The cochlea acts as both a passive
filter and an active filter. The passive filtering properties create
a tonotopic distribution of the frequency spectrum along
the length of the cochlea, based on the inverse relationship
between the mass and the stiffness of the basilar membrane.
The basilar membrane is narrow and stiff at the base of the
cochlea, which corresponds to high-frequency tuning. At the
apex of the cochlea, the basilar membrane is wider and
less stiff, which corresponds to low-frequency tuning. Thus,
each point along the basilar membrane has a characteristic
frequency to which it is tuned.

When a patient with a suspected cochlear sensorineural
hearing loss is first identified, the work-up should try to
identify the si te of the lesion in the cochlea. The major
sites that can be affected are the stria vascularis, the hair
cells (both inner and outer), the afferent nerve fibers (spiral
ganglion cells), and the otic capsule. The area in the
cochlea most often affected is the outer hair cell because it
is the first cell to be damaged by noise trauma, ototoxic
substances, and the effects of aging.

Noise-induced damage in the inner ear is first identified
in outer hair cells. The attachments of outer hair cell stereocilia
to the tectorial membrane can be broken even by
quite mild noise.



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