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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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2
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Structure and Function
of the Auditory and
Vestibular Systems
intRoduction
Hearing is a complex process that is
critical to our everyday functioning. As
humans, we can hear an extremely wide
range of frequencies and intensities. We
can also discriminate small changes in
intensity (on the order of 1 dB) and frequency (on the order of a few hertz). Even
more amazing is the fact that the normal
human auditory system can detect sounds
embedded in noise and process what
someone is saying from across the room
at large social gatherings (i.e., the cocktail
party effect). These hearing skills, which
we all take for granted, are a result of
the ear (peripheral system) and the brain
(central system) working together harmoniously with great precision.
The vestibular system, which shares
some anatomic structures with the auditory system, is responsible for our ability
to perceive changes in head movements
(acceleration and deceleration) and the
orientation of the head with respect to
gravity. Similar to the auditory system,
the vestibular system has both peripheral
as well as central components that work
together to help us maintain our sense
of balance.
Understanding the structure and
function of the peripheral and central components of these two systems is the goal of
both basic and clinical research. It also is
critical for advances in the diagnosis and
treatment of disorders of the auditory system and any comorbid or related vestibular problems. An appreciation and understanding of the anatomy and physiology
of these systems allows one to identify the
location of the disordered region within
the auditory and vestibular systems. This
in turn provides insight as to the nature
of the disorder that can lead to effective
treatment. Therefore, it is safe to say that
understanding disorders of the auditory
and vestibular systems is an essential skill
for the audiologist and hearing care professional that is dependent on a substantial knowledge of the structure and function of these two interrelated systems.
11
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Although the authors can only provide an overview of the anatomy and
physiology of the auditory and vestibular
systems within this chapter, it is hoped that
the reader will be oriented well enough
to fully grasp the essence of the function
and dysfunction of these two systems. The
anatomy and physiology of the auditory
system is discussed first, followed by a
brief overview of the vestibular system.
anatomic teRminology
Prior to a detailed discussion of anatomy
and physiology, it is important for the
reader to have a fundamental under-
Table 2–1 . Commonly Used Anatomic Terminology
Term Anatomic Description
standing of the basic terminology used
to describe the location and orientation
of anatomic structures. Knowledge of this
terminology provides the clinician with
the ability to discuss anatomic position,
eliminate ambiguity, and reduce medical errors. For example, when discussing a tympanic membrane perforation,
if appropriate anatomic terminology is
used, a clear distinction can be made indicating whether the location of the perforation is anterior or posterior and inferior
or superior. These terms, as well as other
common anatomic terms, will be used
throughout this book. Table 2–1 provides
the reader with a list of the more commonly used anatomic terminology along
with a brief definition of each term.
Direction/Location
Anterior or Ventral Toward the front
Posterior or Dorsal Toward the back
Superior or Rostral Toward the head
Inferior or Caudal Toward the feet
Lateral Toward the side
Medial Toward the midline
Proximal Closer to the trunk or origin
Distal Farther from the trunk or origin
Superficial Toward the surface
Deep Away from the surface
Plane
Sagittal Divides the body vertically left and right
Coronal (Frontal) Divides the body into a front and back portion
Transverse Divides the body into an upper and lower portion
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the PeRiPheRal
auditoRy system
The Temporal Bone
The temporal bone either houses or supports most of the structures of the auditory periphery. It is an integral part of the
skull base and is composed of four fairly
distinct segments: the squamous, the tympanic (bony ear canal), the mastoid, and
the petrous segments. The squamous portion is part of the lateral cranium immediately superior to the ear canal. The bony
ear canal is another segment of the temporal bone and constitutes the tympanic portion. Directly posterior to the tympanic
portion is the mastoid segment, which is
characterized by numerous air cells. The
final segment is the petrous portion that
houses the middle ear, the cochlea, and
the vestibular apparatus. It is a wedgeshaped structure that courses medially in
the base of the skull and divides the posterior cranial fossa from the middle cranial
fossa (Figure 2–1).
On the posterior side of the petrous
portion of the temporal bone there are a
number of key structures. The opening to
Figure 2–1. A photograph of the base of the cranium with anterior,
posterior, and lateral aspects depicted. Key: PF = posterior fossa, MF =
middle fossa, FM = foramen magnum, p = petrous portion of the temporal bone which houses the middle and inner ears as well as the internal
auditory meatus. From The Auditory System: Anatomy, Physiology,
and Clinical Correlates
Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 88), by F. E. Musiek and J. A.
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the internal auditory meatus (IAM), also
termed the porous acusticus, is located
about two-thirds of the way along this
structure and courses in a lateral to medial
direction. Through this opening in the
temporal bone exits the auditory, vestibular, and facial nerves that project to their
respective nuclei in the brainstem (lateral,
caudal pons). Also located on the posterior aspect of the petrous bone just lateral
to the IAM opening is the opening for the
vestibular and cochlear aqueducts. These
openings are identified by small recesses
in the posterior side of the petrous bone
(Anson & Donaldson, 1981).
