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G A Manley

Publications and source records attributed to G A Manley.

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

Distortion product otoacoustic emissions in the tree frog Hyla cinerea.

The frog inner ear contains two hearing organs: the amphibian and the basilar papilla. The amphibian papilla is sensitive to low- and mid-frequency stimuli (0.1--0.5 and 0.5--1.3 kHz, respectively, in Hyla cinerea), while the basilar papilla is sensitive to high-frequency stimuli (2.8--3.9 kHz in H. cinerea). Distortion product otoacoustic emissions (DPOAE) were recorded from the ear of the tree frog H. cinerea. In each of six ears investigated, a cubic distortion product (DP) at 2f(1)--f(2) was present when the primary frequencies f(1) and f(2) and the DP frequency were close to either the mid- or the high-frequency range. At frequencies between the sensitive ranges of both papillae, no emissions were observed. For the basilar papilla, the dependence of DP level on the primary tone frequency ratio f(2)/f(1) showed a pattern characteristic of the response of a single nonlinear resonator. Thus, in agreement with neural data, DPOAE from the basilar papilla reflect the contribution of a single auditory filter to emission generation.

Acoustic Stimulation↗
PubMed2001

Evidence for an active process and a cochlear amplifier in nonmammals.

The last two decades have produced a great deal of evidence that in the mammalian organ of Corti outer hair cells undergo active shape changes that are part of a "cochlear amplifier" mechanism that increases sensitivity and frequency selectivity of the hearing epithelium. However, many signs of active processes have also been found in nonmammals, raising the question as to the ancestry and commonality of these mechanisms. Active movements would be advantageous in all kinds of sensory hair cells because they help signal detection at levels near those of thermal noise and also help to overcome fluid viscosity. Such active mechanisms therefore presumably arose in the earliest kinds of hair cells that were part of the lateral line system of fish. These cells were embedded in a firm epithelium and responded to relative motion between the hair bundle and the hair cell, making it highly likely that the first active motor mechanism was localized in the hair-cell bundle. In terrestrial nonmammals, there are many auditory phenomena that are best explained by the presence of a cochlear amplifier, indicating that in this respect the mammalian ear is not unique. The latest evidence supports siting the active process in nonmammals in the hair-cell bundle and in intimate association with the transduction process.

Amphibians↗
PubMed2000

Cochlear mechanisms from a phylogenetic viewpoint.

The hearing organ of the inner ear was the last of the paired sense organs of amniotes to undergo formative evolution. As a mechanical sensory organ, the inner-ear hearing organ's function depends highly on its physical structure. Comparative studies suggest that the hearing organ of the earliest amniote vertebrates was small and simple, but possessed hair cells with a cochlear amplifier mechanism, electrical frequency tuning, and incipient micromechanical tuning. The separation of the different groups of amniotes from the stem reptiles occurred relatively early, with the ancestors of the mammals branching off first, approximately 320 million years ago. The evolution of the hearing organ in the three major lines of the descendents of the stem reptiles (e.g., mammals, birds-crocodiles, and lizards-snakes) thus occurred independently over long periods of time. Dramatic and parallel improvements in the middle ear initiated papillar elongation in all lineages, accompanied by increased numbers of sensory cells with enhanced micromechanical tuning and group-specific hair-cell specializations that resulted in unique morphological configurations. This review aims not only to compare structure and function across classification boundaries (the comparative approach), but also to assess how and to what extent fundamental mechanisms were influenced by selection pressures in times past (the phylogenetic viewpoint).

Animals↗
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

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↗
PubMed1999

Influence of contralateral acoustic stimulation on distortion-product and spontaneous otoacoustic emissions in the barn owl.

