Auditory Systems Flashcards
what are the three parts the ear is composed of?
- outer ear, midde ear and inner ear
what does the outer ear consist of?
the pinna and ear canal
what does the middle ear consist of?
the eardrum and 3 small bones called ossicles
what does the inner ear consist of?
the cochlea
what is the cochlea responsible for?
converting sound waves into electrical signals the the brain can interpret
what is sound?
refers to pressure waves generated by vibrating air molecules, auditory percept
- change in air pressure
4 major features of sound waves
- waveform
- phase
- amplitude
- frequency
what is frequency and amplitude?
frequency (f) = pitch
amplitude (A) = loudness = log of pressure high/pressure low
what is timbre?
the complexity of the sound wave
what are the main challenges of the auditory system?
- detect and code the amplitude and relevant frequencies
- difficult to detect because the environment is changing
- uses information for hearing and sound localization
what does the outer ear do?
function to channel sound, filters sound waves depending on elevation of source, amplify sound
what does the middle ear do?
prevents reflection of sound due to resistance in air/water differences (size of ear drum and lever action)
-tranduces sound
-a little bit of amplification of sound
what does the inner ear do?
converts fluid pressure waves into neural impulses
the pinna ________ sound waves depending on the ______ of their source
differentially filters, elevation
frequency ranges of humans and whales
humans: 20 - 20,000 hz
whales: 20- 100,000 hz
-> some blue whales have changed freq bcuz of ship noises (mating season)
sounds ____ by the outer ear causes ____ of the ear dream
amplified, vibration
what happens to sounds after it passes the pinna?
causes the tympatic membrane or eardrum to vibrate, is transduced to the middle ear through ossicles
where is the middle ear
between tympanic membrane and cochlea
what three tiny ossicles are the middle ear made up?
- malleus
- incus
- stapes
why is the cochlea in an aqueous medium in the ear?
all the soudn energy hits the water and is reflected; the change in resistance doesnt allow sound to go through the water medium very well. something must transduce that energy from air to that aqueous medium. this is the ossicles, which amplify a larger vibration on the stapes, which is attached to the oval window of the cochlea.
ossciles of the middle ear tend to…
stiffen the ear drum and dampen the transfer of sound energy from the outside world -> doesn’t allow sound and protects ear
how does the middle ear control energy that comes through?
stiffen the ossicles, not allowing sound to go through as effectively
the malleus is controlled by…
the trigeminal nerve -> tensor tympani
the stapes is controlled by…
facial nerves -> stapedius
what happens if you lose control of those nerves?
there is no control to stiffen. the smallest sound can cause big vibrations and are quite painful -> hyperacusis
what is hyperacusis?
lesions affecting trigeminal and facial motor nerves can lead to increased acuity and hypersensitivity to sound
vibrations are passed from _____ to the _____
oval window, cochlea
what is the cochlea structure?
a bony, snail-like structure
what is the cochlea filled with?
it is a labyrinth within a labyrinth
- one is filled with perilymph (CSF, low K+)
- the membranous labyrinth contains high k+ and endolymph
what are the chambers of the cochlea?
- scala vestibuli
- scala media
- scala tympani
-> are all fluid filled
what are the membranes that separate the chambers in the cochlea?
- rasiner’s membrane
- basilar membrane
what do hair cells do?
transform mechanical energy into electrical energy
what is the organ of corti made up of?
hair cells (inner and outer)
- tectorial membrane (top)
- basilar membrane (bottom)
- output cells
what are the two sets of hair cells?
inner hair cells and outer hair cells
vibrations on tectorial membrane
traveling wave initiates sensory transduction by displacing hair cells that sit on basilar membrane, motion between two membranes bends sterocilia leading to voltage changes across hair cell membranes.
structure of hair cells
kinocilium - large structure, with stereocilia beside it
this is an example of ______ of ______ in arrangement in the cochlea.
stereocilia of inner hair cells
this is an example of ______ of ______ in arrangement in the cochlea.
stereocilia of outer hair cells
how does vibration depolarize hair cells
a vibration = change in electric membrane potential = release of transmitter = activate spiral ganglion cell afferents
shearing force causes the motion of hair cells to be activated
what happens if we have a big hair cell?
