The Inner Ear

It will be seen that all we have examined so far consists of the vibrations in the outer and middle ear. The vibrations are constantly passed forward, but so far there is still nothing apart from a mechanical motion. In other words, there is as yet no sound.

The process whereby these mechanical motions begin to be turned into sound begins in the area known as the inner ear. In the inner ear is a spiral-shaped organ filled with a liquid. This organ is called the cochlea.

The last part of the middle ear is the stirrup bone, which is linked to the cochlea by a membrane. The mechanical vibrations in the middle ear are sent on to the liquid in the inner ear by this connection.

The vibrations which reach the liquid in the inner ear set up wave effects in the liquid. The inner walls of the cochlea are lined with small hair-like structures, called stereocilia, which are affected by this wave effect. These tiny hairs move strictly in accordance with the motion of the liquid. If a loud noise is emitted, then more hairs bend in a more powerful way. Every different frequency in the outside world sets up different effects in the hairs.

But what is the meaning of this movement of the hairs? What can the movement of the tiny hairs in the cochlea in the inner ear have to do with listening to a concert of classical music, recognizing a friend's voice, hearing the sound of a car, or distinguishing the millions of other kinds of sounds?

The answer is most interesting, and once more reveals the complexity of the design in the ear. Each of the tiny hairs covering the inner walls of the cochlea is actually a mechanism which lies on top of 16,000 hair cells. When these hairs sense a vibration, they move and push each other, just like dominos. This motion opens channels in the membranes of the cells lying beneath the hairs. And this allows the inflow of ions into the cells. When the hairs move in the opposite direction, these channels close again. Thus, this constant motion of the hairs causes constant changes in the chemical balance within the underlying cells, which in turn enables them to produce electrical signals. These electrical signals are forwarded to the brain by nerves, and the brain then processes them, turning them into sound.

Science has not been able to explain all the technical details of this system. While producing these electrical signals, the cells in the inner ear also manage to transmit the frequencies, strengths, and rhythms coming from the outside. This is such a complicated process that science has so far been unable to determine whether the frequency-distinguishing system takes place in the inner ear or in the brain.

At this point, there is an interesting fact we have to consider concerning the motion of the tiny hairs on the cells of the inner ear. Earlier, we said that the hairs waved back and forth, pushing each other like dominos. But usually the motion of these tiny hairs is very small. Research has shown that a hair motion of just by the width of an atom can be enough to set off the reaction in the cell. Experts who have studied the matter give a very interesting example to describe this sensitivity of these hairs: If we imagine a hair as being as tall as the Eiffel Tower, the effect on the cell attached to it begins with a motion equivalent to just 3 centimeters of the top of the tower.358

Just as interesting is the question of how often these tiny hairs can move in a second. This changes according to the frequency of the sound. As the frequency gets higher, the number of times these tiny hairs can move reaches unbelievable levels: for instance, a sound of a frequency of 20,000 causes these tiny hairs to move 20,000 times a second.

Everything we have examined so far has shown us that the ear possesses an extraordinary design. On closer examination, it becomes evident that this design is irreducibly complex, since, in order for hearing to happen, it is necessary for all the component parts of the auditory system to be present and in complete working order. Take away any one of these-for instance, the hammer bone in the middle ear-or damage its structure, and you will no longer be able to hear anything. In order for you to hear, such different elements as the ear drum, the hammer, anvil and stirrup bones, the inner ear membrane, the cochlea, the liquid inside the cochlea, the tiny hairs that transmit the vibrations from the liquid to the underlying sensory cells, the latter cells themselves, the nerve network running from them to the brain, and the hearing center in the brain must all exist in complete working order. The system cannot develop "by stages," because the intermediate stages would serve no purpose.

358 Jeff Goldberg, "The Quivering Bundles That Let Us Hear," Seeing, Hearing, and Smelling the World, A Report from the Howard Hughes Medical Institute, p. 38.