The Implementation of a Photoelectronic Motion Transducer for Measuring the Sub-Micrometer Displacements of Vestibular Bundles

Abstract

The vestibular system is one of our main organs responsible for the sense of balance. This system is located within the inner ear and contains cells with ciliary bundles. These hair cells are transducers that convert a mechanical movement, detected by the bundle of cilia extending from their top surface, into an electrochemical signal to be sent to the brain. The bundles vary structurally within the organs of the inner ear, and this structural difference may play a role in the mechanical properties of each bundle. Analyzing the mechanical properties of the cells will provide information necessary for understanding the transduction process. In an effort to evaluate one of these properties, cell bundle stiffness, a system was designed to mechanically stimulate the bundles within their physiological range and then measure the resulting displacement. The mechanical stimulation was the result of a force applied to the tip of a bundle with the end of a glass whisker. The distance the base of the whisker moves is measured by an extrinsic Fabry-Perot interferometer (EFPI). The magnitude of this movement is compared with the amount the bundle is deflected, detected by a photoelectronic motion transducer (PMT). Knowing these displacements and the stiffness of the glass whisker, simple kinematics is used to determine the bundle stiffness. System tests were conducted on imitation bundles (whiskers of known stiffness) and the experimental stiffness differed from the known value by less than 4.5% for every test. These results lead us to conclude the system was in good working order and could be used to conduct tests on cell bundles. For tissue tests, this work focused on the hair cells located within the utricle, which senses linear accelerations of the head. Within the utricle, we examined two types of hair cells: non-striolar (medial type II) and striolar. Tests on twelve medial type II cells found bundles ranging in stiffness from 0.26 to 2.62 x 10⁻⁵ N/m. Results with striolar bundles provided a range from 2.83 to 27.10 x 10⁻⁵ N/m. The results of the preliminary tissue tests lead us to conclude that the average stiffness of the striolar and non-striolar bundles seems to vary by an order of magnitude. This is consistent with the relative relationship produced through a computer model. However, the model predicted larger stiffness values for both types of cells.