Audition Flashcards
Why must we have ways beyond phase locking to encode frequency of sound?
an action potential lasts 1ms, so we can only enocde 400-500Hz with action potentials
What frequencies does human speech span?
300Hz to 3000Hz
Air conduction uses which part of the ear?
the middle ear
Bone conduction bypasses what part of the ear?
the middle ear
Why do hair cells both hyperpolarize and depolarize?
because air pressure both increases and decreases, so must be able to encode both in the sensory receptors
Why do we need amplification mechanisms in the ear?
sound doesn’t travel well through water, so without amplification most of the signal would be lost
Definition of frequency; perception of frequency
number of waves per second; perception of pitch
Definition of amplitude; perception of amplitude
height of the wave; perception is intensity/loudness
Speed of sound
wavelength x frequency
Why can sound be used to locate stimuli?
because it is slow, so we can “measure” the time it takes to get to either ear, also can measure difference in loudness (intensity) between the ears; can’t use light because it is too fast
What is fourier transform?
the breaking of complex signals into its components (ie into component frequencies that make up a complex sound)
What in the ear performs physical Fourier transform?
basilar membrane
What is the place code of the ear?
axons carry a pulse code of depolarization from each region of the bailar membrane, resulting in a tonotopic map
Structures and their functions of the external ear:
- pinna- help collect sound, also aids in localization because sound hits pinna differently when it comes from different locations
- ear canal- has resonance so collects certain sounds better than others
Structures and their functions of the middle ear:
- tympanic membrane: pushes on bones of middle ear in response to sound waves coming into through ear canal
- bones of middle ear (malleus, incus, stapes): vibrate in response to tympanic membrane movement, help amplify sound because its a lot of pressure onto the tiny area of oval window
- round window- moves in opposition to oval window, allows energy from sound waves to disappate from the cochlea
Structures and functions of inner ear:
- oval window: stapes pushes on this, which causes fluid in cochlea to move
- basilar membrane: vibrates due to movement of cochlear fluid, displacing hair cells
Overview of sound transduction steps:
- sound eaves vibrate tympanic membrane
- ossicles vibrate
- oval window vibration results in fluid movement in vestibular duct
- basilar membrane movement and modulation of hair cell transmitter release
- modulation of sensory neuron firing
Gain control involving the osicles
muslces in the ear can increase joint stiffness so the bones move less, resulting in less energy getting from middle ear to inner ear; allows us to accomplish both high sensitivity and wide dynmaic range
Conductive hearing loss
sound energy doesn’t doesn’t get through to the oval window, problem with osiccles, punctured tympanic membrane, etc
Sensorineural hearing loss
signal gets through, but have loss of the neurons invovled in hearing
Eustachian tube
- connects to the pharynx
- equalizes pressure between external ear/environment and middle ear
Cochlea
region of the inner ear where sound waves are first converted into fluid waves, then into chemical signals, and finally into action potentials
What are the three fluid filled chambers of the inner ear?
vestibular duct, cochlear duct, and tympanic duct
Where are hair cells located?
All are in the basilar membrane, some with hairs that stick up into the tectorial membrane, others with hairs that dont stick into the tectorial membrane
Do hair cells have action potentials?
No, their graded potenitals directly influence the release of neurotransmitter, which directly influences the firing of the neurons that compose the cochlear nerve
Outer hair cell stereocilia are in what structure?
the tectorial membrane
Fluid movement in the inner ear moves the _____
basilar membrane
Basilar membrane movement displaces ______
hair cell bundles
If the basilar membrane moves up, what direction do stereocilia bend? What is the consequence of this?
Stereocilia bend outward toward the kinocilium and the hair cell depolarizes
If the basilar membrane moves down, what direction do stereocilia bend? What is the consequence of this?
Stereocilia bend inward, away from the kinocilium, and the hair cell hyperpolarizes
What does the movement of stereocilia do?
opens or closes K+ ion channels via tip links
Outward bending of stereocilia ____ the tip link, ____ channels, resulting in _____
stretches the tip link, opening channels, resulting in depolarization (cell becomes more positive)
Inward bending of stereocilia _____ the tip link, ____ channels, resulting in _____
relaxes the tip link, closes the channels, resulting in hyperpolarization (cell becomes more negative)
What bathes the stereocilia of hair cells? What are the characteristics of this fluid?
Endolymph, very high in K+
What bathes the “body” of hair cells? What are the characteristics of this fluid?
Perilymph, its like normal CSF
What is the endocochlear potential relative to perilymph?
+80mV
What is the transmembrane potential of hair cells?
-120mV
What is the result of the large difference in K+ concentration between the inside of the hair cell and the outside of the hair cell?
creates a large driving force for K+ and thus a large receptor potential of 25mV
What results from the K+ influx into the hair cell?
