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Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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23
Anatomy, Physiology, and
Relevant Pathologies
Jackie L. Clark and Katharine Fitzharris
Auditory System
Chapter 2
broad and deep knowledge base regarding the anatomy and physiology of the auditory system is
A
a necessary asset for the delivery of comprehensive hearing services both in clinical and research
domains in audiology. This chapter provides descriptions, tables, and figures for the following classical
anatomical terms of orientations as well as anatomy and physiology according to the classic division
of the auditory system: outer ear, middle ear, inner ear, central auditory, and vestibular systems. It also
includes common pathologies associated with each system and an overview of genetics and embryology.
Anatomical Terms of Orientation
It is important to possess a deep understanding regarding the orientation (both visually and in text)
of each anatomical structure according to proximity and possible interactions between auditory and
nonauditory structures. Each anatomical orientation is identified in-situ according to axis (or plane) of
position when referring to each anatomical structure as if the subject were in a standing position with
palms, feet, and head facing in a forward position.
As seen in Figure 2–1, each orientation is referenced accordingly:
n
Sagittal/Longitudinal Plane: body being divided (medially) into left and right (lateral)
sections with proximal being positioned near a reference point and distal being away from a
reference point
n
Transverse/Axial Plane: body being divided into upper (superior) and lower (inferior) sections
n
Coronal/Frontal Plane: body being divided into front (anterior or ventral) and back (posterior
or dorsal) sections
Regardless of body position, a structure within the body can also be described relative to another
body structure. For example, the in-situ human cochlea would be distal from the cortex and anterior
to the vestibule. Conversely, the left tympanic membrane would be lateral to the human cochlea and
proximal to the malleus.
23
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
e
T
n
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Sagittal plane
Coronal plane
Coro
Tranverse plan
FIGURE 2–1. Classic anatomic orientation for humans.
Typical Anatomy of the Auditory Mechanism
Despite the minute size of the auditory system in total, due to its complexity, scientists will continue to
discover new aspects about its anatomy and physiology. The auditory system will be described in four
classic sections with basic anatomic and physiologic functions.
n
The classic anatomical divisions and their appropriate labels for the major components of the
auditory system are seen in Figure 2–2 and described in Table 2–1. Figure 2–2 provides a view
of the anatomical structure of the hearing mechanism’s peripheral system, while Table 2–1
lists the function of those (and other) major structures of the peripheral and central auditory
system (hearing mechanism).
An important task of the peripheral hearing mechanism (comprising the outer ear, middle ear,
cochlea/inner ear, and auditory nerve) is to convert minute, but rapid, pressure variations in the
air (initiated by vibrating objects in and about the environment) into neural impulses. These will
ultimately facilitate a transfer of the information to the central system while preserving the acoustic
characteristics of sound for identification and discrimination. Peripheral hearing mechanisms and
their function within the auditory sensory system are displayed in Table 2–2. Each anatomical section
manages important transduction of energy to varying degrees from mechanical to hydrodynamic to
electroacoustic action of activity. Depending upon the frequency, intensity, distance, and mode of
transmission of sound, there are instances in which pathology or anatomical abnormalities may (or may
not) yield a small to dramatic loss of acoustic energy transmission, resulting in significant disruption
in communication abilities.
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FIGURE 2–2. Main structures of the outer, middle, and inner ear with a cochlear cross section. Key: 1:
helix; 2: antihelix; 3: scaphoid fossa; 4: concha; 5: lobe; 6: triangular fossa; 7: external auditory canal; 8:
tympanic membrane; 9: Eustachian tube; 10: middle ear space; 11: incus; 12: malleus; 13: stapes; 14:
superior semicircular canal; 15: bony cochlea; 16: round window; 17: CN VIII. Source: From The Auditory
System: Anatomy, Physiology, and Clinical Correlates, Second Edition (pp. 1–487) by Musiek, F. E., & Baran,
J. A. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
TABLE 2–1. Auditory System
AUDITORY SENSORY SYSTEM: HEARING MECHANISM
PERIPHERAL SYSTEM CENTRAL SYSTEM
• Outer Ear
• Middle Ear
• Cochlea
• Auditory Nerve
• Cochlear Nucleus
• Superior Olivary Complex
Lateral Lemniscus
•
• Inferior Colliculus
• Medial Geniculate Body
• Auditory Cortex
• Association Areas
Outer Ear
Of the two major features within the outer ear (i.e., pinna/auricle and external auditory canal/meatus),
the typical human pinna is the largest and most visible auditory structure, which facilitates entry of
air-conducted sound into the hearing mechanism (see Figure 2–2).
