Sorry, didn't explain myself carefully enough. I was referring to outer hair
cells, which have a distinctly different means of amplification. I was
running of old memory so checked it out again. Below are some links and
refs, can explain it better than me. These hairs can produce spontaneous
oscillations, hence my interest. However, as noted in one article, some
invertebrates also have acute hearing but lack outer hair cells, so as usual
nothing is straightforward.
Biophys J. 1995 Jul;69(1):138-47. Related Articles,
Modeling the active process of the cochlea: phase relations, amplification,
and spontaneous oscillation.
Markin VS, Hudspeth AJ.
Howard Hughes Medical Institute, University of Texas Southwestern Medical
Center, Dallas 75235-9117, USA.
The high sensitivity and sharp frequency selectivity of acoustical signal
transduction in the cochlea suggest that an active process pumps energy into
the basilar membrane's oscillations. This function is generally attributed
to outer hair cells, but its exact mechanism remains uncertain. Several
classical models of amplification represent the load upon the basilar
membrane as a single mass. Such models encounter a fundamental difficulty,
however: the phase difference between basilar-membrane movement and the
force generated by outer hair cells inhibits, rather than amplifies, the
modeled basilar-membrane oscillations. For this reason, modelers must
introduce artificially either negative impedance or an appropriate phase
shift, neither of which is justified by physical analysis of the system. We
consider here a physical model based upon the recent demonstration that the
basilar membrane and reticular lamina can move independently, albeit with
elastic coupling through outer hair cells. The mechanical model comprises
two resonant masses, representing the basilar membrane and the reticular
lamina, coupled through an intermediate spring, the outer hair cells. The
spring's set point changes in response to displacement of the reticular
lamina, which causes deflection of the hair bundles, variation of outer hair
cell length and, hence, force production. Depending upon the frequency of
the acoustical input, the basilar membrane and reticular lamina can
oscillate either in phase or in counterphase. In the latter instance, the
force produced by hair cells leads basilar-membrane oscillation, energy is
pumped into basilar-membrane movement, and an external input can be strongly
amplified. The model is also capable of producing spontaneous oscillation.
In agreement with experimental observations, the model describes mechanical
relaxation of the basilar membrane after electrical stimulation causes outer
hair cells to change their length.
PMID: 7669891 [PubMed - indexed for MEDLINE]
Geleoc GS, Holt JR. Auditory amplification: outer hair cells pres the issue.
Trends Neurosci. 2003 Mar;26(3):115-7. Review.
PMID: 12591210 [PubMed - indexed for MEDLINE]
Mechanical Amplification by Active Movement of Hair Bundles
Near the threshold of audition, human hearing is about 100 times as
sensitive as would be anticipated from the ear's physical properties. The
cochlea's extraordinary responsiveness stems from its ability to amplify
mechanical inputs. So potent is this active process that even normal human
ears can spontaneously emit sounds when the amplifier is activated in a very
quiet environment! Although cell-body contractions of outer hair cells are
thought to mediate amplification in the mammalian cochlea, vertebrates
without outer hair cells also display highly sensitive, sharply tuned
hearing and spontaneous otoacoustic emissions. In these animals the aural
amplifier must reside elsewhere. We have therefore examined an alternative
mechanism, active hair-bundle movement, to determine its basis and potential
role in mechanical amplification.
When situated in a fluid environment similar to that of the intact inner
ear, hair bundles from the frog's inner ear often oscillate spontaneously.
Because the statistical nature of mechanoelectrical channel gating renders
hair bundles very flexible, and can even make their stiffness negative in
sign, this movement might reflect the thermal motion of very compliant
structures. To examine this possibility, we compared the unprovoked motion
of individual hair bundles to their movement in response to very weak
mechanical stimulation. Making use of the fluctuation-dissipation theorem, a
basic physical relation, we demonstrated that the observed spontaneous
bundle movement exceeds that attributable to brownian motion. Thus each hair
bundle must contain an energy source capable of performing mechanical work,
the active process also responsible for amplification.
