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Exploring human organs with computers 163

the cochlea is functioning normally, the motion of the basilar membrane
near the peak is boosted 1000-fold by forces exerted on it by the organ of
Corti. This process makes it possible for us to hear very quiet sounds, and
it improves our ability to resolve different tones. Furthermore, damage to it
is responsible for 80 per cent of hearing losses. The forces driving cochlear
amplification most probably come from one of the sensory cells inside the
organ of Corti, the outer hair cell. Like heart cells, outer hair cells change
their length in accordance with the voltage across the cell membrane. But
outer hair cells are extra special, in that they are much faster than heart
cells, operating on a timescale of one-millionth of a second, and they work
in both directions, in that they both shorten and lengthen. Furthermore,
outer hair cells are extremely sensitive, generating forces in response to dis-
placements of one-millionth of a millimetre.
When developing a model we must decide what simplifications to use
to retain as much structural realism as possible whilst ensuring that the
model is solvable on present-day computers. In comparison with the heart,
the development of structurally realistic finite-element models of cochlear
mechanics is in its infancy. Most current models reduce the complex struc-
ture of the cochlea to just a handful of independent variables, which is a
bit like simulating car crashworthiness using a Duplo model consisting of
four wheels and a handful of blocks. My approach is to embed an orthogo-
nal organ of Corti into the cochlear fluids, and to restrict the stimuli to
pure tones, which happens to be consistent with most experimental inves-
tigations. These simplifications have made it possible to divide the com-
plete cochlea into 0.01mm pieces. The properties of the individual model
structures in the resulting 1000000 system equations are based on recent
experimental measurements.
The computer model allows use to predict what is going in the real
organ of Corti (Figure 9.6). However, most experimental data currently
relates only to the motion of the basilar membrane. By comparing the
model response under different experimental conditions (Figure 9.7), we
can get valuable insight into how the cochlear amplifier operates. The
when the basilar membrane is moving upwards, the hair cells contract. This
process, known as cochlear amplification, increases the sensitivity and frequency
sensitivity of the auditory system. An accurate understanding of cochlear
amplification requires the characterisation of the interactions between the outer
hair cells and the other structures of the cochlear partition, whilst taking into
account loading by the fluids that surround them.