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TABLE 2–2. Peripheral Anatomy and Physiology of the Hearing Mechanism
AREA OUTER EAR MIDDLE EAR COCHLEA NEURAL
FUNCTION
ENERGY
TYPE
Sound Localization
& Directing Sound
Acoustic Mechanical Hydraulic &
• Pinna
• Tragus
• Helix
• Antihelix
ANATOMY
• Triangular Fossa
• Scaphoid Fossa
• Concha
• Lobe
External Auditory
•
Canal
n
The outer ear is important for sound localization and directing sound into the system. Such
Impedance
Matching
• Tympanic
Membrane
•
Pars Tensa
• Pars Flaccida
• Incus
• Malleus
Stapes
•
• Eustachian
Tube
Convert Mechanical to
Electrical Signal
Electrochemical
• Cochlea/Otic Capsule
• Round Window
• Oval Window
• Spiral Ganglion
• Reissner’s Membrane
• Basilar Membrane
Scala Tympani
•
• Scala Vestibuli
• Scala Media
Tectorial Membrane
•
Translate Electrical
Signals
Electrochemical
•
Auditory/
Vestibulocochlear
(CN VIII) Nerve
Facial (CN VII)
•
Nerve
sound can occur in isolation or multiplicity within varying levels and angles in a listener’s
environment. The pinna is responsible for an approximate 10 dB “boost” (gain) in an auditory
signal around 5000 Hz (Shaw, 1974).
n
Any pathology, absence, or malformation of the outer ear may result in some loss of hearing
acuity in the mid-to-high frequencies (2000–5000 Hz). It is believed that the convolutions
seen in the outer ear
— the helix, antihelix, triangular fossa, and scaphoid fossa — work as
a funnel directing acoustic signals into the external auditory meatus/canal (EAM/EAC).
Physically, the EAC is oriented lateral to the tympanic membrane (TM).
The pinna is attached to the head (specifically, the temporal bone) at a 15º to 30º angle at the
following points:
n
Zygomatic arch anteriorly
n
Mastoid process posteriorly
n
External auditory canal medially
It comprises yellow fibrous cartilage, vestigial musculature, and ligaments and is covered by perichondrium and squamous epithelium. Note that it is common for individuals to develop squamous cell
carcinoma on the pinna; this is often treated with surgical removal of the affected area, which distorts
the shape of the pinna.
On the left pinna, starting superiorly and working clockwise (anterior–inferior–posterior), major
landmarks of the pinna include:
n
Helix; triangular fossa, crura of the antihelix, concha cymba; crus of the helix; supertragic
notch, concha cavum, tragus, intertragal notch, antitragus, lobule, antihelix, Darwin’s tubercle
(not present on every pinna), scaphoid fossa
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The superior pinna is innervated by the auriculotemporal nerve (a portion of the CN V, the
trigeminal nerve); the concha is supplied by CN VII (facial), CN IX (glossopharyngeal), and CN X
(vagus). The lobe and inferior/posterior portion of the pinna is served by the greater auricular nerve
(C2 and C3).
n
There is some debate about the exact innervation patterns of the pinna, but the previously
mentioned nerves are commonly accepted. With regards to vasculature, the pinna receives its
blood supply from the external carotid artery, specifically from two branches: the superficial
temporal artery (STA) and the posterior auricular artery (PAA).
n
The epithelial layer of the pinna is continuous within the EAC. This cartilaginous and bony
tube (typically closed at one end with a resonance of ¼ wavelength frequency) is approximately 1 inch long in adults and about 6 to 8 mm in diameter; in children, it is straighter and
shorter. In adults, it provides an approximate 12 dB “boost” (gain) around 3000Hz.
