The auditory system Flashcards
What do we use sound for?
communication, emotion, recognising different sounds, 3D view of auditorial world, survival
What features of sound need encoding?
frequency, intensity, onset, duration
What are the 3 chambers of the cochlear spiral from top to bottom?
scala vestibuli
scala media
scala tympani
Where is the organ of corti and basilar membrane?
between the scala media and tympani
What parts of the cochlear spiral contain perilymph?
scala vestibuli and scala tympani
What are the contents of perilymph?
low K+, normal Ca2+ and high Na+ (same as extracellualr fluid)
What are the contents of endolymph compared to perilymph?
high K+, low Ca+ and low Na+
What is the purpose of the different ion concentrations in the cochlear chambers?
electrical driving force into hair cells is about 140mV overall which is vital for function
What is tonotopic organisation of the cochlea?
apex detects low frequencies and base detects high frequencies
sound wave travels from base to apex
How is the frequency of sound encoded?
the brain doesn’t receive sound- the position of the hair cell along the membrane that is depolairsed interprets how high or low it is
not coded by firing pattern, represented by location of cell only
What is the characteristic frequency location?
the location on the basilar membrane where there is maximal movement for a certain frequency
lower sounds will be closer to the apex, higher will be closer to the base
determined by width and stiffness of basilar membrane
High frequency sounds
short wavelength, low energy so don’t travel far and peak movement is at the base
Low frequency sounds
long wavelength, high energy so travels further and peak movement is at the apex
What is the role of inner hair cells?
primary sensory recpetors, encode all of the auditory info and pass it onto nerve fibres
What is the role of outer hair cells?
not sensory receptors, act as cochlear amplifiers by shortening and lengthening in time with sound frequency- electromotility
How do inner hair cells initiate action potentials?
mechanosensitive ion channels that are called MET channels are at the tips of the shorter stereocilia
connected to tip links that pull the channels open when sound waves move the IHCs
At rest when no sound is present:
slight tension in tip links
resting inward MET current
K+ enters down large electrical gradient
main driving force pushing K+ into cells
conc gradient for K+ exit is bigger than entry
hair bundle in endolymph, depolarised
When there is excitatory stimulation:
large deflection of hair bundle toward taller stereocilia
increases tension in tip linkes and opens MET channels- high current
depolarises hair cells activating Ca2+ channels
increases activity in nerve fibres
activates K+ channels and K+ exits into perilymph to repolarise cell
When there is inhibitory stimulation:
large deflection in hair bundle toward shorter stereocilia
tip links slacken and MET channels close
turns off MET current and hyperpolarises hair cell below resting potential
none-little neuronal activity
k+ channels open longer to fully repolarise cell
How do hair cells react to sustained sound?
moves hair bundle back and forth at the sound frequency
creates cycle of membrane potential matching the frequency generating pulses of NT release
Outer hair cell structure
do not have many afferents due to lack of sensory role
V shaped hair bundles with tip links and MET channels, VGICs and prestin in their cell membrane
How do outer hair cells amplify stimuli?
shorten when depolarised
elongate when hyperpolarised
movement up and down amplifies movement of basilar membrane over narrow CF region
makes them sharply tuned and highly sensitive
What happens if outer hair cells are damaged?
stimulation of IHC bundle is weaker so loss causes severe hearing loss but not complete deafness
How is sound localised in the horizontal plane?
interaural level differences
interaural timing differences
Interaural level differences
the differences in loudness of the same sound at the two ears as little as 1 or 2 db
head acts as a barrier that reflects or absorbs sound waves
Interaural timing differences
the difference in the arrival time of the same sound at the two ears as small as 10um
sounds are delayed in the far ear compared to the near one
Sound localisation centres
through the cochlear nucleus to the lateral superior olive, medial superior olive and medial nucleus of trapezoid body
one of each either side of the midline so theres always a near and far one
What are the main centres involved in ILD and ILT detection?
lateral superior olive
medial superior olive
How are inputs from the two LSOs integrated?
each LSO receives + from near ear and - from far ear
outputs of LSOs are opposite but balanced, where excitatory input is the highest is where sound is loudest
combined input gives accurate indication of position of sound
How is sound in a central position received?
the more central the sound is the more the two LSOs overlap so accuracy is very high
so small changes in the centre are detected rapidly which is vital for hunting
How are ILDs encoded by the two LSOs?
two LSOs act as two channels broadly tuned to sounds mainly from the same side of the head
made up of many neurons tuned to different ILDs
position recognised by balanced and opposite inputs from LSOs
Tiny differences in a sounds arrival time are detected by what centre?
MSO by the principle neurons
When is the MSO maximally active?
when both inputs arrive simultaneously
How does the MSO detect differences in the timing of sounds?
excitatory inputs from both ears converge on neurons in MSO
neurons are different lengths which is key to function
activity from far ear takes longer to reach MSO than the input from the near ear
mirror image of MSO on other side of head
neurons compare coincident arrival of excitatory inputs
How are the initial circuits for sound localisation calibrated?
using alignment with the visual map- hardwired auditory map is aligned to overlay the visual map
based on geneticd and formed early on, doesn’t depend on sensory function
What is the function of the medial nucleus of the trapezoid body in ILD?
turns input from the far ear into an inhibitory signal
What nuclei are involved in the integration of visual and auditory information?
central nucleus of inferior colliculus
external nucleus of inferior colliculus
optic tectum
Central nucleus of inferior colliculus
neurons tuned to specific ITDs
organised in frequency specific layers
show little adaptive plasticity
External nucleus of inferior colliculus
projections from ICC layers converge on neurons of ICX
forms map of auditory space
site of large scale adaptive plasticity
Optic tectum
combines auditory map from ICX with visual map
overlapping receptive fields
feedback projections to ICX
site of large adaptive plasticity
What is the difference between the auditory and visual maps?
visual map adjusts rapidly but auditory takes longer
visual map may be dominant for space perception over auditory
What part of the brainstem is involved in ITD detection?
medial superior olive
What parts of the brainstem is involved in ILD detection?
lateral superior olive
medial nucleus of trapezoid body