= big membrane = big capacitance = big time constant = cannot respond quickly to inputs
time constant is….
the product of membrane resistance and capacitance. high capacitance from larger hair cells prevents quick movement
what did Georg von Bekesky do?
as a sine tone initiates a traveling wave in the cochlea that propagates from the base toward the apex of the basilar membrane, growing in amplitude and slowing in velocity until a point of maximum displacement is reached
where does the main tuning frequency come from?
basilar membrane
how does the basilar membrane act as a frequency analyzer?
stiff end at the beginning takes higher frequencies (like 500hz), while the pliable apical end takes lower frequencies (like 100z).
what happens in the basilar membrane in terms of vibrations and frequencies?
vibrations peak in different parts of the basilar membrane; lower frequencies travel further down, creating a larger amplitude at the end
spiral ganglion cells encode different frequencies of info along the basilar membrane
the basilar membrane is tuned for…
high frequencies
the apex is tuned for…
low frequencies
what is the best frequency?
the lowest amplitude / frequency you can detect
what is a neurons tuning curve
it is a curve that describes the intensity (dB) at which a neuron fires action potentials at varying frequencies (kHz)
place code
each neuron has a best frequency that depends on its place in the cochlea
how do cochlear implants work??
can retrieve hearing by putting electrode and transducer into cochlea and send current pulses to different parts of membrane
cochlear implants according to law of ______;
due to the law of specific nerve energy, it doesn’t matter if the neuron is stimulated with sound or with electricity. once stimulated, you get the same perception of sound.
analysis of complex sound waves
breaks down complex waves and represents in different places along membrane
sound waves ____ through the _____
spiral, cochlea
afferent and efferents to the organ of corti
95% afferant on inner hair cell (do most of the hearing)
- 20 afferants converge onto each IHC
90% efferant on outer hair cells
how does the organ of corti hair cells sense sound vibrations?
feeds input into spiral ganglion cellls that each have a certain frequency turning and collect different frequency info
are afferents connected to inner or outer hair cells?
> 95% are connected to inner hair cells; very few receive info from outer hair cells
- this tells you immediately that inner hair cells carry sensory info
- more than 20 afferent fibres on each hair cell
are efferents connected to inner or outer hair cells?
> 90% of contacts are on outer hair cells
efferent information doesnt regulate inner hair cells directly in a strong way
brain regulates information by controlling activity of outer hair cells
given a certain tune on the basiliar membrane, you get _____ broad tuning curves compared to those of ganglion cells
broad
tuning curves have _____ components in high intensities but have _____ components and respond to a _____ range of frequencies at lower intensities
broad, narrow, narrow
what happens in tuning curves in fresh vs tired animals?
fresh has a sharp tuning curve, but as the animal tires the tuning curve becomes much broader, similar to that of the basilar membrane
what does the broad tuning curve in neurons in tired animals tell you?
cochlea is most likely an active system with a positive feedback loop that accounts for the high cochlear sensitvity
_____ is likely the molecule motor
prestin
what does prestin do?
can change the length of the outer hair cell by binding chloride
contractile element depends on prestin molecule ->chloride binding sites on prestin
in the resting state, there is lots of negative charge on the inner surface
all the chloride goes into the molecule, which keeps it in an expanded state, since it’s soaked with chlorid
when you get the depolarization upon vibrating the hair cells, the membrane no longer has its neg charge, and nothing pushes the chloride.
chloride then leaks out and the whole molecule contracts.
outer hair cell in hyperpolarized state:
its inactive, elongated conformation (long state)
outer hair cell in a depolarized state:
chloride is forced out, and its in its short state
what is the cochlear feedback loop driven by?
outer hair cells
what is the cochlear feedback loop?
chloride goes out: hair cell shortens
positive loop: shortening of hair cells causes basilar membrane to move, sound causes motion which is amplified, depolarizes, shortens and causes more motion
what do the tuning curves in animals with no outer hair cells or in animals without the prestin gene look like?
similar to tired animals, broad
what degree do outer hair cells aid in cochlear amplification?
up to 1000x
through what efferent innervations can you gain control and frequency tuning in the cochlea?
olivocochlear bundle
what is a stereocilia bundle connected by?
kinocilium, losing these connecting strands would mess up entire auditory system.
at which parts of teh hair cell is there high/low extracellular potassium?
at top, high k+, at bottom low k+
why is there high k+ outside a certain part of hair cells?
the endolymph which surrounds that area is the scala media, which has high potassium levels produced by the scala vascularis
mechanoelectrical transduction mediated by hair cells
spring loaded tip links gate channels physically
- pushes to the right which causes depolarization and potassium rushes in
- high K concentration allows K to flow in (scala media)
how do you repolarize a hair cell?
