Inner ear

The cochlea transforms sounds into action potentials in auditory nerve fibers. This process is known as mechano-electrical transduction. Vibrations of the tympanic membrane (TM) are finally transmitted to those of basilar membrane (BM) in the cochlea through the perilympth. The organ of Corti sits on the BM, and it consists of sensory cells such as outer hair cells (OHCs), inner ear cells (IHCs) and many other kinds of cells.

Structure of the organ of Corti and the OHC lateral membrane

Although the amplitude of the TM vibration is only a few nanometer when we make conversations, we can understand clearly what they say. This is speculated to be caused by the cochlea amplification. This mechanism is considered that the vibration of the BM is amplified by the force which is accompanied with the motility of the OHCs. In addition, the OHC motility is estimated to be due to the deformation of the motor protein, which distributes along the OHC lateral wall.

The IHCs play a role in the discharge of impulses in response to the BM vibration. The IHC has a three-row structured hair bundle called stereocilia on the top of the cell. The stereocilia are thought to be tilted by the BM vibration. Deflection of the stereocilia is supposed to cause opening ion channel in the apical region of the stereocilia and influx of ions into the cell. As a result, action potentials are produced in the auditory nerve fibers and we can finally recognize as a sound.

(i) BM vibration
The mechanism of the cochlear amplification has not been well understood. Therefore, an attempt is made to clarify this mechanism using the finite-element method (FEM) or measurement of the BM vibration directly. In addition, the timing of neural excitation in relation to BM motion is examined.

(ii) OHC
It is speculated that mechanical properties of the OHC lateral wall and the distribution of the motor protein of the OHC are related to the motility and force production of the OHC. Therefore, an attempt is made to measure the motility of isolated OHC in response to the electrical stimulus and the OHC lateral membrane using an atomic force microscope. In addition, the OHC model is constructed using shell theory and the force produced by the cell is analyzed.

(iii) Otoacoustic emissions
When a sound is transmitted into cochlea, low intensity sounds are generated within the normal cochlea and transmitted to the ear canal. This phenomenon is called otoacoustic emissions (OAEs). The OAEs are speculated to be closely associated with the BM vibration. The OAEs are reduced or absent in cases of certain types of hearing losses. Therefore, it is expected that measurement of OAEs is available for screening hearing in difficult-to-test neonates, infants, and young children.