Cochlear amplifier

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Positive feedback mechanism within mammalian ears

title: "Cochlear amplifier" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["otology"] description: "Positive feedback mechanism within mammalian ears" topic_path: "general/otology" source: "https://en.wikipedia.org/wiki/Cochlear_amplifier" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Positive feedback mechanism within mammalian ears ::

The cochlear amplifier is a positive feedback mechanism within the cochlea that provides acute sensitivity in the mammalian auditory system.{{Cite journal | last1 = Matsumoto | first1 = N. | last2 = Kitani | first2 = R. | last3 = Maricle | first3 = A. | last4 = Mueller | first4 = M. | last5 = Kalinec | first5 = F. | title = Pivotal Role of Actin Depolymerization in the Regulation of Cochlear Outer Hair Cell Motility | doi = 10.1016/j.bpj.2010.08.015 | journal = Biophysical Journal | volume = 99 | issue = 7 | pages = 2067–2076 | year = 2010 | pmid = 20923640 | pmc =3042570 | bibcode = 2010BpJ....99.2067M | last1 = Dallos | first1 = P. | title = The active cochlea | journal = The Journal of Neuroscience | volume = 12 | issue = 12 | pages = 4575–4585 | year = 1992 | pmid = 1464757 | pmc = 6575778 | doi = 10.1523/JNEUROSCI.12-12-04575.1992

Discovery

The cochlear amplifier was first proposed in 1948 by Gold. This was around the time when Georg von Békésy was publishing articles observing the propagation of passive travelling waves in the dead cochlea.

Thirty years later the first recordings of emissions from the ear were captured by Kemp. This was confirmation that such an active mechanism was present in the ear. These emissions are now termed otoacoustic emissions and are produced by the cochlear amplifier.

The first modeling effort to define the cochlear amplifier was a simple augmentation of Georg von Békésy's passive traveling wave with an active component. In such a model, a lopsided pressure about the organ of Corti is hypothesized which actively adds to the passive traveling wave to form the active traveling wave. An early example of such a model was defined by Neely and Kim. The existence of otoacoustic emissions is interpreted as implying backward as well as forward traveling waves generated in the cochlea, as proposed by Shera and Guinan.

Contention still surrounds the existence and mechanism of the active traveling wave. An experimental study in 2006 show that, emissions from the ear occur with such a fast response that the slowly propagating active traveling waves can not explain them. Their explanation for fast emission propagation is the dual of the active traveling wave, the active compression wave. Active compression waves were proposed as early as 1980 by Wilson due to older experimental data. An example model of the active compression wave (pressure wave) is defined by Flax and Holmes in 2011.

Other theories for the active processes in the inner ear exist.

Function

Effect of sound waves on the cochlea

In the mammalian cochlea, wave amplification occurs via the outer hair cells (OHCs). These cells sit directly above the basilar membrane (BM), a structure highly sensitive to differences in frequency, and below the tectorial membrane (TM). When sound waves enter and travel through the scala vestibuli, they exert pressure on both BM and TM, which then vibrate the stereocilia of the OHCs are deflected toward the tallest stereocilia. This causes the tip links of the OHC hair bundle to open, allowing the inflow of Na+ and K+ ions, depolarizing the OHC. Upon depolarization, the OHC can then begin its process of amplification through force generated by the hair cell motors.

The somatic motor

The somatic motor is the OHC cell body and its ability to elongate or contract longitudinally due to changes in membrane potential. This function is aptly associated with the OHC structure within the organ of Corti. As seen through scanning electron micrograph imagery, the 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.{{Cite journal | last1 = Frolenkov | first1 = G. I. | title = Regulation of electromotility in the cochlear outer hair cell | doi = 10.1113/jphysiol.2006.114975 | journal = The Journal of Physiology | volume = 576 | issue = Pt 1 | pages = 43–48 | year = 2006 | pmid = 16887876 | pmc =1995623

Prestin is the transmembrane protein underlying the OHC's ability to elongate and contract, a process essential for OHC electromotility. This protein is voltage-sensitive. Contrary to previous research, prestin has also been shown to transport anions; the exact role of anion-transport in the somatic motor is still under investigation.{{Cite journal | last1 = Bai | first1 = J. P. | last2 = Surguchev | first2 = A. | last3 = Montoya | first3 = S. | last4 = Aronson | first4 = P. S. | last5 = Santos-Sacchi | first5 = J. | last6 = Navaratnam | first6 = D. | title = Prestin's Anion Transport and Voltage-Sensing Capabilities Are Independent | doi = 10.1016/j.bpj.2008.12.3948 | journal = Biophysical Journal | volume = 96 | issue = 8 | pages = 3179–3186 | pmc = 2718310 | year = 2009 | pmid = 19383462 | bibcode = 2009BpJ....96.3179B