One of the main functions of the temporal bone is to provide a framework of
support for the outer, middle, and inner
ears, as well as the seventh and eighth
cranial nerves. In addition to support and
stabilization of these structures, the temporal bone provides protection for most of
the anatomic structures within the auditory periphery that support hearing.
The Outer Ear
Structure
The outer or external ear includes the
pinna (also referred to as the auricle)
and the external auditory meatus (EAM)
or ear canal. The pinna is C shaped and
composed of a foundation of cartilage that
is covered with skin. The structure of the
pinna takes on a shape that conforms to
the underlying cartilage, which results in
its distinct appearance as a structure with
numerous folds and recesses. These folds
and recesses constitute specific anatomic
sites or areas within the pinna for which
particular terms are used (Figure 2–2).
Referring back to the C-shaped structure
Helix
Tr iangular
Scaphoid Fossa
Anti-Helix
Concha (Cymba)
Concha (Cave)
Helix
Anti-Tragus
Ear Lobe
Fossa
Crus or “Limb”
of Helix
Tragus
Intertragal
Incisure
Figure 2–2. A photo of a human right pinna. From The Auditory System:
Anatomy, Physiology, and Clinical Correlates
Musiek and J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright
© 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 46), by F. E.
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of the pinna, the outer circular part of the
C is termed the helix and just inside of the
helix is the antihelix. Located superiorly
between the helix and antihelix is a groove
called the scaphoid fossa, which lies next
to the triangular fossa. Immediately inferior to the antihelix is a ridge termed the
crus of the helix. This ridge is responsible
for much of the rigidity of the pinna. The
deepest recess of the pinna is the concha,
which leads to the opening of the ear
canal. Anterior to the concha is a protective flap called the tragus, which can be
pressed into the opening of the ear canal
to serve as protection. Inferior and posterior to the tragus is a prominence termed
the antitragus. At the most inferior aspect
of the pinna is the ear lobe, probably the
softest and most flexible of the pinna’s
structures. Between the ear lobe and the
tragus is the intertragal incisure (Clark &
Ohlemiller, 2008; Musiek & Baran, 2020).
The various structures of the pinna are
innervated by the fifth (trigeminal) and
seventh (facial) cranial nerves (Musiek &
Baran, 2020; Zemlin, 1998).
The anatomy of the pinna becomes
useful when taking earmold impressions
and when fabricating earmolds. Variances
in the pinna’s fine structure can be well
defined using the anatomy discussed previously. This in turn can allow better and
more precise communication between the
audiologist and the earmold manufacturer
concerning the need for attention to a particular area or areas of the earmold to be
created or modified, which should result in
a better fit, and ultimately, a more satisfactory hearing aid experience for the patient.
The second structure in the external
ear is the EAM. This outer ear structure
is shaped like a tube and averages 2.5 cm
to 3.0 cm in length and 0.75 cm in diameter in the adult human. Obviously, these
dimensions would be much smaller in
pediatric patients. The EAM originates at
the concha of the pinna and ends at the
tympanic membrane (Figure 2–3). The
EAM is not straight, but rather curves
like an elongated letter S that is lying on
its side. However, it is important to note
that there is great variation in the shape of
the EAM in humans. Visual inspection
of this structure will reveal some EAMs
that are relatively straight, whereas in
other instances, the EAM will be observed
to be curvier or more tortuous. Pulling on
the posterior aspect of the pinna can help
straighten the ear canal for visual inspection with otoscopy and allow for a better
seal during tympanometry and acoustic
reflex testing. This procedure can prove to
be especially helpful when one is visually
inspecting an ear canal that is quite curvy
in nature.
The foundation of the outer third of
the ear canal is cartilaginous, whereas the
foundation of the medial two-thirds is
bony. The entire length of the ear canal
is covered by an epidermal lining and
the outer third contains hair follicles and
glands that secrete a waxy substance
resulting in the production of cerumen.
Innervation of the EAM comes from the
fifth (trigeminal), seventh (facial), and
ninth (glossopharyngeal) cranial nerves,
making the EAM sensitive to tactile stimulation (Møller, 2000; Musiek & Baran,
2020; Zemlin, 1998).
Function
The pinna’s main function is to help funnel sounds from the environment into
the smaller diameter EAM. Because of
its unique structure and size, the pinna
tends to result in a slight enhancement of
sounds in the vicinity of 5000 Hz (Shaw &
Teranishi, 1968). This enhancement occurs
because the unique configuration of the
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CO
ME
EAM
Figure 2–3. A drawing of the outer, middle, and inner ears showing the location
and configuration of the external auditory meatus. Key: EAM = external auditory
meatus, ME = middle ear, CO = cochlea. Adapted with permission from The Audi-
tory System: Anatomy, Physiology, and Clinical Correlates
by F. E. Musiek and J. A. Baran, 2020, Copyright © 2020 Plural Publishing, Inc.
