# 25 EAR

**OVERVIEW OF THE EAR**

**EXTERNAL EAR**

**MIDDLE EAR**

**INTERNAL EAR**


Structures of the Bony Labyrinth


Structures of the Membranous Labyrinth
Sound Perception

Innervation of the Internal Ear

Blood Vessels of the Membranous Labyrinth

**Folder 25.1** Clinical Correlation: Otosclerosis

**Folder 25.2** Clinical Correlation: Hearing Loss—Vestibular

Dysfunction
**Folder 25.3** Clinical Correlation: Vertigo


**HISTOLOGY**

### **OVERVIEW OF THE EAR**


The **ear** is a three-chambered sensory organ that functions as an **auditory**
**system** for sound perception and as a **vestibular system** for balance.
Each of the three divisions of the ear—the **external ear**, **middle ear**, and
**internal ear** —is an essential part of both systems (Fig. 25.1). The external
and middle ears collect and conduct acoustic energy to the internal ear,
where auditory sensory receptors convert that energy into electrical
impulses. The sensory receptors of the vestibular system respond to gravity
and movement of the head. They are responsible for the sense of balance
and equilibrium and help coordinate movements of the head and eyes.


**FIGURE 25.1.** **Three divisions of the ear.** The three divisions of the ear are
represented by different colors and consist of the external ear (auricle and
external acoustic meatus; _pink_ ), the middle ear (tympanic cavity, auditory
ossicles, tympanic membrane, and auditory tube; _green_ ), and the internal ear
containing the bony labyrinth (semicircular canals, vestibule, and cochlea;
_blue_ ) and the membranous labyrinth (not visible).


**The ear develops from surface ectoderm and components of the**
**first and second pharyngeal arches.**


The internal ear is the first of the three ear divisions to begin development.
At the end of the third week, a thickening of **surface ectoderm** that
appears on each side of the myelencephalon develops into the **otic placode** .
Early in the fourth week, the otic placode invaginates and then pinches off
to form the **otic vesicle (otocyst)**, which sinks deep to the surface
ectoderm into the underlying mesenchyme (Fig. 25.2). The otic vesicle
serves as a primordium for the development of the epithelia that line the
membranous labyrinth of the internal ear. Later, development of the first and
part of the second pharyngeal arch provides structures that augment hearing.
The endodermal component of the **first pouch** gives rise to the
**tubotympanic recess**, which ultimately develops into the **auditory tube**
**(Eustachian tube)** and the **middle ear** and its epithelial lining. The
corresponding ectodermal outgrowth of the **first pharyngeal groove** gives
rise to the **external acoustic meatus** and its epithelial lining (see Fig.


25.2). The connective tissue part of the pharyngeal arches produces the
ossicles (“ear bones”). The **malleus** and **incus** develop from the first
pharyngeal arch and the **stapes** from the second pharyngeal arch. The
sensory epithelia of the membranous labyrinth that originates from the otic
vesicle link with cranial nerve VIII, which is an outgrowth of the central
nervous system. The auricle of the external ear develops from six **auricular**
**hillocks** located at the dorsal ends of the first and second pharyngeal arches
surrounding the first pharyngeal cleft. The cartilaginous, bony, and muscular
structures of the ear develop from the mesenchyme surrounding these early
epithelia.


**FIGURE 25.2.** **Schematic drawings showing development of the ear. a.**
This drawing shows the relationship of the surface ectoderm–derived otic
vesicle to the first pharyngeal arch during the fourth week of development. **b.**
The otic vesicle sinks deep into the mesenchymal tissue and develops into
the membranous labyrinth. Note the development of the tubotympanic recess
lined by endoderm into the future middle ear cavity and auditory tube. In
addition, accumulation of mesenchyme from the first and second pharyngeal
arches gives rise to the auditory ossicles. **c.** At this later stage of
development, the first pharyngeal groove grows toward the developing
tubotympanic recess. The auditory ossicles assume a location inside the
tympanic cavity. **d.** This final stage of development shows how the tympanic


membrane develops from all three germ layers: surface ectoderm,
mesoderm, and endoderm. Note that the wall of the otic vesicle develops into
the membranous labyrinth.

### **EXTERNAL EAR**


**The auricle is the external component of the ear that collects and**
**amplifies sound.**


The **auricle (pinna)** is the oval appendage that projects from the lateral
surface of the head. The characteristic shape of the auricle is determined by
an internal supporting structure of elastic cartilage. Thin skin with hair
follicles, sweat glands, and sebaceous glands cover the auricle. The auricle
is considered a nearly vestigial structure in humans, compared with its
development and function in other animals. Nevertheless, it is essential in
collecting the sound and directing it into the external acoustic meatus.


**The external acoustic meatus conducts and amplifies sounds on**
**the way to the tympanic membrane.**


The **external acoustic meatus** is an air-filled tubular space that follows a
slightly S-shaped course for about 25 mm to the **tympanic membrane**
**(eardrum)** . Because of its length, the external acoustic meatus can amplify
sounds with frequencies of 2,000–5,000 Hz. By passively conducting
sounds at this frequency and acting as a **resonator**, the external acoustic
meatus increases the sound pressure at the tympanic membrane by
approximately a **factor of 2** .

The wall of the meatus is continuous externally with the auricle. The
wall of the lateral one-third of the meatus is cartilaginous and is continuous
with the elastic cartilage of the auricle. The medial two-thirds of the meatus
are contained within the temporal bone. Both parts of the meatus are lined
by skin, which is also continuous with that of the auricle.

The skin in the lateral part of the meatus contains hair follicles,
sebaceous glands, and **ceruminous glands**, but no eccrine sweat glands.
The coiled tubular ceruminous glands closely resemble the apocrine glands
found in the axillary region. Their secretion mixes with that of the
sebaceous glands and desquamated cells to form **cerumen**, or **earwax** .
Because the external acoustic meatus is the only blind pouch of the skin in
the body, the earwax provides the means to evacuate desquamating cells
from the stratum corneum, thus preventing their accumulation in the meatus.
The **cerumen** lubricates the skin and coats the meatal hairs to
impede the entry of foreign particles into the ear. It also provides
some antimicrobial protection from bacteria, fungi, and insects.


