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ROLE OF RESTING MEMBRANE POTENTIAL (RMP) IN HEARING AND THE COCHLEAR -…
ROLE OF RESTING MEMBRANE POTENTIAL (RMP) IN HEARING AND THE COCHLEAR
structure of the auditory system
cochlear
keywords:
scala vestibuli
scala media
scala tympani
vestibular membrane
basilar membrane
endolymph
perilymph
organ of corti
hair cells (IHC & OHC)
tectorial membrane
nerve fibers
hair cells
IHC
fx: Information about the acoustic environment is conveyed to the auditory nerve fibers almost exclusively via the electrical signals of IHC
majority of afferent emanate from the inner hair cells
OHC
fx: boost the stimulus by mechanically amplifying the sound-driven vibrations of the cochlear partition.
the outer hair cells receive synaptic contact with efferent fibres
perilymph
composed of a typical interstitial fluid of K+ (less) and Na+ (more)
endolymph
atypical ECF, K+ (high), Na+ (low) and Ca+
electromechanics of the cochlear are driven by the hair cell RMP
cochlear amplifier:
is a positive feedback mechanism within the cochlea that provides acute sensitivity in the mammalian auditory system.
outer hair cells
play a role in sound amplification and fine frequency selectivity through these mechanisms
somatic electromotility
OHC cell body and its
ability to elongate or contract
longitudinally due to changes in membrane potential. (bcs of the structure of the OHC)
outer hair cell electromotility is indeed modulated by the efferent neurotransmitter,
acetylcholine
.
structure: apical side of the OHC is mechanically coupled to the reticular lamina while the basal side of the OHC is coupled to the Deiter's cell cupula, cell body is not in direct contact with any structure and is surrounded by the fluid-like perilymph,
Prestin
is the transmembrane protein underlying the OHC's ability to elongate and contract, a process essential for OHC electromotility.
active hair bundle
hair bundles are bathed in the ECF, high in K and low in Ca at a +100mV potential, which optimizes the transduction process.
operates by
deflecting hair bundles
in the positive direction and providing positive feedback of the basilar membrane, increasing the movement of the basilar membrane which increases the response to a signal.
force
generated from a mechanical stimulus.
done through the use of the mechanoelectrical transduction
(MET) channel,
which allows for the passage of
Na+, K+, and Ca2+.
mechanism:
fast adaptation,
or channel re-closure, and
slow adaptation.
sound intensity depends on the size of the receptor potential in the hair cells
sound intensity
over a dynamic range may also be assisted by the differential sensitivities of the synapse
high sensitivity being located on the pillar side of the IHC
low sensitivity on the modiolar side
Mechano-electrical transduction and generation of the receptor potential
definition
: the conversion of the mechanical stimulus into an electrical signal called the receptor potential.
rotation of the bundle towards its taller edge was first shown to open specialized mechanosensitive ion channels
allows entry of small positively charged ions, depolarizing the hair cell from its resting potential of approximately −50mV
A small proportion of the mechanosensitive channels are normally open at rest, and rotation of the bundle towards its shorter edge closes those channels already open and hyperpolarizes the cell.
thus, sinusoidal motion of the hair bundle about its resting position, as might be produced by a pure tone, generates a receptor potential oscillating around rest
As the magnitude of the stimulus is increased, the receptor potential grows in size and eventually saturates, reaching the greatest amplitude for hair bundle displacements of only 0.5µm
The maximal stimulus transduced is therefore comparable to the diameter of a stereocilium
At auditory threshold, the smallest displacement of the hair bundle detectable is 1000 times less than this, and is estimated to produce a receptor potential of 0.1mV
At threshold, the hair bundle motion is comparable to the size of a hydrogen atom, testifying to the extraordinary sensitivity of the mechanotransduction process
1 more item...
Hair cells do not generate action potentials bcs lack on the voltage
