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Hair Cell Micromechanics, Biophysics and Pathophysiology

Individual hair cells or epithelial sheets containing many hair cells can be harvested from the chick cochlea and kept alive in culture medium for hours. These hair cells are viewed at high magnification with specially designed and equipped microscopes. An underwater loudspeaker with a 10 micron diameter has been engineered to stimulate the sensory hairs on the isolated hair cells. This speaker produces a water micro jet that can blow the hairs back and forth as fast as 5.0 - 6.0 kHz. The hair motion can be rendered in slow motion by illuminating the microscope with stroboscopic light.

Patch clamp electrodes attached to the plasma membrane of the hair cell allow us to measure receptor currents in response to the hair bundle stimulation.

There are currently two hair cell projects in progress. One of these over-stimulates the hair bundle to examine the consequences of acoustic injury on the hair cell response. These studies examine the consequences of damage to the hair bundle, and its tip links on the physiology of the cell itself. The second area is concerned with the relationship between hair bundle stimulation and neurotransmitter release. Capacitance measures are used to determine the degree of vesicle fusion with the plasma membrane during the injection of depolarizing current into the hair cell. The quantitative relationship between hair cell stimulation and neurotransmitter release remains one of the most poorly understood processes of the hair cells, and these studies address this issue.

In addition, we are examining quantitatively the damage and repair of chick hair cell tip links following exposure to intense sound. The morphologic shapes of the hair bundles when viewed in profile is also being mapped across the sensory epithelium of the chick basilar papilla.

These studies use the scanning electron microscope to examine the hair bundles.

Finally, studies are under way to examine the mRNA of in vitro damaged hair cells to ascertain the response to injury at the molecular level.

Middle-Ear Sound Transmission

Investigations of middle-ear structure and function have been undertaken in the Auditory Laboratory for over 15 years. Most of the effort during the last decade has been directed toward understanding the contribution of middle-ear maturation on the overall maturation of hearing capacity. These investigations studied the changing structure of the conductive apparatus during development in various laboratory animals. Functional development was traced by using laser interferometry to measure the magnitude of tympanic membrane (the eardrum) motion at various test frequencies when the membrane was acoustically stimulated at a constant sound pressure level (i. e. 100 dB SPL).

The current middle-ear work is oriented toward the changes that occur to the conductive apparatus in the aging middle ear. The tympanic membrane contain collagen fibers which impart to the membrane structural properties that are important to its function. Collagen is known to break down with aging. If collagen in the tympanic membrane changes with age it may contribute to a reduction in the efficiency of sound transmission through the conductive system. This possibility is presently being explored.

Recovery of Function Following Exposure to Intense Sound

Excessive and dangerous sound levels capable of producing permanent hearing loss abound in our society. They are more associated with our recreational activities than with the work place where OSHA regulations have set standards that limit the exposure to damaging sound levels.

A number of investigations are directed toward understanding the underlying mechanisms of acoustic injury to the ear and the processes that govern the recovery from acoustic trauma. Young chicks serve as an animal model because of the remarkable ability of this species to regenerate new hair cells in replacement of those lost to the loud sound exposure. Physiologic responses of the inner ear and cochlear nerve are used to trace the process of functional recovery. The inner ear endocochlear potential is studied by using ion selective electrodes to measure post-exposure ionic changes in scala media. Single-nerve fibers of the cochlear nerve are characterized by their response to sound following exposure. Currently we are examining the coding of rate-intensity functions and phase-locking in the responses of these nerves. In addition, distortion product otoacoustic emissions are being measured to determine if there are gross mechanical changes in the chick sound-damaged cochlea. These emissions are studied using the cubic difference tone, which is reflected out of the inner ear and measured as an acoustic signal in the ear canal. Changes in the central nervous system as a consequence of acoustic injury in the cochlea are also being examined. Cells in the brainstem nucleus magnocellularis are studied both morphologically, physiologically and biochemically to determine their response to altered input following cochlear damage.

Finally, we routinely use scanning electron microscopy to examine the details of injury to the surface of the cochlea, and to quantitatively trace structural changes as the chick inner ear recovers.

Sound Motion Analysis by Cells in the Central Auditory Pathway

Several years ago a graduate student thesis undertaken by Dr. Daryl Doan engineered a virtual reality of moving sounds using mathematical equations to dynamically model intensity and time of arrival of sound at the two ears. The dynamic expression of these equations resulted in sound motion along the horizontal azimuth. The motion was either circumferential (around the head) or radial (toward or away from the head).

The equations were modified using a spherical model to fit the interaural distance between the left and right tympanic membranes of the rat head. The sounds were presented to anesthetized animals through ear bars attached to each ear individually. As the moving sounds were presented to the animal, a microelectrode inserted into the primary auditory cortex recorded the cellular response of neurons. Dr. Doan was able to demonstrate that about 30% of his cells were clearly responsive to simulated motion in one direction, but not the other.

These studies are progressing. The stimulus will be modified to cover continuous motion across a 90 degree range of circumferential motion and over a radial distance of 1 to 5 meters from the head.

We also intended to label cells with dye tracers in order to identify the cortical layers from which they originate. Forthcoming studies will record from the inferior colliculus to see if the motion patterns noted in the cortex exist in brainstem nuclei.