Excessive accumulation of cerumen ( **impacted cerumen** ) can plug
the meatus, resulting in **conductive hearing loss** . The medial part of
the meatus located within the temporal bone has thinner skin and
fewer hairs and glands.

### **MIDDLE EAR**


**The middle ear is an air-filled space that contains three small**
**bones, the ossicles.**


The **middle ear** is located in an air-filled space, called the **tympanic**
**cavity**, within the temporal bone (Fig. 25.3). It is spanned by three small
bones, the **auditory ossicles**, which are connected by two movable joints.
The middle ear also contains the **auditory tube (Eustachian tube)**, which
opens to the nasopharynx as well as the muscles that attach to the ossicles.


**FIGURE 25.3.** **Horizontal section of a human temporal bone.** The
relationships of the three divisions of the ear within the petrous part of the
temporal bone are shown. Note the orientation icon that shows the plane of
section. The _tympanic membrane_ separates the _external acoustic meatus_
from the tympanic cavity. Within the tympanic cavity, sections of the malleus


( _M_ ) and incus ( _I_ ) can be seen. The posterior wall of the tympanic cavity is
associated with the mastoid air cells ( _AC_ ). The lateral wall of the cavity is
formed principally by the tympanic membrane. The opening to the internal
ear or oval window ( _OW_ ) is seen in the medial wall of the cavity (the stapes
has been removed). The facial nerve ( _F_ ) can be observed near the oval
window. The _cochlea_, vestibule, and a portion of the lateral semicircular
canal ( _LSC_ ) of the bony labyrinth are identified. The _cochlear_ and _vestibular_
_nerves_ are divisions of cranial nerve VIII and can also be observed within the
_internal acoustic meatus_ . _Inset_ in the upper left of the photomicrograph
shows the plane of the section through the bony labyrinth. ×65.


The tympanic cavity has a roof, floor, and four walls: anterior, posterior,
lateral, and medial. The tympanic cavity contains an opening of the auditory
tube and is bound anteriorly by a thin layer of bone that separates it from the
internal carotid artery. The posterior wall of the tympanic cavity is formed
by the spongy bone of the **mastoid process**, which contains the **mastoid**
**antrum** and other, smaller, air-filled spaces called **mastoid air cells** . The
middle ear is bound laterally by the **tympanic membrane** and medially by
the bony wall of the internal ear. The floor and roof of the tympanic cavity
are both formed by a thin layer of bone, which separates them from the
internal jugular vein and middle cranial fossa, respectively.

The middle ear is a mechanical energy transformer. Its primary function
is to convert sound waves (air vibrations) arriving from the external acoustic
meatus into mechanical vibrations that are transmitted to the internal ear.
Two openings in the medial wall of the middle ear, the **oval (vestibular)**
**window** and the **round (cochlear) window**, are essential components in
this conversion process.


**The tympanic membrane separates the external acoustic meatus**
**from the middle ear.**


The intact **tympanic membrane** is a semitransparent, thin (about 0.1 mm)
membrane approximately 1 cm in diameter that has an average surface area
in humans of about 65 mm [2] . It is shaped like an irregular flat cone, the apex
of which is located at the **umbo** that corresponds to the tip of the
manubrium of the malleus. The tympanic membrane at the end of the
external acoustic meatus is tilted anteriorly and inferiorly. Thus, orientation
of the tympanic membrane has been compared to the position of a miniature
satellite dish tuned to receive signals coming from the ground in front of the
body and to the side of the head. During otoscopic examination of a normal
ear, the tympanic membrane is a semitransparent light gray color and has a
visible concavity toward the external acoustic meatus. Owing to its
concavity, light from the otoscope reflects off the tympanic membrane as a


triangular **cone of light** (light reflex) that radiates anteriorly and inferiorly
from the umbo (Fig. 25.4). The malleus is one of three small auditory
ossicles residing in the middle ear and is the only one that attaches to the
tympanic membrane (see Fig. 25.1).


**FIGURE 25.4.** **The tympanic membrane in otoscopic examination of the**
**external ear.** This diagram and photograph show the left tympanic
membrane seen with an otoscope during examination of the external
acoustic meatus. The landmarks of the tympanic membrane include the
manubrium of the malleus with its visible attachment to the tense part of the
membrane, umbo at the tip of the manubrium, and projecting lateral process
of the malleus. A small, flaccid part of the tympanic membrane is located
above the lateral process of the malleolus. Note the cone of light (light reflex)
that is usually seen extending anteroinferiorly from the umbo of the tympanic
membrane. (Courtesy of Dr. Eric J. Moore.)


The **tympanic membrane** forms the medial boundary of the external
acoustic meatus and the lateral wall of the middle ear (Fig. 25.5). From
outside to inside, the three layers of the tympanic membrane are


**FIGURE 25.5.** **Cross section through a human tympanic membrane.** This
photomicrograph shows the _tympanic membrane_, _external acoustic meatus_,
and _tympanic cavity_ . ×9. **Inset.** Higher magnification of the tympanic
membrane. The outer epithelial layer of the membrane consists of stratified
squamous keratinized epithelium ( _SSE_ ), and the inner epithelial layer of the
mucous membrane consists of low simple cuboidal epithelium ( _SCE_ ). A
middle layer of connective tissue core ( _CTC_ ) lies between the two epithelial
layers. The dense irregular connective tissue core is formed by two layers:
the outer layer in which fibers are radially arranged ( _rad_ ) and the inner layer
with circumferentially arranged fibers ( _cir_ ). ×190.


the skin of the external acoustic meatus (epidermis composed of stratified
squamous keratinized epithelium)
a core of connective tissue with an outer layer of radially and inner layer
of circularly arranged collagen fibers, and
the mucous membrane of the middle ear (composed of simple cuboidal
epithelium).


