Audition 1 Flashcards
Behavioral functions of audition
- Locate predator/prey
- Orient attention
- Identify/recognize
- Communicate
The auditory and visual systems look very different
The auditory and visual systems have a similar architecture
What is sound?
Audible fluctuations in air pressure
What are the physical dimensions of sound?
- Amplitude or intensity
- Frequency
- Speed of sound
Amplitude/intensity of sound
- The magnitude of changes in air pressure
- Units = decibels or dB
What is the unit for amplitude or intensity of sound?
Decibels/dB
Frequency of sound
- The number of cycles of air pressure change per second
- Units = cy/sec or Hertz (Hz)
What is the unit for frequency of sound?
Cy/sec or Hertz (Hz)
Speed of sound
About 340 m/sec (77mph)
Physical dimensions of sound
- Loudness
- Pitch
- Timbre
What is loudness based on?
Intensity of sound
What is pitch based on?
Frequency of sound
What is timbre?
- The distinctive character of a sound
- Sounds with the same pitch and loudness are perceived to be different based on the relative intensities of various frequencies (e.g. same note played on different musical instruments)
Intensity of various sounds
- Sound intensity is measured in decibels (dB)
- The decibel scale is logarithmic
- A 10dB change is a x10 change in intensity; it will sound about twice as loud
- Hearing damage is related to sound intensity, not loudness – damage from
Frequencies heard by different animals
What is fundamental frequency?
The lowest frequency produced
Timbre & fundamental frequency
- Each note has a “fundamental frequency” (the lowest frequency produced). This determines the pitch
we hear. - In addition, there are harmonics (higher frequencies).
- The harmonics and their timing determine the unique qualities of a sound made by a voice,
The human audiogram
Overview of the auditory system
Ear anatomy
Outer ear:
○ Pinna
○ Auditory canal
○ Tympanic membrane
Middle ear:
○ Ossicles
○ Eustachian tube
○ Oval window
Inner ear:
○ Cochlea
Components of the outer ear
- Pinna
- Auditory canal
- Tympanic membrane
Components of the middle ear
- Ossicles
- Eustachian tube
- Oval window
Components of the inner ear
Cochlea
Impedance matching
- Problem: Impedance mismatch occurs between the air in the outer ear and the fluid in the inner ear because they have very different resistances to sound waves
- Solution: The ossicles act as a mechanical lever system to match impedance between air and cochlear fluid
What are the two mechanisms through which impedance matching works?
Area difference
Lever action
Impedance matching- area difference
- The tympanic membrane has a much larger surface area than the oval window (where the stapes connects to the cochlea).
- Sound waves that hit the larger tympanic membrane are concentrated onto the smaller oval window, amplifying the force of the vibrations → like focusing sunlight with a magnifying glass
Impedance matching- lever action
- The ossicles act as a lever system.
- The malleus is longer than the stapes, giving a mechanical advantage that increases the pressure exerted on the oval window.
- This increases force without losing too much sound energy, allowing for efficient transmission of sound into cochlear fluid.
Transferring sound to the inner ear
- Movements of the ossicles transfer sound from the outer ear to the inner ear
- In response to movements in and out at the tympanic membrane, the stapes moves in and out at the oval window
Why bother with the ossicles? Why not have the tympanic membrane push on the
oval window and skip the ossicles?
Because the cochlea is filled with fluid. Without the ossicles, 99.9% of sound energy would be reflected off the eardrum and back into the environment. (when you are underwater it is hard to hear someone outside a pool)
Large amplitude, low pressure movements at tympanic membrane are
transformed into small amplitude, high pressure movements at oval
window:
* 1. Footplate of stapes (oval window) is 30 times smaller in area
han tympanic membrane
* 2. Force at the oval window is larger because of the lever action of the ossicles
* 3. Movements are miniscule! Ossicles move only a few nanometers at hearing threshold!
Attenuation Reflex
- Loud sounds (> 70dB) cause a reflexive contraction of the stapedius muscle (and tensor tympani in some species )
- Reflex in both ears even if sound in only one
- The ossicles become more rigid and movements at oval window are attenuated to protect the inner ear
- Unfortunately, this mainly works at low frequencies and relatively slowly (e.g. little protection against loud concerts, explosions, etc)
- This reflex also occurs when we speak, so we sound quieter to ourselves
- Fluid in the middle ear (e.g. you have a cold) can also dampen ossicle movements and impair hearing
Diagram of inner ear
Cross-section of cochlea
Structure of cochlea
- The cochlea is a tube wrapped about 2 ¾ times
- The basilar membrane and Reissner’s membrane partition the cochlea into 3 parts:
- scala vestibuli (scala = stairway)
- scala media (cochlear duct)
- scala tympani
What are the three parts into which the cochlea is separated?
