What converts sound waves into vibrations

How the ear processes sound

This happens in the ear when we hear: a sound wave first makes the eardrum vibrate - this movement is transmitted via the ossicles as a pressure fluctuation and ultimately sets tiny hairs on so-called hair cells in motion in the inner ear. The hair cells convert the vibrations of the hairs into nerve impulses.

Each hair cell is in contact with up to twenty downstream nerve fibers. Depending on the volume, the hair cell activates a different number of these downstream nerve fibers. The transmission efficiency at the contact points between the hair cell and nerve fiber differs depending on the contact point: some downstream cells react to soft sounds, others only to loud ones.

Scientists from Göttingen at the Bernstein Center for Computational Neuroscience under the direction of Professor Tobias Moser have investigated how the hair cells work in the inner ear of the mouse. In doing so, they were able to uncover a mechanism that is unusual for nerve cells: By deflecting the hairs of a hair cell, the electrical voltage across its cell membrane changes - and the louder the signal, the more so.

This change in voltage opens voltage-regulated calcium channels that are located at the contact points with the downstream nerve fibers. Calcium can flow through these channels into the interior of the cell and causes the signal to be transmitted from the hair cells to cells downstream. The working group was able to show that different amounts of calcium flow into the contact points of a hair cell, although all calcium channels are controlled by the same voltage.

"These differences between the various contact points of a hair cell could explain why weak signals are already being passed on at some contact points, while other contact points only become active when stronger signals are received," says Moser.

MEDICA.de; Source: Göttingen University Medical Center