Biomedical subjects

C Köppl

Publications and source records attributed to C Köppl.

At least 19 recordsLinked to original sources

PubMed2001

In vivo evidence for a cochlear amplifier in the hair-cell bundle of lizards.

Vertebrate sensory hair cells achieve high sensitivity and frequency selectivity by adding self-generated mechanical energy to low-level signals. This allows them to detect signals that are smaller than thermal molecular motion and to achieve significant resonance amplitudes and frequency selectivity despite the viscosity of the surrounding fluid. In nonmammals, a great deal of in vitro evidence indicates that the active process responsible for this amplification is intimately associated with the hair cells' transduction channels in the stereovillar bundle. Here, we provide in vivo evidence of hair-cell bundle involvement in active processes. Electrical stimulation of the inner ear of a lizard at frequencies typical for this hearing organ induced low-level otoacoustic emissions that could be modulated by low-frequency sound. The unique modulation pattern permitted the tracing of the active process involved to the stereovillar bundles of the sensory hair cells. This supports the notion that, in nonmammals, the cochlear amplifier in the hair cells is driven by a bundle motor system.

Acoustic Stimulation↗

PubMed2001

Tonotopic projections of the auditory nerve to the cochlear nucleus angularis in the barn owl.

The nucleus angularis (NA), one of the two cochlear nuclei of birds, plays an important role in the processing of sound intensity. To begin investigating the NA in detail in the barn owl, which is a popular animal model for neural mechanisms of sound localization, a frequency map for this nucleus is presented here. Focal injections of horseradish peroxidase or neurobiotin were placed either in the NA or in the cochlear nucleus magnocellularis, labeling small groups of auditory nerve fibers of known characteristic frequency (CF) from 0.25 to 9.6 kHz. The courses of their axonal branches were used to construct a composite average map of the tonotopic frequency representation in the nucleus angularis. Nucleus angularis in the barn owl, as seen in frontal sections, resembles a sheet of cells bent approximately into an S shape. The lowest frequencies were found represented at the ventromedial extreme. The representation of increasingly higher frequencies then followed the S shape, with the highest frequencies located at the ventrolateral tip. Auditory nerve fibers of a given CF always entered the nucleus angularis within a well-restricted area and then traveled along their isofrequency band within the NA while branching off terminals. The isofrequency bands were typically slanted from caudo-ventro-medial to rostro-dorso-lateral. The basic tonotopic organization is comparable to that found in other birds, the major differences being the large size and unusual shape of the barn owl's nucleus angularis.

Animals↗

PubMed2001

Efferent axons in the avian auditory nerve.

The sensory hair cells of the inner ear receive both afferent and efferent innervation. The efferent supply to the auditory organ has evolved in birds and mammals into a separate complex system, with several types of neurons of largely unknown function. In this study, the efferent axons in four different species of birds (chicken, starling, barn owl and emu) were examined anatomically. Total numbers of efferents supplying the cochlear duct (auditory basilar papilla and the vestibular lagenar macula) were determined; separate estimates of the efferents to the lagenar macula only were also derived and subtracted. The numbers for auditory efferents thus varied between 120 (chicken) and 1068 (barn owl). Considering the much larger numbers of hair cells in the basilar papilla, each efferent is predicted to branch extensively. However, pronounced species-specific differences as well as regional differences along the tonotopic gradient of the basilar papilla were documented. Myelinated and unmyelinated axons were found, with mean diameters of about 1 microm and about 0.5 microm, respectively. This suggests two basic populations of efferents, however, they did not appear to be distinguished sharply. Evidence is presented that some efferents lose their myelination at the transition from central oligodendrocyte to peripheral Schwann cell myelin. Finally, a comparison of the four bird species evaluated suggests that the efferent population with smaller, unmyelinated axons is the phylogenetically more primitive one. A new population probably arose in parallel with the evolution and differentiation of the specialized hair-cell type it innervates, the short hair cell.

Acetylcholinesterase↗

PubMed2000

A quantitative study of cochlear afferent axons in birds.

