Mechanism of Sound Transduction
The role of the inner ear
Even though the structure of cochlea seems to be complicated, its basic operation is relatively simple. Inward motion at the oval window caused by ossicles pushes the perilymph in the scala vestibule towards the apex of the cochlea. The fluid pressure, through the helicotrema, then travels back down the scala tympani to the round window. This causes the membrane at the round window to bulge out. In this process, the flexible basilar membrane that sits in between scala tympani and scala vestibule bends in response to sound.
How does the basilar membrane respond to sound?
Two structural properties of the basilar membrane determine the way it responds to a sound wave:
1. The apex of the membrane is 5 times wider than the base.
2. The base of the membrane is about 100 times stiffer than the apex.
Figure 1: Schematic representation of the basilar membrane within the cochlea (uncoiled for illustration of greater clarity) showing its length variation from the base toward the apex.
Image courtesy of https://openlearn.open.ac.uk/mod/resource/view.php?id=263156 under a Creative Commons license.
The wave travelling along the membrane is analogous to that runs along a rope when you give it a snap while holding it in your hand. The distance the wave travels up from the base to the apex of the basilar membrane depends on its frequency: high-frequency waves cause great deal of vibration at the stiffer base and dissipates most of the energy before propagating further; in contrast, low-frequency waves travel all the way up to the floppy apex before complete dissipation of energy. The unique response of the basilar membrane sets up a place code whereby different locations of membrane are maximally bent or deformed at different sound frequencies.
Figure 2: Displacement of the basilar membrane in reponse to sound waves of low frequency (top) and high frequency (bottom).
Image produced by the author of the website.
How is the place code converted into neural coding of pitch?
Since the organ of Corti is rigidly connected to the basilar membrane, any movement of the membrane in response to sound waves will cause the structures within the organ of Corti, including the hair cells, to move as a unit. When sound causes the basilar membrane to move upward, the reticular lamina moves up and in towards the tectorial membrane, causing the stereocilia of outer hair cells to bend due to their attachment at the tips to the membrane. Likewise, downward movement of basilar membrane causes bending of stereocilia in the opposite direction. In fact, stereocilia from inner hair cells are similarly bent, presumably as a result of endolymph movement. The stereocilia on a hair cell are made to stick together by cross-link actin filaments and hence move as a unit.
Figure 3: The bending directions of the hair cells depend on whether the basilar membrane is moving upward (a), resting (b), or moving downward (c).
Image courtesy of https://openlearn.open.ac.uk/mod/resource/view.php?id=263160 under a Creative Commons license.
Bending of the stereocilia leads to the generation of neural signals
Recording from the bony structure in vivo has technically been a difficult issue. Therefore the transduction mechanism has been revealed mostly by in vitro studies of isolated hair cells from the cochlea. Recordings from these cells have shown that bending of stereocilia in a direction causes depolarization of the hair cell. As they bend in the other direction, the cell hyperpolarizes instead. Consequently, when the sound wave causes the stereocilia to bent backward and forward, hyperpolarization and depolarization of the hair cells occur in alternative manner from the resting potential.
Figure 4: Hyperpolarization and depolarization of the hair cell depending on the bending direction of the stereocilia. The hair cell receptor potential follows the changes in air pressure closely during a low-frequency sound.
Image courtesy of https://openlearn.open.ac.uk/file.php/3373/SD329_1_015i.jpg under a Creative Commons license.
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