It opens voltage-gated Ca channels, which then induces neurotransmitter release
What are the transducers of the auditory system?
hair cells
How are hair cells like photoreceptors?
Both are the transducers of their system and neither projects to the CNS or has action potentials
When a hair cell hyperpolarizes, what happens to neurotransmitter release?
it decreases (so less transmitter is released)
What is the consequence of tip links being able to move through the membrane?
makes the cells more or less excitable by altering the strain on the tip links and so is a form of gain control
How do larger oscillations in air pressure impact the basilar membrane?
result in larger oscillations of the basilar membrane and a bigger/more spread out wave on the basilar membrane
What is a rate code?
an increase in firing rate; increasing intensity results in increased firing rate
What is a population code?
increase the number of neurons firing
What is the senistivity of a hair cell?
on the order of Brownian motion, .3nm movement of hair bundle
What is the operating range of a hiar cell?
120dB (so from 1 to 10^6)
List the mechanisms that contribute to hair cell sensitivity
- accessory structures
- temporal and spatial averaging
- specialized extracellular environment/endocochlear potential
- multiple hair cell specializations in transduction
- motor proteins as mochlear amplifiers
What does signal averaging do?
improves the auditory singal-to-noise ratio; because of this we can detect signals that would otherwise be undetectable in isolation
The cyclic nature of the auditory signal allows for
temporal averaging of responses over several cycles
The wave-like movement of the basilar membrane allows for
spatial averaging
How does the different extracellular environments of apical and basal faces contribute to sensitivity of hair cells?
There is a large electrical response in the cell from a very tiny movement due to this large K+ gradient; the driving force is 2x more than usual
How are endolymph and perilymph kept separate?
by tight junctions
Do hair cells release transmitter at rest?
Yes!
Why would hair cells release transmitter at rest?
- this allows for modulation of transmitter release rather than a threshold
- allows for bi-directionality (increase or decrease nt release)
- is well-suited for the auditory stimulus which is compression and rarefaction of air
Hair cell afferent specializations (presynaptic dense body)
may be involved with unusally rapid release in response to minimal stimulation
Why is it important for the auditory transduction mechanism to be direct and mechanical?
biochemical cascades are slow
Hair cell specializations:
- direct, mechanical transduction mechanism
- tonic release of neurotransmitter
- different extracellular environments of basal and apical faces of hair cells
What contributes to the wide dynmaic range of hair cells?
- parallel pathways (high and low-threshold primary afferents)
- adaptation
- attentuation reflex
- efferent neural control
Outer hair cell convergence
8 OHCs converge onto 1 primary fiber
Inner hair cell divergence
1 IHC to 10 primary fibers
IHCs carry information about ____
sound
Primary afferents located near the cochlear spiral have
low sensitivity and low spontaneous firing rate
Primary afferents located distal to the cochlear spiral have
high sensitivity and high spontaneous firing rate
Parallel channels of auditory primary afferents have
different sensitivities and different dynamic ranges
Adaptation of mechanoelectiral transduction/shift in hair cell sensitivity
hair cells have an initial phasic increase in responding to a stimulus, then reduce firing to a plateau, then have a post-offset response below baseline
- decreased response to a maintained stimulus but still show sensitivity to transient stimuluation
- implies a shift in range of sensitivity
Molecular model of hair cell adaptation (how strain on tip links is impacted by Ca)
- Ca activates calmodulin, which decreases activity of myosin ATPase
- this causes the tip link to migrate downwards on the kinocilium (towards height of shorter stereocilia), decreasing tip link tension and so decreasing channel opening
How does the auditory system have gain before even reaching the hair cells?
muscles in the ear control how much the oscilles can move, thereby altering the gain of the system
Ear attenuation relfex to high intensity stimuli
-contraction of middle ear muscles dapens movements of ossiclles, decreasing sensitivity
Why is the ear attenuation relfex bad for extrinsic explosive sounds, but good for chronic sounds?
there is a 50-100ms delay in dampening the movement of the middle ear bones and so it is too late to help in response to an explosive sound, but it works with sounds that are ongoing because 50-100ms doesn’t make much of a difference at that point
If mechanism X leads to increased gain and so increased sensitivity, how does one increase the dynamic range?
vary the gain
Depolarization evokes contraction of _____ motor proteins
OHC
How does movement of OHC motor proteins amplify cochlear mechanics?
Because OHCs are in both the tectorial membrane and the basilar membrane, contraction of the motor proteins shortens OHCs, bringing the basilar membrane closer to the tectorial membrane
-now the inner hair cells move more, resulting in a larger membrane potential
What happens when OHCs are depolarized?
they shorten, pulling the basilar membrane up, and thus depolarizing inner hair cells
What happens when OHCs are hyperpolarized?
they lengthen
If contractions of OHCs are in-phase with basilar membrane movements
it amplifies oscillations of the bsilar membrane
OHC motor proteins contract in response to changes in _____
voltage
If OHC contractions are out of phase with basilar membrane movements
they attenuate oscillations of the basilar membrane
How can antibiotics damage hearing?
