Length Of Basilar Membrane Research Articles

Morphology and morphometry of the inner ear of the dromedary camel and their influence on the efficiency of hearing and equilibrium

BackgroundThe inner ear morphology and size are linked to hearing and balance ability. The goal of this study was to determine the morphology and morphometrics of the dromedary camel's inner ear and how it influences hearing accommodation and equilibrium in the desert environment.Materials and methodsGross morphology, computed tomography images, and the endocast were used to show the inner ear morphology. A caliper and ImageJ software were used to take measurements on a plastic endocast.ResultsThe presence of the subarcuate fossa, flat cochlea, radii curvature of the semicircular canals, particularly the lateral semicircular canal, orthogonality, and the union between the semicircular canals, along with slightly increased saccule and utricle size, maintains camel balance on sandy ground, even during heavy sandstorms. The cochlear basilar membrane length and cochlea radii ratio aided low-frequency hearing and perception over a wide octave range.ConclusionThe camel's cochlear characteristics revealed a lengthy basilar membrane, a high radii ratio, 3.0 cochlear canal turns, and a very broad cochlea. The orthogonality of the semicircular canals, the high curvature of the lateral semicircular canal, the presence of the subarcuate fossa, and the confluence between the lateral and posterior semicircular canal were particular specifications that allowed the inner ear of the camel to adapt to desert living.

Functional Analyses of Peripheral Auditory System Adaptations for Echolocation in Air vs. Water

The similarity of acoustic tasks performed by odontocete (toothed whale) and microchiropteran (insectivorous bat) biosonar suggests they may have common ultrasonic signal reception and processing mechanisms. However, there are also significant media and prey dependent differences, notably speed of sound and wavelengths in air vs. water, that may be reflected in adaptations in their auditory systems and peak spectra of out-going signals for similarly sized prey. We examined the anatomy of the peripheral auditory system of two species of FM bat (big brown bat Eptesicus fuscus; Japanese house bat Pipistrellus abramus) and two toothed whales (harbor porpoise Phocoena phocoena; bottlenose dolphin Tursiops truncatus) using ultra high resolution (11–100 micron) isotropic voxel computed tomography (helical and microCT). Significant differences were found for oval and round window location, cochlear length, basilar membrane gradients, neural distributions, cochlear spiral morphometry and curvature, and basilar membrane suspension distributions. Length correlates with body mass, not hearing ranges. High and low frequency hearing range cut-offs correlate with basilar membrane thickness/width ratios and the cochlear radius of curvature. These features are predictive of high and low frequency hearing limits in all ears examined. The ears of the harbor porpoise, the highest frequency echolocator in the study, had significantly greater stiffness, higher basal basilar membrane ratios, and bilateral bony support for 60% of the basilar membrane length. The porpoise’s basilar membrane includes a “foveal” region with “stretched” frequency representation and relatively constant membrane thickness/width ratio values similar to those reported for some bat species. Both species of bats and the harbor porpoise displayed unusual stapedial input locations and low ratios of cochlear radii, specializations that may enhance higher ultrasonic frequency signal resolution and deter low frequency cochlear propagation.

Histological structure of Nannospalax xanthodon cochlea tissue

Because of the growth retardation in the eyes of Nannospalax xanthodon blind mole rat, some genetic and environmental adaptations have occurred in the smelling and hearing systems so that they can communicate efficiently in the galleries underground. This study was aimed to determine the histological and morphometric structure of the unique organization of the cochlea, which plays an important role in hearing in N. xanthodon. After the decalcification process, the cochlear tissue was cut in 5 μm thickness after routine histological procedures. Then these sections stained with Hematoxylin & Eosin and Masson trichrome methods were examined histologically. Besides, the data obtained by taking measurements with the Image J program in the basal, media, and apex regions of the cochlea were evaluated statistically. It was observed that basilar membrane length, tectorial membrane length, stria vascularis thickness, and inner-outer hair cell lengths increased, while Reissner’s membrane length and basilar membrane thickness decreased. These data show that the general histological structure of the blind mole rat cochlea is similar to that of other mammals. By evaluating histomorphological findings, it was concluded that cochlea, which plays a primary role in hearing with the effect of living conditions and genetic structures, develops better in blind mole rats than other living species.

