Page 180 - Visions of the Future Chemistry and Life Science

Exploring human organs with computers 161

(a) (b)

Figure 9.4. (a) The cochlea is a liquid-filled duct, typically 30 mm long and 1 mm in
diameter, that is coiled into a spiral. The duct is divided lengthways into two
chambers by the flexible cochlear partition. Mechanical stimulation is provided by
piston-like motion of the stapes bone, which is the last bone in the middle-ear
chain and is the smallest bone in your body. Stapes motion sets up a pressure
differential across the partition which causes it to move. The incompressibility of
the cochlear liquid means that when the stapes moves out the round window
moves in, and vice versa. (b) Most models of the cochlea consider it to be straight,
since the coiling is not thought to influence its operation. This figure shows the
displacement of the partition at one instant in time during sound stimulation at a
single frequency. Note that the displacement is shown greatly exaggerated, as near
the threshold of hearing the maximum displacement is only one-millionth of a
millimetre. The stiffness of the partition decreases away from the stapes, so when
it is excited by a fluid pressure difference, it moves first near the stapes, since
stiffer things respond more rapidly. This is followed by motion at positions
progressively further away, thereby giving the appearance of a travelling wave
propagating away from the stapes. As this wave travels along the length of the
cochlea it increases in amplitude before reaching a peak and then dying away
rapidly. The stiffness gradient also means that the position of the peak is dependent
upon the frequency, so that lower frequency stimuli produce a peak further from
the stapes. The cochlea thereby divides the incoming sound into its constituent
pure tones. The sensory cells that convert partition motion into electrical signals
in the auditory nerve are present along the entire length of the cochlea.

The human cochlea, which derives its name from the Greek word koch-
lias, consists of approximately three turns of a circular duct, wound in a
manner similar to that of a snail shell (Figure 9.4). The ability to resolve dif-
ferent tones is determined by the pattern of vibration of the flexible coch-
lear partition. This partition has three main components (Figure 9.5). When