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PubMed1988

Frequency dependence of synchronization of cochlear nerve fibers in the alligator lizard: evidence for a cochlear origin of timing and non-timing neural pathways.

The dependence of synchronization of spike discharges on tone frequency was measured in cochlear nerve fibers of anesthetized alligator lizards at 21 degrees C. Synchronization measures were based on the fundamental component of a Fourier analysis of the instantaneous discharge rate in response to tone bursts. Measurements were obtained from fibers innervating hair cells in both the region of the cochlea that contains a tectorial membrane (tectorial fibers) and the region where hair-cell stereocilia are free-standing in scala media (free-standing fibers). Both rate and synchronization tuning-curves were measured automatically as a function of tone frequency. For tectorial fibers, the shapes of synchronization tuning-curves are roughly similar to the shapes of rate tuning-curves: the characteristic frequencies (CF's) of both curves are approximately equal. For free-standing fibers, the shape of synchronization tuning-curves differ markedly from those of rate tuning-curves. The CF's of synchronization and rate tuning-curves differ - the ranges are 0.2-0.6 kHz and 1-4 kHz, respectively - and the two CF's are uncorrelated. Synchronization filter-functions, which are contours of synchronization index at constant average discharge rate, were measured as a function of tone frequency for both tectorial and free-standing fibers. These synchronization filter-functions have the shapes of lowpass filters. For the populations of tectorial fibers and of free-standing fibers taken separately, these functions are independent of CF. The corner frequency of these functions is 0.50 +/- 0.038 kHz for tectorial fibers and 0.37 +/- 0.037 kHz for free-standing fibers. We conclude that these populations are characterized by different synchronization filters. For free-standing fibers, synchronization filter-functions measured at average driven discharge rates of about 20 and 40 spikes/s do not differ appreciably, and the high-frequency slope is -80 to -115 dB/decade. The results show that tectorial fibers encode timing information for low-level stimuli, whereas free-standing fibers do not. It is proposed that in the alligator lizard, neural pathways that encode timing information originate in the tectorial region and those that encode non-timing information originate in the free-standing region.

Acoustic Stimulation↗
PubMed1996

[Changes with aging in ECochG].

The present study was designed to determine the standard values of AP, adapted AP and CM (0.5, 1, 2, and 4kHz) of the electrocochleogram using subjects with normal hearing (20 ears, mean subject age 30.4 years). The input-output function, the latency and the threshold were measured in AP, whereas only an input-output curve was made for the adapted AP and the input-output function and the threshold were measused in CM. The CM threshold had the highest positive correlation with the threshold of the conventional pure tone audiometry at 1kHz. These standard curves will be very useful for analysing clinical data. In order to discuss the aging changes affecting electrocochleograms, the test subjects were divided into two groups, i.e. one group was composed of subjects under 30 years of age (mean: 19.7 y/o, 10 ears) and the other of those 30 years old or over (mean: 41.1y/o, 10 ears). There were no differences in hearing threshold or the threshold of the electrocochleogram between the two groups. The CM threshold exhibited the highest positive correlation with the threshold of conventional audiometry at 1kHz in both groups. AP and adapted AP showed no difference in any item between the two groups. Although there was no difference in the CM threshold, there was an obvious difference in the CM input-output curves between these two groups at high stimulus intensities of 2 and 4kHz. It is known that the EP decrement makes CM smaller. However, it is unlikely that this decreased CM response in the older group was due to EP suppression, since this phenomenon was only observed in response to high intensity stimulo and there was no difference in responses to low intensity stimulo. Another possible explanation for this phenomenon is aging changes in the basilar membrane, tectorial membrane and hair cells. Further studies are necessary to elucidate the etiology of the CM suppression revealed in the present study.

Adolescent↗
PubMed1992

[Human type II collagen induced autoimmune inner ear lesions in guinea pigs].

Guinea pigs were immunized with human type I, type II or type III collagen. Inner ear lesions were induced in some guinea pigs by immunizing them with human type II collagen. Histopathologic changes included endolymphatic hydrops, organ of Corti lesions and endolymphatic duct contained a large number of inflammatory cells. These histologic changes resembled those observed in patient with Meniere's disease. We didn't discover any inner ear lesions in guinea pigs by immunizing them with human type I and type III collagen, although their serum antibody titers were high and antibody presented in perilymph. Immunohistochemical study showed the presence of type II collagen in the cochlea tissues--spiral ligament, basilar membrane, tectorial membrane, spiral limb and Rosenthal's canal. We didn't discover type I and type III collagen in these area.

