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22
S. Dayisoylu et al.
2.4 Embryology oftheCochlear Nerve andCentral
Auditory Pathways
Embryologically, the vestibulocochlear ganglion (acoustic vestibular ganglion) and
the seventh nerve ganglion (facial ganglion) rst unite. The three parts separate one
after the other throughout time. Unlike other sensory ganglia in the brain, which
derive neuroblasts from both neural crest placodes and ganglionic cells, the otocyst
epithelium is the sole source of neuroblasts in the vestibular and cochlear (acoustic)
ganglia. All cranial ganglia have the characteristic that their supporting Schwann
and satellite cells are completely neural crest-originated, most likely deriving from
the facial nerve ganglion [17–22].
References
1. Bhatt RA.Ear anatomy. In: Gest TR, editor. Medscape; 2016. Updated: Jun 27, 2016. https://
emedicine.medscape.com/article/1948907- overview#a4. Accessed online on July 23, 2023.
2. Honrado CP, Bradley DT, Larrabee WF. Embryology of the external ear. In: Azizzadeh B,
Murphy M, Johnson C, editors. Master techniques in facial rejuvenation. Saunders; 2007.
p.17–32.
3. Ear PF.Sobotta Atlas of human anatomy, vol. 3. 23rd. ed. Elsevier GmbH; 2010. p.133–60.
4. 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).
5. Whiteld TT.Development of the inner ear. Curr Opin Genet Dev. 2015;32:112–8.
6. Fuchs JC, Tucker AS. Development and integration of the ear. Curr Top Dev Biol.
2015;115:213–32.
7. Anniko M, Wikström SO.Pattern formation of the otic placode and morphogenesis of the
otocyst. Am J Otolaryngol. 1984;5(6):373–81.
8. Patel PK.Head and neck embryology. In: Narayan D, editor. Medscape; 2021. Updated: Dec
21, 2021. https://emedicine.medscape.com/article/1289057- overview#a1 (Accessed online on
July 23, 2023).
9. Moore KL, Persaud TVN, Torchia MG.The developing human: clinically oriented embryology. 10th ed. Philadelphia, PA: Elsevier; 2016.
10. Schoenwolf GC, Bleyl SB, Brauer PR, Francis-West PH.Larsen’s human embryology. 5th ed.
Philadelphia, PA: Churchill Livingstone; 2015.
11. Langman J.Medical embryology. Baltimore, MD: Williams and Wilkins; 1981.
12. Roth DM, Bayona F, Baddam P, Graf D.Craniofacial development: neural crest in molecular
embryology. Head Neck Pathol. 2021;15(1):1–15.
13. Carstens MH. Neural tube programming and craniofacial cleft formation. I. The neuromeric organization of the head and neck. Eur J Paediatr Neurol. 2004;8(4):181–210. discussion 179–80
14. DeMyer WE, Zeman W, Palmer CG.The face predicts the brain: diagnostic signicance of
median facial anomalies for holoprosencephaly (arrhinencephaly). Pediatrics. 1964;34:256–63.
15. Carstens MH.Development of the facial midline. J Craniofac Surg. 2002;13(1):129–87. discussion 188–90
16. Hashimoto C, Kitamura K, Yamamoto M, etal. Auricular cartilage conguration: a histological study using late-stage human fetuses and adult cadavers. Anat Rec (Hoboken). 2021;
17. 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 at July
23, 2023).
2 Outer–Middle–Inner Ear Embryology
18. Kandel ER, Schwartz JH, Jessell TM. Principles of neuroscience. 4th ed. New York, NY:
McGraw-Hill; 2000. p.591–624.
19. Johnston MC. Developmental biology of the ear. In: Tewk TL, Der Kaloustian VM, editors. Congenital anomalies of the ear, nose, and throat. NewYork, NY: Oxford University
Press; 1997.
20. Josey AF, Glasscock ME 3rd, Musiek FE.Correlation of ABR and medical imaging in patients
with cerebellopontine angle tumors. Am J Otol. 1988;9:12–6.
21. Ferguson MA, Smith PA, Lutman ME, Mason SM, Coles RR, Gibbin KP. Efciency of
tests used to screen for cerebello-pontine angle tumours: a prospective study. Br J Audiol.
1996;30(3):159–76.
22. Lonsbury-Martin BL, Martin GK, Coats AC.Chapter 47: The physiology of the auditory and
vestibular systems. In: Ballenger JJ, editor. Diseases of the nose, throat, ear, head and neck.
