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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4455_Библиотеки_им_академика_М_И_Перельмана
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20 Disorders of the Auditory System
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The Acoustic Reflex. Stapedius muscle
contraction is the end point of the acoustic
reflex (AR). Because the stapedius muscle
is located in the middle ear, it is reasonable to discuss this reflex in this section of
this chapter. However, it is important to
note that the AR involves both peripheral
and central mechanisms (Figure 2–5).
The AR pathway starts with sound
entering the external ear and passing
through the middle ear to the cochlea.
The auditory nerve (see Figure 2–5) picks
up the impulses from the cochlea and
directs them to the ventral segment of the
cochlear nucleus, which is located in the
caudal brainstem. The ventral cochlear
nucleus then sends a fiber tract directly
to the ipsilateral facial nerve nuclei
and another branch to the contralateral
superior olivary complex. The cochlear
nucleus also sends fibers to the ipsilateral
superior olivary complex, which then
connects to its contralateral counterpart.
The ipsilateral superior olivary complex
connects to the facial nerve nuclei on both
sides, and the contralateral superior olive
also has input to the facial nerve nuclei.
Hence, there is bilateral input to the facial
nerve nuclei resulting in bilateral acoustic reflexes, even if there is only monaural
stimulation. From the facial nerve nuclei,
the AR follows an efferent course back to
the stapedius muscle in the middle ear
(Borg, 1973).
Møller (2000) and Borg (1973) provide excellent accounts of the nature and
physiology of the acoustic reflex. Probably the most popular explanation offered
for the purpose of the acoustic reflex is
that it serves to attenuate high-intensity
sounds and therefore may help to protect
the inner ear. As has been documented by
Møller (2000), the higher the intensity of
an acoustic signal above the reflex thresh-
old intensity, the greater the contraction of
the AR, at least for individuals with normal auditory and facial nerve function.
The action of the AR could be viewed as
contributing to the nonlinearity of intensity coding within the auditory system
as it results in a compression effect. In
humans with normal auditory function,
the AR is initiated at 70 to 90 dB HL for
tonal stimuli and frequencies from 250
to 4000 Hz have little or no differential
effects on the level of the threshold of the
AR that is measured. The amplitude of
the AR is largest for bilateral stimulation
followed by ipsilateral and then contralateral stimulation (see Musiek & Baran,
2020). The latency of the AR also is variable and depends on a host of factors
including stimulus type, intensity level,
frequency, and how it is recorded.
The Cochlea
Structure: The Bony Cochlea
The cochlea is a bony shell similar in
shape to a snail shell that is located within
the petrous portion of the temporal bone
(Figure 2–6). Inside this snaillike shell
are fluids (endolymph and perilymph),
special cells (hair and supporting cells),
membranes (basilar, tectorial, Reissner’s),
nerve fibers, blood vessels, and special
epithelium. The cochlea makes up part of
the inner ear that also includes the vestibular apparatus. The latter structure has
a highly identifiable external bony structure that includes the three semicircular
canals and the less obvious areas for the
utricle and saccule. The bony cochlea has
two openings in the shell. The more superior is the oval window, which articulates
with the stapes, and inferior to the oval
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21
p. 240), by F. E. Musiek and J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
Figure 2–5. A schematic of the acoustic reflex with the acoustic stimulus presented to the left ear. The vertical dotted line in the middle of the drawing
represents the midline of the head at the brainstem level. From The Auditory System: Anatomy, Physiology, and Clinical Correlates (2nd edition,
22 Disorders of the Auditory System
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Figure 2–6. A photo of the bony cochlea situated in the temporal bone.
Key: 1 = basal turn, 2 = helicotrema. (Courtesy of M. Pinheiro.) From
The Auditory System: Anatomy, Physiology, and Clinical Correlates
(2nd edition, p. 5), by F. E. Musiek and J. A. Baran, 2020, San Diego,
CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All
rights reserved.
window is the round window. As mentioned earlier, a bony eminence between
the two openings is called the promontory
(Zemlin, 1998).