All rights reserved.
pinna in terms of its ridges and recesses
results in a more efficient collection of
sound in the higher frequency range,
whereas the wavelengths of lower frequency sounds are essentially larger than
the pinna and can pass around this structure more readily than those of higher frequency sounds (Musiek & Baran, 2020).
The pinna also helps with sound localization by creating complex resonances that
change as the location of the sound source
changes (Blauert, 1983).
The EAM helps to protect the ear with
its debris-catching cilia and cerumen, and
it also serves as an acoustic resonator. The
EAM peak resonance is around 3000 to
4000 Hz in the adult. At these frequencies,
(2nd edition, p. 1),
there is a gain of 10 to 15 dB in the acoustic
signal with little or no gain at the frequencies below 1000 Hz (Dallos, 1973).
It also should be mentioned that in
addition to the pinna and the EAM, the
head and even the torso can exert differential effects on the sounds reaching the
ear. The combination of these effects provides valuable auditory information that
the normal auditory system uses to help
identify the source of a sound (i.e., directional hearing). Most notable of the directional hearing effects is the head shadow
effect, which can have a significant effect
on the sound reaching the ear when the
sound is originating from a source located
on one side of the head versus the other.
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Take, for example, the situation where one
has a sound being presented in a sound
field to the right side of the head. In this
case, the sound would have almost direct
access to the right ear, but the head would
interfere with the sound that is traveling
to the left ear. The end result would be
that the sound reaching the left ear would
be of a lower intensity (especially at high
frequencies) than the sound arriving at the
right ear. The central auditory system is
capable of detecting and analyzing these
types of intensity differences, as well as
the “small” time of arrival differences that
occur as a result of the further distance
that the signal has to travel to reach one
ear versus the other in order to accurately
identify the source of a sound (for additional discussion of directional hearing
effects, see Musiek & Baran, 2020).
The Middle Ear
Structure
The tympanic membrane (TM) marks the
beginning of the middle ear and is oval
shaped and concave in its appearance
(Figure 2–4). In the adult human, it averages 8 to 10 mm in diameter, with a slightly
larger diameter along the vertical axis
than along the horizontal axis. The membrane has three layers and measures about
0.1 mm in thickness (Gelfand, 1997; Zemlin, 1998). These three layers include the
epidermal (outer), the fibrous (middle),
and the membranous (mucosal lining)
layers. The fibrous layer is thicker in the
center portion of the TM and thinner in the
superior portion of this membrane. This
“thinner” area is called the pars flaccida,
HeadoftheMalleus
Incus
ParsFlaccida
AnteriorMalleus
Prominence
Posterior&
AnteriorMalleusFolds
ChordaTympani
LateralProcessof
theIncus
Umbo
ParsTensa
ConeofLight
LEFT
Annulus
RIGHT
Figure 2–4. An illustration of the tympanic membrane superimposed on the middle ear ossicles
and related ear structures observed from this lateral view. Adapted by Frank E. Musiek and Jane A.
Baran in The Auditory System: Anatomy, Physiology, and Clinical Correlates, 2020. Original
from The Human Ear in Anatomical Transparencies by Stephen Polyak, Gladys McHugh, and
Delbert Judd, 1946.
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which is the most elastic portion of the
TM. For the most part, the remainder of
the TM is stiffer (containing more fibers)
and is referred to as the pars tensa (see
Yost, 2000).
The TM has several important anatomic landmarks in addition to the pars
flaccida and the pars tensa. The annular
ligament is the rim around the TM, which
anchors the membrane to the wall of the
ear canal. The manubrium of the malleus
attaches to the TM in its upper center portion, and the umbo is located in the center
of the TM and marks the point of attachment for the most lateral aspect of the
malleus (see Figure 2–4). Coursing inferiorly and in a slightly lateral direction is
the cone of light (usually at ~5 o’clock for
the right TM and ~7 o’clock for the left
TM), which simply is the light of the otoscope that is reflected off the TM during
otoscopy. With good observation skills
and a translucent tympanic membrane,
it is possible to visualize the long process
of the incus through the TM. If observed,
the location of the long process would be
viewed in the superior-posterior quadrant
of the TM.