This paper is a comparative study of auditory-nerve morphology in birds. The chicken (Gallus gallus), the emu (Dromaius novaehollandiae) and the starling (Sturnus vulgaris) were chosen as unspecialised birds that have already been used in auditory research. The data are discussed in comparison to a similar earlier study on the barn owl, a bird with highly specialised hearing, in an attempt to separate general avian patterns from species specialisations. Average numbers of afferent fibres from 8775 (starling) to 12¿ omitted¿406 (chicken) were counted, excluding fibres to the lagenar macula. The number of fibres representing different frequency ranges showed broad maxima in the chicken and emu, corresponding to hearing ranges of best sensitivity and/or particular behavioural relevance. Mean axon diameters were around 2 microm in the chicken and starling, and around 3 microm in the emu. Virtually all auditory afferents were myelinated. The mean thickness of the myelin sheaths was between 0.33 microm (starling) and 0.4 microm (emu). There was a consistent pattern in the diameters of axons deriving from different regions. Axons from very basal, i.e. highest-frequency, parts of the basilar papilla were always the smallest. In the emu and the chicken, axons from the middle papillar regions were, in addition, larger than axons innervating apical regions.

Afferent Pathways↗

PubMed2000

Auditory processing in birds.

Over the past year, much progress has been achieved in the study of both the peripheral and the central auditory systems of birds. Significant advances have been made in the study of hair cells, including elucidation of the mechanisms of selectivity for sound frequency, functional differentiation, efferent innervation, and regeneration. Most of the studies of central auditory neurones have concerned the developmental and physiological correlates of vocal learning in songbirds and sound localisation in owls.

Acoustic Stimulation↗

PubMed2000

Rate-intensity functions in the emu auditory nerve.

Rate-versus-intensity functions recorded from mammalian auditory-nerve fibers have been shown to form a continuum of shapes, ranging from saturating to straight and correlating well with spontaneous rate and sensitivity. These variations are believed to be a consequence of the interaction between the sensitivity of the hair-cell afferent synapse and the nonlinear, compressive growth of the cochlear amplifier that enhances mechanical vibrations on the basilar membrane. Little is known, however, about the cochlear amplifier in other vertebrate species. Rate-intensity functions were recorded from auditory-nerve fibers in chicks of the emu, a member of the Ratites, a primitive group of flightless birds that have poorly differentiated short and tall hair cells. Recorded data were found to be well fitted by analytical functions which have previously been shown to represent well the shapes of rate-intensity functions in guinea pigs. At the fibers' most sensitive frequencies, rate-intensity functions were almost exclusively of the sloping (80.9%) or straight (18.6%) type. Flat-saturating functions, the most common type in the mammal, represented only about 0.5% of the total in the emu. Below the best frequency of each fiber, the rate-intensity functions tended more towards the flat-saturating type, as is the case in mammals; a similar but weaker trend was seen above best frequency in most fibers, with only a small proportion (18%) showing the reverse trend. The emu rate-intensity functions were accepted as supporting previous evidence for the existence of a cochlear amplifier in birds, the conclusion was drawn further that the nonlinearity observed is probably due to saturation of the hair-cell transduction mechanism.

Animals↗

PubMed1999

Coding of sound pressure level in the barn owl's auditory nerve.

Rate-intensity functions, i.e., the relation between discharge rate and sound pressure level, were recorded from single auditory nerve fibers in the barn owl. Differences in sound pressure level between the owl's two ears are known to be an important cue in sound localization. One objective was therefore to quantify the discharge rates of auditory nerve fibers, as a basis for higher-order processing of sound pressure level. The second aim was to investigate the rate-intensity functions for cues to the underlying cochlear mechanisms, using a model developed in mammals. Rate-intensity functions at the most sensitive frequency mostly showed a well-defined breakpoint between an initial steep segment and a progressively flattening segment. This shape has, in mammals, been convincingly traced to a compressive nonlinearity in the cochlear mechanics, which in turn is a reflection of the cochlear amplifier enhancing low-level stimuli. The similarity of the rate-intensity functions of the barn owl is thus further evidence for a similar mechanism in birds. An interesting difference from mammalian data was that this compressive nonlinearity was not shared among fibers of similar characteristic frequency, suggesting a different mechanism with a more locally differentiated operation than in mammals. In all fibers, the steepest change in discharge rate with rising sound pressure level occurred within 10-20 dB of their respective thresholds. Because the range of neural thresholds at any one characteristic frequency is small in the owl, auditory nerve fibers were collectively most sensitive for changes in sound pressure level within approximately 30 dB of the best thresholds. Fibers most sensitive to high frequencies (>6-7 kHz) showed a smaller increase of rate above spontaneous discharge rate than did lower-frequency fibers.

Acoustic Stimulation↗

PubMed1999

Reversed tonotopic map of the basilar papilla in Gekko gecko.