The larger, lower part of the tympanic membrane ( **tense part** or **pars**
**tensa** ) is tightly stretched and has a thick middle core that contains radial
and circular collagen fibers and gives the membrane its shape and smooth
appearance. The smaller, upper part of the tympanic membrane that lies
superior to the lateral process of the malleolus is loose ( **flaccid part** or
**pars flaccida** ) and lacks a prominent middle fibrous layer (see Fig. 25.4).
Sound waves cause the tympanic membrane to vibrate, and these vibrations


are transmitted through the ossicular chain of three small bones that link the
external ear to the internal ear.

**Tympanic membrane perforations** are caused by a rupture in
the tympanic membrane that creates a connection between the
external auditory meatus and the middle ear. This rapture can be
attributed to infections, mechanical injury, or rapid changes in
pressure, leading to sudden ear pain (otalgia), ear discharge
(otorrhea), ringing in the ears (tinnitus), and a sensation of feeling offbalance (vertigo). Most perforations resolve spontaneously without
complications; however, some may cause transient or permanent

.
**hearing impairment**


**The auditory ossicles connect the tympanic membrane to the oval**
**window.**


The **three auditory ossicles** or bones—the malleus, the incus, and the
stapes—cross the space of the middle ear in series and connect the tympanic
membrane to the oval window (Fig. 25.6). These bones work like a lever
system that increases the force transmitted from the vibrating tympanic
membrane to the stapes by decreasing the ratio of their oscillation
amplitudes. The ossicles help convert sound waves to mechanical vibrations
(hydraulic waves) in tissues and fluid-filled chambers. Movable synovial
joints connect the bones, which are named according to their approximate
shape:


**FIGURE 25.6.** **Photograph of the three articulated human auditory**
**ossicles** . The three ossicles are the malleus, the incus, and the stapes. ×30.


The **malleus (hammer)** attaches to the tympanic membrane and
articulates with the incus.


The **incus (anvil)** is the largest of the ossicles and links the malleus to
the stapes.
The **stapes (stirrup)**, the footplate of which fits into the oval window.
The footplate in the human stapes measures approximately 3 mm × 1 mm
and has an average surface area of 3 mm [2] . It acts like a small piston on
the cochlear fluid, creating hydraulic waves to represent the air-pressure
fluctuations of the sound wave.


Diseases that affect the external acoustic meatus, tympanic
membrane, or ossicles are responsible for **conductive hearing loss**
(see Folders 25.1 and 25.2).





**FOLDER 25.2**


**Two muscles attach to the ossicles and affect their movement.**


The **tensor tympani muscle** lies in a bony canal above the auditory tube;
its tendon inserts on the malleus. Contraction of this muscle increases
tension on the tympanic membrane. The **stapedius muscle** lies in a bony
eminence on the posterior wall of the middle ear; its tendon inserts on the
stapes. Contraction of the stapedius tends to dampen the movement of the
stapes at the oval window. The stapedius is only a few millimeters long and
is the smallest skeletal muscle.

The two muscles of the middle ear are responsible for a protective reflex
called the **attenuation reflex** or **acoustic reflex** . In response to intense
sound, involuntary contraction of the muscles makes the chain of ossicles
more rigid, thus reducing the transmission of vibrations to the internal ear.
The muscles will contract on both sides, regardless of which ear is
stimulated. This reflex protects the internal ear from the damaging effects of
very loud sounds. In certain conditions, such as impulse noise (i.e.,
fireworks or gun fire), the attenuation reflex is ineffective.


**The auditory tube connects the middle ear to the nasopharynx.**


The **auditory (Eustachian) tube** is a narrow flattened channel
approximately 3.5 cm long. This tube is lined with ciliated pseudostratified
columnar epithelium, about one-fifth of which is composed of goblet cells.
It vents the middle ear to nasopharynx, equalizing the pressure of the middle
ear with atmospheric pressure. In addition, the auditory tube is responsible
for draining the secretion produced by the mucous membrane of the middle
ear towards the nasopharynx with the aid of the ciliated pseudostratified
columnar epithelium. The auditory tube is normally closed; its walls are
pressed together but they separate during yawning, chewing,
swallowing, and when individual holds the nose and blows. Children
are more vulnerable to the middle ear infections, due to the immature
development of their auditory tubes which are shorter, narrower, and
more horizontal than in the adults. It is common for infections to
spread from the pharynx to the middle ear via the auditory tube
(causing **otitis media** ). A small mass of lymphatic tissue, the **tubal**
**tonsil**, is often found at the pharyngeal orifice of the auditory tube.


**The mastoid air cells extend from the middle ear into the temporal**
**bone.**


A system of **air cells** projects into the mastoid portion of the temporal bone
from the middle ear. The epithelial lining of these air cells is continuous
with that of the tympanic cavity and rests on periosteum. This continuity
allows infections in the middle ear to spread into **mastoid air cells**,


causing **mastoiditis** . Before the development of antibiotics, repeated
episodes of otitis media and mastoiditis usually led to deafness.


**The middle ear contributes to the amplification of mechanical**
**forces generated by the vibration of the tympanic membrane.**


All three ossicles in the tympanic cavity are involved in the **amplification**
**of the mechanical force** that vibrates the tympanic membrane in two

ways:


The main amplification comes from **differences in the surface area**
between the tympanic membrane and the footplate of the stapes. The
**tympanic membrane** has a surface area of approximately **65 mm** **[2]**,
whereas the footplate of the **stapes** has a surface area of about **3 mm** **[2]** .
Sound waves apply force to every square millimeter of the tympanic
membrane, and this energy is transferred via the chain of ossicles to the
much smaller area of the footplate. Therefore, the pressure applied to the
cochlear fluid by the footplate is **about 22 times** the pressure applied to
the tympanic membrane (Fig. 25.7).