n
The EAC is tortuous (S-shaped) and comes to its narrowest point at the osseocartilaginous
juncture, or isthmus. The outer one third to one half of the EAC is cartilaginous (i.e., mobile),
while the inner one half to two thirds is bony. The cartilaginous portion, in most humans,
runs anterior and superior from the tragus to the first bend, then runs posterior and superior
from the first bend to the isthmus; note that these turns are helpful in protecting the TM from
damage and also play a role in altering acoustic properties of a signal. The osseous portion of
the EAC runs inferior and anterior, ending at the TM. The entire structure is housed within
the tympanic portion of the temporal bone.
Q & A
Question: Why is it important for audiologists to be familiar with EAC anatomy?
Answer: Typically, one of the first clinical procedures conducted during an
audiological appointment is otoscopy, which is done to visualize the anatomical orientation of the EAC prior to inserting probes, earphones, or otoblocks.
During otoscopy, one will pull upward and back (for the adult patient) on the
superior posterior edge of the pinna in order to straighten the cartilaginous
portion of the EAC to better view the TM. Knowing the landscape of the EAC
helps with choosing the appropriately sized probe and insert earphone. It is
equally important to know where each bend of the EAC occurs to achieve a deep
placement of an otoblock for production-ready earmold investments.
The osseous and cartilaginous portions of the EAC vary in terms of their anatomical elements. At
the entry of the EAC, where cerumen is produced, is the cartilaginous portion containing squamous
epithelia (~0.5–1 mm thick), hair follicles (pointing outward to provide obstruction and protection),
vasculature, subcutaneous fat, as well as sebaceous and ceruminous glands. Cerumen is made of a mix
of minerals, lipids, and protein-free amino acids and has protective functions:
n
It maintains a pH balance of ~6.1, which creates a toxic environment for most insects.
n
It is slightly antifungal and antibacterial.
n
It keeps the ear canal lubricated.
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Further beyond the cartilaginous portion resides the bony portion of the canal, which has a thinner
layer of squamous epithelium (~0.02–0.1 mm) and vasculature.
AUDIOLOGY NUGGET
Cerumen is secreted at the base of the hair cells (HCs) in the cartilaginous EAC.
The different types of cerumen are determined by age, genetics, and race. It is
determined by a single gene variant, which creates cerumen along the spectrum
of dry, crumbly/flaky, gray to tan in color to wet, sticky, yellow-brown to dark
brown in color. If there is a blockage of typical epithelial migration exiting the
EAC or an overproduction of cerumen in the canal, an occlusion of the EAC
can create a conductive hearing loss (CHL). Note that the longer the cerumen
remains in the EAC, the drier and more difficult extraction becomes. Audiologists, depending upon state licensure laws, can practice cerumen management.
Several techniques to remove impacted cerumen blockage include mechanical
(using instruments such as curettes and forceps), lavage (forcing warmed water
— sometimes mixed with alcohol, mineral oil, and hydrogen peroxide to flush
the cerumen out of the EAC), and microsuctioning. Different ages and types
of cerumen require specific removal techniques. It is important to recall the
contraindications to cerumen management procedures: If the patient is on
blood thinners or has diabetes, undesirable medical side effects may occur. If the
cerumen is too deep or if the audiologist is uncomfortable proceeding, a referral
to an ENT or other specialty physician is necessary.
Disorders of the Outer Ear
While not exhaustive, Appendix 2–A lists a variety of pathologies, conditions, malformations, or
absence of any mechanism within the outer ear. However, outer ear disorders are often treatable (if
specialized services are available) and hearing loss may be resolved if treated quickly without chronicity
of the etiology in the future.
