L5 - overview of the auditory system Flashcards
what do we use sound for ?
communication emotion recognise different sound topographic view of the auditory world survival
how is sound relayed from the ear to the brain
highly specialised mechanisms in the cochlea
what features of sound need encoding
sound frequency (pitch) - Hz
sound intensity - dB
onset
duration
what range of sound frequency can we hear
x10^3 Hz
achieved by cochlear mechanisms and physiology of the hair cells
what range of sound intensity can we hear
x10^12 dB
what is a rapid onset important for ?
localising different sounds and creating a topographic map of auditory space
structure of human ear
sound enters outer ear
travels across the middle ear using the ossicles
transferred to the cochlea in the inner ear
what is the cochlea
snail shell structure that contains sensory hair cells and nerve fibres that transmits the sound to the brain as a neuronal signal
what are the 3 chambers of the cochlea
scala vestiboli
scala media
scala tympani
what is the organ of corti
sensory organ in the cochlea that contains the sensory hair cells, is located at the bottom of the Scala media
what innervates the cochlea
the auditory nerve
what do the Scala vestibuli and tympani contain
perilymph - solution similar to the ECF - low K+ and normal Ca2+ concentrations
what does the Scala media contain
endolymph - very high K+ conc and very low Ca2+ conc
due to having a very active pumping from cells in the stria vascularis
what is the potential in the Scala. media ?
endocochlear potential - important for cochlea function
difference in potential of. around 140mv to that of solution of hair cells
what are the inner hair cells of organ of corti
single row, sensory cells of. cochlea
outer hair cells of organ of corti
3 rows - function as the cochlea amplifier
what does the frequency range depend on ?
size of animal
pressure to communicate over long distance, hunting and survival
low frequencies can travel greater distances than higher frequencies
specialised functions to do with ultrasonic sounds
echolocation in bats
sonar for dolphins
what is the tonotopic organisation of the mammalian cochlea
spiraled cochlea - allows to hear as much as possible
- hair cells at base respond to high frequency sounds
- hair cells at apex respond to low frequency sounds
what is the reserve piano
the sensory hair cells in the cochlea are like individual keys on a piano. Each key being a narrow frequency range
what is tonotopicity established by
the basilar membrane travelling wave
what is the basilar membrane travelling wave
sound vibrations enter cochlea, there is a pressure wave that travels along the different compartments from the base of the cochlea to the apex. This creates a wave along the basilar membrane that causes a maximal movement that corresponds to the frequency of the sound
describe the width and stiffness of the basilar membrane
narrow and stiffer at the base
wider and more flexible at the apex
therefore the highest sound frequencies cause a peal movement towards the base of the cochlea and lower frequencies cause a peak a the apex
describe the general hair cell
stereocillia hair bundle projecting from apical surface and the mechanoelectrical transducer channels located on shorter rows of stereocilia connected to taller rows by tip links
what happens when force is applied to tip links
pull open the MET channels allowing current into the hair cell
state of tip links at rest
slight tension on a few of the tip links so a few channels are open
what does slight tension on tip links lead to at rest
inward resting transducer current carried by transducer ions, depolarises inside of the hair cell slightly, giving it a more depolarised resting membrane potential
what is the apical surface of the hair cell and steriocillia surrounded by ?
endolymph
what is the body of the hair cell surrounded by
perilymph
what does a slightly depolarised resting potential the hair cell result in
activates some of the calcium channels and leads to a resting action potential firing rate in the nerve fibres called the spontaneous rate
what is the spontaneous rate caused by
slight tension on tip links, activating calcium channels leading to a resting action potential firing rate in the nerve fibres
what is excitatory deflection
when the hair cell is being stimulated - in response to excitatory phase, elicits a large movement/deflection of the hair bundle, pulls tip links, opens all of the transducer channels in the cell. Leads to a large current that depolarises the cell. This depolarisation opens the voltage gated calcium channel. Causes neurotransmitter release and increases the AP frequency in the nerve fibre. The depolarisation also activates K+ channels, means K+ moves out of the cell, helps to depolarise the hair cell back top its resting potential, ready for the next cycle
what is inhibitory deflection
large deflection of the hair bundle in the opposite direction
tip links become slack - closes most of the transducer channel
no MET current
fully hyper polarises the hair cell below its resting potential
resulting in no or very few action potentials
potassium channels in cell are open longer than Ca2+ as have a more negative voltage activation range, allows cell to become fully hyperpolarsied
what are the primary sensory receptors of the mammalian cortex
primary sensory receptors
function of the primary sensory receptors
At rest the is a resting transducer current
Creates spontaneous activity in the nerve fibres
In response to sound, the nerve bundle deflects and the hair cell depolarises
Increases nerve activity
Sound pulls hair bundle back, hyperpolarizes the cell
Reduces the firing load
The membrane pot inside the cell therefore ossiclates between polarised and hyperpolarized values at the same fequency as the stimulating sound
What is the IHC response to low frequencies
the membrane potential follows the sound and does so up to around 2-3KHz
where do oscillations begin to saturate in IHCs
2-3KHz
how do IHCs respond to sounds above 2-3KHz
with a sustained graded receptor potential
what AP do IHC nerve fibres show at low frequency
pulses of AP that match the sound frequency
what AP do IHC nerve fibres show at high frequency
a sustained AP
role of outer hair cells
function as the cochlea amplifier
how do OHCs receive info
contacted by many efferent fibres, these carry the instructions from the brain to the OHCs.
these are inhibitory and turn the outer hair cells off
what are the efferent fibres associated with
postsynaptic cisterns
what cell membrane do OHCs have
prestin in the cell membrane, molecule that allows the cell to contract or elongate in response to changes in the membrane potential, allows the cell to become electrimotile
outline the transduction of OHCs
1) resting transducer current, larger than the IHCs, therefore the resting potential is more depolarised
2) sound stimulation - OHC depolarisation and hyper polarisation matching the sound frequency
3) . causing the cell to shorten and elongate
4) this matches the sound frequency and makes the cell a dancing hair cell
5) shortens with depolarisation and lengthens with hyperpolarsiation
6) as there are 3 layers of cells, this combined with the up and down movement acts as positive feedback to increase the movement of the basilar membrane and increase. stimulation of the inner hair cells
change in movement of the basilar membrane without OHC amplification
movement of basilar membrane is small but still shows a degree of tuning (decreases hearing by 40-50dB)
change in movement of the basilar membrane with OHC amplification
sharp increase of basilar membrane displacement over very narrow region, corresponds to the sound frequency
what does sharp tuning ensure
ensures each hair cell is sharply tuned to narrow frequency bands
type 1 afferent neurons in the cochlea
innervate IHCs
each IHC has 10-30 type 1 afferents - more resilient to nerve damage
each fibre contacts a single IHC
function - to carry all the sound from IHCs to the cochlea nucleus
type 2 afferent neurons in the cochlea
enter the organ of corti and turn basalt to intercalate OHCs
branched contact up to 30 OHCs also synapse in the cochlea nucleus
not sure of function