**FIGURE 25.7.** **Summary of amplification of sound entering the ear.**
This drawing shows the external and middle ear structures and their
contributions to the amplification of sound entering the ear. Note that the
largest amplification comes from the difference in the surface area


between the tympanic membrane (65 mm [2] ) and the footplate of the stapes
(3 mm [2] ). This surface area difference results in ~22 times the amplification
of the pressure applied by the footplate of the stapes. Another source of
amplification comes from the external acoustic meatus and middle ear.
The external acoustic meatus acts as a resonator that increases the sound
pressure acting on the tympanic membrane by _2 times. Finally, the_
_arrangement of auditory ossicles resembles a basic lever that multiplies_
_the applied mechanical force acting on the footplate of the stapes by_ 1.3
times. By multiplying these three amplification factors, the acoustic energy
entering the ear is amplified ~60 times.


Additional amplification comes from the arrangement of **auditory**
**ossicles** that act as **levers** that multiply the mechanical force applied to
the stapes. Because the pivot point of the ossicle chain is located farther
from the tympanic membrane than from the stapes, the amplification of
the mechanical force at the oval window is increased by a factor of
**approximately 1.3** . This lever system is adjustable by the action of
muscles in the tympanic cavity and may attenuate loud sounds to protect
the ear (see Fig. 25.7).


Under normal conditions, the acoustic energy entering the ear is
amplified **approximately 60 times**, allowing humans to detect frequencies
between 2,000 and 5,000 Hz. The degree of amplification is calculated by
multiplying the amplification factors contributed by the external acoustic
meatus (~2 times) as described earlier on pages 1018-1019, the surface area
differences between the tympanic membrane and the footplate of stapes
(~22 times), and the basic lever action of ossicles (~1.3 times). However,
this calculation (2 × 22 × 1.3 = 57.2) must be used with caution owing to
variability in the mechanical function of the middle ear and its components,
such as ossicular joints, ligaments, muscles, and air volumes, as well as
varying frequencies of sound.

### **INTERNAL EAR**


**The internal ear consists of two labyrinthine compartments, one**
**contained within the other.**


The **bony labyrinth** is a complex system of interconnected cavities and
canals in the petrous part of the temporal bone. The **membranous**
**labyrinth** lies within the bony labyrinth and consists of a complex system of
small sacs and tubules that also form a continuous space enclosed within a
wall of epithelium and connective tissue.


There are three fluid-filled spaces in the internal ear:


**Endolymphatic spaces** are contained within the membranous labyrinth.
The **endolymph** of the membranous labyrinth is similar in composition
to **intracellular fluid** (it has a high K [+] concentration and a low Na [+]
concentration). The endolymph is produced in the stria vascularis, a
specialized area of the cochlear duct (see pages 1032-1034). It drains via
the endolymphatic duct to the endolymphatic sac, which terminates in the
epidural space of the posterior cranial fossa.
The **perilymphatic space** lies between the wall of the bony labyrinth
and the wall of the membranous labyrinth. The **perilymph** is similar in
composition to **extracellular fluid** and **cerebrospinal fluid** (it has a low
K [+] concentration and a high Na [+] concentration). Perilymph is produced as
an ultrafiltrate from the periosteal microvasculature within the bony
labyrinth. It drains via a narrow channel within the temporal bone, called
the _cochlear aqueduct_, directly into the cerebrospinal fluid contained
within the subarachnoid space of the cranial cavity.
The **cortilymphatic space** lies within the tunnels of the organ of Corti
of the cochlea. It is a true intercellular space. The cells surrounding the
space loosely resemble an absorptive epithelium. The cortilymphatic
space is filled with **cortilymph**, which has a composition similar to that
of **extracellular fluid** .

### **Structures of the Bony Labyrinth**


**The bony labyrinth consists of three connected spaces within the**
**temporal bone.**


The three spaces of the bony labyrinth, as illustrated in Figure 25.8, are the


**FIGURE 25.8.** **Photograph of a cast of the bony labyrinth of the internal**
**ear.** The cochlear portion of the bony labyrinth appears _blue green_ ; the
vestibule and semicircular canals appear _orange red_ . (Courtesy of Dr. Merle
Lawrence.)


**semicircular canals**,
**vestibule**, and
**cochlea** .


**The vestibule is the central space that contains the utricle and**
**saccule of the membranous labyrinth.**


The **vestibule** is the small oval chamber located in the center of the bony
labyrinth. The **utricle** and **saccule** of the membranous labyrinth lie in
elliptical and spherical recesses, respectively. The **semicircular canals**
extend from the vestibule posteriorly, and the **cochlea** extends from the
vestibule anteriorly. The oval window into which the footplate of the stapes
inserts lies in the lateral wall of the vestibule.


**The semicircular canals are tubes within the temporal bone that lie**
**at right angles to each other.**


**Three semicircular canals**, each forming about three-quarters of a circle,
extend from the wall of the vestibule and return to it. The semicircular


canals are identified as anterior, posterior, and lateral and lie within the
temporal bone at approximately right angles to each other. They occupy
three planes in space—sagittal, frontal, and horizontal. The end of each
semicircular canal closest to the vestibule is expanded to form the **ampulla**
(Fig. 25.9a and b). The three canals open into the vestibule through five
orifices; the anterior and posterior semicircular canals join at one end to
form the **common bony limb** (see Fig. 25.9a).


**FIGURE 25.9.** **Diagrams and photograph of the human internal ear. a.**
This lateral view of the left bony labyrinth shows its divisions: the vestibule,
cochlea, and three semicircular canals. The openings of the oval window and
the round window can be observed. **b.** This photograph of a cast obtained by
injection of polyester resin into the human internal ear shows an authentic
shape of the bony labyrinth. Note that the cast material is pouring out of the
cochlea through the oval and round windows. Also, in this image, the cast of
the facial canal that contains the facial nerve is visible. ×5. (Courtesy of Dr.
Elsa Erixon.) **c.** Diagram of a membranous labyrinth of the internal ear lying
within the bony labyrinth. The cochlear duct can be seen spiraling within the
bony cochlea. The saccule and utricle are positioned within the vestibule,
and the three semicircular ducts are lying within their respective canals. This
view of the left membranous labyrinth allows the endolymphatic duct and sac
to be observed. **d.** This view of the left membranous labyrinth shows the
sensory regions of the internal ear for equilibrium and hearing. These regions
are the macula of the saccule and macula of the utricle, the cristae


ampullares of the three semicircular ducts, and the spiral organ of Corti of
the cochlear duct.