A published model of the frequency responses of different locations on the basilar papilla of the Tokay gecko Gekko gecko (Authier and Manley, 1995. Hear. Res. 82, 1-13) had implied that (a) unlike all other amniotes studied so far, the frequency map is reversed, with the low frequencies at the base and the high frequencies at the apex, and (b) the high-frequency area is split into two parallel-lying hair cell areas covering different frequency ranges. To test these hypotheses, the frequency representation along the basilar papilla of Gekko gecko was studied by recording from single auditory afferent nerve fibers and labelling them iontophoretically with horseradish peroxidase. Successfully labelled fibers covered a range of characteristic frequencies from 0.42 to 4.9 kHz, which extended from 78% to 9% of the total papillar length, as measured from the apex. The termination sites of labelled fibers within the basilar papilla correlated with their characteristic frequency, the lowest frequencies being represented basally, and the highest apically. This confirms the first prediction of the model. The map indicates, however, that one of the two high-frequency papillar regions (the postaxial segment) represents the full high-frequency range, from about 1 to 5 kHz. No functionally identified labelling was achieved in the preaxial segment. Thus the assumptions underlying the proposed model need revision. A good mathematical description of the frequency distribution was given by an exponential regression with a mapping constant in the living state of approximately 0.4 mm/octave.

Animals↗

PubMed1998

Fine structure of the basilar papilla of the emu: implications for the evolution of avian hair-cell types.

The morphology of the basilar papilla of the emu was investigated quantitatively with light and scanning electron microscopical techniques. The emu is a member of the Paleognathae, a group of flightless birds that represent the most primitive living avian species. The comparison of the emu papilla with that of other, more advanced birds provides insights into the evolution of the avian papilla. The morphology of the emu papilla is that of an unspecialised bird, but shows the full range of features previously shown to be typical for the avian basilar papilla. For example, the orientation of the hair cells' sensitive axes varied in characteristic fashion both along and across the papilla. Many of the quantitative details correlate well with the representation of predominantly low frequencies along the papilla. The most distinctive features were an unusually high density of hair cells and an unusual tallness of the hair-cell bodies. This suggests that the evolution of morphologically very short hair cells, which are a hallmark of avian papillae, is a recent development in evolution. The small degree of differentiation in hair-cell size contrasts with the observation that a significant number of hair cells in the emu lack afferent innervation. It is therefore suggested that the development of functionally different hair-cell types in birds preceded the differentiation into morphologically tall and short hair cells.

Animals↗

PubMed1998

Phylogenetic development of the cochlea and its innervation.

Comparative studies of vertebrate hearing organs have enabled an integrated approach to difficult questions related to function. Recent evidence for the independent evolution of similar hearing-organ specializations, in particular hair-cell differentiation, has helped identify common problems of hearing receptors and put them in a new perspective. Evidence that cochlear amplification is an ancient phenomenon has widened the search for the motor mechanism involved. In this regard, different hypotheses are best examined by making optimal use of natural structural variations. Studies on the evolution of the efferent system have provided new routes to investigate its function.

Animals↗

PubMed1998

Effects of testosterone suppression on serum levels of hepatitis B surface antigen and HBV-DNA in men.

AIMS/BACKGROUND: There is epidemiological evidence that progression of hepatitis B virus (HBV)-induced liver disease is adversely influenced by male gender. Furthermore, in male transgenic mice, HBsAg levels increase after puberty, resulting in 4- to 10-fold higher HBsAg levels than in female transgenic mice. Castration reduces HBsAg levels by 90-95%, while substitution of testosterone to castrated animals rapidly increases HBsAg concentrations. We hypothesized that suppression of endogenous testosterone levels may have similar effects on HBsAg serum levels in men, as observed in male mice. METHODS: To test our hypothesis, we studied the influence of reversible testosterone suppression by the LHRH-analog triptorelin on serum concentrations of HBsAg and HBV-DNA. Eight male patients, who were chronically infected with HBV, were studied in a prospective interventional study. RESULTS: Triptorelin decreased serum testosterone levels to castration levels for several weeks. However, this reversible testosterone suppression had no effect on HBsAg or HBV-DNA serum concentrations (p > 0.05). CONCLUSIONS: Suppression of endogenous testosterone levels had no effect on HBsAg levels in men, which points to a different regulation of HBsAg expression in men compared with transgenic mice.