Inside the bony cochlear shell is a
spiral-shaped osseous structure called
the osseous spiral lamina (OSL). The OSL
is shaped like an evergreen tree with the
basal part of the cochlea located where the
lower branches would be, and the apical
part of the cochlea located toward the top
of the tree (Figure 2–7). The bony cochlea
is approximately 1 cm wide at the base
and about 0.5 cm in length from base to
apex. The human cochlea’s “spiral” has
from 2.2 to 2.9 turns. The modiolus is
the central part of the structure, which is
composed of perforated bone that allows
auditory nerve fibers to pass through
small openings in the OSL and connect
to the hair cells. The OSL has a shelf that
spirals up the modiolus. This bony shelf
is the supporting structure for the basilar
membrane and the limbus. This shelf is
wider at the base than at the apical end of
the cochlea (Musiek & Baran, 2020; Zemlin, 1998).
Function: The Bony Cochlea
In general, the bony cochlea serves as a
framework of support and protection for
the membranous cochlea and the auditory
nerve fibers. The oval and round windows supply access to the membranous
inner ear and seal the inner ear fluids with
the stapes and the round window membrane serving these functions. Because of
2. Structure and Function of the Auditory and Vestibular Systems 23
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Figure 2–7. A drawing of the osseous spiral
lamina. The bony shelf spirals around the modiolus from base (bottom of figure) to apex (top
of figure
omy, Physiology, and Clinical Correlates
edition, p. 6), by F. E. Musiek and J. A. Baran,
2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights
reserved.
). From The Auditory System: Anat-
(2nd
its position, the promontory protects the
round window. For example, if one were
to perforate the lower segment of the TM
with a probe, the probe would likely contact the promontory and not the round
window. This is what happens when
transtympanic electrocochleography is
conducted. That is, after piercing the lower
TM, a needle electrode comes into contact
with the promontory where the electrophysiologic recording takes place.
Structure: The
Membranous Cochlea
The bony cochlea provides a spiral framework that is followed by the membranous
cochlea (Figure 2–8). The membranous
cochlea contains three ducts called scalae:
the scala vestibuli (superior), the scala tym-
pani (inferior), and the scala media — the
smallest scala that is situated between the
other scalae (often termed the cochlear
duct) (Figure 2–9). The stapes’ footplate
articulates with the oval window, which
opens into the vestibule of the inner ear
and transfers energy down the scala vestibuli. The round window is located at the
end of the scala tympani. Reissner’s membrane and the basilar membrane (BM)
divide the cochlea into the three ducts.
Reissner’s membrane separates the scala
vestibuli from the scala media, and the
BM divides the scala media from the scala
tympani. These ducts run the length of the
cochlea except at the very end, termed
the helicotrema, where the scala vestibuli
and the scala tympani communicate. The
scala media communicates with the vestibular system (specifically the saccule)
via a narrowed channel called the ductus
reuniens.
Two important fluid channels, the
vestibular and cochlear aqueducts (VA,
CA), play key roles in inner ear function.
The VA is an endolymphatic channel that
communicates from the posterior surface
of the petrous bone to the saccule. The
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SSC
Ampulla
LSC
Cochlear Duct
Utricle
Saccule
Cupula
PSC
Figure 2–8. A drawing of the membranous labyrinth. Key: SSC = superior
semicircular canal, LSC = lateral semicircular canal, PSC = posterior semicircular
canal. From The Auditory System: Anatomy, Physiology, and Clinical Cor-
relates
CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights
reserved.
VA encompasses the endolymphatic duct
and sac. The CA, which also opens to the
petrous bone’s posterior surface, courses
to the scala tympani near the round window. This channel contains perilymph,
a fluid highly similar to cerebral spinal
fluid. The two main fluids in the cochlea
are endolymph (in the scala media) and
perilymph (in the scala vestibuli and
scala tympani). Endolymph is high in
potassium and low in sodium, whereas
perilymph has just the opposite chemical
concentrations (Figure 2–10).
The BM is composed of about 24,000
fibers and is 25 to 35 mm in length in the
adult human. The BM supports the organ
of Corti, the actual end organ for hearing.
