Organ of Corti | Cochlea
Y is the ratio of the distance between basilar membrane and hair-cell apex to the Movement of the organ of Corti and the tectorial membrane, based on. Anatomy of hearing and balance Reissner's/vestibular membrane · Basilar membrane Tectorial membrane; Sulcus spiralis. The tectorial membrane (TM) of the mammalian inner ear is Modes of the tectorial membrane, transversal eigenfunctions, and the origin of the outer of the tectorial membrane moves in phase with the basilar membrane.
The motion of the basilar membrane is generally described as a traveling wave. The basilar membrane is widest 0. Function[ edit ] Sinusoidal drive through the oval window top causes a traveling wave of fluid—membrane motion.
A modeled snapshot of fluid streamlines is shown. The wavelength is long compared to the duct height near the base, in what is called the long-wave region, and short 0. Therefore, they are kept strictly separated. This separation is the main function of Reissner's membrane between scala vestibuli and scala mediaand it is also the function of tissue held by the basilar membrane such as the inner and outer sulcus cells shown in yellow and the reticular lamina of the organ of Corti shown in magenta.
For the organ of Corti the basilar membrane is permeable to perilymph. Here the border between endolymph and perilymph occurs at the reticular lamina, the endolymph side of the organ of Corti. There are approximately 15, hair cells in each human ear see figure. This function as base of the sensory cells gave the basilar membrane its name, and it is again present in all land vertebrates.
Due to its location, the basilar membrane places the hair cells in a position where they are adjacent to both the endolymph and the perilymph, which is a precondition of hair cell function. Frequency dispersion[ edit ] A third, evolutionarily younger, function of the basilar membrane is strongly developed in the cochlea of most mammalian species and weakly developed in some bird species: This hypothesis is further supported by data obtained from two transgenic mice with reduced OHC coupling: An exquisite balance between TM radial stiffness and HB rotational stiffness along the length of the cochlea guarantees that HBs operate at an optimal point on their current-displacement function.
Cochlear microphonics is a useful tool to characterize the aforementioned phenomenon in vivo, because recordings on animals whose HBs are optimally tuned have symmetric receptor potentials with no phase lags. This surprising result shows that an actual protein linkage is not required for proper mechanical interaction between TM and HBs, suggesting that contact of the two structures is enough to guarantee optimal performance of the HBs.
Recent experiments indicate that other types of relative motion between TM and RL may also exist, such as a mode with the RL pivoting over the tunnel of Corti, resembling the movement of a teeter-totter [ 22 ]. Such a mode would create additional fluid flow via a pumping effect, contributing to HB deflection [ 2324 ]. Elevated neural thresholds are observed in mice with an enlarged subtectorial space [ 18 ]. To explain this observation, it is generally accepted that the magnitude of fluid flow and thus HB deflection depend strongly on the TM-RL distance [ 25 ].
TM coupling at the macroscale, dynamics and travelling waves Measurements performed at the macroscale hundreds of microns have shown that the TM is a structure with significant longitudinal coupling [ 2728 ].040 The Role of Hair Cells in Hearing
Accordingly, the sharpness of tuning of basilar membrane and neural recordings Q10db is increased by a factor of 2—3 at mid to high frequencies [ 16 ]. Interestingly, increased tuning arised at the expense of sensitivity, which was reduced by 10dB SPL. These findings challenged previous ideas stating that sensitivity and frequency selectivity increase together, and showed instead that sensitivity and selectivity are two opposing features that need to be balanced through proper longitudinal coupling [ 29 ].
Longitudinal coupling allows a large number of OHCs to work in synchrony, thus promoting sensitivity. Nevertheless, as the number of coupled OHCs grows larger, so does the extent of frequencies excited by a given stimulus, thus reducing frequency selectivity [ 29 ].
TECTORIAL MEMBRANE - Definition and synonyms of tectorial membrane in the English dictionary
Together with sensitivity and frequency selectivity, the third cornerstone of proper hearing is time discrimination. Interestingly, mutations in tectorins or collagen XI do not alter the elastic to viscous ratio, even though global values of impedance are in some cases largely decreased [ 1428 ]. It should be noted that the aforementioned predictions are based on interactions between a BM pressure-driven wave and a TM radial wave [ 29 ].
In this connection, new models including multiple modes of TM and BM vibration may provide further understating on the interaction between TM and BM, and its role on proper hearing. Mechanics of the spiral geometry The purpose of the coiled shape of the cochlea, unique to the mammalian cochleae, has been the focus of research and speculation for over half a century.
