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Part I
Anatomy, Embryology and Physiology of
Auditory System
Outer–Middle–Inner Ear andCentral
Hearing System Anatomy
OguzhanOguz, CemalCingi, andJoaoFlavioNogueira
1.1 Introduction
In terms of anatomy, there are three distinct parts to the ear: the external, middle,
and inner ear. Sound waves emitted by the surrounding environment are received
and transmitted via the outer ear, composed of the pinna, external auditory canal,
and tympanic membrane (TM) [1, 2]. The ossicles—the malleus, incus, and stapes—are bones that transport vibrations from the tympanic membrane to the inner
ear. The middle ear is an air-lled region that contains these bones. The malleus
sends vibrations via the incus to the stapes, which then contacts the cochlear oval
window. The cochlea, semicircular canals, utricle, and saccule comprise the inner
ear, housed within the bony labyrinth of the temporal bone. Within the bony labyrinth, these organs form the membrane labyrinth, with perilymph as the sole divider.
Endolymph, a uid contained in the membranous labyrinth, is essential for stimulating hair cells that transmit vestibular signals and sound [1].
1
O. Oguz
Department of Audiology, Istanbul Nişantaşı University, Health Services Vocational School,
Istanbul, Turkey
Dr. Oğuzhan Oğuz Wellnose Clinic, Istanbul, Turkey
C. Cingi (*)
Faculty of Medicine, Department of Otorhinolaryngology, Eskisehir Osmangazi University,
Eskisehir, Turkey
J. F. Nogueira
Health Sciences Center, Universidade Estadual do Ceará, Fortaleza, Brazil
© The Author(s), under exclusive license to Springer Nature
Switzerland AG 2024
M. T. Kalcioglu et al. (eds.), Otology Updates, Comprehensive ENT,
https://doi.org/10.1007/978-3-031-76173-7_1
3
4
O. Oguz et al.
1.2 The External Ear
The external ear comprises the auricle and external auditory canal, also called the
external acoustic meatus. The inner ear receives and processes sound after the external ear has collected and amplied it. Sound is better localized due to delays introduced by the exterior auricle’s asymmetrical form [3].
1.2.1 The Auricle
The external auditory meatus (external auditory canal) opening is encircled by the
concha, while the auricle is composed of the following parts:
• The helix root or the crus helix
• A helix
• An antihelix structure
• The scaphoid recess
• The triangular fossa
• The superior and inferior crura of the helix
• The antitragus
• The lobule
• The tragus
The anterior auricular branches of the supercial temporal artery, occipital artery,
and posterior auricular artery make up the auricle’s vascular supply. The designated
arteries go hand in hand with the veins [3].
1.2.2 Innervation oftheExternal Auditory Canal
The external auditory nerve, which has been the subject of extensive research, is
comprised of the following three branches:
• The lower two-thirds of the external ear is within the great auricular nerve’s sen-
sory region, which extends from its origin in the cervical plexus (C2–3).
• The auriculotemporal nerve (a branch of V3) provides sensory innervation to the
anterior upper one-third of the ear, including the tragus, crus helix, and supe-
rior helix.
• Sensory data from the back (cranial) surface of the top one-third of the external
ear are supplied by the lesser occipital nerve, which originates from the C2
branch of the cervical plexus.
The oor and concha of the external auditory canal are innervated by the auricular branch of the vagus nerve, also known as Arnold’s nerve.
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
1.2.3 The Eternal Auditory Canal/External Acoustic Meatus
The external auditory canal, or the external acoustic meatus, is created by bone and
cartilage. The canal is curled into an S shape and extends approximately 4cm from
the tragus to the tympanic membrane. The mandibular condyle is located ahead of
the external auditory canal’s bony part. The air cells of the mastoid bone are located
behind the canal’s bony wall [3].
