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SUMMARY:Biophysics of the inner ear and beyond - Tobias Reichenbach\, Impe
 rial College London
DTSTART:20151026T163000Z
DTEND:20151026T180000Z
UID:TALK60081@talks.cam.ac.uk
CONTACT:P.H. Marchington
DESCRIPTION:Number 1\nReichenbach\, T. & Hudspeth\, A.J. 2014. The Physics
  of Hearing: Fluid Mechanics and the Active Process of the Inner Ear. Repo
 rts on Progress in Physics \n10.1088/0034-4885/77/7/076601\nAbstract\nMost
  sounds of interest consist of complex\, time-dependent admixtures of tone
 s of diverse frequencies and variable amplitudes. To detect and process th
 ese signals\, the ear employs a highly nonlinear\, adaptive\, real-time sp
 ectral analyzer: the cochlea. Sound excites vibration of the eardrum and t
 he three miniscule bones of the middle ear\, the last of which acts as a p
 iston to initiate oscillatory pressure changes within the liquid-filled ch
 ambers of the cochlea. The basilar membrane\, an elastic band spiralling a
 long the cochlea between two of these chambers\, responds to these pressur
 es by conducting a largely independent traveling wave for each frequency c
 omponent of the input. Because the basilar membrane is graded in mass and 
 stiffness along its length\, however\, each traveling wave grows in magnit
 ude and decreases in wavelength until it peaks at a specific\, frequency-d
 ependent position: low frequencies propagate to the cochlear apex\, wherea
 s high frequencies culminate at the base. The oscillations of the basilar 
 membrane deflect hair bundles\, the mechanically sensitive organelles of t
 he ear’s sensory receptors\, the hair cells. As mechanically sensitive i
 on channels open and close\, each hair cell responds with an electrical si
 gnal that is chemically transmitted to an afferent nerve fibre and thence 
 into the brain. In addition to transducing mechanical inputs\, hair cells 
 amplify them by two means. Channel gating endows a hair bundle with negati
 ve stiffness\, an instability that interacts with the motor protein myosin
 -1c to produce a mechanical amplifier and oscillator. Acting through the p
 iezoelectric membrane protein prestin\, electrical responses also cause ou
 ter hair cells to elongate and shorten\, thus pumping energy into the basi
 lar membrane’s movements. The two forms of motility constitute an active
  process that amplifies mechanical inputs\, sharpens frequency discriminat
 ion\, and\nconfers a compressive nonlinearity on responsiveness. These fea
 tures arise because the active process operates near a Hopf bifurcation\, 
 the generic properties of which explain several key features of hearing. M
 oreover\, when the gain of the active process rises sufficiently in ultraq
 uiet circumstances\, the system traverses the bifurcation and even a norma
 l ear actually emits sound. The remarkable properties of hearing thus stem
  from the propagation of traveling waves on a nonlinear and excitable medi
 um. \n\nNumber 2\nReichenbach\, T & Hudspeth\, A. J. 2010. A Ratchet Mecha
 nism for Amplification in Low-frequency Mammalian Hearing. PNAS Vol.107. N
 o 11. 4973-4978.\nSummary\nThe sensitivity and frequency selectivity of he
 aring result from tuned amplification by an active process in the mechanor
 eceptive hair cells. In most vertebrates\, the active process stems from t
 he active motility of hair bundles. The mammalian cochlea exhibits an addi
 tional form of mechanical activity termed electromotility: its outer hair 
 cells (OHCs) change length upon electrical stimulation. The relative contr
 ibutions of these two mechanisms to the active process in the mammalian in
 ner ear is the subject of intense current debate. Here\, we show that acti
 ve hair-bundle motility and electromotility can together implement an effi
 cient mechanism for amplification that functions like a ratchet: Sound-evo
 ked forces\, acting on the basilar membrane\, are transmitted to the hair 
 bundles\, whereas electromotility decouples active hair-bundle forces from
  the basilar membrane. This unidirectional coupling can extend the hearing
  range well below the resonant frequency of the basilar membrane. It there
 by provides a concept for low frequency hearing that accounts for a variet
 y of unexplained experimental observations from the cochlear apex\, includ
 ing the shape and phase behaviour of apical tuning curves\, their lack of 
 significant nonlinearities\, and the shape changes of threshold tuning cur
 ves of auditory-nerve fibres along the cochlea. The ratchet mechanism cons
 titutes a general design principle for implementing mechanical amplificati
 on in engineering applications.\n\nNumber 3\nReichenbach\, T.\, Stefanovic
 \, A.\, Nin\, F.\, & Hudspeth\, A. J. 2012. Waves on Reissner’s Membrane
 : A Mechanism for the Propagation of Otoacoustic Emissions from the Cochle
 a. Cell Reports 1 374-384 April 2012.\nSummary\nSound is detected and conv
 erted into electrical signals within the ear. The cochlea not only acts as
  a passive detector of sound\, however\, but can also produce tones itself
 . These otoacoustic emissions are a striking manifestation of the cochlea
 ’s mechanical active process. A controversy remains of how these mechani
 cal signals propagate back to the middle ear\, from which they are emitted
  as sound. Here\, we combine theoretical and experimental studies to show 
 that mechanical signals can be transmitted by waves on Reissner’s membra
 ne\, an elastic structure within the cochlea. We develop a theory for wave
  propagation on Reissner’s membrane and its role in otoacoustic emission
 s. Employing a scanning laser interferometer\, we measure traveling waves 
 on Reissner’s membrane in the gerbil\, guinea pig\, and chinchilla. The 
 results are in accord with the theory and thus support a role for Reissner
 ’s membrane in otoacoustic emissions.\n
LOCATION:The Hodgkin Huxley Seminar Room\, Department of Physiology Develo
 pment and Neuroscience
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