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Biomedical subjects

J C Saunders

Publications and source records attributed to J C Saunders.

At least 91 records · Page 5Linked to original sources
PubMed1986

Growth of threshold shift in hair-cell stereocilia following overstimulation.

Sensory hair bundle micromechanics were measured from all four hair-cell rows in the isolated guinea pig cochlea before and during overstimulation. The stereocilia bundle was stimulated by an oscillating water jet, and stroboscopic illumination slightly offset from the frequency of the stimulus revealed their motion. The intensity of the water jet could be varied in decibel steps and a 'visual detection level' threshold of stereocilia movement served as the criterion response. Pre-exposure thresholds provided a reference and were compared to thresholds sampled at 1-min intervals during a 10-min exposure period. The exposure stimulus consisted of the water jet adjusted to either 8, 13, or 18 dB above the visual detection level. Stereocilia on hair cells in each of the four rows showed a loss in stiffness which systematically increased during the first 5 to 6 min of exposure. Between 6 and 10 min of overstimulation the threshold shift exhibited a plateau whose magnitude was proportional to the exposure level. There were also differences in the magnitude of threshold shift between the hair-cell rows. The results clearly showed that overstimulation changed the micromechanical behavior of the stereocilia bundle, and that there was a complex interaction between exposure duration, level, and hair-cell row.

Acoustic Stimulation↗
PubMed1985

Auditory structure and function in the bird middle ear: an evaluation by SEM and capacitive probe.

The anatomic features of the middle ear in five avian species were identified in the scanning electron microscope. Various aspects of the conductive apparatus were quantitatively measured in a number of specimens from each species. These included the tympanic membrane, columella footplate and oval window area; the length of the columella, extra-stapedius and Platner's ligament: and the angular relations between the columella and the tympanic membrane, extra-stapedius and footplate. The velocity vs frequency response curve, measured from the tip of the concave tympanic membrane, and corrected to a constant stimulus level of 100 dB SPL, was obtained for the neonatal chick and parakeet for frequencies between 0.2 and 10.0 kHz with a capacitive probe. In both species this curve resembled a bandpass filter whose best frequency was in the range of 1.5-3.0 kHz. The low and high frequency roll-off was 6 and 16 dB per octave, respectively. Displacement of the TM in the chick was measured at several frequencies between 70 and 120 dB SPL and was found to be linear. The results of displacement measured from the tip of the tympanum and from a location 1.0 mm more central on the drum membrane revealed a large difference in displacement over all frequencies. A comparison between the shape of the audibility curve and the tympanic membrane velocity function in the parakeet revealed that both curves were nearly the same for the mid-range frequency region. These findings add to our understanding of middle-ear function in the avian ear.

Age Factors↗
PubMed1985

The structure and function of actin in hair cells.

Within the last 7 years, the protein actin has been identified in the stereocilia and cuticular plate region of the hair cell. An intensive effort has been mounted to describe the structural organization of this protein, to identify actin-associated proteins in these regions, and to identify the functional role of these proteins. The paracrystalline array of actin and its cross bridges imparts rigidity and stiffness to the stereocilia, which are important in determining their response properties. It also appears that these properties can be changed if the paracrystalline array is damaged by noise exposure. The functional implications of stereocilia rigidity and stiffness, as well as potential contractile mechanisms in the hair cell, are discussed. Finally, it is suggested that changes in the cross-sectional shape of the stereocilia caused by shearing of actin filaments during stereocilia deflections can be related to the mechano-electrical events in the plasma membrane of the cell. This may be the link between the transmission of vibrational energy through the sensory accessory structures of the peripheral ear, and the initiation of electrochemical events associated with transduction.

Actins↗
PubMed1985

The anatomical consequences of acoustic injury: A review and tutorial.

The anatomic consequences of acoustic overstimulation are explored in this presentation, and attention is directed toward issues where improvements in technology and empirical observation are needed before further advances in our understanding can be achieved. Gains have been made in the last decade in appreciating sound-induced cochlear injury, but there is now a need to evaluate not only cochlear pathology but also the functional state of the surviving structures. There is a wealth of information about the susceptibility of inner or outer hair cells to acoustic injury; however, the etiology of this injury is not yet fully understood. In addition, current ideas concerning the effects of noise on hair-cell stereocilia, hair-cell synapses, the cochlear vascular supply, and the central auditory pathways are in a state of flux and are either undergoing revision or emerging. Other issues, such as the basis of temporary or permanent threshold shift at the cellular level, and the individual differences in susceptibility to injury are in need of a fresh approach. It would seem that the time is now ripe to review our knowledge, recognize its gaps, and develop testable hypotheses concerning the mechanisms of acoustic injury to the ear.