Characterization of the human helicotrema: implications for cochlear duct length and frequency mapping

BackgroundDespite significant anatomical variation amongst patients, cochlear implant frequency-mapping has traditionally followed a patient-independent approach. Basilar membrane (BM) length is required for patient-specific frequency-mapping, however cochlear duct length (CDL) measurements generally extend to the apical tip of the entire cochlea or have no clearly defined end-point. By characterizing the length between the end of the BM and the apical tip of the entire cochlea (helicotrema length), current CDL models can be corrected to obtain the appropriate BM length. Synchrotron radiation phase-contrast imaging has made this analysis possible due to the soft-tissue contrast through the entire cochlear apex.MethodsHelicotrema linear length and helicotrema angular length measurements were performed on synchrotron radiation phase-contrast imaging data of 14 cadaveric human cochleae. On a sub-set of six samples, the CDL to the apical tip of the entire cochlea (CDLTIP) and the BM length (CDLBM) were determined. Regression analysis was performed to assess the relationship between CDLTIP and CDLBM.ResultsThe mean helicotrema linear length and helicotrema angular length values were 1.6 ± 0.9 mm and 67.8 ± 37.9 degrees, respectively. Regression analysis revealed the following relationship between CDLTIP and CDLBM: CDLBM = 0.88(CDLTIP) + 3.71 (R2 = 0.995).ConclusionThis is the first known study to characterize the length of the helicotrema in the context of CDL measurements. It was determined that the distance between the end of the BM and the tip of the entire cochlea is clinically consequential. A relationship was determined that can predict the BM length of an individual patient based on their respective CDL measured to the apical tip of the cochlea.

Additional row of outer hair cells – The unique pattern of the Corti organ in a subterranean rodent, the Gansu zokor (Eospalax cansus)

Acoustic conditions in burrows are different from those aboveground and restrict hearing of subterranean mammals to low frequencies, which is reflected in the ear morphology. While low-frequency adaptations of the middle ear attracted more attention of researches, the inner ear remained rather understudied. Here, we examined the cochlea of the inner ear of the Gansu zokor (Eospalax cansus), a subterranean rodent from the Tibetan Plateau. We focused on the quantitative parameters of the organ of Corti, which are assumed to determine hearing sensitivity and frequency tuning. Apart from the morphological traits common to the ear of subterranean rodents studied thus far, the Gansu zokor shows two unique features: the presence of a fourth row of outer hair cells along 20% to 50% of the basilar membrane length and almost constant width of the organ of Corti over more than 10% of its spiral length. Both these anomalies occur in the middle of the cochlear spiral. These features are unusual in comparative morphology of the organ of Corti and presumably are reflected in the functional specialization. They are expected to affect sensitivity and /or resolution of hearing in the frequency range registered in the given cochlear segment. The Gansu zokor thus profiles to an interesting candidate for hearing research which might provide further insight not only into morpho-functional adaptations in subterranean mammals in particular but also in the function of outer hair cells in general.