Animals↗
PubMed1996

Tectorial membrane-organ of Corti relationship during cochlear development.

The development of stereociliary attachment to the tectorial membrane was investigated in the mouse cochlea using transmission and scanning electron microscopy. At the 18th gestational day, only the major tectorial membrane can be identified covering the greater epithelial ridge and the inner hair cells in all turns. At the 19th gestational day, the minor tectorial membrane was first seen in the basal turn, over the outer hair cells. During early stages of development, the stereocilia of hair cells were surrounded by a loose fibrillar material underneath the tectorial membrane. After the 10th postnatal day, the outer hair cells' stereocilia were attached to Kimura's (or Hardesty's) membrane, while inner hair cells' stereociliary bundles were attached to the undersurface of the tectorial membrane near the Hensen's stripe. Between the 10th and the 14th postnatal days, the space between the inner hair cells and the first row of outer hair cells widened by virtue of the growth of the heads of pillar cells, and the inner hair cells' stereocilia were displaced towards the Hensen's stripe. After the 14th postnatal day, the inner hair cells' stereociliary bundles detached from the tectorial membrane, while the outer hair cells' stereocilia remained attached to it. The tip-link system, which connects the tips of the stereocilia to the next tallest stereocilia, is present at birth in the outer hair cells. The marginal pillar, that anchored the tectorial membrane to the underlying organ of Corti during development, first appeared on the 6th postnatal day and disappeared on the 14th-15th postnatal day. The present data together with other reports support the idea that although some structures, such as hair cells' stereocilia and innervation, are already formed early during development, the cochlear microarchitecture is not fully developed morphologically and ready to function normally until the end of the second postnatal week in the mouse.

Animals↗
PubMed1998

Identification and localization of type IV collagen chains in the inner ear cochlea.

Mutations in the genes encoding the alpha3(IV), alpha4(IV) and alpha5(IV) chains of type IV collagen have been implicated in the pathogenesis of Alport's syndrome, a hereditary disorder characterized by progressive nephropathy and sensorineural deafness. The known expression of these chains in kidney basement membranes supports the contention that they play a crucial role in the ultrafiltration function. Whether they play a role in auditory signal transduction remains unknown as heretofore, they have not been identified in the inner ear. In the present study, the expression of type IV collagen in cochlea of the inner ear of guinea pigs was determined. All six alpha-chains of type IV collagen were identified by biochemical and immunological methods. By indirect immunofluorescence, alpha1(IV) and alpha2(IV) chains were localized to the spiral limbus, basilar membrane and tectorial membrane. The alpha3(IV), alpha4(IV), alpha5(IV) and alpha6(IV) chains localized exclusively to the tectorial membrane and basilar membrane. These results suggest a possible role of type IV collagen chains in the active tuning of the basilar and tectorial membrane, an essential step in frequency discrimination and amplification of auditory signals.

Animals↗
PubMed1985

Developmental morphology of the mouse inner ear. A scanning electron microscopic observation.

The developing inner ears of mice (CBA/CBA), ages ranging from gestational day 12 through postnatal day 21, were examined using scanning electron microscopy following the epoxy-embedding/freeze-fracture technique. This technique provides unique three-dimensional views of surface and fractured structures of the developing inner ear, thus allowing excellent preservation of the relationships between the developing sensory epithelium and the overlying membranes (i.e. the tectorial membrane and cupula) during their development. The tectorial membrane is formed of two distinct parts: the major (medial) and the minor (distal). The major portion is produced by the cells of the greater epithelial ridge prior to the formation of the minor part, which is produced largely by the primordial supporting cells of the lesser epithelial ridge. The developing tectorial membrane has two types of fibers: radial (found mainly in the major part) and slanted (found mainly in the minor part). The slanted fibers become the cover net, which fuses with the marginal band. The marginal zone of the developing tectorial membrane is completely sealed during development by the third row of Deiters' cells. The surfaces of cells that produce the tectorial membrane are characterized by numerous long microvilli which are largely lost when the tectorial membrane completely forms and separates from the supporting cells. The surface of developing auditory sensory cells is initially covered with numerous microvilli, some of which become future stereocilia. Stereocilia form stepped rows in the shape of a "W", with the tallest row located at the periphery of the cell. As the sensory cell matures, the short transitional stereocilia gradually disappear. Kinocilia on hair cells are still seen in the 14-day-old mouse (even though the organ of Corti is morphologically mature) but not in the 21-day-old mouse, indicating that complete maturation of the sensory cells in all turns is attained by 21 days of age. The mouse has upper radial tunnel fibers and basal tunnel fibers. Neural contacts of the upper radial tunnel fibers with the outer hair cells at the apical portions are frequent in the developing organ of Corti. The external sulcus cells undergo drastic changes during development, forming numerous pits that are often covered with mucus-like droplets or grape-like spherical structures of varying sizes. This phenomenon was observed only during postnatal days 6 and 14. The developing cupula starts as a thin amorphous membrane, which later becomes compact and fibrotic-like as the mass increases. By the 6th postnatal day well-developed cupular canals occur.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMedDate not supplied

Mechanical transduction in outer hair cells.

