Auditory system Flashcards
how many newborns have a hearing disorder?
4-6 out of 1000 (most common congenital illness)
why is hearing loss irreversible?
there is no regeneration of hair cells once they die
what % of adults have hearing impairment?
40% of adults over 75
what are the 2 universal newborn hearing screening?
- otoacoustic emissions
- auditory brainstem response
what is otoacoustic emissions test?
you put a speaker in babie’s ear and pick up the sounds that come out of the hair cells
what is auditory brainstem response test?
a more detailed test with electrodes on the forehead and on the auditory brainstem behind the ear
why is auditory brainstem response test better than otoacoustic? 2 reasons
- babies often have fluid in their ear, making the otoacoustic test unreliable
- auditory brainstem responses tells you that the whole auditory pathway is functional
what do vestibular hair cells respond to?
linear acceleration
what part of the ear is fluid filled
inner ear only
what frequencies activate cochlear vs vestibular hair cells?
vestibular hair cells are more sensitive to lower frequenceis
what is in the middle ear?
ossicles, between tympanic membrane and middle ear
what is the pitch of a sound?
the frequency of the sound
what level are audible frequencies?
20-20 000 Hz
what is the loudness of a sound? what can we hear?
the amplitude of the wave.
range 0.002 to 2000 dynes/cm^2
frequency response is determined by what?
the functional anatomy of the ear
what frequencies are human voices? (we are most sensitive to these frequencies)
500-5 000 Hz
what do decibels represent?
Weber-Fechners Law: decibels represent sound intensity in a way that corresponds to perceived loudness
what is Weber-Fechners Law formula?
L (loudness) = 20 x log10 (P/Pstd)
where P/Pstd = pressure / minimum pressure
where does auditory mechano-electrical transduction happen?
inner ear
what is the external auditory meatus?
auditory canal in external ear
what is the meatus function?
resonate the sound waves to ensure reliable transmission of speech (frequencies aren’t amplified, they just loose less energy)
what does the eustachian tube connect?
the middle ear to the pharynx
blocked eustachian tube can lead to what?
otitis media (middle ear infection from build up of fluid that slows down the ossicles)
is the eustachian tube normally open or closed?
normally closed; it opens during yawning to equilibrate pressure
name the 3 ossicles in order from tympanic membrane to the oval window
Malleus, Incus, Stapes
what are the 2 mechanisms of sound amplification? by how much do they each amplify the sound?
- mechanical amplification by ossicles: 1.3x increase
- pressure amplification from large tympanic membrane to smaller oval window: 17x amplification
- together = 1.3 x 17 = 22x apmplification
why do we need such a sound amplification?
can’t hear without that because the fluid in the cochlea is so much denser than the air that all the energy would be lost
remember: pressure = ?? / ???
how does this apply to tympanic membrane and oval window?
pressure = force / area
tympanic membrane is 50 mm2, oval window is 3mm2 -> smaller area = bigger pressure
how does the MIDDLE EAR protect us from loud sounds?
muscles that limit motion of the ossicular chain:
- tensor tympani (malleus)
- stapedius (stapes)
when do the 2 middle ear muscles activate? give examples
activate reflexively for sounds above 80 dB, ex during chewing, or when a truck honks
to what frequencies is the external vs the middle ear sensitive to?
external ear: 2000 - 5000 Hz
middle ear: 500 - 2000 Hz
describe the cochlea
- 33 mm long coiled structure that makes 2.5 turns
- 3 fluid-filled compartments
- sensory transduction at the Organ of Corti
- basilar and tectorial membranes
what is found on the basilar vs tectorial membrane?
basilar = receptor cells
tectorial = stereocilia
what are the 3 fluid-filled compartments of the cochlea called?
scala vestibuli, scala media (cochlear duct), scala tympani
what media are found in endolymph vs perilymph? describe them
- Endolymph → similar to intracellular fluid, found in scala media. high K+ + 80 mV than perilymph – produced by cells in stria vascularis
- Perilymph → similar to extracellular fluid, high Na+ found in scala vestibuli, and scala tympani
how many hair cells do we have and how are they distributed?
16 000 cells per ear: 3 500 inner hair cells, 12 000 outer hair cells
what are inner vs outer hair cells role?
inner: 1 row, send sensory info to CNS
outer: 3 rows, shape response and amplify it
what does the round window act as?
a pressure release
what is the motion along basilar membrane called? how big is it?
traveling wave: 150 nm in height
explain the trajectory of the pressure wave in the inner ear
oval window -> scala vestibuli -> basilar membrane traveling wave -> round window
describe the basilar membrane mechanical properties
the apex is elastic and responds to 20Hz sounds
the base is stiffer and responds to 20 000 Hz sounds
(place coding = tonotopy)
what does upward vs downward deflection of the basilar membrane cause?
upward deflection towards the tallest cilia = excitation
downward = inhibitions
do hair cells fire AP?
