Modes of cellular vibration in the organ of Corti

NUMERICAL SIMULATION ON THE MECHANICAL BEHAVIOR OF OUTER STEREOCILIA IN CORTI

In this paper, a two-dimensional model of the organ of Corti (OC) which includes basilar membrane (BM), tectorial membrane (TM), inner and outer hair cells and reticular lamina (RL) is established by Comsol. Based on experimental data that sinusoidal excitation was applied on the pectinate zone of the BM, the incentives are added to the corresponding position of the model. Transient analysis is made and the displacement of six different positions is achieved. The results are in good agreement with experimental data, which confirms the validity of the FE model. Based on time-domain and frequency-domain analysis, the relative motion between stereocilia and TM and RL is studied under sinusoidal excitation. The results show that, under time-domain analysis, the whole trend of relative displacement and velocity difference of Inner is similar to that of Outer, while there is little different when comparing with Middle. The relative velocity difference of Middle and Inner lag behind Outer with roughly 0.01[Formula: see text]s. Under frequency-domain analysis, at characteristic frequency, the deviation of each stereocilium is the largest. In the meantime, the frequency that the maximum value of outer stereocilia achieved is different. This new finding may cause the difference of lateral vibration of BM and indicate the frequency sensitivity of BM.

Amplification in the apical turn of the cochlea with negative feedback

The apical turn of the anesthetized guinea pig cochlea was opened to examine the basilar membrane optically through the intact Reissner's membrane. Vibrations of the outer Hensen's cell and the basilar membrane (BM) adjacent to and about 130 microm below the level of the Hensen's cell were measured. Outer Hensen's cell vibration at the characteristic frequency was up to 900 times higher compared to the BM amplitude. After sacrifice BM vibration increased while Hensen's cell vibration decreased. The magnitude and sequence of change after sacrifice can best be explained by the presence of negative feedback between reticular lamina and BM. In other experiments using ototoxic drugs that damage outer hair cells, similar changes in Hensen's cell and BM vibration were observed. These results show that the apical turn behavior is different from that observed by other investigators in the basal turn. The potential benefits of the negative feedback are discussed. The presence of negative feedback would explain the linearity at the fundamental frequency observed in the apical turn of cochlea.

Mechanical responses of the mammalian cochlea

Recent findings in auditory research have significantly changed our views of the processes involved in hearing. Novel techniques and new approaches to investigate the mammalian cochlea have expanded our knowledge about the mechanical events occurring at physiologically relevant stimulus intensities. Experiments performed in the apical, low-frequency regions demonstrate that although there is a change in the mechanical responses along the cochlea, the fundamental characteristics are similar across the frequency range. The mechanical responses to sound stimulation exhibit tuning properties comparable to those measured intracellularly or from nerve fibres. Non-linearities in the mechanical responses have now clearly been observed at all cochlear locations. The mechanics of the cochlea are vulnerable, and dramatic changes are seen especially when the sensory hair cells are affected, for example, following acoustic overstimulation or exposure to ototoxic compounds such as furosemide. The results suggest that there is a sharply tuned and vulnerable response related to the hair cells, superimposed on a more robust, broadly tuned response. Studies of the micromechanical behaviour down to the cellular level have demonstrated significant differences radially across the hearing organ and have provided new information on the important mechanical interactions with the tectorial membrane. There is now ample evidence of reverse transduction in the auditory periphery, i.e. the cochlea does not only receive and detect mechanical stimuli but can itself produce mechanical motion. Hence, it has been shown that electrical stimulation elicits motion within the cochlea very similar to that evoked by sound. In addition, the presence of acoustically-evoked displacements of the hearing organ have now been demonstrated by several laboratories.