Animals↗
PubMed1984

Effects of kanamycin on cochlear nuclear evoked responses and behavioral responses in C57BL/6J mice.

The effect of kanamycin on auditory function was investigated in C57BL/6J mice. Results showed that kanamycin treatments (400 mg/kg/day, injected between 10 to 13 days of age) caused severe cochlear damage, particularly to the outer hair cells, and elevation of cochlear nucleus evoked-response thresholds. The treatment also rendered the treated animals susceptible to audiogenic seizures, suggesting a change in the animals' reactivity to intense sounds. However, unlike the effect of noise, exposure to kanamycin did not cause the treated animals to show abnormal augmentation of cochlear nucleus evoked-response amplitude when stimulated at high intensities.

Acoustic Stimulation↗
PubMed1983

Auditory development in the mouse: structural maturation of the middle ear.

The development of middle-ear structures in the mouse was examined in nine groups of pups between 1 and 45 days of age. The area of the tympanic membrane (pars tensa and pars flaccida), the length of the lever arms of the malleus and incus, the surface area of the oval window, and the volume of the bulla all showed systematic changes during neonatal life. The area of the oval window reached maturity first and the lever arms achieved 90% of their adult size on day 11. The tympanic membrane achieved the same criterion on day 18. These data help us further to understand the processes that contribute to the functional ontogeny of the middle ear.

Age Factors↗
PubMed1983

The sensitive period for ototoxicity of kanamycin in mice: morphological evidence.

The ototoxicity of kanamycin was investigated in mice treated with the drug either from 6 to 9, 10 to 13, or 15 to 18 days of age. The results showed extensive damage to the outer-hair-cell (OHC) receptors of the cochlea in the 10-13-day group; some less severe damage to the OHC was found in the 6-9-day group, and little damage was observed in the 15-18 day group. The present finding is consistent with the view that the mouse cochlea is most vulnerable to the ototoxic effect of kanamycin during the period of its rapid development, i.e., between 10 and 13 days of age.

Age Factors↗
PubMed1983

Actin filaments, stereocilia, and hair cells of the bird cochlea. I. Length, number, width, and distribution of stereocilia of each hair cell are related to the position of the hair cell on the cochlea.

Located on the sensory epithelium of the sickle-shaped cochlea of a 7- to 10-d-old chick are approximately 5,000 hair cells. When the apical surface of these cell is examined by scanning microscopy, we find that the length, number, width, and distribution of the stereocilia on each hair cell are predetermined. Thus, a hair cell located at the distal end of the cochlea has 50 stereocilia, the longest of which are 5.5 microns in length and 0.12 microns in width, while those at the proximal end number 300 and are maximally 1.5 microns in length and 0.2 micron in width. In fact, if we travel along the cochlea from its distal to proximal end, we see that the stereocilia on successive hair cells gradually increase in number and width, yet decrease in length. Also, if we look transversely across the cochlea where adjacent hair cells have the same length and number of stereocilia (they are the same distance from the distal end of the cochlea), we find that the stereocilia of successive hair cells become thinner and that the apical surface area of the hair cell proper, not including the stereocilia, decreases from a maximum of 80 microns2 to 15 microns2. Thus, if we are told the length of the longest stereocilium on a hair cell and the width of that stereocilium, we can pinpoint the position of that hair cell on the cochlea in two axes. Likewise, if we are told the number of stereocilia and the apical surface of a hair cell, we can pinpoint the location of that cell in two axes. The distribution of the stereocilia on the apical surface of the cell is also precisely determined. More specifically, the stereocilia are hexagonally packed and this hexagonal lattice is precisely positioned relative to the kinocilium. Because of the precision with which individual hair cells regulate the length, width, number, and distribution of their cell extensions, we have a magnificent object with which to ask questions about how actin filaments that are present within the cell are regulated. Equally interesting is that the gradient in stereociliary length, number, width, and distribution may play an important role in frequency discrimination in the cochlea. This conclusion is amplified by the information presented in the accompanying paper (Tilney, L.G., E.H. Egelman, D.J. DeRosier, and J.C. Saunders, 1983, J. Cell Biol., 96:822-834) on the packing of actin filaments in this stereocilia.

Animals↗
PubMed1983

Morphology of inner hair cell stereocilia in C57BL/6J mice as studied by scanning electron microscopy.

