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Flashcards in Audition Deck (109):

Where does transduction of sound energy into electrical impulse occur

Inner ear by cochlea


Why is audition important

Efficient warning system warning us about changes in environment
Physiological well being --> important for social communication
Only true omnidirectional sense --> can localise sound in all direction from head to direct attention towards unexpected objects


What is sound

Any audible pressure variation that propagates away from source of vibration e.g. loudspeaker cone, tuning fork


Describe pure tone

Simplest sound (single frequency), consists of sinusoidal increases and decreases in pressure and is characterised by frequency (--> pitch) and peak amplitude (--> loudness)


What is timbre determined by

Spectrum of sounds determines tonal quality aka timbre --> distinguish between speech sounds or musical instrunment


Describe innervation of external auditory meatus and tympanic membrane

External auditory meatus: mandibular trigeminal nerve, vagus
Tympanic membrane: both + facial nerve


What are the two roles of the external ear

Resonance in the concha + external auditory meatus produces increase in pressure between 2-7khz --> depending on dimensions of outer eat certain frequencies boosted more
Amplitude of different frequencies modifies in different ways depending on location of sound source --> colouration gives rise to spectral cues for monoaural sound localisation (enhanced by convolution in pinna)


Describe how external ear uses spectral cues for monoaural sound localisation

Amplitude of different frequencies modifies in different ways depending on location of sound source --> colouration gives rise to spectral cues for monoaural sound localisation --> can distinguish sounds in front vs behind (due to shape of pinna --> favours sound coming from ahead) and localise sound at different elevations (enhanced by convolution in pinna)


What nerve innervates middle ear



What happens in middle ear as we swallow/yawn, why

Middle ear connected to nasopharynx by Pharyngotympanic tube (auditory/Eustachian tube) which opens during swallowing/yawning so air enters middle ear cavity to maintain middle ear at atmospheric pressure --> equalise pressure either side of ear drum --> free to vibrate


What are the ossicles and what do they do

Malleus --> incus --> stapedius connect tympanic membrane to oval membrane in wall of cochlea --> transmit vibration from tympanic membrane to movement at oval window


How do ossicles prevent 99.9% of sound waves being reflected due to pressure exerted by cochlea fluid on oval window

Ossicles match impedance (density) of air to much higher impedance of cochlear fluid in 3 ways --> amplify pressure sufficiently to vibrate oval window membrane:
1) Ear drum has greater SA than stapes footplate attaches to oval window --> 17 times increase in pressure
2) Lever action of ossicles (malleus longer than incus so moves more ) --> increases force at stapes footplate applied to oval window
3) Buckling of tympanic membrane as it moves --> increases force at stapes footplate


Contrast roles of tragus and pinna

Tragus faces rearwards and aids collecting sounds from behind
Pinna shaped to make ear more sensitive to sounds coming from ahead


What is the innervation of tensor tympani and stapedius

Tensor tympani --> trigeminal nerve
Stapedius --> facial nerve


What does contraction of stapedius and tensor tympani achieve

Increase stiffness of bones so reduces transmission of sound through middle ear


What triggers middle ear reflex i.e. contraction of stapedius and tensor tympani

Intense low frequency sound --> can reduce background noise to discern high frequency sounds - --> understand speech more easily in noisy environment
Before self vocalisation --> reduce auditory self stimulation during speech
Plays role in protecting ear from sustained loud noises (but reflex activated after external sounds begin)


What happens with facial nerve damage

Stapedius paralysed --> hyperacusis


What is otitis media

Middle ear inflammation diseases leading to hearing loss due to fluid in middle ear/rupture of tympanic membrane


Where is inner ear located

Within petrous temporal bone


What is the endocochlear potential

+80mV across basilar membrane due to electrical potential of endolymph around 80mV more positive than perilymph


What is the basilar membrane situated between

Scala media and scala tympani


What is the spiral/cochlear ganglion

Neuronal cell bodies in modiolus (central axis of cochlea), these bipolar neurones innervate hair cells of organ of corti and project axons to ventral and dorsal cochlear nuclei


Name drugs with ototoxicity



Describe how the nerve fibres innervating inner hair cells arranged

In progressively decreasing frequencies progressing form base to apex of cochlea


Describe the innervation of cochlea

Innervated by auditory nerve fibre which travel to ipsilateral cochlear nucleus in medulla --> each axon branches to synapse onto both dorsal and ventral cochlear nucleus. All parallel pathways converge onto inferior colliculus