Adult↗

PubMed1997

Phase locking to high frequencies in the auditory nerve and cochlear nucleus magnocellularis of the barn owl, Tyto alba.

The auditory system of the barn owl is an important model for temporal processing on a very fast time scale and for the neural mechanisms and circuitry underlying sound localization. Phase locking has been shown to be the behaviorally relevant temporal code. This study examined the quality and intensity dependence of phase locking in single auditory nerve fibers of the barn owl to define the input to the known brainstem circuit for temporal processing. For direct comparison in the same individuals, recordings were also obtained from the relevant next higher center, the nucleus magnocellularis (NM). Phase locking was regularly seen at sound pressure levels (SPL) below those eliciting an increase in spike rate, thus providing an additional cue for signal detection. The quality of phase locking, expressed as vector strength, decreased with increasing frequency. Auditory nerve fibers showed an unusual step-like decline with a prominent plateau in the mid-frequency range (1.5-3 kHz), indicating that some specialization enables the owl to halt the deterioration and extend phase locking to frequencies up to 10 kHz, above the range commonly observed in other species. Phase locking in the NM was consistently inferior to that of auditory-nerve fibers at frequencies above 1 kHz, suggesting that the synapse plays a limiting role in temporal precision. The response delays, or group delays, derived from the phase-versus-frequency functions of auditory nerve fibers were not consistent with the unusual spatial frequency representation in the owl cochlea. This questions the common assumption that group delays reflect cochlear wave travel times.

Acoustic Stimulation↗

PubMed1997

Low-frequency pathway in the barn owl's auditory brainstem.

The cytology of the nucleus magnocellularis and the nucleus laminaris in the barn owl, as well as the axonal pathways connecting them, were studied. The interest was focussed on those regions of both nuclei coding the low-frequency end of the tonotopic spectrum (below approximately 2 kHz) because many previous reports on a variety of bird species had indicated significant differences to higher frequencies, both in morphology and physiology. Standard light- and electron microscopy, as well as immunocytochemistry and tract-tracing techniques, were used. The nucleus magnocellularis contains a distinct stellate cell type in the low-frequency region, in addition to neurons classified as a small version of the principal cell. In the nucleus laminaris, two cell types were characterized as distinct to the low-frequency region: stellate neurons with long, smooth dendrites, and multipolar neurons with thick, spiny dendrites. The low-frequency projections from the nucleus magnocellularis showed two terminal fields in the nucleus laminaris: one containing a rough tonotopic representation and a second one where all low-frequency projections converged. In addition, the anatomical basis for delay lines, which are known to play an important role in the coding of interaural time differences at higher frequencies, was not observed. The morphological differences observed at low frequencies in both nuclei, compared to the well-studied higher-frequency regions, may reflect inherent limitations to the accuracy in the processing of interaural phase disparities at low frequencies.

Animals↗

PubMed1997

Activity of primary auditory neurons in the cochlear ganglion of the emu Dromaius novaehollandiae: spontaneous discharge, frequency tuning, and phase locking.

The spontaneous and pure-tone sound-driven activity of primary auditory units was recorded in the cochlear ganglion of emu chicks aged between post-hatching days 1 and 14. Spontaneous activity tended to increase both with the chick's age and as a function of the unit's characteristic frequency (CF). The CF of 887 units ranged from 0.04 to 4 kHz, the thresholds down to 0 dB SPL. Although the CF range did not change, the thresholds improved with age during the first two weeks after hatching, by up to 18 dB at the highest frequencies. The threshold spread between units of similar CF in single animals was up to 60 dB. Rate-threshold tuning-curve symmetry varied with CF. In low-frequency units, the slope of the low-frequency tuning-curve flank was on average steeper than the high-frequency flank, whereas for high-frequency units, the reverse was true. Units of CF > 0.5 kHz generally showed low-frequency "tails" similar to those seen in mammalian primary auditory fibers. The mean tuning-curve frequency selectivity increased with CF. For units of CF > 0.2 kHz, thresholds were moderately correlated with tuning-curve frequency selectivity. Significant phase locking was observed up to about 4 kHz, the corner frequency of phase locking being at 1.15 kHz.

Animals↗

PubMed1997

Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba.