The spiral shape is not required for hearing, as monotreme mammals have cochleae shaped like a slightly bent tube and physical models of the cochlea have shown that the place frequency map remains the same in straight and toroidal cochlear models [ 1 ]. This supports earlier suggestions that the purpose of the coiled shape is to pack a long organ into a small space in the skull, as well as to provide an efficient organization of the nerve and blood supply to an organ that is protected inside a central shaft.
Other observations, however, suggest a functional role for the coiled geometry. The radial movement of the HB near the maximal amplitude of the traveling wave indicates an effect of fluid motion in a curved channel [ 1 ]. Also, a morphometric analysis of the cochleae of different mammals showed a strong correlation between the number of spiral turns times BM length and the low frequency limit of hearing [ 35 ].
The effect of spiral geometry on the mechanics of the cochlea has been the focus of theoretical studies over many years [ 36 — 41 ]. The spiral shape, which evolved together with the lengthening of mammalian cochleae, allows a wider range of audible frequencies. However, the spiral shape achieves more than that. Like a whispering gallery, which is known for propagating even slight whispers along its concave walls, the spiral cochlea not only propagates, but also focuses acoustic wave energy along its concave outer wall.
This focusing results in an increased vibration of the BM towards the outer wall of the cochlear duct, which creates an effective dynamic tilt of the recticular lamina RL over and above its geometric tilt. Both geometric and dynamic tilts increase as the radius of the spiral decreases. The effect is greatest at the apex, where the radius of curvature is smallest and low frequencies are analyzed [ 43 ].
Organ of Corti: overview
Shear gain amplitude increases as the cochlear radius decreases towards the apex, while shear gain phase becomes such that an upward deflection of the BM bends the OHC HB in the excitatory direction [ 42 ]. The tilted radial distribution of fluid pressure acting on the BM was confirmed by an independent analysis [ 46 ], which calculated different pressure modes by assuming an ideal fluid as done in a previous model [ 47 ].
While the simplest mode of their analysis agrees well with previous predictions [ 43 ], higher modes show a more complex behavior, such as a small shift of the maximum fluid pressure tilt from the apex towards the base of the cochlea.
All of the above results point to a potentially functional role of the spiral geometry on the mechanics of hearing. While genes that affect coiling have been suggested, no experiments so far have been able to control the cochlear coil while keeping the auditory functionality of the cochlea.
Theoretical predictions were thus put to the test indirectly, by comparing the hearing limits and the geometrical characteristics for different land and marine mammals. Conclusions Two often neglected but unique features of the mammalian cochlea, the anisotropic tectorial membrane and the cochlear spiral geometry, conspire to increase sensitivity and selectivity in mammalian hearing.
Key Points Strong tectorial membrane radial coupling to outer hair cell bundles improves hearing sensitivity. Weak tectorial membrane longitudinal coupling to outer hair cell bundles improves frequency selectivity.
Proper spacing between the tectorial membrane and the reticular lamina is necessary for optimal mechanotransduction. Cochlear curvature gradient determines the low frequency hearing limit across mammalian species. Acknowledgements This work was supported by the intramural program project DC in the National Institute on Deafness and other Communication Disorders.
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- Responses to sound of the basilar membrane of the mammalian cochlea
- Tectorial membrane
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Mechanisms of the inner ear. Ann Otol Rhinol Laryngol. Cochlear micromechanics - a physical model of transduction. J Acoust Soc Am. Stereocilin connects outer hair cell stereocilia to one another and to the tectorial membrane. Linkage of a gene for dominant non-syndromic deafness to chromosome Tectorial membrane stiffness gradients. Gavara N, Chadwick RS. Collagen-based mechanical anisotropy of the tectorial membrane: Implications for inter-row coupling of outer hair cell bundles. The authors measure base to apex gradients of these moduli and correlate these gradients with measured gradients of fiber thickness and spacing.
Fine morphology of the tectorial membrane. Its relationship to the organ of Corti. Sound-evoked deflections of outer hair cell stereocilia arise from tectorial membrane anisotropy. Enhancement of sensitivity gain and frequency tuning by coupling of active hair bundles. Col11a2 Deletion reveals the molecular basis for tectorial membrane mechanical anisotropy. Deficient forward transduction and enhanced reverse transduction in the alpha-tectorin CG human hearing loss mutation. Sharpened cochlear tuning in a mouse with a genetically modified tectorial membrane.
Tectorial membrane travelling waves underlie abnormal hearing in Tectb mutant mice. A deafness mutation isolates a second role for the tectorial membrane in hearing.