**The cochlea is a cone-shaped helix connected to the vestibule.**


The spiral lumen of the **cochlea**, called the **cochlear canal** (like the
semicircular canals), is continuous with that of the vestibule. It connects to
the vestibule via two openings, the **round window** and the **oval window**,
both of which are located on the side opposite the openings of the
semicircular canals. Between its base and the apex, the cochlear canal makes
approximately 2.75 turns around a central core of spongy bone called the
**modiolus** (Plate 25.1, page 1042). A sensory ganglion, the **spiral**
**ganglion**, lies in the modiolus. A thin membrane (the secondary tympanic
membrane) covers the round window, whereas the footplate of the stapes is
positioned within the oval window. These two openings are located at the
base of the cochlear canal.

### **Structures of the Membranous Labyrinth**


**The membranous labyrinth contains the endolymph and is**
**suspended within the bony labyrinth.**


The **membranous labyrinth** consists of a series of communicating sacs
and ducts containing endolymph. It is suspended within the bony labyrinth
(Fig. 25.9c), and the remaining space is filled with perilymph. The
membranous labyrinth is composed of two divisions: the **cochlear**
**labyrinth** and the **vestibular labyrinth** (Fig. 25.9d).

The vestibular labyrinth contains the following:


Three **semicircular ducts** lie within the semicircular canals and are

continuous with the utricle.

The **utricle** and the **saccule**, which are contained in recesses in the
vestibule, are connected by the membranous **utriculosaccular duct** .


The cochlear labyrinth contains the **cochlear duct**, which is contained
within the cochlea and is continuous with the saccule (see Fig. 25.9c and d).

###### **Sensory cells of the membranous labyrinth**


**Specialized sensory cells are located in six regions in the**
**membranous labyrinth.**


Six sensory regions of membranous labyrinth are composed of sensory **hair**
**cells** and accessory **supporting cells** . These regions project from the wall


of the membranous labyrinth into the endolymphatic space in each internal
ear (see Fig. 25.9d):


Three **cristae ampullares (ampullary crests)** are located in the
membranous ampullae of the semicircular ducts. They are sensitive to the
angular acceleration of the head (i.e., turning the head).
Two maculae, one in the utricle ( **macula of utricle** ) and the other in the
saccule ( **macula of saccule** ), sense the position of the head and its
linear movement.
The **spiral organ of Corti** projects into the endolymph of the cochlear
duct. It functions as a sound receptor.


**Hair cells are epithelial mechanoreceptors of the vestibular and**
**cochlear labyrinth.**


The **hair cells** of the vestibular and cochlear labyrinths function as
**mechanoelectrical transducers** ; they convert mechanical energy into
electrical energy that is then transmitted via the vestibulocochlear nerve to
the brain. The hair cells derive their name from the organized bundle of
rigid projections at their apical surface. This surface holds a **hair bundle**
that is formed by rows of stereocilia called _sensory hairs_ . The rows increase
in height in one particular direction across the bundle (Fig. 25.10). In the
vestibular system, each hair cell possesses a single true cilium called a
**kinocilium**, which is located behind the row of longest stereocilia (Fig.
25.11). In the auditory system, the hair cells lose their cilium during
development but retain the **basal body** . The position of the kinocilium (or
basal body) behind the longest row of stereocilia defines the polarity of this
asymmetric hair bundle. Therefore, movement of the stereocilia toward the
kinocilium is perceived differently than movement in the opposite direction
(see later).


**FIGURE 25.10.** **Electron micrographs of the kinocilium and stereocilia**
**of a vestibular sensory hair cell. a.** Scanning electron micrograph of the
apical surface of a sensory hair cell from the macula of the utricle. Note the
relationship of the kinocilium ( _K_ ) to the stereocilia ( _S_ ). ×47,500. **b.**
Transmission electron micrograph of the kinocilium ( _K_ ) and stereocilia ( _S_ ) of
a vestibular hair cell in cross section. The kinocilium has a larger diameter
than the stereocilia. ×47,500. ( **a.** Reprinted with permission from Rzadzinska
AK, Schneider ME, Davies C, _et al._ An actin molecular treadmill and myosins
maintain stereocilia functional architecture and self-renewal. _J Cell Biol_ .
2004;164:887–897. **b.** Reprinted with permission from Hunter-Duvar IM,
Hinojosa R. Vestibule: sensory epithelia. In: Friedmann I, Ballantyne J, eds.
_Ultrastructural Atlas of the Inner Ear_ . Butterworth; 1984.)


**FIGURE 25.11.** **Diagram of two types of sensory hair cells in the**
**sensory areas of the membranous labyrinth.** The type I hair cell has a
flask-shaped structure with a rounded base. The base is enclosed in a
chalice-like afferent nerve ending containing several ribbon synapses in
addition to several synaptic boutons for efferent nerve endings. Note the
apical surface specializations of this cell, which include a kinocilium and hair
bundle. The apical cytoplasm of hair cells contains basal bodies for the
attachment of the kinocilium and a terminal web for the attachment of
stereocilia. The type II hair cell is cylindrical and possesses several nerve
terminals at its base for both afferent and efferent nerve fibers. The apical
surface specializations are identical to those of the type I cell. The molecular
organization of the stereocilia is depicted in the diagram on the _right_ . The top
link connects the lateral plasma membrane of the stereocilium shaft (where
K [+] transduction channels are located) with the tip of the shorter stereocilium
(where the mechanoelectrical transduction [ _MET_ ] channel protein is located).
Movement of the stereocilia toward the kinocilium opens the _MET_ channels,
causing depolarization of the hair cell, whereas movement in the opposite
direction (away from the kinocilium) causes hyperpolarization. Note that the
proximal end of each stereocilium is tapered and its narrow rootlets are
anchored within the terminal web (cuticular plate) of the hair cell. Several
other fibrillar connectors between neighboring stereocilia are also shown.