- Scala vestibuli
- Scala media (cochlear duct)
- Scala tympani
Perilymph
- Scala vestibuli and scala tympani are filled with perilymph:
- Similar to other extracellular fluids:
[K+] = 7 mM
[Na+] = 140 mM
Endolymph
Scala media is filled with endolymph which has a very high K+ concentration for an extracellular fluid (more like an intracellular fluid):
[K+] = 140 mM
[Na+] = 1 mM
Function of stria vascularis
Aabsorbs Na+ and secretes K+ into
the scala media
Diagram of fluid-filled spaces of cochlea
Summary of fluid movement in the cochlea
- Sound causes oval window to push in
- Fluids in the cochlea move
- Round window bulges out
More detail on fluid movement in cochlea
- Oval window pushed in
- Fluid moves down scala vestibuli
- Perilymph pushes down on Reissner’s
membrane - This pushes down on endolymph in scala media
- Basilar membrane gets pushed down
- Fluid in scala tympani gets pushed out and
makes the round window bulge out
The fluid moves back and forth as the eardrum and
ossicles go in and out
The cochlea uncoiled (For simplicity, we ignore the scala media and focus on movements of the basilar membrane)
Stapes movement evokes a traveling wave on the basilar membrane
Different frequencies resonate at different locations on the basilar membrane:
- Low frequency: vibration greatest at apex
- High frequency: vibrations greatest at base
- If the frequency is too low, the membranes don’t move, and the fluid flows from scala vestibuli to scala tympani, going through the helicotrema (a sort of safety valve)
What is tonotopy?
- A place code in which sound frequency is mapped along the basilar membrane
- The cochlea is a ‘spectral frequency analyzer’
Basilar membrane vibrations and place code
Location of Organ of Corti
Sits on the basilar membrane
Info on hair cells
What do hair cells release onto auditory nerve axons?
Glutamate (excitatory transmitter)
Explain how basilar membrane vibrations bend hair cell cilia
- Up and down movements of basilar membrane cause sweeping motion of hair cell cilia
- Tips of outer hair cell cilia stick to tectorial membrane
- Tips of inner hair cells move in the endolymph
Tip links
Tip links connect hair cell cilia
Effect of cilia bending on mechanically-gated K+ channels
Cilia bending causes mechanically-gated K+
channels in the tips to open or close
Transduction depends critically on ___
Endolymph
Explain how transduction depends critically on endolymph
- Resting potential is a bit more positive because, at rest, about 25% of the K+ channels are open.
- [K+] is high in the endolymph and inside the hair cells no [K+] gradient at tips of stereocilia
- But the endolymph is about 140 mV more positive than the hair cell interior and this will push K+ inward. This is the largest electrical potential difference anywhere in the brain or body.
- The potential difference between endolymph and perilymph is called the endocochlear potential or the “cochlear battery”.
- In age-related degeneration of the stria terminalis, the [K+] concentration of endolymph is insufficient. Deafness results without the endocochlear potential.
Hair Cell Transduction
- Receptor potential: when the cilia move in one direction, more K+ channels open, the large potential difference pushes K+ into the cell, and it depolarizes. When the cilia move in the other direction, K+ channels close and the cell hyperpolarizes.
- Note that opening K+ channels has the opposite effect it does in neurons
- Depolarization triggers opening of Ca++ channels and the release of glutamate
- The auditory nerve fires action potentials proportional to the amount of depolarization/hyperpolarization
- Auditory transduction is faster (µsec) than would be possible with ion diffusion and faster than visual transduction that uses G-proteins.
Transduction and receptor potentials
Bending hair cells
- Slight bending of hairs causes large changes in membrane potential
- Maximal stereocilia bending ~1o
- Hearing threshold ~0.3 nm
Paradoxical connections of the auditory nerve
4,000 inner hair cells (1 row)
12,000 outer hair cells (3 rows)
50,000 axons in auditory nerve
95% synapse on inner hair cells
5% synapse on outer hair cells
one IHC connects to 10-20 auditory nerve cells
many OHCs connect to a single auditory nerve cell
Why?