Describe the axons from ventral cochlear nucleus

Project to superior olivary nucleus on both sides of brain stem (trapezoid body carries decussating fibres to contralateral superior olivary nucleus) --> first site of binaural convergence (imp for sound localisation)


What happens from superior olivary nucleus onwards

Fibres ascend in lateral lemniscus to inferior colliculus
Info passes form inferior colliculus via medial geniculate nucleus of thalamus to A1 (Broadman's area 41/Primary auditory cortex)on superior temporal gyrus


What does trapezoid body do

Carry decussating auditory fibres from ventral Cochlear nucleus to contralateral superior olivary nucleus


A second frequency lying outside tuning curve….

Suppresses response of auditory nerve fibres to excitatory tone (two tone suppression) --> due to non linear response of basilar membrane (relevant in complex tones with different frequency components)


How is tonotopy preserved in auditory nerve and all primary auditory nuclei and cortical fields

Nearby auditory nerve fibres have similar characteristics, divergence in output from cochlea means each sound frequency represented by single point on basilar membrane but sheet of neurones (isofrequency sheet) at higher brain levels


How is tonotopy maintained in cochlear nucleus. Mention neural inhibition too

Dorsal, anteroventral and posteroventral nuclei all contain neural map of sound frequency, in nucleus there are bands of cells with similar characteristic frequencies, which increase progressively from anterior to posterior. --> location of active neurones in nuclei contributes to sound frequency perception
Neural inhibition seen here for first time --> increases from ventral to dorsal cochlear nucleus --> cochlear neurones sensitive to more complex spectral patterns


How is tonotopy maintained in A1

Low frequencies represented rostrally/laterally and high frequencies represented caudally/medially so isofrequency sheet run mediolaterally across A1
Neurones in same cortical columns have similar characteristic frequencies but vary in degree of frequency tuning , some sharply tuned, some barely tuned (sensitivity to other properties represented within isofrequency sheet e.g. changes in frequency over time)


Why is binaural interaction in superior olivary nucleus relevant

First point of binaural convergence --> important for sound localisation


How are columns arranged in higher cortical areas

Patterns of summation (binaural) and suppression (one ear dominant)


What happens to neuronal response properties as auditory pathway ascends, what's the consequence

Become more diverse and complex --> cortical neurones general respond better to complex sounds not pure tones


What are 2 limitations of tonotopy in encoding frequency

Tonotopic map doesn't contain neurones with very low characteristic frequency --> frequency below this range can't be distinguished by tonotopy
Region of basilar membrane maximally displaced depends on sound intensity as well as frequency (esp apically) --> broadening of tuning curves + saturation of responses --> poorer frequency resolution above threshold


How is information about intensity conveyed to higher brain centres

Infor about intensity conveyed to intensity tuned neurones in central auditory fibres e.g. some A1 neurones intensity tuned --> maximum activity in response to particular sound frequency


Describe the frequency range for young healthy adults and the range they are more sensitive to



What do we use audiometry for

Show variation in threshold sensitivity across different frequencies --> useful for detecting hearing impairments.


Why does an audiogram look the way it does

Resonance of external ear boosts certain frequencies more --> threshold varies with frequency
Distribution of tuning curves of auditory nerve fibres follows shape of audiogram (around 1-2hkz) --> greatest proportion of sensitive auditory nerve fibres --> most sensitive areas of audiogram


What are the two types of deafness and what are there causes

Conductive hearing loss (mechanical damage to outer/middle ear due to wax occluding external auditory meatus, perforated eardrum, fluid in middle ear, immobilised ossicles)
Sensorineural hearing loss --> hair cells die progressively increasing age (presbyacusis), trauma to cochlea and outer hair cell (susceptible to loud noise)


Compare and contrast conductive vs sensorineural hearing loss

Conductive: hearing impaired across sound frequency range -->threshold declines for all frequencies by similar extent and overcome by amplifying input
Sensorineurral: hearing loss greater for some frequencies than others, can't be overcome by amplifying input but some hearing restored with cochlear implant to stimulating remaining nerve fibres


