How is sound amplified in the cochlea?


© Dr. M. Leibovici/Institut Pasteur

The ciliated cells in the cochlea are the motor for hearing. The internal cells transmit sound to the brain, while the external ones amplify the sound. Without them, our surroundings would be nearly impossible to hear (the sensitivity is increased by approximately 60 dB).

It has already been proven that amplification is possible because the external ciliated cells stretch and shorten, causing a vibrating movement in different membranes of the cortical organ (basal membranes and tectorial membranes).

Two scientists from the University of Columbia recently found out how these contractions synchronize in order to produce correct amplification. If the external ciliated cells shorten and stretch with the vibrations caused by the entry of sound waves in the cochlea in a passive matter, this would not have the effect of amplifying the signal, but would attenuate it.

What process do the external ciliated cells use in order to obtain exact synchronization to allow amplification? There is a short time lapse between the passage of the wave, that should lead to contractions in the external ciliated cells, and the moment when the contractions actually take place, according to results obtained by Elizabeth Olson and Wei Dong, the authors of the study.

They use the motion of a child on a swing as an analogy: it is as if the external ciliated cells were waiting for the right moment for optimal amplification, just like when a child stretches his legs out at the right moment in order to amplify his movement on a swing. To be able to demonstrate the time lapse, the researchers inserted an electrode into the cochlea at the level of the interface between tissue and cell, allowing them to measure both the potentials generated by sound stimuli, and the vibrations of the basal membrane.

While this discovery is a great step towards elucidating the process of cochlear amplification, the two scientists have not determined the underlying mechanism, which is yet to be explained.

Source: Dong W and Olson ES. Detection of cochlear amplification and its activation. Biophysical Journal 2013;105:1067-78.