**Stereocilia of hair cells are rigid structures that contain**
**mechanoelectrical transducer channel proteins at their distal ends.**


The **stereocilia of hair cells** have a molecular structure similar to those
described on pages 127-128. Tightly packed **actin filaments** cross-linked
by **fimbrin** and **espin** (actin-bundling proteins) form their internal core
structure. Espins provide the most rigid cross-linking for stereocilia;
mutations that alter their structure cause cochlear and vestibular
dysfunction. The high density of actin filaments and the extensive crosslinking pattern impart rigidity and stiffness to the shaft of the stereocilium.
The shaft tapers at its proximal end near the apical surface of the cell, where
the core filaments of each stereocilium are anchored within the terminal web
(cuticular plate). When stereocilia are deflected, they pivot at their proximal
ends like stiff rods (see Fig. 25.11).

Transmission electron microscope examination of the distal free end of
the stereocilium reveals an electron-dense plaque at the cytoplasmic site of
the plasma membrane. This plaque represents the **mechanoelectrical**
**transducer (MET) channel complex** . A fibrillar cross-link called the **tip**
**link** connects the tip of the stereocilium with the shaft of an adjacent longer
stereocilium (see Fig. 25.11). These tip links are anchored to mechanically
gated ion channels on both ends. The upper insertion of the tip link to the
shaft of neighboring stereocilium contains a cluster of motor proteins
(unconventional myosin VIIa) that maintains a resting tension on the tip
link. The lower insertion to the distal free end of the stereocilium is
connected to the MET channel complex. The tip link is composed of
**cadherin-23** (CDH23) and **protocadherin-15** (PCDH15); however, the
molecular composition of the MET channel complex remains elusive.
Recently, two transmembrane channel-like (TMC) proteins, TMC1
and TMC2, have been identified in the MET channels that are
expressed in developing hair cells. Mutations in the genes encoding
TMC1 cause **deafness** in humans.

The tip link plays an important role in activating the MET channel
complex at the tip of the stereocilia and opening additional transduction K [+]

channels at the site of its attachment to the shaft of neighboring stereocilium
(see Fig. 25.11). The molecular structure of the transduction K [+] channels is
unknown.

A mutation that disrupts the gene that encodes the actin-bundling
protein **espin** causes cochlear and vestibular symptoms in
experimental mice. They lose their hearing early in life; these animals
also spend most of their time walking or spinning in circles. The
stereocilia of these animals do not maintain the rigidity necessary for
the proper functioning of the **MET channels** . In humans, mutations in
a gene located on chromosome 1 that encodes espin are associated
with deafness without vestibular involvement.


**All hair cells use mechanically gated ion channels to generate**
**action potentials.**


All hair cells of the internal ear appear to function by moving (pivoting)
their rigid stereocilia. Mechanoelectrical transduction occurs in stereocilia
that are deflected toward its tallest edge (toward the kinocilium, if present).
This movement exerts tension on the fibrillar tip links, and the generated
force is used to open **mechanically gated ion channels** near the tip of the
stereocilium. This allows for an influx of K [+], causing depolarization of the
receptor cell. This depolarization results in the opening of voltage-gated
Ca [2+] channels in the basolateral surface of the hair cells and the secretion of
a neurotransmitter that generates an action potential in afferent nerve
endings. Movement in the opposite direction (away from the kinocilium)
closes the MET channels, causing hyperpolarization of the receptor cell. The
means by which stereocilia are deflected varies from receptor to receptor;
these are discussed in the sections describing each receptor area.


**Hair cells communicate with afferent nerve fibers through ribbon**
**synapses, a specialized type of chemical synapse.**


Deflection of the stereocilia on hair cells generates a high rate of prolonged
impulses that are quickly transmitted to the afferent nerve fibers. To ensure
rapid release of the glutamate neurotransmitter from synaptic vesicles, hair
cells possess specialized **ribbon synapses** that contain unique organelles
called **ribbons** . In electron microscopy, ribbons appear as ovoid, 30-nmthick, electron-dense plates that are anchored to the presynaptic membrane
by electron-dense structures (Fig. 25.12). This arrangement allows the
ribbons to float just above the presynaptic plate like balloons on a short
leash. The ribbons tether a large number of synaptic vesicles on their surface
that are primed for fusion with the presynaptic membrane, which contains a
high density of voltage-gated Ca [2+] channels (see Fig. 25.12). After
activation of the Ca [2+] channels, the ribbon serves as a fast-moving conveyor
belt, delivering the vesicles to the presynaptic membrane for fusion. The
tethered pool of synaptic vesicles is approximately fivefold greater than the
pool of the remaining vesicles. The ribbons contain several proteins,
including the active-zone protein RIM that interacts with rab3, a GTPase
enzyme expressed on the surface of synaptic vesicles. Other proteins of the
ribbon complex include presynaptic matrix proteins, such as RIBEYE,
Bassoon, and Piccolo. A hair cell typically contains about 10–20 ribbons.
These ribbon synapses are also found in the photoreceptors and bipolar cells
of the retina.


**FIGURE 25.12.** **Diagram and electron micrograph of a ribbon synapse in**
**a hair cell.** Diagram on the _left_ shows a type I hair cell with several ribbon
synapses that are specialized for transmitting long-lasting and high-volume
impulses to the afferent nerve cell endings ( _yellow_ ). **a.** This schematic view
of a ribbon synapse shows the ribbon protein complex that contains several
presynaptic matrix proteins (RIM, RIBEYE, and Piccolo) and is anchored into
the presynaptic plate by another protein called _Bassoon_ . The surface of the
ribbon serves as the tethering platform for multiple synaptic vesicles. Note
the presence of voltage-sensitive Ca [2+] channels in the presynaptic
membrane next to the attachment of the ribbon. Upon influx of Ca [2+], the
ribbon accelerates movement of the attached vesicles toward the
presynaptic membrane for fusion (similar to the action of a fast-moving
conveyor belt). **b.** This electron micrograph of a ribbon synapse from a
mouse cochlear hair cell shows the ribbon protein complex with attached
synaptic vesicles. ×27,400. (Reprinted with permission from Neef A, Khimich
D, Pirih P, _et al._ Probing the mechanism of exocytosis at the hair cell ribbon
synapse. _J Neurosci_ . 2007;27:12933–12944.)