on the bottom of the cell, there’s no high potassium on the outside. when you open potassium channels on the bottom during repolarization, you get hyperpolarization like normal
- endolymph is at +80mV (150mM)
- perilymph is at 0mV (5mM)
- inner hair cell is at -45mV (120mM)
an unusal adaptation of hair cells
k+ reserves both to depolarize and repolarize the cell
-> passive ion movement
what is endocochlear potential?
the sterocilia of hair cells protude into the endolymph which has k+ (due to stria vascularis)
mechanoelectrical transduction currents are ____ and adapt to _____ stimulation
graded, constant
graded mechanoelectrical transduction currents
- adapt to constant stimulation
responds to positive deflection but not to negative - different decays because of adaptation (channels r closing)
slow adaptation
when tiplink is stretched it opens channel and slides down
- reducing tension and closing channels
- mediated by myosin motors
- moves back to starting position to re-establish tension
why would you want adaptation?
- allows you to respond to the most interesting stimulus in the environment
- decreases sensitivity to allow the system to respond to stronger stimuli
what does adaptation of hair cells do?
- shifts the operating range of the channel
- in noisy environment you can still be able to hear sounds
- curve shifts to the right
what myosin is implicated in adaptation?
myosin-1c
how does slow myosin-dependent adaptation in hair cells work?
- the tip link potential increases and you get an opening of channels with deflection
- but, the whole molecular complex marches down the filaments to reduce tension on the tip link
- when it marches down, you lose tension and channels close, even though the hair cells are still deflected
- when you return to rest, you lose the deflection, and there is even more slack than there was in the beginning; they need to march back up and reset the basal tension
processing of natural sound
brain discriminates info from a mixed-up sound and separated out in the cortex
auditory nerve (VIII cranial nerve)
carries neuronal info from IHC - connects to multiple spiral ganglion and sends to cochlear nucleus
do spiral ganglion cells connect to the same IHC have heterogeneous properties?
yes, phasic and tonic activity
what happens to hair cells utilize?
L-type Ca channels
hair cells rely on ____
ribbon synpases
what are ribbon synapses?
ribbon like protein structure that organizes synaptic vesicles for efficient NT release
how do hair cells rely on ribbon synapses?
what are mutations of the otoferlin gene associated with?
deafness map
what is seen with immunolabelling of otoferlin?
in really young mice, you can see the inner and outer hair cells with otoerflin expression, with generally less on the outer hair cells
in the more mature system, there’s very little expression in the outer hair cells
what happens if you knock out otoferlin in mice and measure the auditory brain stem response?
there is no auditory brain stem response
Ca2+ triggered _____ is almost completely _____ in the absence of otoferlin
exocytosis, abolished
what happens to Ca triggered exocytosis in otoferlin KO?
exocytosis is abolished
- ability to cause increase in capacitance is gone and no vesicle release
- can increase capacitance by increasing SA or cage calcium
what does otoferlin interact with? (HCs dont have synaptogamin)
- snares
- endocytic proteins -> AP2 that enables rapid clearance of exocytosed material from the vesicular release site.
what is the importance of otoferlin and Ca?
calcium sensor - is important in sensing calcium incoming -> most neurons use synaptotagmin
what is temperature dependant deafness?
patients have hearing loss with a fever
sounds at _____ frequencies are sounds the hair cells can keep up with
low
what is phase locking?
synchronization of the phase of an oscillating signal (sound wave) with the phase of another, typically in response to periodic stimuli
what frequencies does phase locking occur up to?
10khz, will get an action potential for every cycle at the top. if you go any faster, will miss responses
what are the three divisions in the cochlear nucleus each auditory nerve fiber innervates
- anteroventral CN - AVCN
- postereoventral CN - PVCN
- dorsal CN - DCN
what is the central auditory pathway?
spiral ganglions –> cochelar nuclei of the medulla (dorsal cochlear nucleus, anteroventral CN, posteroventral CN) –> inferior colliculus –> thalamic medial geniculate nucleus (MGN) –> auditory primary cortex
what does tonotopic organization of the CN refer to?
spatial arrangement of neurons, based on the frequency of auditory stimuli
what is the connection from auditory nerve to bushy cells?
end bulb of head
- VCN bushy cells show improve precision of phase locking
- fidelity of firing is stronger downstream then it is upstream
cell types in the VCN
- spherical
- globular bushy
- stellate/multipolar
- octopus cells
- granule cells
discharge patterns in the CN
- much more complexity than auditory nerve fiber
- can respond both tonic and phasic to a tone
- discharge patterns shaped by time dependant interactions of excitation and inhibition
what is two-tone suppression (lateral inhibition)
two different tones, typically a low-frequency tone and a high-frequency tone, are presented simultaneously, the response of the auditory nerve fibers to the low-frequency tone is reduced or suppressed by the presence of the high-frequency tone.