Antibiotics can kill OHCs, thus preventing modulation of basilar membrane movement, thus damaging hearing due to decreased IHC movement
Where do brainstem efferents contact IHCs?
efferent fibers contact the primary afferents that IHCs synapse onto
Where do brainstem efferents contact OHCs?
directly contact OHCs
What does efferent inhibition in the auditory system do?
reduces peak response and disrupts tuning specificity
In response of efferent stimulation of OHCs,
OHC hyperpolarizes and oscillations go away at one frequency, but not necessarily at another freqeuncy
What happens to gain and sensitivity when OHCs recieve efferent stimulation?
loose high gain and spectral sensitivity; can’t distingish between different frequences
Air conduction hearing test requires ____ to function
middle ear
Bones conduction tests for what type of hearing loss?
sensorineural
Rinne test
hold tuning fork up to air and bone
With bone conduction, if one ear is plugged, which ear is the sound louder in? Why?
sound is louder in plugged ear because bone conduction bypasses OHCs, which are respnsible for inhibition
Duplex theory of frequency encoding
place theory applies at higher frequencies while frequency theory applies at lower frequenices, but there is overlap in middle frequenices
Place theory of audition
basilar membrane serves as a frequency analyzer because different parts wiggle differently to different stimuli
Frequency theory of audition
cells fire in phase with auditory signal
Changes in air pressure produce up and down movements of what structure?
basilar membrane
How do the physical characteristics of the basilar membrane contribute to frequency encoding?
the base (near middle ear) is thick and stiff and so sensitive to high frequencies, whereas the apex in thin and floppy and sensitive to low frequencies- helps with tonotopic map of frequencies
High frequency sounds have what kind of wave in the basilar membrane? Low frequency? Why?
sharp wave; spread out wave; because the apex is thin and floppy and so low frequency sounds result in spread out waves
Why do we have ppor frequency discrimination at low frequencies?
due to the spread out wave of basilar membrane at low frequincies due to basilar membrane’s floppiness
What does tonotopic organization mean?
the auditory nerve fibers are laid out like a piano keyboard, with high frequenices on one end and low frequencies on the other (at the apex); retain tonotopy in the cochlear nucleus
How does one discriminate between low frequency sounds?
cells fire in phase with the auditory signal
-the hair cells are phase locked and so respond at the same point in every cycle of the stimulus
What is the volley principle?
different neurons fire at different points of the phase
-so the post synaptic target still recieves temporally placed inputs for temporal summation, its just that the inputs come from multiple cells
What does the volley principle allow for?
allows for frequency encoding at frequencies higher than an individual neuron can fire
Mechanical tuning of hair cells
- at base: hair cell cilia are short and stiff and repond best (move the most) to high frequencies
- at apex: hair cell cilia are long and floppy and respond best (move the most) to low frequencies
- so the mechanical properties of hair cells reinforce mechanical properties of the basilar membrane
How are turtle hair cells different than mammalian hair cells?
each turtle hair cell has a specific electrical-mechanical resonant frequency and so oscillates at a specific frequency
-has resonsance place code, not a mechanical place code
How does depolarizaiton lead to hyperpolarizaiton in hair cells?
- K influx through tip channels results in depolarization
- depolarization opens voltage gated Ca channels in soma, resulting in further depolarization
- Ca opens Ca sensitive K channels, so K flows out of cell, resulting in hyperpolarization
Slow hair cell K channel kinetics results in
slow oscillations
Fast hair cell K channel kinetics results in
fast oscillations
Cochlear protheses and limited contacts
indirecly contacts the basilar membrane and sends signals that hair cells normally would; with limited contacts you get issues with frequency discrimination
What does location encoding depend on?
interaural time delay and interaural time intensity
When is interaural time delay used?
lower frequencies
When is interaural intensity used?
higher frequencies
What does bilateralization early in the auditory pathway allow for?
allows for comparisons between the ears (interaural comparisons)
Interaural time delay
- takes a long time for stimulus to get to the other side
- have neurons that are sensitive to different interaural time delays; neurons in the superior olive are tuned to specific interaural time delays
Interaural time delays at different frequencies and why it is used at low frequencies
- under 2Hz, wavelength is larger than interaural distance so the relative arrival time of peaks at the two ears is dependent on the speed of sound
- greater than 2Hz, wavelength is shorter than interaural distance, so there si no simple relationship between direction of sound and arrival time of of peaks at the ears
Interaural intensity difference and why it is used at high frequencies
sound shadow more salient at high frequenices than at low frequenices; requires cells that are sensitive to differences in intensity