**Two types of hair cells are present in the vestibular labyrinth.**


Both **hair cell** types are associated with **afferent** and **efferent nerve**
**endings** (see Fig. 25.11). **Type I hair cells** are flask shaped, with a
rounded base and thin neck, and are surrounded by an afferent nerve chalice
and a few efferent nerve fibers. **Type II hair cells** are cylindrical and have
afferent and efferent bouton nerve endings at the base of the cell (see Fig.
25.11).

###### **Sensory receptors of the membranous labyrinth**


**The crista ampullaris senses angular movements of the head.**


Each ampulla of the semicircular duct contains a **crista ampullaris**, which
is a sensory receptor for angular movements of the head (Figs. 25.13 and
25.14). The crista ampullaris is a thickened transverse epithelial ridge that is
oriented perpendicularly to the long axis of the semicircular canal and
consists of the epithelial hair cells and supporting cells (Plate 25.1, page
1042).


**FIGURE 25.13.** **Diagram of function and structure of the crista**
**ampullaris within a semicircular duct. a.** As shown in this drawing, the
crista ampullaris functions as the sensor for angular movement of the head.
For example, when the head of the individual shown in this diagram rotates
toward the left side, the bony labyrinth also rotates at the same speed
together with the head. However, the endolymph lags behind due to its own
fluid inertia. Because the crista ampullaris is attached to the wall of the bony
labyrinth, it will be swayed by the lagging endolymph in the opposite direction
to the movement of the head. **b.** The structure of the crista ampullaris
includes sensory epithelium and large cupula made of a gelatinous protein–


polysaccharide mass that projects toward the nonsensory wall of the
ampulla. Note that the membranous ampulla is filled with endolymph and is
surrounded by perilymph. **c.** The sensory epithelium of the crista ampullaris
is composed of both type I and type II hair cells and supporting cells. The
stereocilia and kinocilium of each hair cell are embedded in the cupula. Their
mechanical deflection opens the K [+] channels, causing depolarization of the
cell.


**FIGURE 25.14.** **Photomicrograph of the crista ampullaris and macula of**
**the utricle of the internal ear. a.** This low-magnification view of a horizontal
section of the temporal bone reveals several regions of the internal ear. The
prominent _cochlea_ contains a well-preserved cochlear duct with a _cochlear_
_nerve_ emerging from the base of the modiolus. Note the cross section of the
_stapedius muscle_ and _facial nerve_ . The central cavity of the slide represents
the vestibule that contains three parts of the membranous labyrinth: the
_utricle_, _saccule_, and _ampulla of the anterior semicircular canal_ . The locations
of sensory receptors (macula of utricle, macula of saccule, and crista
ampullaris) are enclosed within the rectangles. ×20. **b.** This highmagnification view of the crista ampullaris from the anterior semicircular
canal shows a thick _sensory epithelium_ that contains two types of cells: the
_hair cells_ in the upper layer and the _supporting cells_ in the basal layer. Note
that the sensory hair processes of the cells are barely discernable and are


covered by the _cupula_ . The underlying loose _connective tissue_ extends to the
wall of the bony labyrinth and contains nerve fibers with associated Schwann
cells, fibroblasts, capillaries, and other connective tissue cells. ×380. **c.** This
high-magnification view of the macula of the utricle shows _sensory epithelium_
similar to that of the crista ampullaris. The sensory epithelium is overlaid by
the _otolithic membrane_ containing a darker stained layer of _otoconia_ (otoliths)
on its surface. ×380. (Copyright 2010 Regents of the University of Michigan.
Reprinted with permission.)


A gelatinous protein–polysaccharide mass, known as the **cupula**, is
attached to the hair cells of each crista (see Fig. 25.13). The cupula projects
into the lumen and is surrounded by endolymph. During rotational
movement of the head, the walls of the semicircular canal and the
membranous semicircular ducts move, but the endolymph contained within
the ducts tends to lag behind because of inertia. The cupula, projecting into
the endolymph, is swayed by the movement differential between the crista
fixed to the wall of the duct and the endolymph. Deflection of the stereocilia
in the narrow space between the hair cells and the cupula generates nerve
impulses in the associated nerve endings.


**The maculae of the saccule and utricle are sensors of gravity and**
**linear acceleration.**


The **maculae** of the saccule and utricle are innervated sensory thickenings
of the epithelium that face the endolymph of the saccule and utricle (see
Figs. 25.14 and 25.15). As in the cristae, each macula consists of **type I** and
**type II hair cells**, supporting cells, and nerve endings associated with the
hair cells. The maculae of the utricle and saccule are oriented at right angles
to each another. When a person is standing, the macula of the utricle is in a
horizontal plane and the macula of the saccule is in a vertical plane.


**FIGURE 25.15.** **Diagram of function and structure of the macula within**
**the utricle. a.** As shown in this drawing, the macula of the utricle (as well as
macula of the saccule) functions as a sensor for gravity and linear
acceleration. For example, when the head of the individual shown in this
diagram is tilted forward, tiny crystals of calcium carbonate called _otoconia_
are shifted on the surface of the otolithic membrane. This movement is
detected by the underlying hair cells. **b.** The macula is composed of a
sensory epithelium containing both type I and type II hair cells. The hair cell
processes are embedded in the gelatinous polysaccharide otolithic
membrane. The luminal surface of the membrane is covered by otoconia that
are heavier than endolymph. **c.** As visible on the map below the macula, the
hair cells are polarized with respect to the striola, an imaginary plane that
curves through the center of each macula. Note that on each side of the
striola, the kinocilia of the hair cells are oriented in opposite directions facing
toward the striola (see direction of the _blue_ and _green arrows_ on the
polarization map of the utricle). This arrangement is only seen in the utricle
because in the macula of the saccule, the kinocilia of the hair cells are turned
away from the striola.


