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Chapter 7
which extends across the lumen of each semicircular canal at the
ampulla,
a slight bulge in the wall of each duct (
Figure 7–41
).
Whenever the head moves, the semicircular canal within its
bony enclosure and the attached bodies of the hair cells all move
with it. The fl
uid fi lling the duct, however, is not attached to the
Figure 7–39
A tunnel in the temporal bone contains a fl
uid-fi lled membranous
duct system. The semicircular canals, utricle, and saccule make up
the vestibular apparatus. This system is connected to the cochlear
duct. The purple structure within the ampullae are the cupulae,
which contain the hair (receptor) cells.
Redrawn from Hudspeth.
Semicircular
canal
Cupula
Saccule
Vestibulocochlear nerve
Vestibular branch
Utricle
Ampulla
Cochlear duct
Cochlea
Cochlear branch
Figure 7–40
Orientation of the semicircular canals within the labyrinth. Each
plane of orientation is perpendicular to the others. Together they
allow detection of movements in all directions.
Neural Pathways in Hearing
Cochlear nerve fi bers enter the brainstem and synapse with
interneurons there. Fibers from both ears often converge on
the same neuron. Many of these interneurons are infl uenced
by the different arrival times and intensities of the input from
the two ears. The different arrival times of low-frequency
sounds and the different intensities of high-frequency sounds
are used to determine the direction of the sound source. If, for
example, a sound is louder in the right ear or arrives sooner at
the right ear than at the left, we assume that the sound source
is on the right. The shape of the outer ear (the pinna; see
Figure 7–34) and movements of the head are also important
in localizing the sound source.
From the brainstem, the information is transmitted via
a multineuron pathway to the thalamus and on to the audi-
tory cortex in the temporal lobe (see Figure 7–14). The neu-
rons responding to different pitches (frequencies) are arranged
along the auditory cortex in an orderly manner much as sig-
nals from different regions of the body are represented at dif-
ferent sites in the somatosensory cortex. Different areas of the
auditory system are further specialized; some neurons respond
best to complex sounds such as those used in verbal commu-
nication. Others signal the location, movement, duration, or
loudness of a sound.
Electronic devices can help compensate for damage to
the intricate middle ear, cochlea, or neural structures.
Hearing
aids
amplify incoming sounds, which then pass via the ear
canal to the same cochlear mechanisms used for normal sound.
When substantial damage has occurred, however, and hearing
aids cannot correct the deafness, electronic devices known as
cochlear implants
may restore functional hearing. In response
to sound, cochlear implants directly stimulate the cochlear nerve
with tiny electrical currents so that sound signals are transmitted
directly to the auditory pathways, bypassing the cochlea.
Vestibular System
Hair cells are also found in the
vestibular apparatus
of the
inner ear. This connected series of fl uid-fi lled, membranous
tubes also connects with the cochlear duct (
Figure 7–39
).
The hair cells detect changes in the motion and position of the
head. The vestibular apparatus consists of three membranous
semicircular canals
and two saclike swellings, the
utricle
and
saccule,
all of which lie in tunnels in the temporal bone on
each side of the head. The bony tunnels of the inner ear that
house the vestibular apparatus and cochlea have such a com-
plicated shape that they are sometimes called the
labyrinth.
The Semicircular Canals
The semicircular canals detect angular acceleration during
rotation
of the head along three perpendicular axes. The three
axes of the semicircular canals are those activated while nod-
ding the head up and down as in signifying “yes,” shaking the
head from side to side as in signifying “no,” and tipping the
head so the ear touches the shoulder (
Figure 7–40
).
Receptor cells of the semicircular canals, like those of
the organ of Corti, contain hairlike stereocilia. These stereo-
cilia are closely ensheathed by a gelatinous mass, the
cupula,
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