Inner Ear Morphology of Basal‐Most Mammaliaform Morganucodon

Extant therians are unique in their ability to hear at higher and at greater ranges of frequencies than most other vertebrates. This ability has been associated with changes in inner ear morphology, including elongation of the cochlear canal following loss of the lagena maculae, coiling of the cochlear canal, and stabilization of the hearing membrane through bony laminae. Presence and absence of these features is variable across Mesozoic mammaliaforms. To gain insights into the evolution of the inner ear and hearing in mammaliaforms, we document inner ear morphology in the basal‐most mammaliaform Morganucodon in unprecedented detail. We μCT scanned 30 petrosals of Morganucodon from several Jurassic fissure fillings in the UK. Surface reconstructions of the inner ears demonstrate that the cochlear canal gently varies in curvature and length (1.64–1.80 mm, measured from the posterior aspect of the cochlear foramen to the apex). All specimens show a distinct expansion of the apex of the cochlear canal suggesting the presence of a lagenar macula. Interestingly, the size of the apical expansion varies within our sample. None of the specimens preserve complete ossified laminae, but a shallow groove is visible on the ventrolateral surface of the cochlear canal endocasts, extending from the base of the canal (between the perilymphatic foramen and the fenestra vestibule) anteriorly towards the apex. We identify this groove as the attachment for the basilar membrane (termed here “base of secondary lamina”), as seen in extant monotremes. If the base of the secondary lamina is indeed reflective of basilar membrane length in Morganucodon then this could represents the first accurate reconstruction of the length of the basilar membrane in a fossil mammaliaform that retains a lagena. The base of the secondary lamina ends shortly before the apex of the canal and ranges between 1.17 and 1.45 mm. It moderately correlates (R2=0.58) with total cochlear canal length in our sample. Most intriguingly, the bony wall of the promontorium contains a network of canals that surrounds the cochlea, previously described as the circumpromontorium plexus. The plexus enters the cochlear canal next to the base of the secondary lamina and extends along the laminas full length. This strongly suggests that the blood supply to the cochlea might have been substantially different in Morganucodon compared to extant therians where blood vessel enter the cochlea through the cochlear foramen along with the cochlear nerve.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Evolutionary origins of ultrasonic hearing and laryngeal echolocation in bats inferred from morphological analyses of the inner ear

IntroductionMany mammals have evolved highly adapted hearing associated with ecological specialisation. Of these, bats possess the widest frequency range of vocalisations and associated hearing sensitivities, with frequencies of above 200 kHz in some lineages that use laryngeal echolocation. High frequency hearing in bats appears to have evolved via structural modifications of the inner ear, however, studying these minute features presents considerable challenges and hitherto few such attempts have been made. To understand these adaptations more fully, as well as gain insights into the evolutionary origins of ultrasonic hearing and echolocation in bats, we undertook micro-computed tomography (μCT) scans of the cochleae of representative bat species from 16 families, encompassing their broad range of ecological diversity. To characterise cochlear gross morphology, we measured the relative basilar membrane length and number of turns, and compared these values between echolocating and non-echolocating bats, as well as other mammals.ResultsWe found that hearing and echolocation call frequencies in bats correlated with both measures of cochlear morphology. In particular, relative basilar membrane length was typically longer in echolocating species, and also correlated positively with the number of cochlear turns. Ancestral reconstructions of these parameters suggested that the common ancestor of all extant bats was probably capable of ultrasonic hearing; however, we also found evidence of a significant decrease in the rate of morphological evolution of the basilar membrane in multiple ancestral branches within the Yangochiroptera suborder. Within the echolocating Yinpterochiroptera, there was some evidence of an increase in the rate of basilar membrane evolution in some tips of the tree, possibly associated with reported shifts in call frequency associated with recent speciation events.ConclusionsThe two main groups of echolocating bat were found to display highly variable inner ear morphologies. Ancestral reconstructions and rate shift analyses of ear morphology point to a complex evolutionary history, with the former supporting ultrasonic hearing in the common bat ancestor but the latter suggesting that morphological changes associated with echolocation might have occurred later. These findings are consistent with theories that sophisticated laryngeal echolocation, as seen in modern lineages, evolved following the divergence of the two main suborders.