(2nd edition, p. 96), by F. E. Musiek and J. A. Baran, 2020, San Diego,
Ductus Reuniens
The adult BM is wider at its apex than
at its base (0.36
mm), and it is both thicker and stiffer at
the base than at the apex (Buser & Imbert,
1992; Musiek & Baran, 2020) (Figure 2–11).
Reissner’s membrane is essentially avascular, has two layers of cells, and is permeable. From base to apex, Reissner’s
membrane becomes wider like the BM;
however, the scala media and the “shelf”
of the spiral lamina become smaller and
narrower, respectively, as one moves from
the base to the apex.
The organ of Corti is composed of
supporting structures, sensory cells, membranes, and nerve fibers (Figure 2–12). This
structure courses the length of the cochlea.
–0.5 mm versus 0.04–0.1
2. Structure and Function of the Auditory and Vestibular Systems 25
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Figure 2–9. A drawing of a cross-section of the cochlea showing the three scalae.
OHC = outer hair cells, IHC = inner hair cell. From The Auditory System:
Key:
Anatomy, Physiology, and Clinical Correlates
and J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural
Publishing, Inc. All rights reserved.
Directly superior to the hair cells is the
tectorial membrane, which interacts with
the stereocilia of the outer hair cells and is
anchored to the upper lip of the limbus. It
appears that the tectorial membrane does
not articulate with the stereocilia of the
inner hair cells. On its underside and just
above the inner hair cells’ stereocilia is a
bulge termed Hensen’s stripe. This bulge
(2nd edition, p. 97), by F. E. Musiek
in the membrane’s underside reduces the
distance between the tops of the stereocilia and the tectorial membrane and may
play a role in stimulation of the inner hair
cells (see discussion later).
The reticular lamina is composed of
tightly packed cells and forms a ceiling
above the sensory and supporting cells of
the organ of Corti (Figures 2–12 and 2–13).
Figure 2–10. A sketch of the membranous inner ear with a focus on the cochlear and vestibular aque-
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ducts. Key: A = endolymphatic sac, B = vestibular aqueduct, C = endolymphatic duct, D = saccule, E =
utricle, F = semicircular canals, G = ductus reunions, H = cochlear aqueduct, I = round window, J = stapes (oval window), K = scala vestibuli, L = scala media, M = scala tympani. From The Auditory System:
Anatomy, Physiology, and Clinical Correlates
2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 97), by F. E. Musiek and J. A. Baran,
Figure 2–11. A drawing looking down on the basilar membrane showing the relative width and stiff-
ness dimensions of the basilar membrane from its apical to basal end (A), and the relationships between
the basilar membrane and the bony shelf of the cochlea from apex to base (B). Adapted from The Audi-
tory System: Anatomy, Physiology, and Clinical Correlates
2016, San Diego, CA: Plural Publishing. Copyright © 2016 Plural Publishing, Inc. All rights reserved.
(p. 85), by F. E. Musiek and J. A. Baran,
26
Figure 2–12. A drawing showing the major structures within the organ of Corti. (Based on a
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drawing by Polyak, 1946.) From The Auditory System: Anatomy, Physiology, and Clinical
Correlates
Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 101), by F. E. Musiek and J. A. Baran, 2020, San Diego, CA: Plural
Figure 2–13. A micrograph from above the reticular lamina. (Cour-
tesy of M. Pinheiro.) From The Auditory System: Anatomy, Physiol-
ogy, and Clinical Correlates
J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020
Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 105), by F. E. Musiek and
27
28 Disorders of the Auditory System
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The stereocilia of the hair cells protrude
through the lamina. The reticular lamina
is composed of phalanges of Deiter’s cells
and inner and outer border cells, as well
as the tops of the pillar cells. It keeps the
endolymph from penetrating the internal
structures of the organ of Corti. Hence,
only the stereocilia and not the hair cells
themselves are bathed in endolymph,
which has a +80 mV charge (see Figure
2–13).
The lateral wall of the cochlear duct is
discussed next. The lateral wall is the location of the stria vascularis and the spiral
ligament (see Figure 2–9). The stria vascularis is thought to play key roles in the
production and absorption of endolymph.
It is composed of three layers of cells.