NO they produced GRADED POTENTIAL by releasin glutamate
what part of hair cells are in contact with the endolymph (+80mv)? why?
stereocilia on the apical membrane. if we bathed the whole cell in endolymph it would depolarize
what and where is the synaptic ribbon?
found at the synapse of the hair cell with the afferent nerve; helps to block out background firing
what links stereocilia?
cross-linked actin filaments
why are tight junctions between hair cells basolateral sides important?
separates endolymph (in which stereocilia bathe) and perilymph
steps of signal transduction happening in the hair cell?
- stereocilia deflection
- hair cell depolarization
- Ca2+ influx and glutamate release
- afferent nerve firing to CNS
what cranial nerve for afferent sending of auditory signal?
8 vestibulocochlear lol throwback anat
what type of cells are hair cells?
neuroepithelium cells
how many stereocilia do outer hair cells have?
only 3 rows
where air hair cells found? what are they for in the other locations?
- cochlea (sound)
- semicircular canal organs (angular head movement)
- utricle and saccule (detect linear acceleration)
hair bundle displacement causes a change in what?
receptor potential (membrane potential)
what causes the membrane potential change produced by hair bundle displacement?
opening (depol) or closing (hyperpol) of mechanoelectrical transduction channels
how is the “problem” of saturation of hair bundles solved?
adaptation step during long stimulus allows the reset the channels opening and closing, so the same current can be produced as before adaptation
how does adaptation of the hair bundle happens? explain
change in tension on the gating springs that link stereocilia together
- controlled by myosin motor that move according to calcium levels in the stereocilia
- ex: sound -> stereocilia move, channels open -> Ca+ flow in -> myosin attachment of tip link slides down the stereocilia to DECREASE the tension
- low calcium = myosin climbs and increase tension
give 2 reasons why mechanical gating is better
- faster than second messengers (10s microsecond for hair mechanotransduction vs 10s milliseconds for phototransduction)
- speed is critical for hearing,
a) sound of 100 kHz = hair bundle must move at 100 kHz = channels open and close at 100 kHz
b) sound localization is based on temporal delay of sometimes 10 microseconds
the basilar membrane is more finely tuned for what frequencies? how?
high frequency sounds:
- bundles at the base of the membrane are shorter and stiffer = higher freq
- bundles at the base have faster mechanically-gated channels
what are the cochlear amplifiers?
hair cells
much of the sound stimulus energy is needed to overcome what?
to overcome damping by the cochlear fluid
what are spontaneous otoacoustic emissions?
In a quiet environment, human ears spontaneously emit pure tones
what happens to the shape of the outer hair cells OHC themselves when depolarized/hyperpolarized? why?
depolarized = OHCs rapidly shortens
hyperpolarized = OHCs rapidly lengthen
- this increases the motion of basilar membrane which amplifies the receptor potential in inner AND outer HCs
what are responsible for otoacoustic emmissions?
OHCs
Reducing OHC function with efferent activation, drugs or ablation decreases what?
decreases cochlear sensitivity and frequency discrimination
what is required for electromotility of OHCs?
Prestin: protein in walls of OHCs
what does hypoxia do to OHC?
it eliminates OHC sharp tuning, making the sound pressure level passive response
what is the evidence for the cochlear amplifier to exist?
evoked otoacoustic emissions: return sound contains different frequency components that could not be generated just by the mechanical shape of the tympanic membrane
how many afferent nerves do we have par cochlea and how are they distributed?
- 30 000 afferents per cochlea
- 95% of afferent come from IHC (about 10 afferent per IHC)
- 5% come from OHCs (multiple OHC per afferent)
how are efferent distributed?
95% of efferent go to OHCs
where do afferents go?
type I neuron -> 8th nerve -> cochlear nucleus in brainstem
where are efferent cell bodies located?
in the superior olivary complex in the brainstem
what do neurons coming from lateral vs medial olivary complex each do?
- lateral OC neurons (5% of efferents) synapse on type I afferent neurons to decrease the brain’s own firing sound
- medial OC neurons (95% of efferents) synapse on OHCs and depolarize them
how does the afferent response adapt?
it declines over time to adapt for louder sustained sounds
what happens to the firing rate of afferents when there is a pure tone and noise playing?
due to adaptation, the neurons responds less than when only a pure tone is played
what role do efferents play in afferents response to sounds in noisy environment?
medial OC efferents increases the response of a neurons (brings operating range back to normal)
sound perception by the brain relies on what information being extracted from the afferent?
- which nerve fiber is activated
- rate of spiking
- temporal pattern of spiking
why is afferent information from one INC not redundant?
frequency is tonotopically arranged in place code, each of the 10 afferent that come from one IHC can cover their own part of the amplitude to cover the whole 0 to 120 Db range together
different afferent with the same characteristic frequency can cover the entire _______ range for a given frequency
amplitude
afferent have different spontaneous rates. what does that change?
afferents with higher spontaneous rates fire more easily and therefore are more sensitive, need less sound energy to be activated
what is the afferent response temporal code? explain
phase locking: neuron can not fire at every peak of a sound wave; so it fires at the same phase of the wave but not at each cycle
why is phase locking most important for lower frequency sounds, if the waves of those sounds are faster??
because low freq sounds activate a wider area of the basilar membrane, therefore the place code is not as useful
what is the sensitivity range of each type 1 afferent?