13th ed. Philadelphia, PA: Lea and Febiger; 1985. p.952–4.
23
Physiology ofthePeripheral andCentral
Hearing System
MustafaBaran, ErdoğanBulut, ŞerefErdoğan,
andAtrurLorens
3.1 Introduction
This chapter provides a comprehensive overview of the auditory system, with special emphasis on the peripheral and central parts of hearing. The journey of sound
perception begins with the synchronized movement of anatomical structures. This
orchestrated sequence is critical to the transformation of sound waves into perceptible sounds.
The conduction phase of hearing primarily involves the outer and middle ear.
The outer ear, consisting of the auricle (pinna) and the external ear canal, acts as the
initial receiver of sound waves. The pinna, with its unique shape and structure, helps
capture the sound waves and directs them through the ear canal to the eardrum. This
stage involves not only the transmission of sound but also the initial amplication
and localization of these sound waves, preparing them for further processing. The
middle ear, which contains the ossicular chain (malleus, incus, and stapes), serves
3
M. Baran
Medical Faculty, Department of Physiology, Istanbul Medeniyet University, Istanbul, Turkey
e-mail: mustafa.baran@batman.edu.tr
E. Bulut (*)
Faculty of Health Sciences, Department of Audiology, Trakya University, Edirne, Turkey
Faculty of Medicine, Department of Physiology, Trakya University, Edirne, Turkey
Trakya University, Mirko TOS Ear & Hearing Research Center, Edirne, Turkey
e-mail: erdoganbulut@trakya.edu.tr
Ş. Erdoğan
Medical Faculty, Department of Physiology, Cukurova University, Adana, Turkey
e-mail: serdogan@cu.edu.tr
A. Lorens
Implants and Auditory Perception Department, World Hearing Center, Institute of Physiology
and Pathology of Hearing, Nadarzyn, Poland
e-mail: a.lorens@ifps.org.pl
© 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_3
25
26
as a mechanical amplier, transmitting vibrations from the eardrum to the oval window of the cochlea. Efcient transmission of these vibrations is essential for the
subsequent conversion stage, and any disruption in this stage can result in conductive hearing loss.
The conversion stage takes place in the inner ear, specically in the cochlea, a
uid-lled spiral organ that is the cornerstone of auditory transduction. In the
cochlea, the mechanical energy of sound vibrations is converted into electrical signals by delicate hair cells. These signals are then transmitted via the eighth nerve to
the auditory cortex, where sound is perceived and interpreted. This conversion from
mechanical to electrical energy is a critical step in the auditory process, and disruptions in this stage can result in sensorineural hearing loss. Understanding these
stages and their components is crucial for diagnosing different types of hearing loss.
For example, problems in the outer or middle ear that affect sound conduction can
lead to conductive hearing loss, while problems in the inner ear, particularly with
hair cells or the auditory nerve, can cause sensorineural hearing loss. In addition,
this detailed understanding of the function of the auditory system facilitates the
development of targeted interventions, ranging from hearing aids that amplify sound
to cochlear implants that stimulate the auditory nerve directly, bypassing damaged
parts of the ear.
M. Baran et al.
3.2 The Peripheral Auditory System: APerspective
The perception of sound stimuli occurs through the synchronized movement of anatomical formations, beginning with the outer ear and extending into the middle and
inner ear. [1]. Within the auditory system, the outer and middle ear participate in the
conduction phase, while the inner ear participates in the conversion phase. These
stages are critical for classifying hearing loss.
3.3 The Outer Ear
3.3.1 Anatomy
The outer ear consists of the auricle (pinna), which contains a resonance cavity
called the concha, and the external ear canal. The external ear canal begins at the
pinna and runs to the tympanic membrane (TM) and is in the form of a tube closed
at one end (Fig.3.1) [2]. The lateral (one-third) part of the external ear canal consists of brocartilage tissue, and the medial (two-third) part consists of the temporal
bone. The isthmus region, where the cartilage and bone parts meet, is the narrowest
part of the external ear canal and is the area where external objects are most likely
to get stuck. The cerumen of the external ear canal, which is the skin portion of the
sweat and sebaceous glands that form earwax, is covered with hair. As the hair
grows outward, it joins the cerumen to form a disinfectant and defensive barrier
within the external ear canal [3, 4]. The dimensions and shape of the external ear
3 Physiology ofthePeripheral andCentral Hearing System
Fig. 3.1 The outer ear
27
canal determine the sound pressure in front of the TM and the transmission function
of sound. Knowledge of its anatomy is essential to compare, optimize, and advance
acoustic measurement methods [3]. It also allows for good hearing aid tting [5].