Under resting conditions, it is thought that chloride is bound to allosteric sites in prestin. Upon deflection of the basilar membrane (BM) upwards and subsequent deflection of the hair bundles toward the tallest stereocilia, channels within the stereocilia open allowing the inflow of ions and depolarizing the OHC results. Intracellular chloride dissociates from the allosteric binding sites in prestin, causing contraction of prestin. Upon BM deflection downwards hyperpolarization of the OHC results, and intracellular chloride ions bind allosterically causing prestin expansion. The binding or dissociation of chloride causes a shift in prestin's membrane capacitance. A nonlinear capacitance (NLC) results which leads to a voltage-induced mechanical displacement of prestin into an elongated or contracted state as described above. The larger the voltage nonlinearity, the larger prestin's response; this shows a concentration specific voltage-sensitivity of prestin.

Prestin densely lines the lipid bilayer of the outer hair cell membranes.{{Cite journal | last1 = Santos-Sacchi | first1 = J. | title = Harmonics of outer hair cell motility | doi = 10.1016/S0006-3495(93)81247-5 | journal = Biophysical Journal | volume = 65 | issue = 5 | pages = 2217–2227 | year = 1993 | pmid = 8298045 | pmc =1225953 | bibcode = 1993BpJ....65.2217S | last1 = Nam | first1 = J. H. | last2 = Fettiplace | first2 = R. | doi = 10.1016/j.bpj.2010.03.052 | title = Force Transmission in the Organ of Corti Micromachine | journal = Biophysical Journal | volume = 98 | issue = 12 | pages = 2813–2821 | year = 2010 | pmid = 20550893 | pmc =2884234 | bibcode = 2010BpJ....98.2813N

Below the hair bundle is an actin-rich cuticular plate. It has been hypothesized that the role of actin depolymerization is crucial for regulation of the cochlear amplifier. Upon actin polymerization, electromotile amplitude and OHC length increase. These changes in actin polymerization do not alter NLC, showing that actin's role in the cochlear amplifier is separate from that of prestin.

The hair bundle motor

The hair bundle motor is the force generated from a mechanical stimulus. This is done through the use of the mechanoelectrical transduction (MET) channel, which allows for the passage of Na+, K+, and Ca2+.{{Cite journal | last1 = Sul | first1 = B. | last2 = Iwasa | first2 = K. H. | doi = 10.1016/j.bpj.2009.08.039 | title = Effectiveness of Hair Bundle Motility as the Cochlear Amplifier | journal = Biophysical Journal | volume = 97 | issue = 10 | pages = 2653–2663 | year = 2009 | pmid = 19917218 | pmc =2776295 | bibcode = 2009BpJ....97.2653S

Fast adaptation

This model relies upon a calcium gradient generated by the opening and closing of the MET channel. Positive deflection of the tip links stretches them in the direction of the tallest stereocilia, causing MET channel opening. This allows the passage of Na+, K+, and Ca2+.{{Cite journal | last1 = Choe | first1 = Y. | last2 = Magnasco | first2 = M. O. | last3 = Hudspeth | first3 = A. J. | title = A model for amplification of hair-bundle motion by cyclical binding of Ca2+ to mechanoelectrical-transduction channels | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 26 | pages = 15321–15326 | year = 1998 | pmid = 9860967 | pmc = 28041 | doi=10.1073/pnas.95.26.15321 | bibcode = 1998PNAS...9515321C | doi-access = free | last1 = Chan | first1 = D. K. | last2 = Hudspeth | first2 = A. J. | doi = 10.1038/nn1385 | title = Ca2+ current - driven nonlinear amplification by the mammalian cochlea in vitro | journal = Nature Neuroscience | volume = 8 | issue = 2 | pages = 149–155 | year = 2005 | pmid = 15643426 | pmc =2151387