Single-unit recordings were obtained from the brain stem of the barn owl at the level of entrance of the auditory nerve. Auditory nerve and nucleus magnocellularis units were distinguished by physiological criteria, with the use of the response latency to clicks, the spontaneous discharge rate, and the pattern of characteristic frequencies encountered along an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic frequency for both auditory nerve and nucleus magnocellularis units. The average difference between these populations was 0.4-0.55 ms. The average most sensitive thresholds were approximately 0 dB SPL and varied little between 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the unusual and greatly expanded mapping of higher frequencies on the basilar papilla of the owl. This observation is contrary to the equal-distance hypothesis that relates frequency selectivity to the spatial representation in the cochlea. On the basis of spontaneous rates and/or sensitivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one frequency, this being only 10-15 dB in individual owls. Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the known properties of endbulb-of-Held synapses, suggests a convergence of approximately 2-4 auditory nerve fibers onto one nucleus magnocellularis neuron. Some auditory nerve fibers as well as nucleus magnocellularis units showed a quasiperiodic spontaneous discharge with preferred intervals in the time-interval histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.

Acoustic Stimulation↗

PubMed1995

Quantitative anatomical basis for a model of micromechanical frequency tuning in the Tokay gecko, Gekko gecko.

The basilar papilla of the Tokay gecko was studied with standard light- and scanning electron microscopy methods. Several parameters thought to be of particular importance for the mechanical response properties of the system were quantitatively measured, separately for the three different hair-cell areas that are typical for this lizard family. In the basal third, papillar structure was very uniform. The apical two-thirds are subdivided into two hair-cell areas running parallel to each other along the papilla and covered by very different types of tectorial material. Both of those areas showed prominent gradients in hair-cell bundle morphology, i.e., in the height of the stereovillar bundles and the number of stereovilli per bundle, as well as in hair cell density and the size of their respective tectorial covering. Based on the direction of the observed anatomical gradients, a 'reverse' tonotopic organization is suggested, with the highest frequencies represented at the apical end.

Acoustic Stimulation↗

PubMed1994

Auditory nerve terminals in the cochlear nucleus magnocellularis: differences between low and high frequencies.

Primary auditory nerve fibers were labelled in the barn owl by localized horseradish peroxidase (HRP) injections into the cochlear nucleus angularis. They were followed to their terminal sites in the hearing organ (basilar papilla), confirming that they were auditory, and to the cochlear nucleus magnocellularis. The terminal sites of low-frequency fibers within nucleus magnocellularis always included an area previously described as the lagenar part, i.e., an area receiving primary input which is probably only vestibular. Furthermore, a number of differences were recognized between these low-frequency (up to 0.64 kHz) and the high-frequency (1.8 kHz and above) auditory nerve projections to nucleus magnocellularis. Most importantly, the collaterals given off by low-frequency fibers into the nucleus typically showed multiple terminal branching, with both en passant and terminal bouton-like swellings. High-frequency fiber collaterals, in contrast, terminated unbranched in a single endbulb of Held. Nucleus magnocellularis is the first station in a brainstem auditory pathway processing stimulus timing information, coded through neuronal phase locking. The prominent difference in terminal shape found between its low- and high-frequency input fibers is interpreted as reflecting different requirements of the absolute temporal precision for significant phase locking. Terminals in the shape of endbulbs of Held are probably a specialization to improve the temporal precision of synaptic transmission, allowing phase locking to higher frequencies.

Acoustic Stimulation↗

PubMed1994

Spontaneous otoacoustic emissions in the bobtail lizard. II: Interactions with external tones.

The response of spontaneous otoacoustic emissions to the presentation of external tones was studied in the Australian bobtail lizard. Three basic types of effects were observed: suppression (a reduction in the emission's amplitude), facilitation (an increase in the emission's amplitude) and frequency shifting. The suppressive effect was highly frequency selective. Iso-suppression tuning curves resembled the rate-threshold tuning curves of the high-frequency population of VIIIth nerve fibres in this species. The frequency with the lowest threshold for suppression corresponded, on average, to the emission's own frequency and did not show any systematic deviation from it. Facilitation of between 2 and 10 dB occurred, but only in response to frequencies within certain narrow ranges, and at sound pressure levels below those that suppressed. The most commonly-observed facilitation range lay between 0.2 and 0.6 octaves above the emission's own frequency and coincided in frequency with a characteristic notch in the iso-suppression tuning curve. In the same narrow frequency range, the input/output functions of amplitude suppression always showed a pronounced increase in slope. The emissions moved their own frequency away from that of an external tone. The observed shifts were comparatively large (up to -330 Hz) and were more pronounced in the downward direction.

Acoustic Stimulation↗