Describe a central cause of hearing loss

Acoustic neuroma (aka vestibular schwannoma) benign tumour growing on cranial nerve 8 compresses auditory nerve fibres


What is Rinne's test used for, how does it work

Conductive hearing loss test --> tuning fork placed on mastoid process; sensitivity to air conducted sound usually much higher than for bone conduction but other way round for conductive hearing loss


What is Weber's test used for, how does it work

Unilateral conductive/sensorineural hearing loss --> tuning fork placed on top of skull: normally --> sound equally loud in both ears. Conductive hearing loss: sound louder in defective ear. Sensorineural: sound louder in unaffected ear


Describe the role of the superior colliculus in sensory integration

Many superior colliculus neurones receive converging multisensory inputs from more than one sensory modality --> receive visual, auditory and tactile inputs arranged ---> topographically aligned maps of sensory space ----> different sensory representations share common coordinates in superior colliculus
Super imposed sensory and motor maps so sensory stimulus can feed into premotor circuit and direct movement towards same location (e.g. saccadic eye movement)


Different sensory inputs converge at many levels of brain including...

Superior colliculus,
Association areas in temporal/parietal/frontal cortex


What does superior colliculus integrate and what does this allow

Sensory inputs to localise novel sensory stimuli and direct orientating orienting movements of peripheral sense organs, esp saccadic eye movements to shift attention towards certain stimuli


Describe how different stimuli are represented in superior colliculus

Received visual, auditory and tactile inputs which are arranged to form topographically aligned maps of sensory space
Many sup colliculus neurones receive multisensory inputs form more than one sensory modality --> response enhanced when multisensory stimuli are delivered at the same time and spatial location
Superimposed sensory and motor maps so a sensory stimulus can feed into premotor circuits to direct movement towards same location


In the superior colliculus how are the visual, auditory and tactile inputs arranged

Topographically aligned maps of sensory space i.e. in a given region of sup colliculus, receptive fields for each sensory mortality are found at approx location in space --> different sensory representations share common coordinates in superior colliculus


Describe what the external ear consists of

Pinna, fold of skin reinforced by cartilage from which auditory membrane extends to tympanic membrane


Tympanic membrane is connected wall of cochlea by ossicels

Oval membrane


What sort of joint between 3 ossicles

Synovial joints


What does inner ear contain , describe the structure

Hearing organ (cochlea) and sense organs of balance (semi circular canals and otolith organs)
Cochlea spirals for 2.5 turns around central modiolus in which cochlear nerve travels in


What is cochlea divided into

Outer 2 compartments (scala vestibuli and scala tympani)
Middle chamber (scala media)


Describe the outer two compartments of cochlea

Scala vestibuli and scala tympani bordered by labyrinth
Filled with perilymph - continuous with CSF through perilymphatic duct so ionic composition resembles ECF i.e. High Na+, low K+


Describe middle chamber of cochlea

Scala media bordered by membranous labyrinth within bony labyrinth
Contains endolymph which resembles ICF (high K+, Low Na+) due to active Na+K+ exchanger in stria vascularis lining outermost wall of scala media


Scala media separated form scala vestibuli by,,, and scala tympani by..

separated form scala vestibuli by Reisnner’s membrane and from scala tympani by basilar membrane


Where does Organ of Corti sit and what is its function

Sits on basilar membrane and is auditory transducer (convert mechanical oscillation to electrical impulse)


Describe structure of Organ of corti

containing hair cells (along with their nerve supply and supporting cells) with bundle of stereocilia (arranged in graded height order) extending form their apical surface. Hair cells categorised into inner/outer based on position with respect to modiolus;


Cochlear contains single row of what type of hair cell vs 3 rows of what hair cells

single row of inner hair cells and three rows of outer hair cells.


Describe process of auditory transduction

1) Vibrations of tympanic membrane transmitted via ossicles in middle ear to oval window of cochlea
2) Induces corresponding movement of round window membrane, because cochlea fluids are incompressible
3) Pressure gradient set up between scala vestibulur and scala tympani --> basilar membrane to move up and down
5) Movement of basilar membrane generates shearing force between hair cells and stiff overlying tectorial membrane which displaces sterocilia bundle
6) Bundle movement in direction of tallest stereocilium opens transduction channels located at tips of sterocilia due to increased tension in tip link connecting each channel to upper wall of neighbouring cilium
7) Opening of transduction channel results in K+ entry (endolymph has similar ionic composition to ICF  no chemical gradient) into hair cell down
8) Depolarisation of hair cells
9) Ca2+ entry through VGCC and glutamate released onto spiral ganglion fibres of auditory nerve (CN VIII)


What sort of channel found in auditory transduction important for transduction

Stretch activated non selective cation channel


Tuning of basilar membrane relies on...