Marginal cells make up the first layer and
are linked to ion channels and pumps.
The intermediate cell layer is lateral to the
marginal cells and contains melanin, and
the basal cell layer is next to the spiral ligament. The cells in all three cell layers are
critical for the function of the cochlea. The
stria vascularis also provides blood supply
to the cochlea and maintains the metabolism of the inner ear (Slepecky, 1996).
The spiral ligament extends throughout the cochlea and covers the lateral wall
of the scala media. It also extends inferiorly to the upper scala tympani. This
structure supplies support to the lateral
aspect of the BM as well as for Reissner’s
membrane.
Sensory Cells. There two types of sen-
sory cells in the cochlea: inner and outer
hair cells (IHCs, OHCs). The OHCs are
organized in multiple rows (3–5) and
the IHCs have only one row. These hair
cells are located on the lateral and medial
sides of the pillar cells, respectively (Figure 2–14). There are about 3,500 IHCs
and 12,000 OHCs that course the length
of the cochlea. Stereocilia are located at
the top of both types of hair cells. The
OHC’s structure is different from that
of the IHC (Figure 2–15). The OHCs are
rather elongated tubelike structures that
vary greatly in their length. The OHCs
in the high-frequency regions of the cochlea are shorter than those located in
the low-frequency regions (Geisler, 1998).
Located within the OHCs are contractile
proteins such as actin, myosin, prestin,
and tubulin. There are also special structures along the outer walls of the OHCs
called cisterns. These cisterns and chemicals allow for rapid expansion and contractile movements of the cells (discussed
later). The OHCs usually have a −60 mV
electrical charge and potassium ion channels. These cells have a cuticular plate on
top that supports the stereocilia, which
connect to the underside of the tectorial
membrane. Each OHC has three rows of
stereocilia with a greater number of stereocilia per cell at the base (approximately
150) than at the apex (approximately 50)
of the cochlea. These stereocilia are stiff
and form a W shape when looking down
on the top of the hair cell.
The IHCs have a different structure
than the OHCs (see Figure 2–15). They
have more mitochondria and both calcium and potassium channels, and they
do not have contractile proteins and cis-
terns. The IHCs have a −40 mV charge and
about 50 to 70 stereocilia per cell, which
are arranged in a U shape. The IHCs typically have three rows of stereocilia that are
graded in length, and the stereocilia are
longer at the apex of the organ of Corti
than at its base.
Both the OHCs and IHCs have stereocilia that have pores that open when
stimulated. This allows K+ ions to pass
2. Structure and Function of the Auditory and Vestibular Systems 29
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Figure 2–14. A photo showing the tunnel of Corti. Key: IHC = inner
hair cell, OHC = outer hair cell. (Courtesy of R. Mount, Hospital for
Sick Children, Toronto.) From The Auditory System: Anatomy, Physi-
ology, and Clinical Correlates
and J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright ©
2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 101), by F. E. Musiek
into the cell resulting in depolarization.
Tip-links are small filament bands that
open the pores on movement in a certain
direction (see Figure 2–15). The tip-links
are likely to be located toward the top of
the stereocilia. Cross-links are similar in
structure to tip-links, but they connect to
the stereocilia closer to their midpoints
(see Figure 2–15). This not only allows the
stereocilia to move in unison, but it also
supplies some support for this movement
(Geisler, 1998). The hair cells are tuned so
that cells located more basally respond to
high-frequency sounds, whereas the more
apically situated cells respond to lower
frequency sounds. This arrangement of
hair cells leads to the tonotopic representation of frequencies within the cochlea,
with the higher frequency sounds being
represented at the basal end of the cochlea
and the lower frequency sounds at the
apical end of the cochlea.
Supporting Cells. Supporting cells are
located along the organ of Corti and include pillar cells, Deiter’s cells, phalangeal cells, Hensen’s cells, Claudian cells,
border cells, and Bottcher’s cells (Musiek
& Baran, 2020; Slepecky, 1996) (see Figures
2–12 and 2–14). Next to the limbus are the
border cells that divide the IHCs from
the inner sulcus. The phalangeal cells
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