The external auditory canal, also known as the external acoustic meatus, receives
sensory innervation from the following three sources:
The auriculotemporal nerve: Data from the front wall and roof are sent by the
auriculotemporal nerve, which originates from the mandibular branch of the trigeminal nerve.
The nervous intermedius: This is a branch of cranial nerve (CN) VII.
The auricular branch of the vagus nerve (Arnold’s nerve): carries the sensibility
for the back of the room’s walls and oors. One such structure that contributes is the
tympanic plexus. The posterior auricular artery, the supercial temporal artery, and
the deep auricular branch of the maxillary artery are the arteries that supply the
area [3].
1.3 The Middle Ear (Tympanic Cavity)
5
Bony conduction of sound, which involves the transmission of airborne sound
waves from the auricle to the inner ear uid, is the principal function of the middle
ear (tympanic cavity). The middle ear is located in the petrous part of the temporal
bone and receives air pressure via the auditory (Eustachian) tube’s connection to the
nasopharynx [4].
The middle ear is known as the tympanic cavity from the tympanic membrane to
the oval window. Inside this cavity are the bony conduction elements of the malleus,
incus, and stapes. Important relationships with the tympanic cavity walls are complicated and include the following [3]:
• The tympanic membrane is located on the side wall.
• The mastoid antrum and the mastoid air cells are both located on the poste-
rior wall.
• The oval window is located on the medial wall, while the round window is on the
labyrinthine wall, which is behind the oval window and separated by the
promontory.
• The auditory tube is housed in the tympanic cavity, which is connected to the
sympathetic plexus of the carotid and the tympanic plexus by the deep petrosal
nerve. The tympanic branch of the internal carotid artery and a thin bone plate
called the carotid wall perforate this wall.
6
To distinguish the epitympanic recess—which houses the incus and malleus—
from the middle cranial fossa, the tegmental wall serves as the roof of the tympanic cavity.
Separating the internal jugular vein from the tympanic cavity, the jugular wall
serves as the oor of the middle ear.
O. Oguz et al.
1.3.1 The Tympanic Membrane
A thin, semitransparent, oval membrane called the tympanic membrane (TM) separates the external ear from the middle ear. The pars accida and pars tensa are the
two sections of the tympanic membrane. Concavity is formed when the malleus
manubrium medially drags the medial tympanic membrane and is securely linked to
it. The term “umbo” describes the point at which this concavity meets. The pars
accida is the part of the tympanic membrane above the umbo, whereas the pars
tensa forms the rest of the membrane [3].
The process begins with the auricle picking up vibrations in the air and sending
them to the ossicles through the movable tympanic membrane [3].
The tympanic membrane receives its sensory nerve supply from the following [3]:
• The auriculotemporal nerve (the trigeminal nerve’s mandibular branch).
• “Arnold’s nerve” or the auricular branch of the vagus nerve
• “Jacobson’s nerve” or the tympanic branch of the glossopharyngeal nerve.
Aneurysms get blood ow via the maxillary artery’s stylomastoid, deep auricular, and anterior tympanic branches. The external jugular vein is a part of the venous
drainage system that originates from the surface of the tympanic membrane and
extends to the dural veins and transverse sinus [3].
1.3.2 The Middle Ear or theTympanic Cavity
The middle ear, or the tympanic cavity, is home to several structures such as the
auditory tube, muscles, and nerves. The mucoperiosteum [3] covers the cavity.
1.3.3 Ossicles
The ossicles form a series of movable bones extending from the tympanic membrane’s deep surface to the oval window. The names of the ossicles are as follows [3]:
• Malleus (hammer)
• Incus (anvil)
• Stapes (stirrup)
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
The internal ear’s perilymph receives and amplies sound waves transmitted by
these bone structures. A pressure wave is generated in the inner ear uid when the
following steps are taken: the tympanic membrane is pushed medially by sound
waves; the malleus is pulled laterally by the incus through its synovial joint; the
incus causes the footplate of the stapes to displace the oval window (the fenestra
vestibuli); and, nally, one can hear a sound. Bony conduction increases the strength
of an incoming sound wave by a factor of 10 [3].
