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1.4.6 Cochlea Nerve Anatomy
The cochlea’s spiral ganglion is home to hair cells that initiate impulses. When hair
cells depolarize, the signal travels to the cochlear nerve [1].
1.4.7 Vestibular Nerves
The ampulla, utricle, and saccule are home to hair cells that initiate impulses. Within
the internal acoustic meatus, where the vestibular and cochlear nerves converge, is
a vestibular ganglion called Scarpa’s ganglion. The dendritic processes of the bipolar cells that make up Scarpa’s ganglion are responsible for directly retrieving electrical impulses from the hair cells. The utricle, the higher and lateral semicircular
canals, and the superior vestibular nerve are all areas that receive innervation from
this nerve. The inferior vestibular nerve innervates the saccule and the inferior/posterior semicircular canal. The electrochemical impulse is then sent to the vestibular
nerve by the bipolar cells through axonal bers [7].
1.4.8 The Vestibulocochlear Nerve
The vestibulocochlear nerve is the name given to the nerve when the vestibular and
cochlear nerves merge at the internal auditory meatus. Synapses on the nuclei within
the pons are made by CN VIII when they enter the brainstem through the posterior
cranial fossa and continue into the brainstem between the medulla and pons. The
ventral and dorsal cochlear nuclei are synapses for the cochlear nerve. All four vestibular nuclei—the superior, inferior, medial, and lateral ones—have connections
with the vestibular nerve [7].
1.5 The Central Hearing System
The spiral organ (of Corti) sends impulses to the cerebral cortex via the ascending
pathway [8].
1.5.1 First-Order Neurons oftheAuditory System
A cluster of nerve cell bodies in the cochlear modiolus’s spiral ganglion is the
source of the cochlear nerve’s bers. Spiral ganglion neurons are the initial fourorder neurons connecting the cochlea to the brain. Their two sets of processes, or
bers, radiating from the cell bodies’ polarities, identify them as bipolar cells. Short
peripheral bers reach the bases of the inner and outer hair cells, while longer central bers—also known as primary auditory bers—form the cochlear nerve.
Radiating outward from the spiral ganglion, they reach the habenula perforata, a
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
13
network of microscopic openings underneath the inner hair cells. Demyelination
occurs as they approach the spiral organ (of Corti) [8].
There are only around 30,000 of these bers, and nearly 95% of them innervate
the cortex’s hair cells. To innervate the outer hair cells, the rest travel through the
tunnel of Corti. The cochlear nerve trunk is formed when the longer central processes of bipolar cochlear neurons come together. These principal auditory bers
leave the modiolus and enter the medulla oblongata through the internal meatus.
Porus acusticus is the name given to the “mouth” portion of the internal auditory
canal (IAC). The cerebellopontine angle and the anatomical link of the seventh and
eighth nerves in the inferior auditory canal (IAC) are signicant areas of anatomy
for neurotologists and skull base surgeons [8].
As they travel through the brain, the seventh and eighth nerves are surrounded by
glial tissue. Beginning in the inferior auditory canal (IAC) near the porus acusticus,
Schwann cells encircle these nerves. The Obersteiner–Redlich zone is located at the
glia–Schwann junction [8].
Near the IAC’s midpoint is Scarpa’s ganglion, also known as the vestibular ganglion. Either the subarachnoid space or the medial section of the inferior auditory
canal (IAC) is where cranial nerve (CN) VIII splits into the vestibular and cochlear
branches. The vestibular nerve’s superior and inferior branches make up the back
half of the inferior auditory canal (IAC). The cochlear nerve can be found toward
the back of the canal. Anterosuperior to the internal auditory canal (IAC) is where
the seventh nerve can be found. At the top portion of the inferior ampulla (IAC), the
facial and superior vestibular nerves are separated by a vertical crest called a bill
bar. The lower vestibular and cochlear divisions are located in the lower half of the
inferior auditory canal (IAC), separated from the upper half by the transverse
crest [8].
One possible opening in the back of the skull is the cerebellopontine angle. The
following [8] are its boundaries:
• The temporal bone’s posterior fossa is located anteriorly.
• Lower half: Olive.
• The inferior border between the cerebellar peduncle and the pons is located
anteriorly.
• Below: The tonsil on the cerebellum.
• Above the cerebellopontine angle, one can see the trigeminal nerve, and, below
it, one can see the ninth, tenth, and eleventh nerves running their respective
courses. Along with the foramen of Luschka, occulus, and anterior inferior cer-
ebellar artery (AICA), the cerebellopontine angle also include other signicant
structures.
