455
CHAPTER TWELVE
Nervous System III
Y
olanda Santana, of Rochester, New York,
probably lost her hearing when she
was only eight weeks old and suffered
a high fever. But it wasn’t until she was nine
months old, when Yolanda didn’t babble like
her age-mates, that her parents suspected she
might be deaf. With hearing aids she did well at
a preschool for the deaf. Then Yolanda’s parents
read about the cochlear implant—it does not
magically restore hearing, but enables a person
to hear certain sounds. Teamed with speech
therapy and use of sign language, the cochlear
implant enables a person to make enough
sense of sounds to speak.
When Carlos and Beth Santana read about
the implants, Yolanda was already approaching
three years of age. Before age three is the best
time to receive a cochlear implant because this
is when the brain is rapidly processing speech
and hearing as a person masters language. Of
the thousands of people in the United States
who have received cochlear implants since they
became available in 1984, about half have had
them since early childhood.
The implant consists of a part inserted under
the skin above the ear that leads to two dozen
electrodes placed near the auditory nerve in the
cochlea, the snail-shaped part of the inner ear.
Yolanda wears a headset that includes a micro-
phone lodged at the back of her ear to pick up
incoming sounds and a fanny pack containing
a speech processor that digitizes the sounds
into coded signals. A transmitter on the headset
sends the coded signals, as FM radio waves, to
the implant, which changes them to electrical
signals and delivers them to the cochlea. Here,
the auditory nerve is stimulated and sends neu-
ral messages to the brain’s cerebral cortex, which
interprets the input as sound.
Yolanda’s audiologist turned on the speech
processor a month after the surgery. At first,
the youngster heard low sounds and some-
times responded with a low hum. She grabbed
at the processor, realizing it was the source of
the sound. Gradually, the little girl learned from
context what certain sounds meant. One day
when Carlos signed “father” and said “poppy,”
Yolanda signed back and tried to say the word!
Able to connect mouth movements to sounds
to concepts, Yolanda was well on her way to
hearing.
12.4
CLINICAL APPLICATION
Getting a Cochlear Implant
Scala vestibuli
filled with perilymph
Vestibular
membrane
Basilar
membrane
Scala tympani
filled with
perilymph
Round window
Membranous
labyrinth
Helicotrema
Cochlear duct
filled with endolymph
Stapes vibrating in
oval window
FIGURE 12.12
The cochlea is a coiled, bony canal with a membranous tube (labyrinth) inside. If the cochlea could be unwound, the membranous
labyrinth would be seen ending as a closed sac at the apex where the bony canal makes a u-turn.
FIGURE 12D
Yolanda Santana received a
cochlear implant when she was three years
old. The device enables her to detect enough
sounds to e±
ectively communicate.
are dissipated into the air in the tympanic cavity by move-
ment of the membrane covering the round window.
The
spiral organ
(organ of Corti), which contains about
16,000 hearing receptor cells, is on the superior surface of the
basilar membrane and stretches from the apex to the base of
the cochlea. The receptor cells, called
hair cells,
are in four
parallel rows, with many hairlike processes (stereocilia) that
extend into the endolymph of the cochlear duct. Above these
hair cells is a
tectorial membrane,
attached to the bony shelf
of the cochlea. It passes like a roof over the receptor cells,
contacting the tips of their hairs
(f
gs. 12.13
and
12.14)
.
Different frequencies of vibration move different parts of
the basilar membrane. A particular sound frequency bends
the hairs of a specific group of receptor cells against the
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