TSLIM imaging and a morphometric analysis of the mouse spiral ganglion

Thin-sheet laser imaging microscopy (TSLIM) was used to serially section five whole cochleas from 4-wk-old CBA/JCr mice. Three-dimensional reconstructions of Rosenthal’s canal (RC) were produced in order to measure canal length and volume, to generate orthogonal cross sections for area measurements, and to determine spiral ganglion neuron (SGN) number. RC length averaged 2.0 mm ± 0.04 (SEM) as measured along the centroid of the canal compared to an average basilar membrane (BM) length of 5.9 ± 0.05 (SEM). RC volume averaged 0.036 mm 3 ± 0.009 (SEM). Significant increases in the radial area of RC were observed at the base (13%), middle (62%), and apex (90%) of its length. The total number of spiral ganglion neurons (SGNs) in RC in each of the five animals averaged 8626 ± 96 (SEM). SGN number increased at the expanded regions of RC. Increased area and cell number at the base and apex are likely related to extensions of the organ of Corti past the length of RC in these areas. The increase in area and cell number in the middle of the RC appears to be related to the most sensitive frequency region of the organ of Corti. Volume imaging or tomography of the cochlea as provided by TSLIM has the potential to be an efficient and accurate semi-automated method for the quantitative assessment of the number of SGNs and hair cells of the organ of Corti.

Explaining High‐Frequency Hearing

Kirk and Gosselin-Ildari (2009) have documented a reliable correlation between the volume of the cochlear labyrinth and high-frequency hearing limit among those Primates for which data are available. Specifically, the correlation shows that primates with small cochleas have better high-frequency hearing than those with larger cochleas. In discussing the implications of their finding, they provide a quote from one of our articles (Heffner, 2004) that they claim states that the size of the cochlea and the length of the basilar membrane are not functionally related to high-frequency hearing limit. However, that is a mistaken interpretation. The paragraph from which the quote is taken is making the point that the relationship between functional head size and high-frequency hearing limit is not explained by the size of the middle and inner ear. Functional head size is defined as the maximum time it takes for a sound to travel in air or water from one ear to the other. This measure indicates how high a mammal must hear to use the two high-frequency cues for sound localization—the binaural frequency-intensity spectral cues and pinna cues. The smaller the functional head size, the higher a mammal must hear for its head and pinnae to generate the high-frequency cues needed to locate the source of a sound. Because both functional head size and the dimensions of the ear tend to increase with the size of the animal, it is necessary to perform a partial correlational analysis to determine how much of the variance in high-frequency hearing is accounted for by each factor. Restricting our analysis to those mammals that localize sound, there are 21 species for which we have data on functional interaural distance, basilar membrane length, and high-frequency hearing limit (Table 1). For these species, controlling for the length of the basilar membrane causes the correlation between functional head size and high-frequency hearing, r = −0.870 (P < 0.0001), to fall only slightly to r = −0.814 (P < 0.0001). However, the correlation between high-frequency hearing and the length of the basilar membrane, r = −0.578 (P = 0.006), falls to chance, r = +0.269 (P = 0.257), when controlling for functional head size. This is why we say that the relationship between functional head size and high-frequency hearing is not explained by the size of the inner ear. Kirk and Gosselin-Ildari have found that among the 10 primates for which data are available, the size of the cochlea is a good predictor of high-frequency hearing; indeed, in this sample, it is a slightly better predictor of high-frequency hearing (r = −0.780) than functional head size (r = −0.660). Thus, we agree that when looking at fossil primates of similar size to those in the sample, the size of the cochlea may give a good estimate of their high-frequency hearing. However, the correlation between the size of the cochlea and high-frequency hearing among a broader sample of mammals is low enough that, contrary to a natural inclination to equate anatomy with behavior, it remains risky to do so. Note that we do not include mammals that do not localize sound (subterranean rodents) in the correlations with high-frequency hearing. This is because we explain high-frequency hearing as evolving in mammals under selective pressure to localize sound. Thus, we expect that any mammal that does not localize sound will not hear high frequencies, and subterranean rodents do not—thus serving as exceptions that prove the rule. However, the poor high-frequency hearing in these species poses a problem for the view that high-frequency hearing is determined by the size of the cochlea as their cochleas are relatively small yet their high-frequency hearing is extremely limited compared with other species with similar-sized cochleas (see Table 2 in Kirk and Gosselin-Ildari). For a presentation of our views on the evolution of high-frequency hearing, see Heffner and Heffner (2008). Finally, we should note that the explanation of the variation in high-frequency hearing offered by Kirk and Gosselin-Ildari addresses proximate mechanisms underlying hearing, that is, how mammals hear high frequencies. Such explanations are complementary to ultimate explanations that look for selective pressures acting on hearing, that is, why mammals hear high frequencies (Mayr, 1961). Failing to recognize the difference between proximate and ultimate explanations in biology and their complementary nature has been the source of many needless conflicts (Mayr, 1961). It is important to recognize that both types of explanations are important for a full understanding of any biological function. Rickye S. Heffner*, Henry E. Heffner*, * Department of Psychology University of Toledo Toledo, Ohio.