The cochlea and labyrinth are supplied by the labyrinthine artery, which is often
a branch of the AICA [8]. Cochlear nerve bers travel to the cochlear nucleus after
passing via the medulla. Five cell types comprise the cochlear nucleus, and their
morphology and physiology vary in important ways, including how they react to the
beginning and end of a stimulus and how they modulate its frequency. Parts of the
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cochlear nucleus are located inside and outside the organ. There are two primary
bundles of cochlear bers. One bundle goes laterally and dorsally to the restiform
body, while the other stays somewhat ventral and medial to the restiform body and
ends in the ventral cochlear nucleus. The dorsal portion of the dorsal cochlear
nucleus is where bers originating from the cochlea’s basal coils terminate. The
ventral nucleus and the dorsal cochlear nucleus are the nal destinations for the
bers originating at the cochlea’s top. However, before they synapse, some bers go
on to higher-order neurons [8].
1.5.2 Hearing Neurons ofDifferent Orders
Second-order neurons have cell bodies in their ventral and dorsal cochlear nuclei.
To reach the cells of the superior olivary complex, some bers from the ventral
cochlear nucleus travel across the midline. In contrast, other bers connect to the
olivary cells on the same side. Inputs from both ears converge at the superior olivary
complex, the initial center in the ascending auditory system. These bers combine
to create the trapezoid body, home to third-order neurons [8].
The dorsal cochlear nucleus sends bers that terminate on the lateral lemniscus
nuclei after they have traveled across the midline. Fibers from the olivary complex
and the ventral cochlear nuclei on either side connect. Each ear can send signals to
the auditory nuclei above the superior olivary complex, either stimulating or inhibiting the auditory pathway. Most of the lemniscus’s bers terminate in the inferior
colliculus, the midbrain’s auditory center [8].
There are several different kinds of cells in the inferior colliculus—at least 18
main cell types and 5 different areas of specialization. They play a role in binaural
hearing, loudness, differential sensitivity to frequency and intensity, and all other
aspects of auditory behavior. The medial geniculate body of the thalamus is home
to fourth-order neurons, albeit some bers may go beyond the colliculus and terminate there and at the next higher level. A section of the temporal lobe cortex receives
a well-organized projection of bers from the medial geniculate body [8].
1.5.3 Auditory Input
The cerebral cortex receives audio input rst in the primary auditory cortex.
Primates, including humans, have a ridge in the temporal lobe called the superior
transverse temporal gyri of Heschl. It is located on the bottom lip of the lateral sulcus, also called the Sylvian ssure, which is a profound gap between the temporal
and parietal lobes [8].
Each ear is represented in the right and left cortices because specic auditory
channels ascend on the same side of the brain while others cross the midline.
Because of this, binaural hearing may remain mostly unaffected despite damage to
the auditory cortex area on one side due to a stroke or some other traumatic event [8].
1 Outer–Middle–Inner Ear andCentral Hearing System Anatomy
15
The superior temporal gyrus (STG) on the right side of the brain is responsible
for hearing. Brodmann 41 and 42, which demarcate the site of the central auditory
cortex—the cortical area accountable for the perception of the fundamental aspects
of sound like rhythm and pitch—are among the numerous signicant brain structures found in the superior temporal gyrus [8].
Area 22, also known as the Wernicke area, is in the brain’s temporal lobe and
responsible for auditory association processing. To differentiate between speech,
music, and noise, this region, which is located close to the lateral cerebral sulcus, is
crucial for processing audio signals [8].
In keeping with the standard practice for thalamocortical connections, the medial
geniculate body nuclei that project to the auditory cortex also receive bers from the
same region. Rarely has a case of impaired hearing, resulting from bilateral cortical
damage, affected both auditory regions. The perception of faint, short-duration signals, speech discrimination, and sound localization are among the behaviors
impacted by cortical deafness, which can be caused by bilateral lesions of the temporal lobe [8].
1.5.4 The Auditory Nerve’s Descending Routes
Descending efferent pathways are an additional auditory route alongside reex and
conscious afferent pathways. Perception of sound also involves efferent connections
from the brain to the cochlea. Typically, the auditory system can self-regulate owing
to the descending routes, which suppress the ascending bers. Every auditory relay
station is believed to be doubly innervated, which allows for the internal modication, cancellation, or inuence of incoming impulses [8].
Two pathways run parallel: one goes up from the cochlear nuclei to the brain,
while the other goes down from the cortex to the cochlear nuclei. Both routes
include a portion of the nerve bers staying on one side of the brain and a portion
crossing over to the other. Additionally, there is evidence of a “spur” line that
descends from the inferior colliculus to the cerebellum and one that ascends from
the dorsal cochlear nucleus to the cerebellum [8].
