A Study on the Organ of Corti Kinematics Using Optical Coherence Tomography

Sultan Sabha, MS Defense, Advised by Professor Jong-Hoon Nam

https://rochester.zoom.us/j/97076178529

Meeting ID: 970 7617 8529

The cochlea is the hearing organ that encodes acoustical stimulations into neural impulses. The cochlea consists of three cavities filled with lymphatic fluids and partitioned by sensory epithelium. Sound waves result in pressure difference in the ear which are transduced into neural impulses. The neural impulses are carried to the brain to interpret as different frequencies and amplitudes of sound through the auditory nerve. The monotonic variation of the cochlear partition geometry along its length accounts for the steep longitudinal gradient in its mechanical properties. The mechanical gradient in the cochlea is known to determine the most prominent feature of the cochlea: the frequency-location relationship.

Despite its importance, existing data regarding the organ of Corti geometry is insufficient to explain recent observations. It is known that the observed geometry of the organ of Corti could be distorted depending on the methods of histological preparation. In some studies, the cochlear tissues were fixed and dehydrated for histological analyses which resulted in deformation. In other studies, fresh tissues were imaged without fixation, but the physiological two-fluid compartments were compromised. These procedures could affect the tissue morphologies, such as swelling or uncontrolled separation of structures such as tectorial membrane detachment. We studied the organ of Corti geometry in the middle to apical turns of the gerbil cochlea using a custom-designed microfluidic chamber that preserves the two-fluid condition of the cochlea.

According to recent observations, vibrations in the organ of Corti are more complicated than existing theories have anticipated. There are a few intrinsic challenges in recent cochlear mechanics research. First, due to insufficient optical resolution, it has been difficult to observe the motion of individual cells. Second, active vibrations due to outer hair cell motility cannot be isolated from the passive vibrations. Third, it was difficult to observe the mechanics in the middle turn of the cochlea. By modifying commercial imaging system for our micro-chamber preparation, we were able to overcome these three difficulties. That is, we were able to resolve the motion of individual cells. We were able to measure vibrations caused by outer hair cell motility as well as acoustic pressure. Finally, our measurements are taken from the cochlea's middle to apical turns.

Mechanotransduction is the central function of the cochlea, and its sensitivity is a pivotal attribute. However, the sensitivity is still poorly defined. Different studies reported the sensitivity with variation by an order of magnitude. The variation has been ascribed to the complication of the artificial stimulation condition of ex situ preparations. For example, in most mechanotransduction measurements, the tectorial membrane through which the hair cells are agitated is removed. We have established an alternative method to record the organ of Corti mechanotransduction without compromising the natural stimulation through intact tectorial membrane using our microfluidic chamber preparation.