Biomedical subjects

Alfred Nuttall

Publications and source records attributed to Alfred Nuttall.

4 recordsLinked to original sources

PubMed2006

Cochlear transducer operating point adaptation.

The operating point (OP) of outer hair cell (OHC) mechanotransduction can be defined as any shift away from the center position on the transduction function. It is a dc offset that can be described by percentage of the maximum transduction current or as an equivalent dc pressure in the ear canal. The change of OP can be determined from the changes of the second and third harmonics of the cochlear microphonic (CM) following a calibration of its initial value. We found that the initial OP was dependent on sound level and cochlear sensitivity. From CM generated by a lower sound level at 74 dB SPL to avoid saturation and suppression of basal turn cochlear amplification, the OHC OP was at constant 57% of the maximum transduction current (an ear canal pressure of -0.1 Pa). To perturb the OP, a constant force was applied to the bony shell of the cochlea at the 18 kHz best frequency location using a blunt probe. The force applied over the scala tympani induced an OP change as if the organ of Corti moved toward the scala vestibuli (SV) direction. During an application of the constant force, the second harmonic of the CM partially recovered toward the initial level, which could be described by two time constants. Removing the force induced recovery of the second harmonic to its normal level described by a single time constant. The force applied over the SV caused an opposite result. These data indicate an active mechanism for OHC transduction OP.

Acoustic Stimulation↗

PubMed2004

Organ of Corti potentials and the motion of the basilar membrane.

During sound stimulation, receptor potentials are generated within the sensory hair cells of the cochlea. Prevailing theory states that outer hair cells use the potential-sensitive motor protein prestin to convert receptor potentials into fast alterations of cellular length or stiffness that boost hearing sensitivity almost 1000-fold. However, receptor potentials are attenuated by the filter formed by the capacitance and resistance of the membrane of the cell. This attenuation would limit cellular motility at high stimulus frequencies, rendering the above scheme ineffective. Therefore, Dallos and Evans (1995a) proposed that extracellular potential changes within the organ of Corti could drive cellular motor proteins. These extracellular potentials are not filtered by the membrane. To test this theory, both electric potentials inside the organ of Corti and basilar membrane vibration were measured in response to acoustic stimulation. Vibrations were measured at sites very close to those interrogated by the recording electrode using laser interferometry. Close comparison of the measured electrical and mechanical tuning curves and time waveforms and their phase relationships revealed that those extracellular potentials indeed could drive outer hair cell motors. However, to achieve the sharp frequency tuning that characterizes the basilar membrane, additional mechanical processing must occur inside the organ of Corti.

Acoustic Stimulation↗

PubMed2002

Alterations of basilar membrane response phase and velocity after acoustic overstimulation.

To investigate the physiology of noise-induced hearing loss, the sound-induced vibrations of the basilar membrane (BM) of the inner ear were measured in living anesthetized guinea pigs before and after intense sound exposure. The vibrations were measured using a laser Doppler velocimeter after placing reflective glass beads on the BM. Pseudo-random noise waveforms containing frequencies between 4 and 24 kHz were used to generate velocity tuning curves. Before overstimulation, sharp response peaks were seen at stimulus frequencies between 15 and 17 kHz, consistent with the expected best frequency of the recording location. The response to low level stimuli lagged the high level ones by up to 90 degrees at the characteristic frequency. Following exposure to loud sound, the BM vibrations showed a pronounced reduction in amplitude, primarily at low stimulus levels, and the best frequency moved to approximately 12 kHz. At higher levels, the reduction was either absent or much smaller. In addition to the amplitude changes, increased phase lags were seen at frequencies near the characteristic frequency. In animals with more severe exposures, response phases were altered also at frequencies showing no change of the amplitude. The phase was independent of stimulus level after severe exposures.

Acoustic Stimulation↗

PubMed2002

Loud sound-induced changes in cochlear mechanics.

To investigate the inner ear response to intense sound and the mechanisms behind temporary threshold shifts, anesthetized guinea pigs were exposed to tones at 100-112 dB SPL. Basilar membrane vibration was measured using laser velocimetry, and the cochlear microphonic potential, compound action potential of the auditory nerve, and local electric AC potentials in the organ of Corti were used as additional indicators of cochlear function. After exposure to a 12-kHz intense tone, basilar membrane vibrations in response to probe tones at the characteristic frequency of the recording location (17 kHz) were transiently reduced. This reduction recovered over the course of 50 ms in most cases. Organ of Corti AC potentials were also reduced and recovered with a time course similar to the basilar membrane. When using a probe tone at either 1 or 4 kHz, organ of Corti AC potentials were unaffected by loud sound, indicating that transducer channels remained intact. In most experiments, both the basilar membrane and the cochlear microphonic response to the 12-kHz overstimulation was constant throughout the duration of the intense stimulus, despite a large loss of cochlear sensitivity. It is concluded that the reduction of basilar membrane velocity that followed loud sound was caused by changes in cochlear amplification and that the cochlear response to intense stimulation is determined by the passive mechanical properties of the inner ear structures.

Acoustic Stimulation↗