3.3.2 Resonance andAmplification
The external ear canal runs from the top and back to the bottom and front. The shape
of the canal corresponds to an S-shaped conduction path in the coronal plane [3].
Therefore, during otoscopic examination, the external ear canal is straightened by
pulling the auricle backward and upward and an image of the TM is obtained [6].
The geometry of the external ear canal is determined by the parameters of length,
radius, and cavity shape, which vary from person to person. From birth to 7years of
age, a really big change in canal volume and length is observed in the external ear
canal, and, after that, the canal characteristics are comparable to those of adults [3].
Information on the geometry of the external ear canal in adults is based on measurements made by Stinson and Lawton on the right external ear canal of 15 human
cadavers, which will serve as a source for future studies [7]. Although there are
biological differences between individuals, the outer ear canal is approximately
25–32mm in length. The cross-sectional area ranges from 90.13 to 96.16mm2 at the
entrance of the canal and from 65.45 to 75.53mm2 at the TM [8]. The average ear
canal volume is 0.62ml, and, in children under 7years of age, it is 0.37ml [2]. The
effect of the outer ear on the incoming sound can be analyzed in two ways: First, it
is the resonance effect of the outer ear on the sound pressure at the TM and the
resulting amplication. In addition, it affects directional hearing by providing
28
M. Baran et al.
information about the localization of the sound source [9]. Under the resonance
effect, the auricle and concha collect sounds over a wide area and transmit them to
the narrowed external ear canal. In humans, these sounds produce a peak at around
2.5kHz with an amplication of about 15–20dB, causing an increase in pressure on
the TM.However, the maximum increase is approximately 17–22dB at 3 kHz.
Subsequently, this increase creates a maximum resonance within the TM at 3kHz
and its multiples [10]. In infants, the resonance frequency is approximately 6–7kHz
[9, 11]. This is the result of the combined resonance effects of the concha and the
pinna. The resonant frequency of the concha alone is about 5.5kHz, showing that
the concha is dependent on high frequencies for sound pressure changes. For sound
transmission in air, the wavelength is 1.56mm at 20kHz and 15.6 mm at 20 Hz
[11]. This is usually an indication of how important the pinna is to us in the audible
frequency range. The resonant frequency of the external ear canal is between 2.5
and 4kHz. Depending on pressure changes, the resonance of the outer ear changes
between 1.5kHz and 7kHz [1, 11].
3.3.3 Localization
A sound source is located in three spatial dimensions. First, it can be located within
the horizontal plane (azimuth-horizon angle), that is right and left, and, second, it
can be located above or below within the vertical plane. Third, there is distance, i.e.,
near and far sound sources [11]. For this reason, time (interaural time difference)
and intensity (interaural level difference) differences within the sound waves arriving at the two ears provide critical cues within the localization of the sound source
[1]. In the horizontal plane, time and phase differences in the time it takes for the
sound to reach the ear closest to the source versus the ear farthest away.
Considering that sound has no spatial dimension and its speed in air is independent of frequency (sound speed at 20 °C; 1atm pressure is 343m/s), when a tonal
stimulus is presented to one ear (0.5kHz and 1kHz), the sound is delayed by 0.5ms
(interaural time difference) to the other ear and arrives with a phase difference [9,
11]. This allows low-frequency sounds to be localized to the nearest ear. In humans,
the auricle provides a large directional selectivity for high frequencies, and at 6kHz
and above, it provides a gain of 10–15dB at an angle of 70°, depending on the frequency, with the head in a straight position [9]. Although it is assumed that the
sound stimulus is collected by the auricle and directed to the TM, the head and body
of the individual also play a role in this mechanism. The head, the chest, the auricle,
where the incoming stimulus strikes; before the sound reaches the TM, it amplies
the sounds adapted to the resonance frequency or creates a barrier and reduces their
intensity. The angle of incidence and the frequency of the sound stimulus are factors
in this mechanism [1]. It is known that the azimuth effect produces a gain of about
6dB for sounds coming from 45° and 90° angles. When the intensity between the
ears is known, the shadow effects of the head on the sound are critical. On the side
where the sound is coming from, the sound waves that hit the head are reected and
then refracted. This increases the sound pressure in the ear on that side (bafe effect)
3 Physiology ofthePeripheral andCentral Hearing System
and decreases the sound pressure by blocking the head sound waves in the ear on the
opposite side of the sound direction (acoustic shadow). The shadow effect depends
on the frequency of the sound. For high-frequency sounds, the wavelength is short
and the shadow effect of the head is large. The intensity difference between the ears
can be a factor in determining the localization of the sound source at high frequencies [1, 9, 11].
