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
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
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?
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
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)
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
Describe IHC connection to neurites
A single IHC synapses onto multiple neurites (one neurite gets input from one IHC)
Location of outer hair cells
Farther than rods of Corti
The stereocilia of OHCs terminate in the ___
Tectorial membrane
Describe OHC connection to neurites
Multiple OHCs synapse onto a single neurite (one neurite gets input from multiple OHCs)
Which of the ossicles is attached to the tympanic membrane?
Malleus
Which of the following structures is most medial
a) Incus
b) Malleus
c) Oval window
d) Stapes
e) Tympanic membrane
c) Oval window
Reissner’s membrane
Between scala vestibuli and scala media