Middle Ear
By the very resistive anatomical nature of the middle ear mechanisms, not all airborne sound molecules
traveling from the peripheral outer ear mechanism will be transformed into mechanical energy within
the middle ear space. Fortunately, the impedance matching mechanisms found in the middle ear are
critical to make up for the power loss of sound traveling through absorbent human tissue blanketing
the cartilaginous pinna and EAC. It is important to recall that impedance is defined as the resistance
to the flow of energy.
n
One border of the middle ear is the TM, which is the commencement of a series of actions
through the connectiveness of the ossicular chain (malleus, incus, and stapes) that results in
amplifying acoustic signals via mechanical energy (ossicular chain shown in Figure 2–2).
n
The TM is surrounded by an annular ligament that attaches to the temporal bone (tympanic
sulcus) and is thought to comprise three to five layers, depending upon the school of thought,
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from lateral to medial: the epithelial layer, fibrous layer (radial, circular, radial), and mucosal
layer. The pars flaccida (or Shrapnell’s membrane) is the superior portion while the pars tensa
makes up the majority of the TM.
n
Otoscopic inspections allow visualization of the cone of light typically seen with a normal
TM, which appears gray to pearly white, and connects to the manubrium of the malleus that
is bisected from the superior to midportion, ending in the umbo. Typically, the cone of light
will point to the side that matches the ear being observed (i.e., if the cone of light and umbo
are pointing right, it is the right ear; if they are pointing left, it is the left ear).
Q & A
Question: Describe the changes in modes of vibration of the stapes according
to sound intensities.
29
Answer:
As the ossicular joints (malleoincudal and incudostapedial) pivot, the
footplate of the stapes will rotate side-to-side with moderate-intensity signals
and rock front to back with high-intensity signals. The TM also changes its
vibratory patterns depending on frequency and intensity of the acoustic signal.
The stapes does not act like a piston, but has rather more complex movement
patterns concomitant with the intensity of the incoming signal.
There are three primary ratios (impedance matching mechanisms) responsible for dealing with
the impedance mismatch:
1. Area ratio: also known colloquially as the high-heel principle (think walking in flat shoes on a
soft surface versus stiletto heels — the latter sinks in a soft surface), the area of the TM is about
17 times the area of the stapes footplate. When the same force is applied to a smaller area, there
is a dramatic increase in pressure; this gives the area ratio a value of 17:1.
2. Ossicular lever ratio: there is a lever formed by the manubrium of the malleus and the long
process of the incus. These ossicles are tightly bonded at their articulation point, causing them
to move as a unit, thereby resulting in a gain of approximately 1.3:1.
3. Curved membrane ratio: also known as the buckling effect, there are two layers of TM tissue
vibrating around the fulcrum (malleus). As a result of the curvature, there is greater movement
of the curved membranes and less movement of the manubrium, which can increase the force
on the manubrium to move with a gain from this ratio of approximately 2:1.
When combined, these three ratios create a force that is 46 times greater at the stapes footplate than
at the TM and translates into approximately 33 dB SPL of gain. Without the middle ear mechanism,
there would be a loss of 33 dB SPL of the acoustic signal reaching the oval window.
The middle ear anatomy is described below and shown in Figure 2–2, which is much like a six-
sided box with the ossicles suspended in the center with the following landmarks.
1. Lateral wall: TM
2. Medial wall: oval window, round window, promontory (covered by branches of CN IX), semicanal of the tensor tympani, bony eminence of the facial nerve, lateral semicircular canal
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AUDIOLOGY NUGGET
When there are concerns regarding middle ear function negatively contributing
to hearing, it is possible to monitor middle ear status over a long period of time
through immittance testing. Not only is there value in documenting any changes
in the outer ear canal volume but also the amount of flexibility (or specific
periods of limited mobility) at the TM from infancy and through the lifespan
(from infancy through geriatric ages). For example, it is expected that infants to
young children will experience more otitis media than older children and adults.
When coupled with the pure-tone and speech audiometry, any information
yielded about the middle ear system through immittance measures can be used
to confirm the type (i.e., conductive [CHL], mixed [MHL], or sensorineural
[SNHL]) of hearing loss.