The avian auditory papilla provides an interesting object on which to study efferent influences, because whereas a significant population of hair cells in birds is not afferently innervated, all hair cells are efferently innervated (Fischer, 1992, 1994a, b). Previous studies in mammals using contralateral sound to stimulate the efferent system demonstrated a general suppressive effect on spontaneous and click-evoked, as well as on distortion-product otoacoustic emissions (DPOAE). As little is known about the effects of contralateral stimulation on hearing in birds, we studied the effect of such stimuli (broadband noise, pure tones) on the amplitude of the DPOAE 2f(1)-f(2) and on spontaneous otoacoustic emissions (SOAE) in the barn owl, Tyto alba. For the DPOAE measurements, fixed primary-tone pairs [f(1)=8.875 kHz (ratio=1.2), f(1)=8.353 kHz (ratio=1.15) and f(1)=7.889 kHz (ratio=1.1)] were presented and the DPOAE measured in the presence and absence of continuous contralateral stimulation. The DPOAE often declined in amplitude but in some cases we observed DPOAE enhancement. The changes in amplitude were as large as 9 dB. The influence of the contralateral noise changed over time, however, and the effects of contralateral tones were frequency-dependent. SOAE were suppressed in amplitude and shifted in frequency by contralateral broadband noise. Control measurements in animals after middle-ear muscle resection showed that these phenomena were not attributable to the acoustic middle-ear reflex. The finding of DPOAE enhancement is interesting, because a type of efferent fiber that suppressed its discharge rate during stimulation has been described in birds (Kaiser and Manley, 1994).

Acoustic Stimulation↗
PubMed1998

General characteristics and suppression tuning properties of the distortion-product otoacoustic emission 2f1-f2 in the barn owl.

The distortion-product otoacoustic emission (DPOAE) 2f1-f2 was measured in the ear canal of the barn owl. DPOAE were elicited by primary tones in 11 frequency regions from 1 to 9 kHz. The highest DPOAE output levels and best thresholds were found for f1 frequencies of 4 to 7 kHz and additionally at the lowest f1 frequency investigated. In some cases, the DPOAE sound pressures were only 37 dB below the primary-tone levels (PTL). The optimal primary-tone frequency ratios ranged from 1.05 to 1.45 and varied strongly among the different frequency regions investigated. The largest optimal ratios were measured in the middle frequency range for f1. At lower and higher f1, the optimal ratios decreased. DPOAE levels could be suppressed in a frequency-selective way by adding a third tone. As in other non-mammals, the best suppressive frequencies were near f1, suggesting DPOAE generation near the frequency place of this primary tone. This is in contrast to what is known for mammalian species, where the DPOAE is thought to be generated near f2. To obtain 6 dB of suppression of the DPOAE level, suppressor-tone levels ranging from 13 dB below to 4 dB above the primary-tone level were necessary. The Q10dB-values of suppression tuning curves increased as a function of frequency up to a value of 15.8. This tendency resembled the increase in frequency selectivity of auditory nerve fibers in this species.

Acoustic Stimulation↗
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↗
PubMed1997

Spontaneous otoacoustic emissions in the barn owl.

Spontaneous otoacoustic emissions (SOAE) were studied in a bird, the barn owl. They were found in 79% of the ears investigated, and each emitting ear generated on average 1.9 emissions. Their peak sound-pressure levels lay between -5.8 and 10.3 dB, and their centre frequencies between 2.3 and 10.5 kHz. The SOAE originated primarily in the upper quarter of the animal's hearing range, and derived from a specialized area previously described as being within an auditory fovea. Indeed, 93% of the emissions had centre frequencies above 7.5 kHz. The median of the frequency distances between neighbouring SOAE was 406 Hz (0.058 oct). The 3 dB bandwidth of the emissions depended on their amplitude above the noise: for SOAE whose level exceeded 10 dB above the noise floor, the 3 dB bandwidths ranged between 4.5 and 11.4 Hz. SOAE frequencies were temperature sensitive. Raising the temperature shifted the emissions to higher frequencies, and vice versa (the frequency shifted on average 0.039 oct/degrees C). External tones could suppress the level of SOAE, an effect that was highly tuned. For SOAE with frequencies between 2.5 and 10.5 kHz, the Q(10dB) values of 2 dB iso-suppression tuning curves (STC) varied from 1.07 to 10.40. The best thresholds of 2 dB STC were generally below 15 dB SPL.

