In contrast, the inner ear is filled with fluid, which
translates vibrations into pressure waves that move rapid-
ly through the channels in the cochlea and stimulate audi-
tory receptors. The inner ear also contains the
vestibular
apparatus
(
semicircular canals
, utricles, and saccules),
which senses positions and movements of the head. Let’s
take a closer look at how these structures of the ear work
together to perceive the ballerina’s music (
Figure 8.12
).
Receptors for Hearing Are
Found in the Cochlea
The outer ear directs incoming sound waves to the ear canal
and eardrum, which vibrates with the sound (
).
Vibrations of the eardrum cause the auditory ossicles to vi-
brate and move the
oval window
of the inner ear. The vibra-
tions set up pressure waves in the fluid of the
cochlea
, which
moves hair cells in the
organs of
Corti
and elicits action potentials
in sensory neurons.
Hair cells
are
the receptors for hearing. Nerve
impulses travel via the
vestibulo-
cochlear
nerve
(cranial nerve
VIII)
to the brain’s auditory cortex in the temporal lobe.
Along the middle of the cochlea, there runs a mem-
brane called the basilar membrane. Sound waves of various
frequencies cause certain regions of the basilar membrane
within the cochlea to vibrate more than others. Each seg-
ment of the
basilar membrane
is “tuned” for a particular
pitch. Higher-frequency sounds vibrate the membrane near
the oval window, while lower-frequency sounds vibrate the
membrane near the tip of the cochlea. Louder sounds cause
larger vibrations of the membrane and higher-frequency
nerve impulses reaching the brain.
cochlea
(KOK-le-a)
A winding, cone-
shaped tube that
forms a portion of the
inner ear.
The m echanism o f h earing
• Figure 8.13
Malleus
Incus
Stapes vibrating
Cochlea
Sound waves
Ear drum
Round window
Basilar
membrane
Spiral organ
(organ of Corti)
Middle ear
Auditory tube
Sound waves hit the eardrum,
causing the auditory ossicles to
move. The ossicles pull the oval
window in and out, thereby
setting up fluid pressure waves
in the cochlea.
The pressure waves move
through the upper chamber of
the cochlea and stimulate the
hair cells in the spiral organs.
The stimulation of the hair cells
initiates action potentials in the
cochlear branch of the
vestibulocochlear nerve (CN VIII).
The pressure waves shift into
the lower chamber of the
cochlea and travel to the round
window where they are
eliminated from the inner ear.
The vestibulocochlear nerve
conducts the impulses to the
medulla, midbrain, thalamus,
and temporal lobes.
"\
Suppose you had an ear infection and had fluid in your
middle ear that was pushing on the eardrum and making it
somewhat stiff. Why would you have trouble hearing?
The Ear is involved in Hearing and Equilibrium 243
p r o c e ss
diagram
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