Merle Lawrence

Effects of perilymphatic perfusion with neomycin on the cochlear microphonic potential in the guinea pig

The effects of three concentrations of neomycin, administered by a method of acute perilymphatic perfusion of the guinea pig cochlea, on the cochlear microphonic potential (CM) at 4 kHz and 500 Hz are described. A concentration-dependent reduction in CM occured during the 60 minute perfusion period. Neomycin at 10-4 M did not change the CM magnitude, while at 10-3 and 102 M it caused 4 kHz (and 500 Hz) CM reductions which began within 24 (for both frequencies) minutes and 10 (and 12) minutes of drug application respectively. CM reduction proceeded at a higher rate for greater neomycin concentration. The perfusion technique, the implication of the frequency indifference, and the potential of the perfusion technique for inner ear biochemical analysis are discussed.

Some physiological factors in noise-induced hearing loss.

Abstract Loss of auditory sensitivity following exposure to noise is the result of metabolic and structural alteration within the sensory cells of the organ of Corti. Similar changes can be caused by other agents which do not produce a recognizable change in hearing. However, noise is always superimposed upon the physiological state of the sensory epithelium, and this may determine the final affects of the noise. The source of nutrients for the sensory cells is the arcade of vessels lying beneath the basilar membrane. Localized occlusion of these vessels eventually produces degeneration of these sensory cells. Certain conditions produce constriction of some of these vessels, resulting in diminished blood supply and reduction in the metabolic state of the sensory cells. Superimposing overstimulation on these cells at this time would most likely have a destructive effect.

Inner hair cell responses to the velocity of basilar membrane motion in the guinea pig

Triangular wave acoustic stimulation at 200 Hz produced the expected square wave cochlear microphonic at the round window membrane and within the scala media. Intracellular recordings from inner hair cells (IHC) of the first cochlear turn showed a combination waveform having both spike impulse and square wave features. The IHC response suggests a sensitivity of these cells to both the displacement and to the velocity of basilar membrane motion.

Cochlear inner hair cells: Effects of transient asphyxia on intracellular potentials

Intracellular potentials were recorded from inner hair cells in the guinea pig cochlea. Transient asphyxia was induced by interrupting respiration for brief periods. Asphyxia caused a hyperpolarization of the resting membrane potential (resting Em). The hyperpolarization averaged 2.9 mV for 30 s asphyxias and 5.7 mV for 45 s asphyxias. The membrane potential recovered quickly after normal ventilation was resumed. Asphyxia also induced a rapid and profound decrease of the d.c. receptor potential in response to moderate intensity tone bursts at the characteristic frequency of the inner hair cell. At maximal depression, the receptor potential was reduced about 60% for a 30 s asphyxia and 100% for a 45 s asphyxia. The receptor potential recovered slowly after normal ventilation was resumed. A similar percent reduction and time course of recovery were observed for the a.c. receptor potential. In recordings from the same animals, the round window compound action potential (CAP) was as severely depressed by asphyxia as the hair cell receptor potentials. The time course of recovery for the CAP was similar to the slow recovery of the d.c. receptor potential. In contrast, the round window cochlear microphonics (CM) and the endolymphatic potential (EP) were affected less by asphyxia and recovered quickly after ventilation was resumed. Frequency tuning curves (FTCs) for the d.c. receptor potential were measured during the period of maximal receptor potential depression. These FTCs showed decreased tip sensitivity and a decrease in sharpness of tuning, as measured by the Q10. These changes were fully reversible. Low frequency (tail) segments of the FTCs were much less affected by asphyxia. The inner hair cell FTC changes during asphyxia were compared with neural FTC changes reported by other investigators. The similarities lead us to the conclusion that the inner hair cell and the auditory neural response to sound are equally sensitive to asphyxia.

Does loud sound influence the intracochlear oxygen tension

The effect of loud sound on the perilymphatic oxygen tension was studied in anesthetized guinea pigs. Pure tone (4 kHz) and broad-band noise were given at 85-130 dB SPL for 3-8 min. No effects were seen either in the animals exposed to pure tone or in the animals exposed to 85 dB broad-band noise. In the animals exposed to noise at 130 dB SPL both increases and decreases of perilymphatic oxygen were measured but the changes were only of about 12% or less. The response to anoxia was normal. In animals with hypotension ( less than 8 kPa) the perilymphatic PO2 fluctuated with the blood pressure. When the sound was delivered directly into the opened bulla the measured PO2 dropped immediately but was found to be caused by the cooling effect of an air current produced by the noise. Flushing the opened bulla with nitrogen, air or oxygen caused the same temperature-induced drop of measured PO2. The results and the artifacts are discussed.

Oxygen Reserve and Autoregulation in the Cochlea

Endolymphatic O2-concentration was measured in the guinea pig cochlea with a microelectrode inserted through the zona pectinata of the basilar membrane and the cells of Claudius, so as not to interfere with the blood supply to the organ of Corti. The O2-level and the amplitude of the cochlear potentials were followed during hypoxia produced by lowering the pO2 of the inspired air. Persistence of a normal endolymphatic O2-level following the fall in arterial pO2 indicated an O2-reserve and au autoregulation of the cochlear circulation.

