The Puzzle of Sound Amplification in the Inner Ear

One of the extraordinary features of the mammalian sound detection system is the range over which it works. This extends from 11 KHz in birds to 200 KHz in marine mammals.

This is only possible because the inner ear amplifies sounds by a factor of up to 4000. That’s a huge amount of gain. So much, in fact, that it’s hard to square with conventional thinking about mechanical amplification. So there is much head scratching among biologists over how the ear achieves this amplification.

Part of the puzzle is that the amplification is not entirely passive. The inner ear is essentially a fluid-filled tube, divided along its length by a thin elastic membrane. This membrane is covered in hair cells, which come in two types.

The so-called inner hair cells convert pressure waves within the fluid into electrical signals the brain can interpret. However, the outer hair cells act like mechanical amplifiers. When struck by a pressure wave, the cells themselves begin to vibrate at the same frequency, thereby boosting the wave as it passes.

The trouble is that measurements using outer hair cells indicate that they amplify pressure waves by a factor of about 10, a gain that falls far short of what’s required.

Today, however, Tobias Reichenbach and James Hudspeth at The Rockefeller University in New York city say they’ve worked out what else is going on to boost the signal.

Sound enters the inner ear as a pressure wave which travels through the fluid filled chamber, causing the membrane that divides it along its length to vibrate, like a sheet of rubber. Since the hair cells sit on this membrane they also move.

Reichenbach and Hudspeth calculate that the vibration of the outer hair cells not only amplifies the pressure wave, but also increases the displacement of the membrane, like a child bouncing on a trampoline.

When these effects combine, they result in a positive feedback effect, which creates a huge gain. This easily explains the 4000x amplification. In fact, the team says the gain can be even larger: “The overall cochlear gain, the product of these two components, can exceed 10,000,” say Reichenbach and Hudspeth.

All that remains is for the experimentalists to devise a way of showing that this is actually the process that achieves the gain, admittedly not the easiest of tasks. But one that could soon lead to this conundrum being settled once and for all.

Ref: arxiv.org/abs/1009.2034: Dual Contribution To Amplification In The Mammalian Inner Ear