Download Cochlear potentials

Lecture 9
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Electrical potentials can be recorded from
various parts of the cochlea
Resting potentials are the positive and
negative direct current (DC) of the
polarisation of the various tissues and their
surrounding fluid
Receptor potentials are electrical responses
from a receptor cell (cochlear hair cell) that
result when the cell is stimulated
May involve alternating current or direct
current
 The presence of a receptor potential does not
mean that the nervous system is aware of the
stimulus, it reflects the fact that the HC itself
has responded
- It is the transmission of chemical mediator
across the synapse and the resulting neural
firing that indicates that the signal has now
activated the nervous system
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Can be steady (DC) or fluctuating (AC) over
time
‘Resting’ potentials (DC)
◦ Endocochlear potential (+80 mV)
◦ HC intracellular resting potentials
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HC receptor potentials
◦ See figure – responses to 50-ms tone bursts (at
various frequencies as marked)
◦ Receptor potential is the intracellular voltage
change in response to sound
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Hair cell receptor
potentials
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Have both a fluctuating (AC) and a steady
(DC) component
◦ AC component broadly follows waveform of
stimulus (see upper traces of figure)
◦ But the average amplitude of the AC waveform is
not the same as when the stimulus is off (at the end
of the burst)
◦ The slight positive shift in the average AC response
is equivalent to a DC (steady) component
superimposed on it
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◦ Note that DC component is depolarising (positivegoing)
(Recall that in IHCs, depolarisation causes
neurotransmitter release)
◦ Also note that the AC component of the receptor
potential gets smaller with increasing stimulus
frequency & the DC relatively larger
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Massed effects of electrical activity of
individual cells. Can be recorded at various
sites in & around cochlea. (Potentially clinically
useful)
◦ Cochlear microphonic – voltage recorded as
extracellular correlate of AC component of receptor
currents/potentials. Predominantly due to activity of
OHCs
◦ Summating potential – extracellular correlate of DC
component of receptor currents. Due to both IHCs &
OHCs(?)
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Cochlear microphonic (CM) is an alternating
current (AC) voltage that mirrors the
waveform of the acoustic stimulus. It is
dominated by the outer hair cells of the organ
of Corti. The magnitude of the recording is
dependent on the proximity of the recording
electrodes to the hair cells. The CM is
proportional to the displacement of the
basilar membrane
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The cochlea transduces sound waves into an
energy form that is useful to the auditory nerve
This resembles the action of the microphone
which converts the pressure wave from the
speakers mouth into an alternating electrical
current
The CM is the measurable electrical response of
the hair cells of the cochlea
The CM is probably the result of the changes in
polarisation caused by the bending back and
forth of the hair cell cilia
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For every up and down cycle of the BM there
is one in and out cycle of the stereocilia of
the OHCs causing them to become polarised
and hyperpolarised
The size of the CM has been measured by
placing a pickup electrodes over the round
window and in some cases within the cochlea
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The summating potential (SP), first described
by Tasaki et al. in 1954, is the direct
current (DC) response of the hair cells as they
move in conjunction with the basilar
membrane. The SP is the stimulus-related
potential of the cochlea. Although historically
it has been the least studied, renewed
interest has surfaced due to changes in the SP
reported in cases of endolymphatic hydrops
or Ménière's disease.
◦ Compound action potential – summed activity of a
number primary afferent neurons firing together at
the onset of a stimulus
 Typically recorded in response to clicks or tone-bursts
 Distinct negative-going peak within ~ 1 ms of
transient stimulus, denoted N1 (secondary N2 peak
sometimes seen)
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The basilar membrane and the hair cells of the
cochlea function as a sharply tuned frequency
analyser.
Sound is transmitted to the inner ear via vibration
of the tympanic membrane, leading to movement
of the middle ear bones (malleus, incus, and
stapes). Movement of the stapes on the oval
window generates a pressure wave in
the perilymph within the cochlea, causing the
basilar membrane to vibrate.
Sounds of different frequencies vibrate different
parts of the basilar membrane, and the point of
maximal vibration amplitude depends on the
sound frequency.
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As the basilar membrane vibrates, the hair
cells attached to this membrane are rhythmically
pushed up against the tectorial membrane,
bending the hair cell stereocilia. This opens
mechanically gated ion channels on the hair cell,
allowing influx of potassium (K+) and
calcium(Ca2+) ions.
The flow of ions generates an AC current through
the hair cell surface, at the same frequency as the
acoustic stimulus. This measurable AC voltage is
called the cochlear microphonic (CM), which
mimics the stimulus.
The hair cells function as a transducer,
converting the mechanical movement of the
basilar membrane into electrical voltage.