HOW THE EAR'S WORKS WORK:
TRANSDUCTION AND AMPLIFICATION BY HAIR CELLS
Uniquely among vertebrate sensory receptors, the hair cell amplifies its
inputs. An active process in auditory organs increases responsiveness to
sound by over one-hundredfold and sharpens frequency selectivity. Two
epiphenomena arise from the active process, nonlinearity in transduction and
spontaneous otoacoustic emissions (SOAEs). Although changes in the length of
outer hair cells are thought to mediate amplification in the mammalian
cochlea, the auditory receptor organs of non-mammalian tetrapods, which lack
electromotile hair cells, display essentially identical sensitivity, tuning,
nonlinearity, and SOAEs. The active process necessary to explain the
properties of hearing in these animals may therefore constitute part or all
of the mammalian cochlear amplifier as well.
Science. 1995 Mar 31;267(5206):2006-9. Related Articles,
High-frequency motility of outer hair cells and the cochlear amplifier.
Dallos P, Evans BN.
Department of Neurobiology and Physiology, Northwestern University,
Evanston, Illinois 60208, USA.
Outer hair cells undergo somatic elongation-contraction cycles in vitro when
electrically stimulated. This "electromotile" response is assumed to
underlie the high sensitivity and frequency selectivity of amplification in
the mammalian cochlea. This process, presumably operating on a
cycle-by-cycle basis at the frequency of the stimulus, is believed to
provide mechanical feedback in vivo. However, if driven by the receptor
potential of the cell, the mechanical feedback is expected to be severely
attenuated at high frequencies because of electrical low-pass filtering by
the outer hair cell basolateral membrane. It is proposed that
electromotility at high frequencies is driven instead by extracellular
potential gradients across the hair cell, and it is shown that this driving
voltage is not subject to low-pass filtering and is sufficiently large. It
is further shown that if the filtering properties of the cell membrane are
canceled, taking advantage of the electrical characteristics of isolated
outer hair cells in a partitioning glass microchamber, then the lower bound
of the motor's bandwidth is approximately 22 kilohertz, a number determined
only by the limitations of our instrumentation.
PMID: 7701325 [PubMed - indexed for MEDLINE]
Outer hair cells have a special function within the cochlea. They are
shaped cylindrically, like a can, and have stereocilia at the top of the
cell, and a nucleus at the bottom. When the stereocilia are bent in response
to a sound wave, an electromotile response occurs. This means the cell
changes in length. So, with every sound wave, the cell shortens and then
elongates. This pushes against the tectoral membrane, selectively amplifying
the vibration of the basilar membrane. This allows us to hear very quiet
sounds. The electromotile response of an outer hair cell is shown in the
"Didier A. Depireux" <didier at bluenote.isr.umd.edu> wrote in message
news:bd741h$jv7$1 at grapevine.wam.umd.edu...
> John H. <john at faraway.com> wrote:
>> > The other issue I'd like to raise here is that hearing has two primary
> > processes. Most sound is heard through non-amplification in the inner
> > but there is a mechanism where some hairs are acutely sensitive to v.
> > levels
>> I am not Tony, but... what are you talking about?
> First, hearing is always amplified. In your ear, you have 1 row of inner
> hair cells, and 3 rows of outer hair cells. The role of the outer hair
> is to amplify any incoming sound. Anything below 40 dB or so (it's
> dependent) will be majorly amplified by the OHCs. Objective tinnitus,
> produced inside the cochlea, is due to a group of OHCs spontaneously
> oscillating, thereby causing fluid movement near the inner hair cell and
> causing the sensation of a sound.
>> The ear is a very active system indeed, not a passive frequency
> decomposition system. Unfortunately, it gets so active that it sometimes
> goes awry and generates sounds.
> Didier A Depireux ddepi001 at umaryland.edudidier at isr.umd.edu> 685 W.Baltimore Str http://neurobiology.umaryland.edu/depireux.htm> Anatomy and Neurobiology Phone: 410-706-1272 (off)
> University of Maryland -1273 (lab)
> Baltimore MD 21201 USA Fax: 1-410-706-2512