Sound amplification by outer hair cells
Q. What are the more numerous outer hair cells doing?
A. They amplify the movements of the basilar membrane
- Outer hair cell membrane contains prestin protein. K+ entry activates the prestin “motor protein” which shortens the OHC
- The changing length of the OHC pushes and pulls the basilar membrane
How do outer hair cells amplify vibrations of the basilar membrane?
The cochlear amplifier changes hair cell shape
Otoacoustic emissions
Sounds generated by the ear
* Can be either evoked or spontaneous
* Spontaneous emissions occur in over 50% of people
* In rare cases, they are loud enough to be heard by others
Evoked otoacoustic emissions
We could produce otoacoustic emissions by electrically stimulating outer hair cells:
1. Depolarize outer hair cells
2. Motor proteins expand and contract
3. Basilar membrane moves up and down
4. The fluid movement moves the oval window, ossicles, and tympanic membrane pushing sound out of the ear (the ear drum becomes a speaker)
We can also produce otoacoustic emissions by playing a pure tone or click into ear. A microphone picks up the stimulus going in plus anything added by the cochlear amplifier (that is the otoacoustic emission)
Otoacoustic emissions are used to test hearing
- Sounds are played into the ears of newborns and a microphone records the sound
- This tests cochlear function long before a
hearing deficit could otherwise be detected,
allowing early intervention
Effects of raising sound intensity
Raising sound intensity causes increases in:
* Amplitude of basilar membrane vibrations
* Hair cell depolarization and hyperpolarization
* The number of activated hair cells and neurons
* Firing rates of subsequent neurons
Localizing sounds in space
- Azimuth: horizontal angle or
direction (1-2 deg accuracy) - Elevation: vertical angle or direction (4 deg accuracy)
Bilateral auditory projections
- Starting at the superior olive, ascending auditory structures get input from both ears (i.e. binaural input)
- Inputs from two ears are combined for sound localization
- Like inputs from two eyes are combined for depth perception
Locating sound azimuth at high frequencies
- At higher frequency sounds (>1.5 KHz), the head casts a “sound shadow”
- We infer sound direction from differences in intensity between the left and right ears and move our heads around to aid this process.
Locating sound azimuth at low frequencies
Lower-frequency sounds do not produce a sound shadow because of diffraction. Direction is determined by the time (or phase) difference between sound waves reaching the two ears (i.e. the “interaural delay” or “interaural time difference”)
- Given the width of the head and the speed of sound, it takes
about 660 microseconds for sound to move from one ear to the other - Humans can detect time differences down to about 10-30 microseconds
- Experiments in guinea pigs found that behavioral performance
is matched by the sensitivity of neurons in the inferior colliculus.
Note that sound sources ahead vs. behind at any frequency can be ambiguous as to direction (timing and sound shadow are equal in the two ears)
Locating sound elevation
- Locating the elevation of a sound depends on the timing of reflections off the surfaces of the pinna.
- Moving the head around is also very important for localizing elevation
Localizing Sound Azimuth Summary
- High frequencies: interaural intensity difference
- Low frequencies: interaural time difference
- Sound sources ahead vs. behind at any frequency can be confused because sounds arrive at the two ears with same intensity and timing
- Hearing loss in one ear disrupts horizontal localization. If you plug one ear, you will localize sounds toward the open ear
Brainstem coincidence detectors respond to interaural time difference
- Sound from the left initiates activity in the left cochlear
nucleus. Activity is then sent to the superior olive - Shortly later, the sound reaches the right ear,
activating the right cochlear nucleus - Both inputs reach neuron 3 at the same time (the
neuron detects “coincidence” based on the delay from
the right ear) and an action potential is generated - The neuron activated indicates the left/right sound
location
Sound azimuth localization mechanisms
Starting in the cochlear nuclei and in subsequent
structures, neuron responses depend on:
* Interaural intensity difference
* Interaural time difference
Coincidence detectors in the superior olive
Primary auditory cortex (A1):
* Many cells not affected by sound location
* Others prefer sounds to one side or near
the midline
* Inactivating A1 disrupts sound localization
* Areas beyond A1 more sensitive to location
But there are no maps of auditory space in this
pathway (there is in the superior colliculus – recall
sound draws attention and the eyes move
Locating sound elevation
- Locating the elevation of a sound depends on
the timing of reflections off the surfaces of the
pinna. - The pinna alters the intensities of different sound
frequencies reaching the ear drum – “spectral
shaping”. Note shifting notches (arrows) - Filling the pinna folds disrupts vertical sound
localization. - We learn our own ears!