Cochlear Labyrinth Volume and Hearing Abilities in Primates

The primate cochlea is a membranous, fluid-filled receptor organ that is specialized for sound detection. Like other parts of the inner ear, the cochlea is contained within the bony labyrinth of the petrous temporal bone. The close anatomical relationship between the bony cochlear labyrinth and the membranous cochlea provides an opportunity to quantify cochlear size using osteological specimens. Although mechanisms of cochlear frequency analysis are well studied, relatively little is known about the functional consequences of interspecific variation in cochlear size. Previous comparative analyses have linked increases in basilar membrane length to decreases in both the high and low frequency limits of hearing in mammals. However, these analyses did not consider the potentially confounding effects of body mass or phylogeny. Here, we present measurements of cochlear labyrinth volume in 33 primate species based on high-resolution computed tomography. These data demonstrate that cochlear labyrinth volume is strongly negatively allometric with respect to body mass. Scaling of cochlear volume in primates is very similar to scaling of basilar membrane length among mammals generally. Furthermore, an analysis of 10 primate taxa with published audiograms reveals that cochlear labyrinth volume is significantly negatively correlated with the high frequency limit of hearing. This result is independent of body mass and phylogeny, suggesting that cochlear size is functionally related to the range of audible frequencies in primates. Although the nature of this functional relationship remains speculative, our findings suggest that some hearing parameters of extinct taxa may be estimated using fossil petrosals.

Anatomical predictions of hearing in the North Atlantic right whale

Some knowledge of the hearing abilities of right whales is important for understanding their acoustic communication system and possible impacts of anthropogenic noise. Traditional behavioral or physiological techniques to test hearing are not feasible with right whales. Previous research on the hearing of marine mammals has shown that functional models are reliable estimators of hearing sensitivity in marine species. Fundamental to these models is a comprehensive analysis of inner ear anatomy. Morphometric analyses of 18 inner ears from 13 stranded North Atlantic right whales (Eubalaena glacialis) were used for development of a preliminary model of the frequency range of hearing. Computerized tomography was used to create two-dimensional (2D) and 3D images of the cochlea. Four ears were decalcified and sectioned for histologic measurements of the basilar membrane. Basilar membrane length averaged 55.7 mm (range, 50.5 mm-61.7 mm). The ganglion cell density/mm averaged 1,842 ganglion cells/mm. The thickness/width measurements of the basilar membrane from slides resulted in an estimated hearing range of 10 Hz-22 kHz based on established marine mammal models. Additional measurements from more specimens will be necessary to develop a more robust model of the right whale hearing range.

The cochleogram of the guinea pig

The cochleogram is an important tool to relate properties of the cochlea (e.g. hair cell loss, damaged hair cells) to their position in the cochlear turns, to calculate the average hair cell density, and to measure the length of the whole cochlea. In this work different methods of plotting cochleograms are compared. We suggest that a sector-wise division of the cochlea for counting a cochleogram has advantages over line diagrams that provide a higher spatial resolution but might lead to misinterpretations of the degree of missing hair cells. The scanning electron microscopic analysis of 171 guinea pig cochleas revealed a mean basilar membrane length of 16.4 +/- 1.4 mm (mean +/- standard deviation) with sector lengths of 6.9, 4.2, 3.2, and 1.9 mm, thus adding relevant information to the morphology of the guinea pig cochlea.