Electrical potentials and fluid boundaries within the organ of Corti

With careful preservation of the capillary blood supply and by direct visualization of the cells and spaces of the organ of Corti, electrolyte‐filled glass micro‐electrodes are passed through the structures while the following are recorded: the dc resting potentials, ac potentials generated in response to a 200‐Hz microvibrator vibrating either the stapes or basilar membrane directly, and tissue‐electrode impedance at 250 Hz. The electrode paths are verified visually followed by surface preparation and scanning electronmicroscopy. So far, electrophoretically injected dyes have been too diffuse to be useful for this purpose. It is observed that the fluid spaces of the organ of Corti, including the subtectorial space and inner sulcus, are at, or only slightly more negative than, the potential of the scala tympani perilymph and that all of the observed large negative potentials are intracellular. The tectorial membrane also is at the same potential as the organ of Corti spaces and the perilymph. The membrane is easily dislodged from its attachment allowing endolymph to cover the hair cells and fill the internal sulcus. When the stapes is vibrated at 200 Hz the vibration of the basilar membrane is such that the generated ac potentials are radiated from the organ of Corti in the apical region of the cochlea and are recorded maximally in the endolymph of the scala media by the micro‐electrode in the basal hook region. When the basilar membrane is vibrated at 200 Hz in the region of electrode placement, the electrode records the largest ac response at the basilar membrane and at the tectorial membrane‐endolymph and at the Claudius cell‐endolymph interfaces. Impedance measures made simultaneously with the ac and dc measures indicate that the regions of different potentials observed are not the results of changing tissue‐electrode impedance. A unique observation is that there is a marked impedance increase as the electrode passes through the area of Claudius cells. Boettchers cells may be responsible.

Stethoscope Acoustics II. Transmission and Filtration Patterns

This paper describes a fully calibrated and standardized acoustical test method for evaluating the transmission patterns and the filtration patterns of intact stethoscopes. An essential component of the test system is the artificial ear which duplicates the acoustical contribution of the average human ear to the stethoscopes acoustics. The transmission patterns of bell-type stethoscopes fall into four distinct groups which correspond to their basic design features. Shallow bells and single tubing design both result in attenuation at higher frequencies. A deep, trumpet-shaped bell with double tubing design may provide amplification at higher frequencies. Diaphragms attenuate the transmission acoustics of stethoscopes. When the low frequencies are selectively attenuated, high frequencies are heard more distinctly. Some diaphragms were found to attenuate at all frequencies.The acoustical performance of any stethoscope is critical. Any attenuation of clinically significant sounds of low intensity may render them totally inaudible. The majority of stethoscopes tested (bell and diaphragm chestpieces) attenuate high frequency sounds. The adoption of stethoscopic performance criteria is urged. Few modern stethoscopes show any significant acoustical improvement since the time of Laennec.

The Effect of Cervical Sympathectomy on Cochlear Electrophysiology

The purpose of this study was to examine the electrophysiologic responses to sound from guinea pig cochleas, 3--6 days after a unilateral cervical sympathectomy. After recovery from surgery the guinea pigs developed a Horners syndrome on the sympathectomized side. Some days later their bullas were opened and electrodes were placed bilaterally on the round windows. Most of the sympathectomized cochleas showed signs of decreased sensitivity to sound. They had a smaller dynamic range for the click- and tone-burst evoked compound action potential (CAP) compared with the non-sympathectomized cochleas. The threshold sound levels for the CAPs and the sound levels to produce 1 muV of cochlear microphonic potentials were unaffected. In other animals with chronic implanted electrodes, the same electrical responses were measured with only light sedation before and after sympathectomy and showed the same results. In additional animals the crossed olivocochlear bundle (COCB) was electrically stimulated and a similar inhibition of the CAP was registered on both the intact and the sympathectomized side. The results suggest that the sympathetic nerves to the cochlea, coming from the ipsilateral cervical ganglion and ending near the habenula perforata, may be important for the sound perception as they influence the CAP.

Acute perilymphatic perfusion of the guinea pig cochlea

A method for the continuous perfusion of the perilymphatic space of the inner ear in the guinea pig is described. Artificial perilymph is supplied to the cochlea and drained away through a tubing system while flow rates from 10 microliters/min to 0.3 ml/min are established by gravity syphon pressure. Techniques are also presented which allow control over the temperature of the perfusate and over the level of dissolved oxygen in the perfusate. Alone with these variables, the pH of the artificial perilymph can be manipulated and various drugs can be added to the perfusate to test their effect on the inner ear. The function of the inner ear is monitored by continuous recording of the sound evoked bioelectric potentials, the cochlear microphonic and the compound action potential. The cochlear perfusion technique has many applications in the study of cochlear physiology and metabolism, and in testing the sensitivity of the inner ear to ototoxic drugs.