cortical cells
tuned to precise sequence of complex sounds
primary language areas of the brain
brocas: important for creating speech
lesion- understand language but cant produce speech
wernickes: how we understand speech
lesion- can speak but cannot understand
what is adjacent to the primary auditory cortex?
auditory association cortex
- auditory info is interpreted
- important part of language processing
phase locking low vs high frequency
low- neurons phase lock and fire at each phase of cycle
- gives additional information about timing
high- cannot keep up to phase lock instead use tonotopy
interaural time difference measuring
for sound source infront: ITD = 0
can accurately localize 1 degree
directly to the side: around 600us diff (size of head)
can accurately localize 15 degrees
what is the jeffress model for sound localization
inputs from both ears need to be compared by a bunch of coincidence detectors receiving info from both ears
only when sound is coincident to that detector would the detector respond
medial superior olive role in jeffress model
substrate for sound localization based on interaural timing differences
what is the MSO excited by and inhibited by?
excited by inputs from ipsilateral and contralateral AVCN (spherical bushy cells)
- inhibited by MNTB and LNTB
what kind of frequencies does the MSP emphasize?
low frequencies (tonotopically organized with a dorso-vental tonotopic gradient) -> high frequencies are ventral
neural response vs interaural time difference graph
when ear leads by too much there is no response, as it gets closer you see response
- positive ITD then you see firing
- neurons sensitive to small window time (200us)
in phase will respond, out of phase, will not respond
interaural phase difference
when you delay an input on one ear, the stimuli go out of phase
that’s what the cell is detecting
if you further the delay, it comes back into phase
changing the timing between teh presentation of sound in left and right ear results in a best ITD
localization cues for high-frequency sounds make use of…
head shadows - the interaural intensity differences (IIDs)
head shadows - IIDs
rather than focusing on the timing of the input, for high freqs we focus on the intensity of the input on one side vs the other
which part of the brain favours high freqs?
LSO, the lateral superior olive
how does the LSO function
compares inhibition from one side and excitation from the other.
- on the ipsilateral side, the end bulb synapses, cochlear nucleus sends projections and excitatory input to the LSO neurons
- on contralateral side, end bulbs made on globular BS, and goes to MNTB
- release glcyine which inhibits on the LSO
what is the MNTB excited by
individual globular bushy cell inputs from the contralateral VCN
what is the largest synapse in the brain called
calyx of held
what does the MNTB do
inhibits the LSO and MSO with glycine
where does the lateral superior olive get its excitatory and inhibitory input from?
- excitatory from ipsilateral VCN
- inhibitory from contralateral VCN via the ipsilateral MNTB
what determines the LSO output
the balance between excitation and inhibition
- response = (ipsi) excitation - (contra) inhibition)
- if you make the right stronger, you get more and more inhibition that suppresses the output of the left
summary of how we determine sound localization on the horizontal/azimuth (aural plane) vs the vertical (sagittal plane)
horizontal/azimuth (aural plane)
- monoaural cues
- static binaural cues –> interaural time differences (ITD), interaural intensity/level differences (ILD, IID)
- dynamic binaural cues –> ILDs and IIDs change when the head rotates only for objects in the azimuth plane, not for those aboe
vertical (sagittal plane)
- pinna filtering
VCN morphologically distinct cell types
- spherical bushy cells (rostral AVCN)
- globular bushy cells (caudal AVCN)
- stellate/multipolar cells (AVCN/PVCN)
- octopus cells (PVCN)
- granule cells (similar to cerebellum)
transformation of discharge patterns in the CN
show much more complexity than ANFs
- both tonic (sustained) and phasic (brief) responses occur to CF tones
-many discharge patterns are shaped by time-dependent interactions of excitatio and inhibition
two-tone suppression
there is a center of excitation and a surrounding of inhibition
when you stimulate outside, you get suppression of background activity
you stimulate the receptor fields in the center and get a very robust response, but outside the center, you get inhibition
this is really important for detecting sharp edges in the freq domain
how many classes of spectal response areas are found in the CN
5
cortical cells respond well to
precise sequences of complex sounds
if you play a mating call backwards with the same freq components, the neuron wont fire as much and doesnt care about it