1.3.4 The Eustachian Tube
The auditory tube, called the Eustachian tube, connects the middle ear to the nasopharynx. Reducing the pressure differential across the tympanic membrane is its
primary role. When the salpingopharyngeus and tensor veli palatini contract outside
of the middle ear’s tympanic chamber, it opens the auditory tube [3].
1.3.5 Muscles
The stapedius muscle is included in the middle ear’s (tympanic chamber) important
musculature, which links the stapes neck to the posterior tympanum. The facial
nerve supplies innervation to the stapedius. Contraction moves the stapes posteriorly to protect the inner ear from damagingly loud sounds. When the facial nerves
are paralyzed, the stapes footplate can move uncontrollably, leading to heightened
sensitivity to sound and potentially inner ear injury [3].
The manubrium of the malleus is where the tendon of the tensor tympani attaches.
Above the auditory tube, in a semicanal, lies its muscular belly. Tension in the tensor tympani, which is innervated by the mandibular branch of the trigeminal nerve,
causes the malleus and the tympanic membrane to move medially, which, in turn,
tenses the tympanic membrane and reduces the vibration of sound. Along the top of
the auditory tube, this muscle is embedded in the temporal bone [3].
7
1.3.6 Innervation
Just above the stapes footplate, in a bony canal, the facial nerve’s horizontal segment passes through the tympanic cavity on its labyrinthine wall [3].
Before the facial nerve emerges from the stylomastoid foramen, it gives out the
chorda tympani, a recurrent branch of the nerve. After emerging from a bony canal,
it travels medially to the neck of the malleus embedded in the mucous membrane,
emerges in the carotid wall, and nally reaches the tympanic cavity, also known as
the middle ear. The submandibular and sublingual salivary glands, as well as the
front two-thirds of the tongue (which convey information about taste), are innervated by the chorda tympani [3].
8
O. Oguz et al.
Several nerves that branch from the tympanic plexus on the medial wall’s promontory convey the following sensory information regarding the tympanic canal
(middle ear):
• A portion of the glossopharyngeal nerve, known as Jacobson’s nerve, branches
out into the tympanic plexus.
• The sympathetic carotid plexus sends branches that contribute to the tympanic
plexus, including the superior and inferior caroticotympanic nerves, which con-
nect the tympanic branch of the glossopharyngeal nerve.
• There is a communication with a branch from the greater petrosal nerve.
Several areas, including the auditory tube, the fenestra vestibuli, and the mucous
membrane of the tympanic cavity (middle ear), receive branches from the tympanic plexus.
A superior course through the oor of the middle cranial fossa is the way the
lesser petrosal nerve exits the tympanic plexus [3].
1.3.7 Vascular Supply
The tympanic branch of the maxillary (tympanic membrane), the stylomastoid
branch of the posterior auricular (posterior cavity and mastoid), the petrosal branch
of the middle meningeal, a branch of the ascending pharyngeal, the tympanic branch
of the internal carotid, and a branch from the artery of the pterygoid canal (follows
the auditory tube) are the arterial supply sources for the cavity. The pterygoid plexus
and the superior petrosal sinus are the destinations of venous drainage [3].
1.4 The Inner Ear (Labyrinthine Cavity)
The inner ear, also known as the labyrinthine cavity, helps with balance and transmits sounds to the brain (see rst image below). Within the intricate chamber, a
process known as auditory transduction occurs, which involves the transformation
of mechanical acoustic energy into electrochemical energy [3].
The bony osseous labyrinth encases the membranous labyrinth, constituting the
labyrinthine cavity. The osseous labyrinth and membranous labyrinth are found
within the petrous temporal bone. The former consists of a network of bony cavities,
while the latter comprises connecting sacs and ducts contained within the former.