29
3.4 The Middle Ear
3.4.1 Structural Elements andTheir Functions
The middle ear (Fig.3.2a) is located in the cavity within the temporal bone (tympanic cavity—tympanum) and may be a site where there is a continuous ow of air
connected to the air-lled mastoid cells via the Eustachian tube and to the external
environment via the aditus ad antrum. This is important for the conductive function
of the middle ear [9, 11]. It is structurally separated from the outer ear by the
TM.The TM is connected to the oval window of the inner ear by the ossicular chain.
There are two muscles and ligaments on the ossicular chain that are critical to its
physiological function [11–13].
3.4.2 The Tympanic Membrane andtheOssicular Chain
The TM is located in the last part of the external ear canal and, due to the different
lengths of the canal walls, forms an angle of 140° with the upper and posterior canal
walls and an angle of 30° with the lower and anterior canal walls. This tilt or orientation gives the TM a usable area of approximately 85mm2. The periphery of the
TM (Fig.3.2b), approximately more than 270°, is tightly connected to the tympanic
cavity/annular sulcus by a brocartilaginous ring called the tympanic annulus or the
annular ligament [9, 14]. The lower part of the TM that attaches to the annular sulcus is called the pars tensa and constitutes three-fourth of the membrane. The pars
tensa shows a multilaminar layering with circular bers extending throughout the
tympanic layer and radial bers extending to a lesser extent and appearing as a
species-specic variety. The pars accida is located above the TM and is anatomically and functionally distinct from the pars tensa. It attaches to the Rivinus notch
within the annular sulcus at the superior part of the tympanic ring. It constitutes
one-fourth of the membrane, and, although its dimensions are different in mammals, it is thicker than the pars tensa and does not have brous organization. In rapid
vibration of the tympanic membrane, mass distribution, such as thickness, may be a
critical property for vibrational behavior [8, 12]. In their study of three fresh human
cadavers, Kuypers etal. found that the mean thickness values of the TM showed
incredible interindividual variation (40, 50, and 120μm). In any case, the authors
expressed that variations in thickness are common to all individuals [15].
Appropriately, they expressed that the inferior-posterior quadrant of the TM is
30
M. Baran et al.
a
b
c
Fig. 3.2 (a) Middle ear (tympanic cavity of guinea pig) magnication: 15×1.5×11.1. (b) Guinea
pig tympanic membrane with an annular structure; magnication: 15×1.5×11.4. (c) Morphology
of the tympanic membrane
largely thinner and becomes progressively thicker toward the superior part, with the
anterior part being thicker than the posterior part. In histological sections of the TM,
it has been suggested that the membrane may be a multilayered brous structure
containing different layers of varying thickness and density [16]. The TM is threelayered. Its innermost layer includes the inner mucosal epithelial layer, which contains an extremely thin layer of cells on its medial side, the outer epidermal layer,
which is the continuation of the external ear canal epidermis on the lateral side, and
the brous layer or lamina propria in the middle. The lamina propria of the pars
tensa contains two subepidermal layers and two collagen layers organized into
radial and circular bers; while radial bers extend from the TM toward the
3 Physiology ofthePeripheral andCentral Hearing System
31
periphery (converging at the manubrium), circular bers thicken and lengthen
toward the periphery [8]. In a study by O’Connor etal., as shown in Fig.3.2c, they
expressed that radial collagen bers play an important role in high-frequency
sounds, especially 4kHz and above, and circular bers play a critical role in lowfrequency responses [17]. The TM is attached to the arm of the malleus (manubrium) and the umbo but is more loosely attached to the manubrium. The ossicular
chain extends medial to the TM [18].
The primary ossicle consists of the malleus (anvil), head, neck, manubrium, and
umbo. The head extends toward the epitympanum and articulates with the body of
the incus. The anterior projection of the neck is the point of attachment of the tensor
tympani muscle. The incus is the median bone and has short and long arms and a
lenticular process. The lenticular process may be a attened bone that connects to
the head of the stapes [12]. The stapes is the smallest bone in the body. It consists of
a head, two legs (anterior and posterior crus), and a base called the footplate, which
is connected to the oval window by the annular ligament. Thus, the ossicular chain
connects the outer ear to the inner ear directly through the TM [11].
3.4.3 Middle Ear Muscles
The middle ear muscles control the transmission of sound through the middle ear.