The observation that hair cell tuning curves exhibit frequency selectivity as sharply tuned as that seen in auditory nerve fibers has prompted closer examination of the sensory hairs or stereocilia. The present study was designed to examine the morphologic organization of inner hair cell stereocilia in a mammalian species, the neonatal C57BL/6J mouse. The cochleae of mice were fixed in OSO4, dehydrated, dissected, and prepared for scanning electron microscopy. An examination of the number of stereocilia per inner hair cell revealed an orderly decrease from base to apex. Conversely, there was a 300% increase in the height of the tallest stereocilia, a 100% increase in the height of the middle row stereocilia, and a 30% increase in shortest stereocilia from base to apex. The total surface area of the stereocilia, per hair cell, was shown to increase by approximately 250% from the base to the apex of the cochlea.

Animals↗
PubMed1982

Changes in the organization of actin filaments in the stereocilia of noise-damaged lizard cochleae.

Alligator lizards exposed to 105 dB broadband noise for 24 h showed a 33 dB loss in hearing which was almost completely recovered 11 days after removal from the noise. Two lesions were found in the actin filament organization which could affect the rigidity of the stereocilia and thus could account for this hearing loss. These lesions preferentially affect the tallest stereocilia. The more common one is a depolymerization of the actin filaments at the base of the stereocilium where it makes contact with the cuticular plate. This results in a displacement and detachment of the stereocilium from its rootlet, thereby affecting the orientation of the tallest stereocilium. The other lesion involves a loss in crossbridges between adjacent actin filaments in the stereocilium. We demonstrate that such a loss will dramatically affect the rigidity of the stereocilia.

Actins↗
PubMed1982

Sensitive periods of susceptibility to auditory trauma in mammals.

Evidence is presented to support the hypothesis that the cochleae of young animals are more susceptible to auditory trauma than the cochleae of the adult. A sensitive period of heightened susceptibility to acoustic trauma from noise exposure has been demonstrated in three mammalian species. The cochlear pathology associated with this trauma is severe damage to the outer hair cell system. Abnormal growth of auditory evoked responses recorded in central auditory nuclei accompanies the receptor damage during the sensitive period. There is evidence of a similar sensitive period of susceptibility to cochlear insult from ototoxic drugs. The time frame of the sensitive period may be different for drug or noise insult to the cochlea, but the principal pathology of outer hair cell loss remains the same in both cases. The implication of these sensitive periods to auditory trauma, for human development is considered.

Auditory Fatigue↗
PubMed1980

The maturation of frequency selectivity in C57BL/6J mice studied with auditory evoked response tuning curves.

The present study was designed to examine the ontogeny of frequency selectivity in the neonatal auditory system. Mice were tested between 12 and 65 days of age. At each age two measures of auditory sensitivity were made from cochlear nucleus evoked responses. Tone-burst evoked-response thresholds in the quiet were determined for frequencies between 1.0 and 39.0 kHz. A two-tone simultaneous masking procedure was then used to obtain evoked response tuning curves. The frequency selectivity of the tuning curves was quantified by calculating a Q ratio. The results show that tuning is poor in neonates but rapidly improves to adult-like levels within 5-16 days after the inception of auditory function. The data also indicate that the development of frequency selectivity varies directly with the maturation of threshold sensitivity.

Aging↗
PubMed1980

Displacement of the malleus in neonatal golden hamsters.

A capacitive probe was used to measure displacements of the umbo of the malleus in neonatal golden hamsters for discrete frequencies from 0.6 to 35.0 kHz. Displacement, extrapolated to a sound pressure of 100 dB SPL, plotted as a function of frequency demonstrated low-pass characteristics with a cutoff frequency near 9.0 kHz. The amplitude of displacement increased with age to a plateau at all frequencies. Analysis of the data below the cutoff frequency indicated that low frequency displacements were dominated by a compliance which increased with age. It was also found that high frequency responses showed evidence of increased mass limitation in younger as compared with older subjects. Comparisons between the inverse of velocity and evoked response threshold curves indicated that the middle ear plays an important role in determining thresholds throughout development.

Acoustic Stimulation↗
PubMed1979

Frequency selectivity in bird and man: a comparison among critical ratios, critical bands and psychophysical tuning curves.

The relative frequency selectivity of the internal auditory filters, as measured by critical bands or critical ratios, and psychophysical tuning curves, was compared in an avian species (parakeet) and human listeners. The results indicate that the critical band predicts the frequency selectivity of psychophysical tuning curves and vice versa. These findings suggest that there are similar mechanisms which contribute to the frequency selectivity measured by these two procedures and that these mechanisms operate in both the human and avian ear.

Adult↗