Variation in stiffness --> different frequencies induce motion of basilar membrane at different points along its length
Active amplification


How does variation in stiffness affect tuning of basilar membrane/frequency analysis

distance propagated by wave depends on sound frequency – higher frequency sounds (i.e. 20000Hz) vibrate the stiff base more (I.e. peak amplitude of wave occurs near base of cochlea), dissipating energy so the wave doesn’t propagate as far compared to lower frequency sounds


Describe how the shape of the basilar membrane along the length of it changes and why is this important

Increase in width and floppiness from base to apex so distant propagated by wave depends on sound frequency


Describe how a placecode is set up by variation in basilar membrane stiffness

which different locations of membrane are maximally displaced at different sound frequencies, forming a tonotopic map responsible for neural pitch coding.


How do hair cells contribute to frequency tuning

Hair cells begin sound discrimintion (don’t respond to all auditory sounds) as hair cells are tuned to certain frequencies due to surrounding accessory cells


How does active amplification lead to frequency tuning

Motion of basilar membrane during low intensity sound stimuli actively amplified by outer hair cells – as the depolarise/hyperpolarise in response to sound waves, prestin motor proteins in basolateral membrane undergo conformational change so shorten/lengthen the cells respectively, amplifying vibrations + basilar membrane motion thus enhancing auditory transduction


What does prestin do to cell length as outer hair cells depolarise/hyperpolarise respectively

Depolarise --> shorten cell
Hyperpolarise --> lengthen cell


Electromotility of hair cells improves...

sensitivity, resolution and thus frequency selectivity --> OHC for cochlear amplification


Describe how innervation to outer hair cells enhances active amplification

Innervated by efferents from superior olive, which contribute to sharpening frequency along basilar membrane


What impairs amplification of certain frequencies

loop diuretics (e.g. furosemide), aspirin, quinine , antibodies (e.g. kanamycin or gentamicin) which damage outer hair cells--> drug induced ototoxicity


Explain the effect of kanamycin on hearing

increases threshold so less sensitive and more broadly tuned basilar membrane


Describe innervation to inner hair cells , how are the afferent nerve fibres arranged

95% of auditory nerve fibres are myelinated afferents each innervating one inner hair cell , with each inner hair cell innervated by ~ 10 fibres --> divergence in output from cochlea


Describe innervation to outer hair cells

unmyelinated efferents from super olive which innervate ~20 outer hair cells each


Each neurone's characteristic frequency is determined by..

region of basilar membrane it innervates, so characteristic frequency varies topographically within auditory nerve --> tonotopy


Tonotopy means that nerve fibres innervating hair cells near apical basilar membrane...compared to those innervating hair cells near basal basilar membrane

nerve fibres innervating hair cells near apical basilar membrane have low characteristic frequencies as low frequencies maximally deform this part, whereas nerve fibres innervating hair cells near basal basilar membrane have high characteristic frequencies.


What is phase locking and why does it occur

consistent firing of a neurone synchronised to same phase of sound wave, so that sound frequency is encoded by temporal firing pattern – occurs because hair cells are depolarised or hyperpolarised according to whether basilar membrane moves up or down respectively


Increasing intensity of sound increases number of AP but how is this not confused for phase locking

temporal nature of AP stays the same


Why does phase locking still work even if a single phase locked neurone doesn't fire an AP on every cycle due to limited refractory period

their responses occur at times that are integral multiples of period of stimulus, so population response will follow sound waveform --> volley theory of pitch perception


Phase locking doesn't occur when? How is frequency represented

Higher frequencies >4Hz- frequency represented by tonotopy alone


Why does phase locking not occur at frequencies > 4Hz

membrane capacitance of hair cells cannot follow charge transfer at such high rates so hair cell receptor potential no longer follows waveform of sound – overall depolarisation instead of alternating depol/hyperpolarisation


How can intensity of stimulus be encoded

Firing rates of neurones in auditory nerve
Number of active neurones in auditory nerve


How does firing rate of neurones in auditory nerve encode stimulus intensity

as stimulus intensity increases, amplitude of basilar membrane vibration increases, so sterocilia deflected more -->activated hair cells are depolarised/hyperpolarised more and firing rate of nerve fibres innervating them increases


Firing rates of neurones in auditory nerve increases as intensity increases, however...

