Download Structure and Function of the Inner Ear

Structure and Function of the Inner Ear
The inner ear is entirely enclosed within the temporal bone. It has two
separate regions, the cochlea and vestibule, which are responsible for
hearing and balance, respectively. The neural signals from the two
regions of the inner ear are relayed to the brainstem through separate
fiber bundles, but which run together as the vestibulocochlear nerve.
Sound information is transmitted from the middle ear to the inner ear
via the stapes attachment to the oval window, which is a membrane at the
beginning of the cochlea. As the tympanic membrane vibrates from
sound waves, the ossicles amplify that vibration, and then the oval
window moves with the same vibrations. The oval window is at the
beginning of a tube that runs the length of the cochlea to its tip
(helicotrema) and back alongside itself to end at another membrane
called the round window (secondary tympanic membrane).
As the oval window is pushed in by sound waves, fluid within this tube is
pushed along its length and the round window at its other end can bulge
out as a result of that movement. Likewise, when the oval window is
pulled back, the fluid inside this tube is drawn back and the round
window can pucker in to compensate. As vibrations of the tympanic
membrane are transmitted through the ossicles, a wave (often referred
to as standing wave because of its properties) is created within the fluid
in the cochlea that displaces sections of the cochlear
partition (cochlear duct and basilar membrane). It is these
waves that are detected by the sensing cells found attached to the basilar
membrane.
The tube running from the oval to the round window in the cochlea is
separated into two spaces. From the oval window to the tip of the
cochlea the tube is referred to as the scala vestibuli and from the tip
of the cochlea back to the round window it is the scala tympani.
These spaces can be seen in a cross-section of one turn of the cochlea.
The two spaces are on either side of the cochlear duct, which is the
space that contains the structures that transduce sound into the neural
signal. Those structures are contained within the spiral
organ or organ of Corti, which lies on top of the basilar
membrane that separates it from the scala tympani. The spiral organ
contains hair cells with stereocilia on their apical membrane. The
stereocilia bend in response to movement of the basilar membrane
relative to the partially fixed tectorial membrane. Depending on
which direction the stereocilia bend, they open or close ion channels,
leading to signal changes in the cochlear nerve.
As a standing wave is set up within the scala vestibuli and scala tympani
in response to movement at the oval window, the basilar membrane
responds by moving at a specific spot, dependent on the frequency of the
standing wave.
The cochlea encodes auditory stimuli based on frequency between 20 Hz
and 20,000 Hz, the range of human hearing. Higher frequencies (higher
pitch) cause the basilar membrane close to the base of the cochlea to
move and lower frequencies (lower pitch) cause the basilar membrane
closer to the tip of the cochlea to move. The number of hair cells in an
area that are stimulated determines loudness, or sound intensity. A
louder sound produces greater displacement of the basilar membrane,
leading to a greater number of stereocilia responding.