Page 13 - Winter Issue 2018
P. 13

with normal hearing and a typical older adult with hearing middle ear fluid or fusing of the middle ear bones, which
loss. The y-axis plots the levels (in dB hearing level [HL]) prevents their movement in response to sound. These are ex-
at which the listener can just barely hear the sound. In this amples of pathologies that prevent the conduction of sound
case, the reference for decibels is the average thresholds for a through the middle ear. In contrast, a sensorineural hearing
large group of normal-hearing individuals who have no ear loss suggests a pathological condition in either the cochlea
diseases, so smaller values (near 0 dB HL) indicate normal or the auditory nerve. The “sensory” component refers to the
hearing and larger values indicate hearing loss. Note that cochlea, and the “neural” component refers to problems pri-
the y-axis is reversed from the typical plotting convention marily in the auditory nerve.
so that the audiogram plots better thresholds nearer the top The Sensory Part of the inner ear is the cochlea that Com
and poorer thresholds nearer the bottom. . . . .
tains the organ of Corti. Lying on the organ of Corti are two
/\Udi09|'am types of sensory hair cells, the outer and inner hair cells. The
F'eq”e"°V (KHZ) inner hair cells transduce the sound and convert the sound
0.125 0.25 0.5 1 2 3 4 5 8 . _ . _ .
vibrations into electrical energy. The outer hair cells serve as
0   cochlear amplifiers to control the function of the inner hair
10 cells. The gain of this amplifier can be increased or decreased
A 20 &_A by efferent neural connections that bring control of signals
i:' 30 0 from the brain via olivocochlear reflexes that project from
E
3 40 O
E 50  Olwacuchlea
|— 60 O Agdltory fl  ‘LJC/JCS % Andllory
E erve % /' ‘¢";" 1 x A .\ ewe
 7° . ‘  ll  l  \\
so -O—Younger Right Ear  /MoC\ \ \_\"
—x—Younger Left Ear 4‘ N‘*“T°"‘ \
9° 0 Older Right Ear 6)) i @
100  Older Left Ear _ _
Contralaleral Bralnstern Cross—seCtIon lpsllateral
Cochlea cochlea
Figure 1. Pure—tone thresholds as a function of frequency (“audio— IWIHW
gram”) for a typical younger adult with normal hearing (solid lines) Ce" \ °H“:r'
and typical older adult with hearing loss (dotted lines). When Ariltdllory “"5
thresholds are plotted on an audiogram, lower thresholds (near 0 dB <—
hearing level [HL]) are near the top, indicating better hearing, and \ Easilar Membrane
higher thresholds are near the bottom, indicating hearing loss (see ( M«?':\1/|%I
text). According to the audiogram, the older adult has a mild hear— o.iVE,i‘§gi‘,'.ea,
ing loss in the low frequencies, sloping to a moderately severe hearing (LOC) Eflerems
1055 in the h‘ghf’9‘1“9"5i95» whim ’5P“553"t5 “ t}’Pi5“l P“tt5”" "f“g9' Figure 2. A schematized anatomic view of the olivocochlear reflexes
(filmed h5‘m”g 1955- to the right cochlea. Top: outline ofa transverse section ofa cat brain-
  stem showing the locations of lateral olivocochlear (LOC; green) and
medial olivocochlear (MOC; blue and red) neurons. The pathways
From the audiogram and one or more measures of speech for the ipsilateral (stimulated cochlea; blue) and contralateral (un-
recognition, the audiologist interprets the results to assess 5fim“l“t5‘l ‘°‘hl9“»' "54) MOC reflex“ to the right 5”’ joi" the 91‘
the type of hearing loss and the integrity 01,-each Part of the ivocochlear bundle (OCB; gold). The axons from LOC and MOC
, _ , neurons form the OCB, which is composed of crossed (COCB) and
Per_1Phera1 audltory System‘ The  15 Composed of tinge uncrossed (UOCB) components. The COCB is accessible near the
mam Parts that Contnbute to audmoni the Outer ear (Puma fourth ventricle. CN, cochlear nucleus. The S—shaped gray structure is
and 681‘ Canal), the middle ear (eardrum, all‘-filled middle the lateral superior olivary nucleus, and the gray structure medial to
ear cavity, and middle ear bones), and the inner ear (cochlea it is the medial superior olivary nucleus. Bottom: schematic of the or-
and auditory nerve)_ gan of Corti showing the main terminations of MOC neurons (blue)
on outer hair cells and of LOC neurons (green) on the dendrites of
One issue might be a conductive hearing loss that usually auditory nerve fibers. From Guinan (2006), with permission.
indicates a pathology in the outer or middle ear, such as  
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