The middle ear is identified, in part,
by an air-filled cavity within the temporal
bone that is bordered by the bony capsule
of the cochlea medially and aspects of the
temporal bone superiorly (attic), anteriorly, and posteriorly. The medial wall of
the cochlea has two openings: the oval
window (superior) and the round window (inferior), which are separated by the
promontory. The ossicular chain, perhaps
the most obvious structure in the middle
ear, is composed of three bones: the malleus, the incus, and the stapes. Each of
these bones has a detailed anatomy (see
Musiek & Baran, 2020, for review). The
stapes, the final bone in the ossicular
chain, covers the oval window, and the
round window is covered by a flexible
membrane (also known as a secondary
tympanic membrane) (Musiek & Baran,
2020; Zemlin, 1998).
In the anterior, inferior middle ear
cavity is the opening to the Eustachian
tube. This tube connects the middle ear
cavity with the posterior aspect of the
nasopharynx. Also located in the middle
ear is the stapedius muscle tendon, which
arises from the posterior wall of the middle ear and connects to the head of the
stapes, as well as the tendon of the tensor
tympani muscle, which courses through
the middle ear cavity to connect to the
malleus. In addition, a branch of the facial
that innervates the stapedius muscle (the
smallest muscle in the body) transverses
the middle ear space.
Function
Due to its concave structure, the TM has a
rather complex displacement pattern that
accommodates a wide range of frequencies and intensities (see Zemlin, 1998, for
more in-depth explanation). Maximum
displacement of the TM occurs in different
areas and is dependent on the frequency of
the sound stimulus. At high frequencies, the
TM tends to vibrate in segments, whereas
at low frequencies, there is less segmental
vibration and the TM tends to vibrate more
as a single unit (Gelfand, 1998).
A variety of functions are associated with the middle ear. The middle
ear is an air-filled cavity. The volume of
air within this cavity acts as a filter that
limits or alters the transmission of some
low-frequency sounds through the system. The structure also contributes to
some complex interactions between the
ear canal and pinna resonances, which
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alter the transmission of sound across the
frequency range. The end result is primarily an increase in the intensity of the highfrequency components reaching the cochlea
when these are compared to the levels
represented in the original signal arriving
at the outer ear.
The ossicular chain transmits vibrations from the eardrum to the cochlea.
The stapes, the final bone in the ossicular
chain, transmits these vibrations to the
cochlea by horizontally rotating around
an axis (a rockinglike movement) at high
intensities, whereas a more pistonlike
movement of the stapes occurs for low
intensities. The former type of movement
may be related to stapedius muscle contraction, which occurs at high intensities
(see Musiek & Baran, 2020, and Gelfand,
1998, for details). The structure of the
ossicular chain and its ligaments serve to
dampen high intensity sounds and stabilize the middle ear transmission process
(Musiek & Baran, 2020).
The Eustachian tube’s primary function is to allow fresh air into the middle
ear cavity and balance the air pressure in
the middle ear to that of the atmosphere.
This is done when one opens the mouth
or swallows. These actions result in contractions of the tensor veli palatini and
levator veli palatini muscles, which function to open the Eustachian tube (Zemlin,
1998). The Eustachian tube also protects
the middle ear from fluid or solids arising from the nose and/or mouth as this
structure remains closed at rest.
Transformer Action. Sound travels read-
ily through air, which is a low impedance
medium. However, in hearing, sound (i.e.,
changes in air pressure) must be directed
to a fluid-filled system, the cochlea, where
the impedance is quite high. Therefore,
without some help from the transformer
action of the middle ear, most of the acoustic energy reaching the cochlea would be
reflected back out of the ear. Three mechanisms contribute to the middle ear transformer effects: (1) an area ratio advantage
between the TM and the stapes’ footplate,
(2) a lever advantage created by the middle
ear bones, and (3)
a “buckling” advantage
due to the concave structure of the TM.
The area ratio advantage relates to the fact
that the TM has a much greater area than
the stapes’ footplate, which is the point of
energy transfer to the inner ear. The area
ratio of the TM to the stapes’ footplate is
about 22:1, but the pars flaccida of the TM
likely contributes little to this area ratio;
hence, the effective ratio is estimated to be
about 17:1. This area ratio focuses energy
at the stapes’ footplate, thus increasing the
input greatly at this point in the auditory
system (Gelfand, 1998; von Békésy, 1960).
The lever advantage is created by the way
the malleus and incus interact. The gain
in force is related to the longer handle
of the malleus (manubrium) moving the
shorter handle of the incus (long process)
for about a 1.3:1 ratio advantage. Finally,
the buckling action relates to the inward
curvature of the TM, which on vibration
imparts energy to the malleus. When this
happens, the TM moves proportionately
more than the malleus (i.e., a “buckling
effect”). The smaller displacement at the
malleus in reference to the TM creates a
greater force or a gain in force (Musiek
& Baran, 2020). These three transformer
mechanisms allow greater energy to be
directed to the fluid-filled cochlea than
would be the case if these mechanisms
did not exist or did not function appropriately. If these mechanisms are compromised individually or collectively, a conductive hearing loss is expected.
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