It is unclear what role these connections played in the brain’s evolution, but they
may have existed before the cortex. The olivocochlear bundle is one further ber
tract that comes from the superior olivary complex, which is a part of the medulla
oblongata (refer to the image below). The neural impulses that are believed to be
inhibitory make it to the hair cells through this feedback loop, which is an efferent
system [8].
References
1. Bruss DM, Shohet JA.Neuroanatomy, ear. [Updated 2023 Apr 3]. In: StatPearls. Treasure
Island (FL): StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/
NBK551658/ (Accessed online on July 23, 2023).
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2. Ekdale EG.Form and function of the mammalian inner ear. J Anat. 2016;228(2):324–37.
3. Bhatt RA.Ear anatomy. In: Gest TR, editor. Medscape; 2016. Updated: Jun 27, 2016. https://
emedicine.medscape.com/article/1948907- overview#a4 (Accessed online at July 23, 2023).
4. Kahrs LA, Labadie RF.Freely-Available, true-color volume rendering software and cryohistol-
ogy data sets for virtual exploration of the temporal bone anatomy. ORL J Otorhinolaryngol
Relat Spec. 2013;75(1):46–53.
5. Khan S, Chang R. Anatomy of the vestibular system: a review. NeuroRehabilitation.
2013;32(3):437–43.
6. Zhang K, Wang F, Zhang Y, Li M, Shi X.Anatomic investigation of the labyrinthine artery.
Zhonghua Er Bi Yan Hou Ke Za Zhi. 2002;37(2):103–5.
7. Landau ME, Barner KC.Vestibulocochlear nerve. Semin Neurol. 2009;29(1):66–73.
8. Tewk TL.Auditory system anatomy. In: Gest TR, editor. Medscape; 2017. Updated: Dec 08,
2017. https://emedicine.medscape.com/article/1948643- overview#a2 (Accessed online on July
23, 2023).
O. Oguz et al.
Outer–Middle–Inner Ear Embryology
SelinDayisoylu, NurayBayar Muluk, andTaflineC.Arbor
2.1 Introduction
Merging the six auricular hillocks is a complicated process that occurs during external ear development. Abnormal growth and development can occur in numerous
areas due to the inherent intricacy of this process. During the third week of gestation, the otic disc is formed when the ectoderm thickens. An otic pit develops from
the maturing otic disc. At about 32weeks of gestation, the developing auricular
complex reaches its nal location, level with the eyes, after having moved dorsocranially with the mandible’s growth from its ventrocaudal origins near the neck’s
base. Chromosomal aberrations are linked to the ears that are positioned more posteriorly on the skull [1, 2].
The auricle’s structures are formed by the merging of the six hillocks that originate from the rst and second branchial arches. The tragus, helical crus, and helix
most likely originate from the mandibular arch, with the initial arch spanning the
rst three hillocks. The second arch, also called the hyoid arch, is where the lobule,
antitragus, and antihelix develop. This arch is formed by the fourth through sixth
hillocks. Ear anomalies are most common before week 7 of gestation, and most
growth is nished by week 20. The convergence of these hills with the ear’s intrinsic
and extrinsic musculature, which creates numerous folds and curves, determines the
nal ear contour [1–3].
2
S. Dayisoylu
Department of Otorhinolaryngology, Tekirdag City Hospital, Tekirdag, Turkey
N. Bayar Muluk (*)
Faculty of Medicine, Department of Otorhinolaryngology, Kırıkkale University,
Kırıkkale, Turkey
T. C. Arbor
Wake Forest University School of Medicine, Winston-Salem, NC, USA
e-mail: tarbor@marian.edu
© 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_2
17
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S. Dayisoylu et al.
2.2 Embryology
The pre-placodal area of the ectoderm, located at the anterior border of the neural
plate, starts to thicken during week 4 of embryologic development [4, 5]. The inner
ear is gradually built from structures that originate in the otic placode, which is a
derivation of the ectoderm [6]. Otic vesicles and neuroepithelial cells are formed
when the ectoderm invaginates toward the mesoderm [7]. The otic vesicles are the
progenitors of the utricle and saccule. The formation of the cochlear duct from the
otic vesicle and the accumulation of endolymph within the membranous labyrinth
are both observed by week 5. The construction of the cochlear duct and the scala
vestibuli, two distinct cavities, follows the subsequent building of an internal wall
within the cochlea [6]. The scala tympani is formed when the basilar membrane
further separates the cochlear duct. The formation of hair cells, which are located
within the tectorial membrane, starts in the cochlear duct [4].