This response saturates at intensities more than 30-50db above threshold for each fibre because each hair cell has a finite number of transduction channels --> but perceptual dynamic range is much larger because fibres with same characteristic frequency but different thresholds are activated as intensity increases


How does number of active neurones in auditory nerve encode stimulus intensity

increasing stimulus intensity induces movements in basilar membrane over greater distance, so more hair cells are activated broadening the frequency range to which single single auditory nerve fibre responds


With increasing stimulus intesntiy, frequency resolution gets worse, why

Saturation of responses and broadening of tuning curves


Info about stimulus intensity is conveyed to...neurones in central auditory pathway

Intensity tuned neurones e.g. in A1 some neurones intensity tuned --> respond maximally at given intensity


Sound localisation in the horizontal plane requires...

Sounds reaching both ears --> binaural localisation cues used to distinguish sounds up to 1 degree apart


In the horizontal plane, sound localisation occurs either through....or....for what frequencies does this apply

interaural intensity difference to localise >3khz
Interaural phase difference to localise <1.5kHz


How are interaural time differences used to localised sound source in the horizontal plane

When sound source nearer to one side of head, resulting sound waves reach closer ear first producing ITD in arrival time which is detected by specialised phase locked neuron in brainstem (for periodic sounds, timing difference produces interaural phase difference)


What feature enables localisation of sound source through ITD for low frequency sounds

low frequency sound waves are much longer than distance between the ears


Why is ITD not applicable for higher frequencies

Sound wave is smaller than distance between ears so sound wave passes through more than half a cycle before reaching other ear --> phase ambiguity


How do we localise high frequency sounds in horizontal plane

Interaural intensity difference - exists because sound intensity is attenuated at far ear by acoustical shadow cast by head (extent determined by location of sound source)


Why can't IID be applied for low frequency sounds

these sound waves diffract around the head so the intensities at both ears are similar.


How are binaural localisation cues initially processed

initially processed by binaural neurones in superior olivary nucleus


What is the first site of binaural convergence in auditory pathway

binaural neurones in superior olivary nucleus


Superior olivary nucleus receives inputs from the ...

Cochlear nerve on both sides of brainstem


Neurones in the medial superior olive are sensitive to...

Interaural time difference i.e. initially responsible for neural processing of binaural localisation cues


Why are neurones in the medial superior olive sensitive to interaural time difference

because they receive excitatory input from cells from bilateral CN, which have responses phase locked to lower frequency sounds.


Each superior olive neurones responds maximally to...

specific interaural time difference, and because each interaural delay varies with sound location, each superior olive neurone corresponds to a specific position in the horizontal plane


What is one possible mechanism for processing interaural time differences in medial superior olive

Jeffress coincidence model (seen in birds only):
axonal delay lines e.g. input from contralateral ear travel along longer path so MSO neurones are coincidence detectors, responding/firing when there is just the right amount of ipsilateral delay so that inputs from other side arrive at same time, producing two EPSPs which summate which more strongly excite an olivary neurone that with just one EPSP from each ear.


Why was Jefferson's model for ITD processing proved wrong

there is no evidence of axonal delay lines
Instead, MSO neurones are sensitive to phase of waveform due to phase locking, so relative activity of neurones in left/right MSO reflects interaural time difference (ITD)


Neurones in lateral superior olive sensitive to...

Interaural intensity difference


How are neurones in lateral superior olive sensitive to interaural intensity difference

receive excitatory input from ipsilateral cochlea but inhibitory input from contralateral cochlea -->only respond when sound is louder on ipsilateral side


Binaural interactions occurring at brainstem levels are transmitted to...

Cortex - so some A1 neurones sensitive to specific ITD or IID


In vertical plane, can ITD and IID be used



How do we localise in vertical plane

vertical localisation and front/back discrimination relies on monaural spectral cues generated by external ear