The Mechanics of Hearing

Cartoon of Mechanically Activated Nerve Channel*

Flies such as Drosophila hear with their antennae. The base of the antennae, the Johnston’s organ vibrates in response to vibrations in the air (sound). The vibrations mechanically move hairs that are innervated by sound detectors. How is the vibration in hairs and neurons converted into nerve signals?

Nerve signals are generated by ions such as sodium and potassium moving through channels in the nerve membrane. Channels are made from proteins that span the membrane from inside to outside. Ions can cross the membrane through a pore in the center of a protein channel. The pore in the channel has a protein that acts as a gate to opens and close the pore. (See Cartoon) In mechanically activated channels, the protein gate is attached to the actin cytoskeleton of the nerve cell. Deforming the nerve cell causes the actin cytoskeleton to move, which moves the gates and opens pores.

Mechanical coupling acts rapidly. A nerve impulse may be generated in less than a nanosecond after the nerve is deformed. This is much faster than any known chemical activation. The inability of chemical coupling to act rapidly enough led scientists to search for other mechanisms that couple vibration to nerve impulses. The mechanism identified in Drosophila is similar to the mechanisms found in human ear hairs. Thus, Drosophila may be a good model to explore hearing with application to humans.

*Susanne Bechstedt, Jonathon Howard. Hearing Mechanics: A Fly in Your Ear, Current Biology, Volume 18, Issue 18, 23 September 2008, Pages R869-R870.
http://dx.doi.org/10.1016/j.cub.2008.07.069