest firing rate.

• Békésy determined this in two ways:

– Direct observation of the vibrational properties of the basilar membrane in cadavers.

– By doing this in different locations on the basilar membrane he built a model of the cochlea using the physical properties of the basilar membrane.

Question 22

Base of the membrane (by stapes) is:

place theory

Answer 22

• Three to four times narrower than at the apex.

* 100 times stiffer than at the apex.

Question 23

Both the model (from the creep Békésy) and direct observation showed

place theory

Answer 23

showed that the vibrating motion of the membrane is a traveling wave.

Question 24

Envelope of the traveling wave

place theory

Answer 24

to best capture the frequency of the wave coming in:

– Most of the basilar membrane vibrates in response to a traveling wave, but the displacement is largest in one place.

– The envelope shows the entire displacement caused by a traveling wave and its peak (maximum).

– Hair cells at this maximum point are stimulated the strongest (than any other hair cells), leading to the nerve fibers firing the most at this location.

– Position of the peak (P) is purely a function of frequency.

Question 25

Envelopes at different frequencies
show different maximum vibration
points. This indicates…..

place theory

Answer 25

…..the place where most hair cell activity is predicted.

Question 26

Low frequencies activate toward

place theory

Answer 26

the apex

Question 27

high frequencies activate hair cells near

place theory

Answer 27

the base

envelope closer to the base than the apex

Question 28

If I play a pure tone at 100 Hz, it will be transduced by

__________ hair cells nearest the _________ of the cochlea.

Answer 28

inner, apex

Question 29

Cellular recordings
[implementation level] have
supported Place Theory.

Answer 29

recording from one spot in the apex: play a low sound and the cell responds a lot, play a high sound and the cell doesn’t respond or responds very little

– Cochlea shows an orderly map
of frequencies along its length

• Apex responds best to low
frequencies

• Base responds best to high
frequencies

– This is called a tonotopic map.

guinea pigs!

Question 30

tonotopic map

place theory

Answer 30

responses to characteristic frequencies that activate specific places on the cochlea

similar frequencies are represented in brain

Of or being a structural arrangement, as in the auditory pathway, such that different tone frequencies are transmitted separately along specific parts of the structure.

Question 31

Physiological support for place theory: Neural frequency tuning curves

Answer 31

Pure tones are used to determine the threshold for specific frequencies measured at single neurons.

– Record action potentials from one cochlear neuron and play tones at different frequencies.

– Plotting thresholds for frequencies results in tuning curves.

– Frequency to which the neuron is most sensitive is the characteristic frequency.

Question 32

Psychophysical support for place theory:

auditory masking experiments

Answer 32

– First, thresholds for a number of frequencies are determined.

– Then, an intense masking frequency is presented at the same time that the thresholds for the original frequencies are re-determined.

– The masking effect (raised
threshold) is seen at the masking
tone’s frequency and spreads to
higher frequencies more than lower ones.

– greatly affects the tone closes the the mask and the ones right above it

Question 33

Masking effects spread to….

Answer 33

higher frequencies more than

lower ones.

Question 34

The masking effect matches

the predicted …

Answer 34

….envelope for higher frequencies tones.