2.3 Embryologic Development oftheHead andNeck
andtheEar
The head and neck begin to take shape around the third to eighth weeks of gestation.
Embryonic development of the head and neck begins on day 22 with the formation
of the pharyngeal foregut and continues through the ve sets of branchial arches,
which correlate with the primitive vertebral gill bars. An ectoderm layer covers the
outside of the arch, an endoderm layer covers the inside, and a mesenchyme layer
forms the center core. Pharyngeal clefts and pouches lined with the endoderm and
ectoderm, respectively, further divide these arches. Concerning the branchial arches,
this chapter provides a synopsis of the embryologic progression of the head and
neck [8–10].
2.3.1 First Week
Fertilization is complete when the sperm penetrates the uterine tube’s oval cytoplasm, and the maternal and paternal chromosomes merge to create a zygote. The
zygote is guided to the uterus by the ciliary activity of the uterine tube. Once there,
it changes into a ball of tiny cells called blastomeres during a series of mitotic divisions. When blastomeres make it to the uterus in about 3days, they undergo four
mitotic divisions, at this point, they develop into the morula, a 16-cell sphere, and
start to carve out an interior space [11].
The blastocyst comprises two layers of cells: the embryoblast, which develops
into the embryo, and the trophoblast, which creates the embryonic placenta. By the
time the blastocyst fuses with the endometrium in the uterine posterior wall around
the fth day of gestation, the trophoblasts have differentiated into syncytiotrophoblasts and cytotrophoblasts by about the sixth day. The blastocyst’s supercial
implantation is nished by the end of the rst week as the syncytiotrophoblasts
2 Outer–Middle–Inner Ear Embryology
19
invade the underlying endometrial stroma after penetrating the endometrial epithelium [8].
2.3.2 SecondWeek
In the second week, trophoblasts differentiate and proliferate rapidly. In preparation
for the blastocyst implantation, the endometrial tissues’ decidual reaction speeds up
this week. Around day 9, the blastocyst is fully buried beneath the endometrial epithelium, and, around 2days later, the lacunae network is formed by the fusion of
spaces in the syncytiotrophoblasts. On days 11 and 12, the endometrial blood vessels are penetrated by syncytiotrophoblast erosion [8].
Then, the syncytiotrophoblast lacunae networks can receive maternal blood
through the uteroplacental circulation. Days 11 and 12 also see the formation of the
main yolk sac and the differentiation of certain cytotrophoblasts into the extraembryonic mesoderm. The chorionic cavity is lled with the extraembryonic coelom
that develops from the extraembryonic mesodermal space [8].
At the conclusion of the second week, the amniotic cavity forms as a space
between the inner cell mass and the cytotrophoblasts, the secondary yolk sac
replaces the primary one, and the formation of the primary chorionic villi is noticeable. A bilaminar embryonic disc develops from the inner cell mass by further differentiation. The amniotic cavity is associated with the epiblast, the outermost layer.
The hypoblast, the innermost layer, is next to the blastocyst cavity; the prochordal
plate, a localized thickening within it, will eventually form the embryonic cranial
area [8].
2.3.3 Third Week
The embryonic disc, which consists of three layers—the endoderm on the inside,
the mesoderm in the middle, and the ectoderm on the outside—is formed during
gastrulation during the third week. The embryo starts off as a at, two-dimensional
structure, but, soon before the third week, a thickened region of the embryo’s ectoderm called the neural plate [8] makes way for the formation of the central nervous system.
Ectodermal invagination in the middle and elevation of ectodermal tissue along
the groove to create neural folds give rise to a midsagittal groove. At the midline,
where the future brain and spinal cord will meet, these folds unite to form the neural
tube. With the fusion of its folds, the neural tube grows apart from its ectoderm
source [8].
The neural crest develops from a group of ectodermal cells close to the neural
fold but not part of the somatic ectoderm covering the surface. The current thinking
is that these cells from the neuroectodermal crest travel extensively in the embryo in
a mostly cell-free, enriched extracellular matrix and, depending on their
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S. Dayisoylu et al.
surroundings, differentiate into a diverse range of cell and tissue types. Derived
from neural crest cells, most cranial, facial, skeletal, and connective tissues eventually form [12].
A thickened area with a shallow sulcus on the lateral forebrain of the neural tube
in the region of the future diencephalon is the rst morphologic evidence of the
optic primordia. Invagination occurs during this week when the optic sulcus deepens, and the walls around it come into contact with the ectoderm that lies above.
