Lecture 6 Auditory system Flashcards
Evolution importance of sound
- Early mammals small and nocturnal.
- Evolved massive range of frequency and intensity sensitivity
ALSO evolved independently 3 times
Range of sound intensity that needs to be encoded
x10^12
Perilymph concentration of K+/Ca2+
K+ 5mM
Ca2+ 1.3mM
Endolymph concentration of K+/Ca2+
K+ 150mM
Ca2+ 20 micromolar
Stria vascularis
Create unusual composition of ICF as actively pump K+ into SM creating endocochlear potential
Tonotopic organisation - describe shape of cochlea
Spiral
Tonotopic organisation - describe base/apex
Base HC = hi freq
Apex = lo freq
preserved in org of nerve fibres and along auditory pathway
Where is tonotopicity preserved
cochlear BS Midbrain Thalamus Cortex
Describe reverse piano
SHC in cochlea organised like keys on piano
Each IHC = individual key
Reverse as sound of 1 freq activates key detected by body
Pitch apical vs base
Base = high pitch
vice versa
How much stiffer is base of basilar membrane
100x
Describe general hair cell
Stereocilia on apical surface
Mechanoelectrical transducer channels with tip links
VG K+ channels - basolateral - recycle K+ to repolarise, also cause depolarisation
VG Ca2+ channels for Ca2+ influx for release of NT
Nerve fibres
AT REST GHC
VM = -55mV slight tension on tip links rest MET current large elect grad K+ entry depolarises rest Vm resting (tonic activity) of nerve fibres due to slight dep meaning Ca2+ in
Excitatory deflection
Large deflection hair bundle Max tip link tension Large MET current Fully dep HC -30mV rapid train of AP
Describe repolarisation after excitatory deflection
VG K+ channels move K+ out to repolarise HC to rest Vm
Low K+ in perilymph vs HC means rapid
Inhibitory deflection
Large deflection opp direction Min tip link tension No MET current Fully hyperpolarise HC below rest Vm -65mV No or very few action potentials
Explain hyperpolarisation by K+ channels
Unusual K+ channels dep and hyper
Because K+ channels open longer vs Ca2+
IHC function and follows…
Primary sensory receptor of cochlea
same sequence as GHC
IHC K+ channels
Vary in activation ranges, size current, kinetics
fast, slow, negative
Explain change in Vm inside IHC
Vm oscillates as between dep/hyper at same frequency as stimulus
low freq = AP pulse matches sound frequency
high freq = sustained AP train
Describe OHC
V shape hair bundle
Majority of nerve contacts are efferent fibres
Prestin in cell membrane required for electromotility
explain efferent OHC fibres
inhibitory and assoc with postsynaptic cisterns
Transduction of OHC
rest = -40mV
with sound get dep/hyper matching sound freq
Shortening of OHC
BM moves up tip links tighten OHC dep motor proteins contract OHC contract shorter amplifies movement of BM
Importance of cochlea amplification
causes sharp increae in BM displacement over narrow region
so each IHC tuned to narrow frequency band
Type I afferent carry
all sound into from IHC to cochlea nucleus
Type II turn
basally to innervate higher freq
Type II structure
branched to conact up to 30 OHC + synapse onto cochlear nucleus
Type II function
- Thought to be related to nociception in the cochlea caused by damage/overstimulation.
Movement of the BM displaces
the hair bundles of HCs in the organ of Corti, opening MET channels that depolarise the cells.
The OHC RP activates and enhances
activates electromotility that enhances the mechanical stimulation of IHCs and improves tuning in the cochlea.