The approximate scaling law of the cochlea box model

The hydrodynamic box-model of the cochlea is reconsidered here for the primary purpose of studying in detail the approximate scaling law that governs tonotopic responses in the frequency domain. “Scaling law” here means that any two solutions representing waveforms elicited by tones of equal amplitudes differ only by a complex factor depending on frequency. It is shown that this property holds with excellent approximation almost all along the basilar membrane (BM) length, with the exception of a small region adjacent to the BM base. The analytical expression of the approximate law is explicitly given and compared to numerical solutions carried out on a virtually exact implementation of the model. It differs significantly from that derived by Sondhi in 1978, which suffers from an inaccuracy in the hyperbolic approximation of the exact Green’s function. Since the cochleae of mammals do not exhibit the scaling properties of the box model, the subject presented here may appear to be just an academic exercise. The results of our study, however, are significant in that a more general scaling law should hold for real cochleae. To support this hypothesis, an argument related to the problem of cochlear amplifier-gain stabilization is advanced.

Human Cochleae With Three Turns: An Unreported Malformation

The objective of this histologic study of archival temporal bone sections was to describe the morphology of human cochleae found with three turns, a previously unreported anomaly, found in three pairs of temporal bones. The authors conducted histopathologic processing and measurement of basilar membrane length. Basilar membrane length was compared with that found in six normal control bones. Cochleae with three complete turns, rather than the usual two and a half turns, are described for the first time. All had longer than normal basilar membranes, with a mean length of 40.6 mm compared with a mean of 33.8 mm in the normal bones. Human cochleae with three turns exist as an unreported anomaly. This is a new category of anomaly, not likely based on interruption of development.

A physiological place–frequency map of the cochlea in the CBA/J mouse

Genetically manipulated mice have gained a prominent role in in vivo research on development and function of the auditory system. A prerequisite for the interpretation of normal and abnormal structural and functional features of the inner ear is the exact knowledge of the cochlear place–frequency map. Using a stereotaxic approach to the projection site of the auditory nerve fibers in the cochlear nucleus, we succeeded in labelling physiologically characterized auditory nerve afferents and determined their peripheral innervation site in the cochlea.From the neuronal characteristic frequency (CF) and the innervation site in the organ of Corti a place–frequency map was established for characteristic frequencies between 7.2 and 61.8 kHz, corresponding to locations between 90% and 10% basilar membrane length (base=0%, apex=100%, mean length measured under the inner hair cells 5.13 mm). The relation between normalized distance from the base (d) and frequency (kHz) can be described by a simple logarithmic function: d(%)=156.5−82.5×log(f), with a slope of 1.25 mm/octave of frequency. The present map, recorded under physiological conditions, differs from earlier maps determined with different methods. The simple logarithmic place–frequency relation found in the mouse indicates that mice are acoustic generalists rather than specialists.

The guide to plotting a cochleogram

The cochleogram is commonly used for illustrating hair cell loss after insult, yet standardized procedures for plotting either individual or averaged cochleograms are lacking despite more than 40 years of use. Due to the intra-species variation in basilar membrane (BM) length, it is important that length is plotted on the cochleogram in percent and not millimeter. It is also of interest to correlate the location of lesion to frequency by using a frequency-place equation. However, there is no consensus as which equation is most suitable for the species under study. This is an important issue since two different equations can result in significantly different frequency-place maps for the same cochlea. The purpose of this presentation is to suggest procedures for standardizing the cochleogram. The guidelines include: (i) basilar membrane length should be plotted as percent instead of millimeter due to the biological variation that exists in BM length within a particular species and strain, and the total length in millimeter stated on the cochleogram; (ii) the equations used for frequency-place maps should be stated on the cochleogram; (iii) different basilar membrane lengths should be normalized to percent before averaged cochleograms are made. These procedures are illustrated and discussed.