- Artificially “using
someone else’s ears” also disrupts elevation
estimation (head transfer functions)
Sound elevation localization mechanisms
- Neurons in the MGN and A1
are sensitive to sound elevation - This is probably based on
sensitivity to spectral shape
(i.e. the ratio of different sound frequencies)
Some owls hunt in darkness with precise azimuth and elevation information
Function of ossicles
Amplify and transfer vibrations from the tympanic membrane to the oval window of the cochlea
Function of Eustachian tube
Equilibriates air pressure in the middle ear so that it matches the outer ear
Function of oval window
Transfers vibrations from the middle ear to the inner ear
Location of inner hair cells
Between modiolus and rods of Corti
Organization of inner hair cells
~4k in a single row
The stereocilia of IHCs extend into the ___
Endolymph
IHCs make up __% of input to spiral ganglion neurons
95%
Describe IHC connection to neurites
A single IHC synapses onto multiple neurites (one neurite gets input from one IHC)
Purpose of IHCs
Auditory transduction
Location of outer hair cells
Farther than rods of Corti
Organization of OHCs
12-20k in 3 rows
The stereocilia of OHCs terminate in the ___
Tectorial membrane
OHCs make up __% of input to spiral ganglion neurons
5%
Describe OHC connection to neurites
Multiple OHCs synapse onto a single neurite (one neurite gets input from multiple OHCs)
What is the purpose of outer hair cells?
Cochlear amplification
Which of the ossicles is attached to the tympanic membrane?
Malleus
Endolymph has all the following properties EXCEPT
a) has a lower voltage relative to perilymph
b) bathes the stereocilia atop hair cells
c) has a high concentration of potassium
d) has ionic concentrations similar to intracellular fluid
a) has a lower voltage relative to perilymph
The following statements about sound localization are correct EXCEPT:
a) high frequency sounds do not generate sound shadows
b) plugging one ear will degrade the perception of sound azimuth more than sound elevation
c) interaural delay is most useful for localizing lower frequency sounds
d) sound reflections off the bumps and ridges on the pinna appear to be involved in estimating
elevation
a) high frequency sounds do not generate sound shadows
Inner hair cells
a) Have motor proteins that amplify movements of the basilar membrane
b) Are innervated by many more auditory nerve fibers than outer hair cells
c) Are organized into 3 rows in the organ of Corti
d) Sit atop Reissner’s membrane
b) Are innervated by many more auditory nerve fibers than outer hair cells
Which of the following structures is most medial
a) Incus
b) Malleus
c) Oval window
d) Stapes
e) Tympanic membrane
c) Oval window
Mechanical coupling in the middle ear transforms movements produced by sound in which
way?
a) Increases the amplitude and energy/pressure
b) Increases the amplitude and decreases the energy/pressure
c) Decreases the amplitude and increases the energy/pressure
d) Decreases both the amplitude and the energy/pressure
c) Decreases the amplitude and increases the energy/pressure
Decreases the amplitude of the vibrations: The large surface area of the eardrum (tympanic membrane) is reduced to the much smaller surface area of the stapes footplate, which contacts the oval window. This reduction in surface area means that the amplitude of the vibrations decreases.
Increases the pressure: By focusing the energy onto a smaller area (from the tympanic membrane to the oval window), the middle ear increases the pressure of the sound wave entering the cochlea. This is essential because it helps overcome the impedance difference between air (in the outer ear) and fluid (in the inner ear), allowing effective transmission of sound energy into the cochlea.
The endocochlear potential is the voltage difference between ___ and ___
Endolymph and perilymph
The attenuation reflex works by minimizing movement of the ___
Ossicles
The following statements about outer hair cells in the cochlea are all correct EXCEPT
a) Outnumber inner hair cells
b) Are innervated by more auditory nerve axons than inner hair cells
c) Make us more sensitive to sound than if they were not present
d) Are involved in otoacoustic emissions
(b) Are innervated by more auditory nerve axons than inner hair cells
Are there more outer hair cells or inner hair cells?