Anterior to the optic vesicle and the underlying ectoderm are neural crest cells,
which differentiate into specic neuroectodermal components of the eye and
adnexum [13].
A mesencephalic exure happens when the neural tube merges cranially, sealing
off the front neuropore. This allows us to start distinguishing between the prosencephalon (the forebrain), the mesencephalon (the midbrain), and the rhombencephalon (the hindbrain). The frontal nasal process and the branchial arches, the initial
features of a distinguishable face, are born toward the conclusion of the third week
due to the continued development of the cephalic neural tube [8, 14].
2.3.4 Fourth Week
The ve primordia that surround a central depression, also known as the stomodeum
or the oral pit, are the building blocks of the distinctive face. The frontal nasal process, which is placed atop the skull, and the maxillary and mandibular processes on
each side of the face are the three primordia. The rst branchial arch is the origin of
the mandibular and maxillary processes. A slight depression on the ectoderm surface, the stomodeum, is formed by the rst branchial arch, also called the mandibular arch. It serves as the base of the primitive mouth and creates the lateral wall of
the stomodeum [15].
The primordial oropharynx’s lateral and anterior walls are formed by the surviving branchial arches, with the appropriate pharyngeal pouches between them. The
face’s characteristics are born from these processes or masses as they develop differently, eventually obliterating the ectodermal plates or grooves that connect
them [15].
Eventually, the mandible, the bottom portion of the face, and the tongue are born
from the mandibular processes, which rst fuse at the midline. The malleus and
incus of the middle ear are formed when the dorsal end of the rst arch cartilage—
also called Meckel cartilage—ossies. The stapes of the middle ear and the styloid
process of the temporal bone are formed when the dorsal end of the second arch
cartilage, also called Reichert cartilage, ossies. The external ear’s auricle is formed
when the little projections at the dorsal ends of the rst and second arches, which
surround the rst branchial groove, come together. Regarding ear development, the
third branchial arch plays no role [15].
Among the branchial cleft congenital anomalies, remains of the second cleft are
the most prevalent. At the frontal meeting point of the sternocleidomastoid muscle’s
middle and lower thirds is the external component. Its course typically goes via the
2 Outer–Middle–Inner Ear Embryology
21
tonsillar fossa, between the internal and external carotid arteries, and over the glossopharyngeal nerve. It is necessary to remove the entire fossa and track. When
excising a mass above the hyoid bone, it is crucial to evaluate the less common cysts
of the rst branchial cleft. Its path could take it through the facial nerve’s branches.
Even though the third branchial cleft is exceptionally unusual, it might appear outwardly in the same spot as the second. It runs beneath the carotid artery on the inside
of the body.
The primitive trisegmented brain continues to divide throughout week 4. The
forebrain, or the prosencephalon, splits into the endbrain, or the telencephalon,
which has the cerebral hemispheres and the diencephalon, which is where the optic
vesicles are born. While in the exed cephalic exure, the mesencephalon stays
undivided. The myelencephalon (medulla) and mesencephalon (cerebellum, pons)
are the offshoots of the rhombencephalon. The basic structure of the future brain
becomes apparent by the conclusion of the fourth week [15].
During development, the ectoderm that covers the optic vesicle thickens to
become the lens placode. As the diencephalon and the optic stalk become one continuous structure, the optic vesicle undergoes a process of deepening and eventually
forms a goblet-shaped, double-layered optic cup during this week. At this point in
development, the distance between the two eye cups is almost 180° and the frontal
nasal process acts as an intermediary [8].
2.3.5 Sixth Week
Six mesenchymal swellings (hillocks) surround the rst branchial cleft during the
development of the external ear during the sixth week. Two branches, one from the
hyoid arch and the other from the mandibular arch, give rise to the hillocks. The rst
arch’s hillocks transform into the tragus, helix, and cymba concha as they run dorsal
to ventral. In contrast, the second arch’s hillocks become the antitragus, antihelix,
and concha as they run transverse to the rst. The external auditory canal’s primordium develops from the branchial cleft as it grows in length. The auricle, which was
initially low on the side of the neck, is progressively moved to a more lateral cephalic
position as the face grows [8].
Regarding the ear’s latter stages of development, in their research, Hashimoto
etal. found that a cartilage loop commonly forms the core of the skin folds covering
the concha, scapha, and triangular fossa in the embryo. However, there were cases
where the loop was absent; the scapha and triangular fossa loops were the most
common examples. This indicates that cartilage repair or replacement will occur
after birth, with newborns seemingly displaying “signicant region-specic variation in the postnatal growth of the auricular cartilages” [8, 16].