Page 181 - Visions of the Future Chemistry and Life Science

162 P. J. KOLSTON

Figure 9.5. An oblique, simplified view of a 150- m-long portion of the cochlear
partition, approximately to scale. The partition has three main components,
namely the basilar and tectorial membranes, and a collection of cells known as
the organ of Corti. The tectorial membrane has been partially removed to reveal
the tops of the outer hair cells and the reticular lamina. The organ of Corti
contains two types of sensory hair cell: outer and inner, and a number of
supporting cells. The inner hair cells, which are not shown here, are positioned
adjacent to the pillar cells. At each position along the cochlea, the pillar cells
couple vertical motion of the basilar membrane to a shearing motion between the
tectorial membrane and the reticular lamina. This bends the stereociliary bundles
that project from the top of both types of hair cell. These bundles contain ion
channels whose electrical resistance changes with mechanical stimulation. The
mechanical motion of the bundle thereby modulates the flow of ions into the cell,
causing changes in the voltage across the cell membrane. This in turn modulates
the release of neurotransmitter onto the nerve fibres that innervate the cell. Due
to the pattern of innervation, it is the inner hair cells that are primarily
responsible for providing the higher auditory centres with information about
basilar membrane motion, whereby different frequencies are encoded onto
different nerve fibres.
The outer hair cells have a quite different role. Experiments on isolated cells
have shown that their length is proportional to the voltage across the cell
membrane. As in inner hair cells, this voltage is modulated by mechanical motion
of the bundle. But the resulting length changes influence the mechanics of the
basilar membrane. When the basilar membrane moves down, during one half-
cycle of a pure-tone stimulus, the outer hair cells increase their length so as to
push downwards on the membrane, thereby increasing its displacement. And