More outer hair cells
The cochlear amplifier
a) Amplifies sound by the action of motor proteins in the basilar membrane
b) Is based on changes in the length of inner hair cells
c) Increases movements of the basilar membrane
d) Selectively increases our sensitivity to the highest frequency sounds but not to lower
frequency sounds
c) Increases movements of the basilar membrane
(a) Amplifies sound by the action of motor proteins in the basilar membrane
This statement is incorrect. The motor proteins involved in the cochlear amplifier are not in the basilar membrane itself; they are located in the outer hair cells. The motor protein prestin in the outer hair cells enables them to contract and expand, which actively amplifies movements of the basilar membrane
Where is prestin located?
In outer hair cells
Reissner’s membrane
Between scala vestibuli and scala media
Your roommate has a pair of ear buds and you have hacked into them so you can independently
control the amplitude of sound in each ear. To most accurately simulate the real situation in
which a sound comes from the right side, you would
a) Use a 500 Hz frequency sound and make it louder in the left ear
b) Use a 3000 Hz frequency sound and make it louder in the left ear.
c) Use a 500 Hz frequency sound and make it louder in the right ear
d) Use a 3000 Hz frequency sound and make it louder in the right ear
(d) Use a 3000 Hz frequency sound and make it louder in the right ear
Low-frequency sounds (e.g., around 500 Hz) are localized primarily through interaural time differences (ITD), where the brain detects the slight delay between the sound reaching each ear.
High-frequency sounds (e.g., around 3000 Hz) are localized through interaural level differences (ILD), where the head casts a “sound shadow,” making the sound quieter in the ear opposite to the source. This difference in loudness is most prominent at higher frequencies.
Inner hair cells
a) Are more numerous than outer hair cells
b) Have motor proteins that make the cells change in length
c) Provide a larger fraction of the input to the auditory than outer hair cells
d) Are located between the tectorial membrane and Reissner’s membrane.
c) Provide a larger fraction of the input to the auditory than outer hair cells
Which ossicle is directly attached to the tympanic membrane?
Malleus
Which of the following is a
sound level you might encounter in your daily life that can damage your hearing?
a) 1 dB
b) 10 dB
c) 100 dB
d) 1000 dB
c) 100 dB
The organ of Corti sits upon the ___
Basilar membrane
The following statements about transduction in auditory hair cells are all correct EXCEPT
a) Calcium entry into the cell body is required for transmitter release
b) Hair cells both depolarize and hyperpolarize in response to sound waves
c) Auditory transduction is faster than visual transduction
d) There is a large driving force pushing potassium out of hair cell stereocilia
d) There is a large driving force pushing potassium out of hair cell stereocilia
The tonotopic map present in the basilar membrane is a result of
a) variations in the physical property of stiffness along the basilar membrane
b) variations in the frequency sensitivity of inner hair cells along the basilar membrane
c) variations in the frequency sensitivity of outer hair cells along the basilar membrane
d) the musical ability of a tiny little horse named Peter, who plays your basilar membrane like a
marimba.
b) variations in the frequency sensitivity of inner hair cells along the basilar membrane
The structure that allows very low frequency sound waves to flow back and forth in the fluid
between the scala vestibuli and the scala tympani without moving the basilar membrane is the
a) middle ear
b) helicotrema
c) oval window
d) auditory canal
b) helicotrema
Depolarization of hair cells results from
a) Entry of potassium at mechanically-gated channels
b) Entry of potassium at voltage-gated channels
c) Entry of sodium at mechanically-gated channels
d) Entry of sodium at voltage-gated channels
a) Entry of potassium at mechanically-gated channels
Mechanically-gated because bending of stereocilia!
What would most likely happen if the helicotrema was completely closed?
a) The basilar membrane would no longer move when the membrane at the oval window pushed
inward
b) Hair cells would bend significantly more in response to a high frequency sound than with an open helicotrema
c) The basilar membrane would bend more in response to very low frequency sound than with an open helicotrema
d) The cochlear amplifier would maintain the same hair cell movements that occur with an open
helicotrema
c) The basilar membrane would bend more in response to very low frequency sound than with an open helicotrema
The helicotrema is a small opening at the apex of the cochlea that connects the scala vestibuli and scala tympani, two fluid-filled chambers. Its primary role is to allow low-frequency sound waves to bypass the narrow basilar membrane by traveling from the scala vestibuli to the scala tympani without significantly moving the basilar membrane at low frequencies. This effectively reduces the responsiveness of the cochlea to very low frequencies, preventing them from displacing the basilar membrane as much as higher frequencies do.