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The depolarised hair cell releases
neurotransmitters across a synapse to primary
auditory neurons of the spiral ganglion. Upon
reaching receptors on the postsynaptic spiral
ganglion neurons, the neurotransmitters induce
a postsynaptic potential or generator potential in
the neuronal projections. When a certain
threshold potential is reached, the spiral
ganglion neuron fires an action potential, which
enters the auditory processing pathway of the
brain.
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Process whereby vibrations of the BM at the
characteristic frequency (CF) (for a particular
site) are amplified by an intrinsic forcegenerating mechanism
‘Active’ – energy-consuming
Implemented by OHCs
◦ Isolated OHCs in vitro (artificial environment) have
electromotile properties (Brownell, 1985)
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Mechanosensory outer hair cells play an essential
role in the amplification of sound-induced
vibrations within the mammalian cochlea due to
their ability to contract or elongate following
changes of the intracellular potential. This unique
property of outer hair cells is known as
electromotility.
Driven by potential across cell membrane
OHC exhibits length changes of up to ~5%
(Depolarisation -> shortening)
Process appears fast and strong enough to
amplify BM/organ of Corti vibrations in vivo at
acoustic frequencies
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Cellular mechanism?
◦ Although cell bodies contain actin, probably not the
same as typical actin-myosin mechanism as in
muscle fibres
 Estimated factor of 10 faster than muscle fibre
activation
◦ ‘Motors’ probably in or very near cell membrane
itself
◦ Candidate motor protein recently identified –
‘prestin’
◦ OHC electromotility represents an electromechanical (‘reverse’) transduction process
◦ Recall mechano-electrical (‘forward’) transduction
common to both types of HCs
◦ Could complete a positive feedback loop in vivo, to
amplify BM vibrations
◦ IHCs in contrast simply detect these amplified BM
vibrations
◦ Entire chain of events that amplifies input to IHCs
also called the ‘cochlear amplifier’
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The cochlear amplifier is a positive feedback
mechanism within the cochlea that provides
acute sensitivity in the mammalian auditory
system. The main component of the cochlear
amplifier is the outer hair cell (OHC) which
increases the amplitude and frequency selectivity
of sound vibrations using electromechanical
feedback
Upon depolarization, the OHC begins its process
of amplification through positive feedback.
This positive feedback mechanism is achieved
through a somatic motor and a hair bundle
motor which operate independently of one
another.
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Emission of sound energy from the ear
Originate as pressure disturbances within the
cochlea
Middle ear mechanism is bi-directional
Hence, acoustic waves generated in ear canal
Can be spontaneous or evoked
Working definition - ‘return or release of
acoustic energy from the cochlea’
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SOFT-TIPPED PROBE
(HOUSING TRANSDUCERS)
STIMULUS
GENERATION
SIGNAL
ANALYSIS
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Exhibited by all mammalian (and many nonmammalian!) species
In all cases, generation associated with
mechanical processes of the normal cochlea
◦ Window on cochlear function, at least down to
OHCs and cochlear potentials associated with their
function
Give evidence of active processes and/or nonlinearity
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Note that exact generation mechanisms are
unclear!
Spontaneous (SOAE)
◦ Probably represent uncontrolled oscillations of
‘overactive’ cochlear amplifier at one or more
specific frequencies
◦ Not usually audible to individual => not usually a
source of tinnitus
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Evoked
◦ Transient-evoked otoacoustic emission (TEOAE)
 Stimulus –clicks or tone bursts
 Residual (decaying) pressure disturbances within
cochlea that persist for some time following the
transient stimulus
(Sustained by cochlear amplifier)
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Distortion product otoacoustic emission (DPOAE)
 Intermodulation (distortion) product (e.g. at frequency
2f1-f2 ) due to nonlinear interactions between a pair of
pure-tone stimuli (of frequencies f1 and f2)
 Subjective (perceptual) correlate long-known
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◦ Stimulus frequency otoacoustic emission (SFOAE )
 Low-level pure tone in response to a stimulus pure
tone of the same frequency
 Measured by observing the interaction between
stimulus and response, as stimulus frequency is swept
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Although all types rely on normal cochlear
function (mechanics)
◦ Response levels are quite variable between types
(and highly variable between subjects)
◦ Exact response patterns highly individual to subject
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May be regarded as a ‘by-product’
(‘epiphenomenon’) of normal workings of
cochlea, but
Sensitive to physiological insults / pathologies
that are primary causes of cochlear hearing
losses
◦ Noise-induced hearing loss
◦ Majority of ototoxic drugs
◦ Majority of congenital disorders
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All evoked OAEs are usually measurable in
normal ears
Tend to be reduced or absent in
conductive/‘cochlear’ impairment
=> Valuable as a non-invasive screening test
◦ However, relationship to thresholds is not
straightforward => little value in quantifying
(estimating) thresholds
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