The ossicles are connected to two small, striated muscles. The rst is the tensor
tympani, which is 25-mm-long in adults and runs parallel to the Eustachian tube,
most of which lies within the bony canal. Its tendon is attached to the malleus, and
its innervation is the trigeminal nerve known as the fth cranial nerve. It reduces the
tension of the TM to high intensity sounds, making the inner ear less sensitive to
sound. The second is the stapes muscle and is located in a bone canal, and its tendon
is attached to the stapes head. Its length is 6mm. It is innervated by the facial nerve,
known as the seventh cranial nerve. Its contraction moves the base of the stapes
away from the oval window, preventing loud sounds from reaching the inner ear [9,
11, 12]. Contraction of the middle ear muscles increases the stiffness of the ossicu-
lar chain. Sound transmission through the middle ear below 1 and 2kHz is provided
by the stiffness of the ossicular chain. Stiffness also results from the compression
and expansion of air in the middle ear as well as the elasticity of the TM and the
ligaments of the ossicular chain. Stiffness decreases with transmission of lowfrequency sounds. In other words, as the stiffness of the ossicular chain increases,
the response to low-frequency sounds decreases. Above 1 and 2kHz, the middle ear
muscles are less effective in transmitting sound and transmission does not occur
with ossicular chain stiffness [9]. Middle ear muscle contraction may occur as a
reex (stapes or tympanic reex) in response to high-intensity sounds (75dB and
above the hearing threshold), or this response may also occur during speech, tactile
stimulation of the head, swallowing, and general body movements. Many functions
of the middle ear muscles have been proposed [1, 9, 11].
32
M. Baran et al.
– The reex contraction of the middle ear muscles at high-intensity sounds pro-
tects the inner ear from acoustic trauma caused by noise.
– For high-intensity, low-frequency stimuli, the middle ear muscles keep the high-
intensity sound constant in the lower part of the intensity range, providing near-
perfect automatic amplication of low-frequency responses as the sound enters
the cochlea. In other words, to protect the inner ear from high-intensity stimuli,
the contraction of the stapes muscle provides a 10–20-dB gain reduction for
80–90-dB stimuli. The maximum contraction of the stapes muscle occurs on
average 63ms after the stimulus reaches the ear. It is suggested that the tensor
tympani muscle protects the inner ear by limiting the movement of the ossicular
chain with its contractile motion against loud sounds.
– The middle ear muscles also affect the frequency response of the middle ear.
Muscle contraction varies with frequency.
– High-intensity, low-frequency stimulation can mask the high-frequency stimula-
tion response over a wide frequency range by contracting the middle ear muscles.
3.4.4 The Eustachian Tube
The Eustachian tube was rst described in detail by Bartolomeo Eustachius in
1552. However, the real discoverer was a scientist named Alcmaeon in 1500BC.He
studied the Eustachian tube, middle ear, and pharynx and stated that the tensor tympani and the ossicular chain play a critical role in the sound transmission mechanism [18]. The middle ear consists of a cavity within the bones of the head. It is
closed medially by the TM and laterally by the oval window. In order for the TM to
achieve some exibility during sound transmission, pressure must be generated by
lling and emptying the middle ear with air. This function is performed by the
Eustachian tube [1]. The Eustachian tube is a canal that connects the middle ear to
the nasopharynx (Fig.3.3). It is 35–40-mm-long. Two-thirds of its anteromedial
part is cartilage (24mm), and one-third of its posterolateral part is bone (11mm).
The part called the isthmus is the narrowest part of the Eustachian tube. Although it
is horizontal at birth, it has an angle of 450in adults. In infants, it is about 18mm
and more horizontal. The tensor and levator veli palatini are the muscles of the
Eustachian tube. The tensor veli palatini is the muscle that plays an active role in
opening the tube. This muscle is made up of white bers that are strong and fast
twitching. These bers make up 60% of the muscle, while the remaining 40% is
made up of red, slow, long-twitch bers. The levator veli palatini muscle does not
have a direct effect on the opening and closing of the Eustachian tube because it is
located at a considerable distance from the tube. In this muscle, white muscle bers
and slow and long twitch red bers are found equally. In some people, the Eustachian
tube is constantly closed, which is pathological. In this case, movements of the TM
can be observed with an otoscope with every breath [1, 12, 19]. When the Eustachian
tube is open, the air in the middle ear and the air in front of the TM are equalized,
making the TM receptive to incoming stimuli. At high altitudes, the external environmental pressure, and therefore the external ear pressure, decreases. In this case,