Enclosed within the membrane labyrinth is endolymph, cushioned by surrounding
perilymph. The membrane labyrinth has semicircular, cochlear, and vestibular
parts [3].
The semicircular canals, vestibule, and cochlea make up the osseus labyrinth.
Perilymph and the periosteum line these bone canals. An aperture in the lateral wall
of the osseous (bony) labyrinth’s vestibule is known as the fenestra vestibuli, also
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
9
called an oval window. It opens into the inner ear, is lled with uid, and articulates
with the stapes footplate in the middle ear [3].
1.4.1 The Vestibule
Positioned medial to the tympanic cavity (middle ear), in front of the semicircular
canals, and behind the cochlea, the vestibule forms the central section of the osseous
labyrinth [5]. Among its anatomical boundaries are the following three points:
The oval window, or the fenestra vestibuli, is located on the lateral tympanic
wall. It receives information from the base of the middle ear’s stapes.
The acoustic nerve bers that go to the saccule (a portion of the membranous
vestibule) are housed on the medial wall of the vestibule and communicate with it
through holes in the depression called the spherical recesses. Similarly, the acoustic
nerve bers that supply the vestibular end of the ductus cochlearis go through the
fossa cochlearis.
A set of perforations in the ceiling carries nerve impulses to the ampullae (a portion of the membranous semicircular canals) and the utricle (a portion of the membranous vestibule).
Openings to the semicircular canals are located posteriorly, while the communication with the cochlea’s scala vestibuli is located anteriorly.
The utricle and saccule comprise the vestibular apparatus. These “otolithic
organs” detect linear acceleration in horizontal and vertical dimensions. The saccule
is near the cochlea, and the utricle is near the semicircular canals. Because of its
mostly axial orientation, the utricle detects acceleration in the horizontal plane. On
the other hand, saccule orientation is coronal and acceleration is vertical. The ductus
reuniens connects the saccule to the cochlear duct and the utriculosaccular duct to
the utricle. Dilated endolymphatic sacs are the nal destination of the endolymphatic duct, which originates as a branch of the utriculosaccular duct. When taking
in and releasing endolymph, the endolymphatic duct and sac are crucial players [3].
Hair cells, specically saccule macula and utricle macula, are located within
these organs. The cilia of these cells are closely linked to a membrane-bound material that contains calcium carbonate granules or “otoliths.” When the head moves,
the movable otoliths shear the hair cells. The vestibular nerve’s upper division
reaches the utricle, while its inferior division reaches the saccule, both of which
signal this direction change to the brain. The otolithic organs in both ears play a
crucial role in direction perception [3].
1.4.2 Semicircular Canals
The three semicircular canals are on the body’s front, back, and sides. An ampulla,
an enlargement of the canal, joins each semicircular canal to the utricle; these canals
are positioned in distinct planes (x, y, and z). Cristae are sensory epithelia found
within the ampulla; they house projections of hair cells. Located above the hair cells
10
O. Oguz et al.
and cristae is a cupula made of gelatin. The gelatinous cupula rests above the cristae, and, as the head rotates in different directions, endolymph owing through the
semicircular canals moves it, thus stimulating the hair cells contained therein.
Depending on the direction of endolymph ow, the hair cells can become depolarized or hyperpolarized [1, 6].
1.4.3 The Saccule andtheUtricle
The macula, found in both the utricle and the saccule, is the basic end organ responsible for detecting linear acceleration. It is similar to the crista in the ampulla, which
was discussed earlier. The saccule is responsible for acceleration along the vertical
axis, and the utricle controls the longitudinal acceleration. An otolithic membrane
covers each macule, which comprises hair cells and supporting cells surrounded by
a gelatinous layer. Heavy crystals of calcium carbonate called otoconia rest on the
otolithic membrane. The hair bundles are displaced due to a shear force between the
otolithic membrane and macula when the head is linearly accelerated. Displacement
of hair cells in the macula, like the hair bundles in the semicircular canals’ ampulla,
leads to the generation of a potential depending on the direction of movement. As
one approaches the kinocilium, channels open and the cell becomes depolarized.