Sound preprocessing by ac and dc movements of cochlear outer hair cells

In inner and outer hair cells, a sound event results mechano-electrically in a receptor potential from the hair cells by the functioning of apical and lateral K(+)-channels. However, after this point, the signal transfer is divided. Inner hair cells (IHC) release an unknown afferent transmitter. By contrast, outer hair cells (OHC) are proposed to produce mechanical ac and dc responses. In our model, the ac components of the sound signal, the carrier frequencies, determine the response of the OHC. Usually, they respond by ac and dc movements. The rapid ac movements of OHC, for which the underlying mechanism is unknown, may respond cycle-by-cycle to and interfere with the carrier frequency of the traveling wave. Near hearing threshold, they could drastically amplify the traveling wave thus contributing to the postulated cochlear amplifier. Active dc movements of the cytoskeleton of the cell body, as well as of the cuticular plate with the sensory hairs, are proposed to respond to millisecond changes of the sound stimulus over time. Such changes could be a modulation of the amplitude (AM), i.e., an increase or decrease of the sound pressure level (SPL), which is reflected in the envelope of the traveling wave. The active mechanical dc response of OHC to the amplitude (AM) and frequency modulation (FM) pattern is then expected to result in dc position changes of the reticular lamina (RL). These should control the operation point of the stereocilia, thus influencing their transfer function and sensitivity. In addition, experimental data suggest that there are modulations of the compliance of the organ of Corti (OC) and changes of its geometry. This dc modulation of micromechanical properties and geometry of the OC by active force generation of OHCs might contribute to automatic gain control, adaptation, TTS, as well as to the homeostasis of the basilar membrane location. In particular, the motile mechanism may protect the vulnerable cochlear partition against high sound pressure levels. Moreover, according to this model, changes of the sound signal with time are expected to be encoded in the actively produced dc movements of the RL. As the signal changes may carry important information (e.g., complex sound signal modulations such as formant transitions in speech), this is extracted and mechanically encoded by the proposed active dc mechanism. It cannot be excluded that the information-carrying dc signal is transferred to inner hair cells contributing to their adequate stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

[The phenomenon of hearing: an interdisciplinary discussion. II]

This part of the paper deals with the neurophysiological background of speech analysis and hearing of music. Single vowel- and consonant-detector cells could be clearly separated at the colliculus and geniculate level (Kallert, Keidel). Musical stimulation is decoded at three levels: hair cells, geniculate, and auditory cortex. Human cortical evoked potentials represent rhythm, consonance, and (as DC-shift) the emotional content of music. Marked harmonics, even of single hair-cell vibrations and in single units of medial geniculate in combination with clock-cell networks, are the physiological basis of "harmony" in music. Dissonant stimuli are detectable at the cortical level in man (Finkenzeller, Keidel).

[The phenomenon of hearing: an interdisciplinary discourse. I]

For the last century hearing has been considered a purely passive process. G. S. Ohm's and H. v. Helmholtz's resonance theory of hearing was widely accepted until Wien's fundamental objection led to G. v. Békésy's and O. F. Ranke's travelling wave theory, the hydrodynamics of the inner ear that distributes the frequencies along the basilar membrane, which acts as the "first filter". Seebeck, Licklider, Schouten, and deBoer emphasized also the time domain and developed the concept of periodicity hearing and the "residue phenomenon". However, it was not until the concept of the "cochlear amplifier" and "active hearing" with energy generation during the transduction process within the ear itself was introduced that the "second filter" could be understood. Kemp's echo was the key to this revolutionary step in the theory of hearing.

High frequency radial movements of the reticular lamina induced by outer hair cell motility

Recently, it was shown in cochlear explants from the guinea pig cochlea that electrokinetic motile responses of outer hair cells can induce radial and transverse motion of the reticular lamina. Here we demonstrate, that the radial component of these motions can be measured up to high frequencies (15 kHz). Cochlear explants were taken from guinea pig inner ears and exposed to a sinusoidal electric field. A double photodiode was used as a linear position detector with high spatial and temporal resolution to detect radial movements in the plane of the reticular lamina. The organ of Corti of the second, third and fourth cochlear turns was stimulated with frequencies of the electrical field between 0.5 Hz and 20 kHz. Sinusoidal movements of up to 15 kHz were recorded. At higher frequencies the signal-to-noise ratio became too small. The largest responses were measured at the three rows of outer hair cells. If the strength of the electrical field was 2 kV/m, into which the cochlear explants were placed, the amplitudes of outer hair cell movements were around 1 micron at 1 Hz and 10 nm at 10 kHz. Uncoupling of the outer hair cells from the tunnel of Corti and from the inner hair cells decreased the oscillations of inner hair cells but did not affect outer hair cells. The movements showed frequency dependent amplitudes like a complex low-pass filter but no best frequency was observed.

The effects of quinine on the cochlear mechanics in the isolated temporal bone preparation

Quinine is known to induce a reversible hearing loss and to evoke motile responses of isolated outer hair cells. To study the effect of quinine, mechanical tuning curves of the Hensen's cells were measured in the isolated cochlea preparation in response to acoustical stimuli applied to the ear before and after application of the drug. It was shown that 0.5-4 mM quinine increased the vibration amplitude at the peak of the mechanical resonance curves and increased the sharpness of tuning. The time course of the event depended on whether the scala media was opened or not. The results show that quinine alters the micromechanical tuning of the organ of Corti.