30 dB
how is the loudness of a sound coded for?
increasing the firing rate of one afferent and recruiting additional afferents
what is the cochlear amplifier?
outer hair cells lengthening / shortening
most efferent contact which cells?
OHCs, some contact afferents
where do efferents arise from?
Medial Olivary Complex of the superior olivary complex
remember: where do efferent from the lateral olivary complex synapse?
on afferents
the efferent control the gain of what?
the gain of the cochlear amplifier
how is frequency vs loudness encoded by afferents?
frequency:
- high frequency encoded by location of basilar membrane that is maximally excited
- low freq encoded via temporal coding (phase locking)
loudness: number of spikes and recruitment of afferents
explain the auditory pathway from afferent to auditory cortex
afferent -> cochlear nucleus in brainstem -> superior olivary nucleus -> inferior colliculus -> medial geniculate nucleus -? auditory cortex
where does auditory information cross?
at the brainstem, same level as the cochlear nuclei
give the characteristic of the cochlear nuclei
- 3 subnuclei
- each has an orderly tonotopic map
- has different cell types with different response properties (different firing response)
what part of the auditory pathway is responsible for sound localization?
superior olivary nucleus
what are the 3 cues used for sound localization?
1) interaural time differences
2) interaural intensity difference
3) miniature echoes
what part of the brain is responsible for computing interaural time difference? for what sounds can it be used?
- medial superior olive
- used to localize sounds below 1500 Hz
what part of the brain is responsible for computing interaural level difference? for what sounds can it be used?
- lateral superior olive
- for sounds above 3000 Hz
what cells in the cochlear nucleus mimic the auditory nerve fiber firing activity?
spherical bushy cells
spherical bushy cell maintain what type of information?
timing information of the sound
what are other names for auditory cortex?
Area A1 or Brodmann’s areas 41 and 42 on Heschl’s gyrus
A1 has a tonotopic map of what characteristic of sound?
kiloHertz (frequency)
neurons in what part of the auditory path have receptive field for locations?
auditory cortex A1
most regions around the auditory cortex are involved in what?
speech processing and other unknown functions
A1 is arranged in what?
columns
what was the paper about?
recording from A1 neurons in rats to measure their frequency selectivity and see how noise exposure affected their auditory cortex development
Previous experiments showed that playing 7 kHz tones during development led to overrepresentation of _____ _____ ____
but did not affect the ______ _______
7kHz cortical space;
critical period (11-13days development period)
what were the results of the study?
Early (day 7) broadband noise exposure prevents refinement of tonotopic map
then they tested if the noise just extended the critical period or actually altered the mature pattern of A1 (no tonotopic map forever). results?
- played noise for 90 days: the auditory cortex remained plastic!
- after the 90 days: rats were able to form a tonotopic map that overrepresented 7kHz because they listened to 7Khz sound after 90 days
Normal sound exposure (p50-p120) after noise exposure (p7-p50) leads to what?
a normal A1 tonotopic gradient!
give examples of conditions that could contribute to auditory and language-related delays in children
- chronic otitis media
- cleft palate that closes the eustachian tube
Human critical period for auditory cortex development continues until what age?
6 to 7 years of age
in human when does the perception of word sounds and syllables mature? what about word meaning?
sounds and syllables: 8-10 months
word meaning: 2-4 years
how can speech processing be influenced by our sense of touch?
researchers at UBC showed that the puffs of air that accompany sounds starting with “p”, “t” and “k” influence our perception of sound.
congenital deafness affects what % of the population?
0.1%; half of cases are genetic
what is most age-related hearing loss caused by? what are other causes?
- hair cells (cochlear) damage
- genetics, bacterial meningitis, noise, ototoxic drugs (aminoglycoside antibiotics, cisplatin)
how does our hearing range change through life?
- newborn = 20 - 20 000 Hz
- 20 yo: 20 - 16 000 Hz
- retirement: 20 - 8 000 Hz
what is presbycusis?
bilateral hearing loss
what hair cells are most susceptible to damage?
OHCs
what characteristic of sounds can be rescued by an hearing aid?
loudness, but not frequency selectivity
what is conduction deafness?
sound can not get to the middle or inner ear
ex: wax accumulation, otitis media, otosclerosis (bones can’t move)
what is sensorineural hearing loss?
damage to the organ of Corti or the 8th nerve
causes:
- persistent loud noises
- toxic drugs (streptomycin)
- old age
- tumors of VIII nerve
- infections
what are 3 clinical tests for audition?
- Audiometry: test with pure tones
- Bone Conduction (Rinne test)
- Otoacoustic Emissions