When one moves away from the kinocilium, channels close and nerve bers become
hyperpolarized [1].
1.4.4 The Cochlea
The inner ear’s cochlea is the most crucial part of hearing. Like a snail’s shell, the
osseous cochlea has a central modiolus and a 2.5-turn canal that coils around it. The
bony vestibule and bony cochlea are in constant contact with one another. A perforation in its base allows the laments of the vestibulocochlear nerve’s (cranial nerve
VIII) cochlear division to pass through [3].
The bone cochlea is surrounded by a membrane cochlea, which transfers sound
waves’ energy to the cochlear endolymph in the middle ear via pressure waves. The
cochlear nerve carries the transformed mechanical energy from this liquid medium
to the central nervous system [3].
The cochlea has three chambers: the scala vestibuli, the cochlear duct, and the
scala tympani [3]. To understand this further, refer to the image below.
The vestibular membrane (Reissner’s membrane) separates the cochlear duct
from the scala vestibuli, also known as the superior chamber, which contains perilymph. Beginning near the oval window—which connects to the middle ear and the
stapes footplate—this chamber winds its way up the modiolus to the helicotrema,
the apex, and then out to the scala tympani [3].
The vestibular membrane from above and the basilar membrane from below
demarcate the cochlear duct, also known as the scala medium, and is the center
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
11
chamber of the cochlea. The spiral organ (of Corti) and endolymph are contained
inside this membrane-bound organ [3].
Perilymph is located in the inferior chamber, also known as the scala tympani,
which is isolated from the cochlear duct by the basilar membrane. Starting at the
helicotrema, the top of the modiolus, the scala tympani winds its way down to the
fenestra cochleae, the round window covered by the secondary tympanic membrane, in contrast to the scala vestibuli. The middle ear, or the tympanic canal, is
accessible through the spherical opening [3].
The spiral organ (of Corti) is responsible for sound transduction in the membranous cochlea. It comprises four rows of hair cells resting on the basilar membrane.
Stereocilia on these hair cells extend into the tectorial membrane, which is gelatinous. Once sound passes through the tympanic membrane and reaches the middle
ear, it can be transmitted [3].
The energy of a sound wave travels from the middle ear, where it is guided by the
osseous tympanic cavity, to the vestibule near the base of the cochlea via the footplate of the stapes, which moves the oval window medially. After that, uid moves
through the cochlea’s scala vestibuli, applying pressure to the basilar membrane.
Eventually, the uid moves to the scala tympani, which moves the round window
membrane back into the middle ear. The spiral organ’s hair cells can move due to
the perilymph ow within the cochlea [3].
Uncoiled, the basilar lamina has a point at the top and a point at the bottom, with
the former being wider. As an acoustic signal rises from the base to the peak, it
excites the base with higher-frequency stimuli and the peak with lower-frequency
stimuli. These are the data sent by the hair cells in the cochlear nerve’s spiral ganglion to the cell bodies in the spiral organ. These nerve terminals leave the spiral
organ by the modiolar side [3].
1.4.5 Innervation
The principal inner ear nerve is the vestibulocochlear nerve, often known as cranial
nerve VIII.The internal acoustic meatus is the entry point for this nerve into the
petrous temporal bone from the brain’s pons and medulla oblongata. The cochlear
branch, two vestibular nerves (superior and inferior), and the nerve that supplies the
auditory system all originate in the temporal bone. The saccule, utricle, and ampullae provide signals to the vestibular (Scarpa) ganglion through the vestibular nerves.
The spiral ganglion transmits auditory information from the cochlea to the cochlear
nerve [3].
The vestibulocochlear nerve [7] carries an electrochemical impulse from the vestibule, the semicircular canals, and the cochlea to the posterior cerebral fossa via the
internal acoustic meatus.