The outer hair cells are responsible for the exquisite sensitivity, frequency selectivity and dynamic range of the cochlea. These cells are part of a mechanical feedback system involving the basilar membrane and tectorial membrane. Transverse displacement of the basilar membrane results in relative motion between the tectorial membrane and the reticular lamina, causing deflection of the stereocilia and modulation of the open probability of their transduction channels. The resulting current causes a change of membrane potential, which in turn produces mechanical force, that is fed back into the motion of the basilar membrane. Experiments were conducted to address mechanical transduction mechanisms in both the stereocilia and the basolateral cell membrane, as well as modes of coupling of the outer hair cell force to the organ of Corti.

Animals↗
PubMed1980

Cochlear anatomy related to cochlear micromechanics. A review.

The sensory cilia of the mammalian cochlea show an orderly gradation in height along the length of the cochlea. The cilia are taller in the apex and shorter in the base. Differences in ciliary gradation also exist between the different rows of outer hair cell cilia along the length of the cochlea. There are gradations in width and thickness in the moveable portion of the tectorial membrane paralleling those in the basilar membrane. There are also gradations in the relationship between the tectorial membrane and the slope of the reticular lamina along the length of the cochlea. This suggests that there may be additional mechanical fine-tuning capability built into the organ of Corti besides the basilar membrane. The tectorial membrane is firmly attached to the outer hair cell cilia along the entire length of the cochlea in all species examined. The inner hair cell cilia do not have the same firm attachment to the tectorial membrane as outer hair cell cilia. This suggests that the modes of mechanical coupling between the tectorial membrane and the inner and outer hair cell cilia are different.

Animals↗
PubMed1976

Sudden deafness. A histopathological study.

Histopathological studies were performed in the temporal bones from a 77-year-old woman who had bilateral sudden deafness 31 years before her death. The common findings in both cochleas were atrophy of the organ of Corti and stria vascularis, and degeneration of the tectorial membrane. Pathology of the tectorial membrane consisted of atrophy and droplet formation of the middle and marginal zones. In one area, an atrophied tectorial membrane was lifted up by a mass on the limbus. The saccular wall in the left temporal bone was ruptured. There was no pathology in the vascular system in the labyrinths except for moderate degeneration of sensory epithelium. The pathogenesis of sudden deafness was discussed in the light of the present findings. Alteration of the tectorial membrane seemed to be the main factor as a possible cause of sudden deafness. Histopathological findings were reviewed from the literature.

Adult↗
PubMed1991

The organ of Corti in the bat Hipposideros bicolor.

The bat Hipposideros bicolor (Hipposideridae, Microchiroptera) is the mammalian species with the highest upper limit of hearing in which the structure of the organ of Corti has been studied. H. bicolor emits pure tone echo-locating signals of 153 kHz, compensates for Doppler shifts in the echo and hears ultrasonic frequencies up to 200 kHz (Neuweiler et al., 1984). The organ of Corti was investigated qualitatively and quantitatively using the technique of semi-thin sectioning. Some complementary ultra-thin sections were also examined. Length, width and cross-sectional area of the basilar membrane, the tectorial membrane, the hair cells with their stereocilia and the organ of Corti were measured at equi-distant positions on the basilar membrane. The organ of Corti of H. bicolor is composed of elements similar to those found in the cochleae of other eutherian mammals studied. However, in H. bicolor some of these elements show species-specific differences when compared to auditorily unspecialized mammals. The most basal region of the cochlea is characterized by miniaturization and re-inforcement of macro- and micro-mechanically important elements. This is interpreted as an adaptation for hearing extremely high frequencies. Specialized structures as well as local maxima of 'normal' elements in the basal and middle cochlear region are associated with evaluation of the echos of emitted pure tones. Besides the basal specializations. Hipposideros also shows specializations in the apical, low frequency, region which can be correlated with passive acoustic orientation.

Animals↗