Acoustic Stimulation↗
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

Otoacoustic emissions, hair cells, and myosin motors.

The stereovillar bundles of hair cells show active movements that may be generated by the putative myosin-actin interactions underlying hair-cell adaptation. Such movement is a possible candidate for the generation of spontaneous otoacoustic emissions (SOAE) in the ear canal of nonmammals. In the basilar papilla of certain lizard families, most hair cells are not coupled by a tectorial membrane, making it easier to assign the energy in emission peaks to defined groups of hair cells. We have studied 62 SOAE in the Bahamian Anole Anolis sagrei, which has about 140 hair cells with "free-standing" bundles in the high-frequency area of its papilla. Individual SOAE peaks were traced to between 3 and 38 hair cells, and the mean power output per hair cell was calculated to be 141 aW. The number of bundle myosins putatively involved in the generation of each SOAE was estimated and the force generated by each myosin at 1 kHz calculated to be approximately 0.1 pN. The data support the idea that hair cells generate emissions and suggest that myosin produces sufficient power to be the emission motor.

Animals↗
PubMed1996

Brainstem connections of the macula lagenae in the chicken.

The macula lagenae, an otolithic hair-cell organ with probable vestibular function, lies close to the apical end of the avian auditory hair-cell epithelium, the papilla basilaris. In an earlier study in the pigeon in which lesioning techniques were used, Boord and Rasmussen ([1963] J. Comp. Neurol. 120:463-473) reported finding a projection of lagenar fibers to parts of the cochlear nuclei (nucleus magnocellularis and nucleus angularis). Subsequent to this report, it has been generally assumed that at least part of the cochlear nuclei has a vestibular or a combined vestibular-auditory function. In this study, we labeled fibers innervating the macula lagenae of the chicken by using a lipophilic fluorescent tracer. The analysis of Vibratome sections of the brainstem with epifluorescence illumination showed no projection to the cochlear nuclei. In cases in which the apical part of the papilla basilaris was contaminated with tracer, however, we found labeling of the cochlear nuclei in the same areas as described with the lesioning technique in the pigeon. Our results thus imply that there is no processing of information from the macula lagenae in the cochlear nucleus of the chicken. In addition, we studied the origin of the few labeled efferent neurons in the brainstem. The location of all somata encountered was restricted to an area medial to the nucleus facialis dorsalis and corresponded to the dorsal efferent cell group, from which efferents to other vestibular organs also originate.

Afferent Pathways↗
PubMed1996

Surface morphology of basilar papilla of the tufted duck Aythya fuligula, and domestic chicken Gallus gallus domesticus.

Quantitative details of the surface morphology of the hearing organ, the Papilla basilaris, as seen in the scanning electron microscope are described for the tufted duck Aythya fuligula and for comparison for the domestic chicken Gallus gallus domesticus, for which some published information is already available. As in the other avian species investigated to date, each papilla shows a unique constellation of features. The papilla of the tufted duck is 3.5 mm long in the unfixed state and contains 8,200 sensory hair cells. It shows systematic changes in its surface features along the length and across the width of the sensory epithelium. In general, its features and those of the chicken Papilla basilaris can be described as relatively primitive in comparison with other species. The tufted duck papilla does, however, show one feature that has so far been found to be well developed only in advanced papillae; the number of stereovilli per hair cell bundle is generally much higher on hair cells of the neural than those on the abneural side. This difference is only weakly developed in the chicken. It is clear that features considered to be evolutionarily advanced were acquired independently of one another during evolution and that each bird species can show a mosaic of primitive and advanced features.

Animals↗