Slow adaptation

As opposed to the fast adaptation model, slow adaptation relies on the myosin motor to alter the stiffness of the tip links leading to alterations of channel current. First, the stereocilia are deflected in the positive direction opening the MET channels and allowing for inflow of Na+, K+, and Ca2+. The entering current first increases and then quickly decreases due to myosin's release of tension of the tip link and subsequent closing of channels.{{Cite journal | last1 = Hacohen | first1 = N. | last2 = Assad | first2 = J. A. | last3 = Smith | first3 = W. J. | last4 = Corey | first4 = D. P. | title = Regulation of tension on hair-cell transduction channels: Displacement and calcium dependence | journal = The Journal of Neuroscience | volume = 9 | issue = 11 | pages = 3988–3997 | year = 1989 | pmid = 2555460 | pmc = 6569946 | doi = 10.1523/JNEUROSCI.09-11-03988.1989 | last1 = Howard | first1 = J. | last2 = Hudspeth | first2 = A. J. | title = Mechanical relaxation of the hair bundle mediates adaptation in mechanoelectrical transduction by the bullfrog's saccular hair cell | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 84 | issue = 9 | pages = 3064–3068 | year = 1987 | pmid = 3495007 | pmc = 304803 | doi=10.1073/pnas.84.9.3064 | bibcode = 1987PNAS...84.3064H | doi-access = free

Calcium has also been shown to play a crucial role in this mechanism. Experiments have shown that in reduced extracellular calcium, the myosin motor tightens, resulting in more open channels. Then, when additional channels are opened, the inflow of calcium acts to relax the myosin motor, which returns the tip links to their resting state, causing channels to close. This is hypothesized to occur via the binding of calcium to the myosin motor. The timecourse of this event is 10-20 milliseconds. This time scale reflects the time that is needed to amplify low frequencies. Although the largest contributor to slow adaptation is the tension-dependence, calcium-dependence acts as a useful feedback mechanism.

This mechanism of myosin's reaction to hair bundle deflection imparts sensitivity to small changes in hair bundle position.

Integration of electromotility and hair bundle dynamics

Electromotility of the OHC by prestin modulation produces significantly larger forces than the forces generated by deflection of the hair bundle. One experiment showed that the somatic motor produced a 40-fold greater force at the apical membrane and a sixfold greater force at the basilar membrane than the hair bundle motor. The difference in these two motors is that there are different polarities of hair bundle deflection for each motor. The hair bundle motor uses a positive deflection leading to a generation of force, while the somatic motor uses negative deflection to generate force. However, both the somatic motor and the hair bundle motor produce significant displacements of the basilar membrane. This, in turn, leads to augmentation of bundle movement and signal amplification.

The mechanical force that is generated by these mechanisms increases the movement of the basilar membrane. This, in turn, influences the deflection of the hair bundles of the inner hair cells. These cells are in contact with afferent fibers that are responsible for transmitting signals to the brain.

References

References

  1. Ashmore, Jonathan Felix. (1987). "A fast motile response in guinea-pig outer hair cells: the cellular basis of the cochlear amplifier". [[The Journal of Physiology]].
  2. Ashmore, Jonathan. (2008). "Cochlear Outer Hair Cell Motility". [[Physiological Reviews]].
  3. T. Gold 1948 : Hearing. II. The Physical Basis of the Action of the Cochlea
  4. D. T. Kemp 1978 : Stimulated acoustic emissions from within the human auditory system
  5. Neely, S. T. and Kim, D. O., 1986. A model for active elements in cochlear biomechanics. ''The Journal of the Acoustical Society of America'', 79(5), pp. 1472–1480.
  6. Shera, C. A. and Guinan, J. J. Jr., 1999. Evoked otoacoustic emissions arise by two fundamentally different mechanisms: a taxonomy for mammalian OAEs. ''The Journal of the Acoustical Society of America'', 105(2), pp. 782–798.
  7. Ren, T., He, W., Scott, M. and Nuttall, A. L., 2006. Group delay of acoustic emissions in the ear. ''Journal of Neurophysiology'', 96(5), pp. 2785–2791.
  8. Wilson, J.P., 1980. Evidence for a cochlear origin for acoustic re-emissions, threshold fine-structure and tonal tinnitus. ''Hearing Research'', 2(3–4), pp. 233–252.
  9. Flax, M. R., & Holmes, W. H. (2011, November). A mixed mode cochlear amplifier including neural feedback. In AIP Conference Proceedings (Vol. 1403, No. 1, pp. 611-617). American Institute of Physics.
  10. e.g.: Bell, A. and Fletcher, N. H., 2004. The cochlear amplifier as a standing wave: "Squirting" waves between rows of outer hair cells?. ''The Journal of the Acoustical Society of America'', 116(2), pp. 1016–1024.

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otology