Postnatal development of the organ of Corti in cats: a light microscopic morphometric study

This study quantitatively characterizes the development of the major morphological features of the organ of Corti during the first 2 weeks postnatal, the period when the cat auditory system makes the transition from being essentially non-functional to having nearly adult-like responses. Four groups of kittens ( n=3) were studied at one day postnatal (P1), P5, P10, P15, and compared to adults. Measurements were made of the organ of Corti at 3 cochlear locations: 20%, 60% and 85% of basilar membrane length from the base – cochlear locations which in the adult correspond to best frequencies of ≈20 kHz, 2 kHz and 500 Hz, respectively. In addition, measurements of basilar membrane length and opening of the tunnel of Corti were made in 20 cochlear specimens from kittens aged P0–P6. Results indicate that: (i) at P0 the basilar membrane has attained adult length, and the tunnel of Corti is open over approximately the basal one-half of the cochlea; (ii) the initial opening of the tunnel of Corti occurs at a site about 4 mm from the cochlear base (best frequency of ≈25 kHz in the adult cochlea); (iii) the thickness of the tympanic cell layer decreases markedly at the basal 20-kHz location; (iv) the areas of the tunnel of Corti and space of Nuel and the angulation of the inner hair cells (IHC) relative to the basilar membrane all show marked postnatal increases at both the middle and apical locations; (v) IHC are nearly adult-like in length and shape at birth, whereas the OHC (at 2-kHz and 500-Hz locations) undergo marked postnatal changes; (vi) disappearance of the marginal pillars and maturation of the supporting cells are not yet complete by P15.

The location of the cochlear amplifier: spatial representation of a single tone on the guinea pig basilar membrane.

Acoustic stimulation vibrates the cochlear basilar membrane, initiating a wave of displacement that travels toward the apex and reaches a peak over a restricted region according to the stimulus frequency. In this characteristic frequency region, a tone at the characteristic frequency maximally excites the sensory hair cells of the organ of Corti, which transduce it into electrical signals to produce maximum activity in the auditory nerve. Saturating, nonlinear, feedback from the motile outer hair cells is thought to provide electromechanical amplification of the travelling wave. However, neither the location nor the extent of the source of amplification, in relation to the characteristic frequency, are known. We have used a laser-diode interferometer to measure in vivo the distribution along the basilar membrane of nonlinear, saturating vibrations to 15 kHz tones. We estimate that the site of amplification for the 15 kHz region is restricted to a 1.25 mm length of basilar membrane centered on the 15 kHz place.

Cochlear place-frequency map in the marsupial Monodelphis domestica

In order to determine the place-frequency map of the cochlea in the marsupial Monodelphis domestica, iontophoretic HRP-injections were made at several locations in the ventral cochlear nucleus. Prior to iontophoresis the auditory neurons at these locations were characterized electrophysiologically. The resulting distribution of retrogradely labeled cochlear spiral ganglion cells was analysed by means of a three dimensional reconstruction of the cochlea. The map was established for frequencies between 2.4 and 44.5 kHz, corresponding to positions between 95 to 14% of basilar membrane length (base = 0%). The maximum slope amounted to 1.8 mm/octave. Over the basal-most 60% of the cochlea the slope of the place-frequency map was larger than 1.5 mm/octave, further apically the slope rapidly decreased to values below 0.8 mm/octave. The shape of the cochlear place-frequency map is similar to that described in placental mammals.

Frequency representation and spiral ganglion cell density in the cochlea of the gerbil Pachyuromys duprasi

The tonotopic map of the cochlea in the gerbil Pachyuromys duprasi was analysed by local iontophoretic HRP-application into physiologically defined regions of the cochlear nucleus and mapping of subsequent HRP transport patterns in cochlear spiral ganglion cells. Furthermore the spiral ganglion cell density along the cochlear duct was determined. The cochlear tonotopic map was established in the frequency range between 0.6 and 17.5 kHz. These frequencies corresponded to locations between 86 and 3% basilar membrane length (0% = cochlear base). It was found that the slope of the place-frequency map varied with frequency, the maximum slope being found between 1 and 4 kHz. This frequency range corresponds to the frequency range of highest auditory sensitivity as determined from cochlear microphonic recordings (Plassmann et al., 1987). The density of spiral ganglion cells also varied along the cochlear duct. A pronounced maximum (1927 cells/mm) was located at around 70% basilar membrane length, compared to values of 800 cells per mm near the cochlear apex and base. This region of high ganglion cell density also corresponds to the frequency range of highest auditory sensitivity.