3. Inferior wall (floor): jugular vein (inferior to the inferior wall)
4. Superior wall (roof): tegmen tympani (tegmental wall/roof), the thin petrous part of the
temporal bone, separating intracranial compartment and middle ear
5. Anterior wall: Eustachian/auditory tube and carotid canal, as well as the chorda tympani exit
6. Posterior wall: aditus ad antrum, pyramid of the stapedius canal, bony eminence of the chorda
tympani (entrance), fossa incudus
Vascularization of the middle ear is via the internal carotid artery (which occasionally may be a cause
of pulsatile tinnitus), external carotid, and sometimes subclavian arteries. For specific branches of
these arteries, see Musiek and Baran (2020). Venous drainage occurs via the jugular bulb and sigmoid
sinus. The middle ear is innervated by CN IX. Although the chorda tympani branch of CN VII travels
through the middle ear space, it carries taste and pain fibers from the tongue and face and does not
provide sensory innervation.
KNOWLEDGE CHECKPOINT
The Eustachian tube passively opens during natural middle ear pressure changes
and actively opens from tensor palatini muscle contraction that elevate the soft
palate, at which point the epiglottis will fold down. Pressure within the Eustachian
tube will occur when middle ear pressure exceeds the nasopharyngeal pressure, thus
forcing the Eustachian tube to open. When air and gasses exit the ear through
the Eustachian tube, pressure is then reduced. Though significantly small, the
Eustachian tube can become problematic and influential in proliferation of pathologies
(such as otitis media, necrotizing otitis media, cholesteatoma) when chronically
closed due to thick phlegm plugging the tube secondary to allergies, upper respiratory infections, dramatic/excessive weight loss, or chronic acid reflux disorder.
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Disorders of the Middle Ear
Disorders of the middle ear mechanism are shown in Appendix 2–B. Much like disorders of the outer
ear, most disorders of the middle ear if attended to early may be medically managed and result in
transient CHL. However, when these disorders are ignored or co-occur with disorders of the inner ear,
mixed losses can occur. Most often the TM is implicated in disorders of the middle ear. Consequently,
completing inspection and otoscopy of the outer ear provides insight into possible etiologies that create
hearing loss. Immittance testing (discussed in more detail in Chapters 5 and 6) can also provide useful
information regarding the status of the middle ear system.
Auditory Inner Ear Structure and Function
The primary function of the cochlea (seen in Figure 2–2) is to convert the acoustic signal into electrical
energy that can be utilized by the auditory/cochlear portion of the vestibulocochlear nerve (CN VIII).
Mechanisms found at the junction of the stapes and cochlea (the oval window) are the last waystations
critical for translating the mechanical energy to hydrodynamic energy to electrical energy. Between
the vibrations of the TM to the movement of the stapes in the oval window, frequency and intensity
information is preserved for processing within the cochlea via hydrodynamic and electrical events.
Temporal Bone Anatomy
Auditory and vestibular structures are housed within the temporal bone, which is located on the lateral
position of the skull. There are four major portions of the temporal bone:
1. Tympanic: contains the osseous EAM and tympanic sulcus
2. Mastoid: point of attachment for several muscles; contains mastoid air cells and tympanic
ad antrum
3. Squamous: thin and fairly smooth portion that contains the zygomatic process
4. Petrous: houses the middle and inner ear; canals for nerves and vasculature
Cochlear Anatomy
Located in the petrous portion of the temporal bone is the bony inner ear, which contains the cochlea,
vestibule, and semicircular canals. The human cochlea, shown as a cross section in the right side of
Figure 2–2, comprises ~2¾ spiraling turns in its membranous form. At its core of the turns is the
modiolus, where the spiral ganglia nerve fibers merge to form CN VIII as the fibers exit the inner
ear. In addition to the nerve fibers, the modiolus also contains blood vessels for vascular support of
each cochlea.
n
Each turn can be further divided into three sections or scalae: the scala vestibuli (top;
articulates with the oval window), scala media (middle; also known as the cochlear duct),
and scala tympani (bottom; articulates with the round window). These three scalae contain
cochlear fluids: perilymph (high in sodium, low in potassium; similar to cerebral spinal
fluid) in the vestibuli and tympani and endolymph (high in potassium and low in sodium)
in the media. There are two fluid systems within the cochlea: cochlear aqueduct (containing
perilymph) and the vestibular aqueduct (containing endolymph). The scala tympani and
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