Hair cells are polarized with respect to the **striola**, an imaginary plane
that curves through the center of each macula (see Fig. 25.15). On each side
of the striola, the kinocilia of the hair cells are oriented in opposite
directions, facing toward the striola in the utricle and turning away from the
striola in the saccule. Owing to polarization of the hair cells, the maculae of
the saccule and utricle are sensitive to multiple directions of linear
accelerations.


The gelatinous polysaccharide material that overlies the maculae is
called the **otolithic membrane** (see Fig. 25.15). Its outer surface contains
3-to 5-μm crystalline bodies of calcium carbonate and a protein (Fig. 25.16).
**Otoliths**, also called **otoconia**, are heavier than endolymph. The outer
surface of the otolithic membrane lies opposite the surface in which the
stereocilia of the hair cells are embedded. The otolithic membrane moves on
the macula in a manner analogous to that by which the cupula moves on the
crista. Stereocilia of the hair cells are deflected by gravity in the stationary
individual when the otolithic membrane and its otoliths pull on the
stereocilia. They are also displaced during linear movement when the
individual is moving in a straight line and the otolithic membrane drags on
the stereocilia because of inertia. In both cases, movement of the otolithic
membrane causes the stereocilia to move toward the kinocilium, activating
MET channels. This depolarizes hair cells and generates an action potential.
Displacement of stereocilia in the opposite direction away from the
kinocilium causes hyperpolarization of hair cells and inhibits the generation
of the action potential.


**FIGURE 25.16.** **Scanning electron micrograph of human otoconia.** Each
otoconium has a long cylindrical body with a three-headed facet on each
end. ×5,000.


**The spiral organ of Corti is the sensor of sound vibrations.**


The **cochlear duct** divides the cochlear canal into three parallel
compartments or scalae:


**Scala media**, the middle compartment in the cochlear canal
**Scala vestibule**
**Scala tympani**


The **cochlear duct** itself is the **scala media** (Fig. 25.17). The scala
vestibuli and scala tympani are the spaces above and below, respectively, the
scala media. The scala media is an **endolymph-containing space** that is
continuous with the lumen of the saccule and contains the spiral organ of
Corti, which rests on its lower wall (see Fig. 25.17).


**FIGURE 25.17.** **Schematic diagram and photomicrograph of the**
**cochlear canal. a.** Cross section of the basal turn of the cochlear duct is


shown in the _box_ on the smaller orientation view. This view of a midmodiolar
section of the cochlea illustrates the position of the cochlear duct within the
2.75 turns of the bony cochlea. Observe that at the top of the cochlea, the
scala vestibuli and scala tympani communicate with each other at the
helicotrema. The scala media and the osseous spiral lamina divide the
cochlea into the scala vestibuli and the scala tympani, which are filled with
perilymph. The scala media (the space within the cochlear duct) is filled with
endolymph and contains the organ of Corti. **b.** This photomicrograph shows
a section of the basal turn of the cochlear canal. The osseous spiral lamina
( _OSL_ ) and its membranous continuation, the basilar membrane ( _BM_ ) as well
as the vestibular membrane ( _VM_ ) are visible. Note the location of the _scala_
_vestibuli_, the scala media ( _SM_ ) or cochlear duct, and the _scala tympani_ . The
three walls of the scala media are formed by the basilar membrane inferiorly,
the stria vascularis ( _SV_ ) and underlying spiral ligament ( _SL_ ) laterally, and the
vestibular membrane superiorly. The spiral organ of Corti resides on the
inferior wall of the cochlear duct. Dendrites of the cochlear nerve ( _CN_ ) that
originate in the spiral ganglion ( _SG_ ) enter the spiral organ of Corti. The axons
of the spiral ganglion cells form the cochlear part of the vestibulocochlear
nerve. ×65.


The **scala vestibuli** and the **scala tympani** are **perilymph-**
**containing spaces** that communicate with each other at the apex of the
cochlea through a small channel called the **helicotrema** (see Fig. 25.17b).
The scala vestibuli begins at the **oval window**, and the scala tympani ends
at the **round window** .


**The scala media is a triangular space with its acute angle attached**
**to the modiolus.**


In transverse section, the **scala media** appears as a triangular space with its
most acute angle attached to a bony extension of the modiolus, the **osseous**
**spiral lamina** (see Fig. 25.17). The upper wall of the scala media, which
separates it from the scala vestibuli, is the **vestibular (Reissner)**
**membrane** (Fig. 25.18). The lateral or outer wall of the scala media is
bordered by a unique epithelium, the **stria vascularis** . It is responsible for
the production and maintenance of endolymph. The stria vascularis encloses
a complex capillary network and contains three types of cells (Fig. 25.19).
The marginal cells, primarily involved in K [+] transport, line the
endolymphatic space of the scala media. Intermediate pigment-containing
cells are scattered among capillaries. The basal cells separate stria vascularis
from the underlying spiral ligament. The lower wall or floor of the scala
media is formed by a relatively flaccid **basilar membrane** that increases in
width and decreases in stiffness as it coils from the base to apex of the


cochlea. The spiral organ of Corti rests on the basilar membrane and is
overlain by the **tectorial membrane** .


**FIGURE 25.18.** **Transmission electron micrograph of the vestibular**
**(Reissner) membrane.** Two cell types can be observed: a mesothelial cell,
which faces the scala vestibuli and is bathed by perilymph, and an epithelial
cell, which faces the scala media and is bathed by endolymph. ×8,400.


**FIGURE 25.19.** **Transmission electron micrograph of the stria**
**vascularis.** The apical surfaces of the marginal cells ( _M_ ) of the stria are
bathed by endolymph ( _E_ ) of the scala media. Intermediate cells ( _I_ ) are
positioned between the marginal cells and the basal cells ( _B_ ). The basal cells
separate the other cells of the stria vascularis from the spiral ligament ( _SpL_ ).
×4,700.


**The spiral organ of Corti is composed of hair cells, phalangeal**
**cells, and pillar cells.**


The **spiral organ of Corti** is a complex epithelial layer on the floor of the
scala media (Fig. 25.20 and Plate 25.2, page 1044). It is formed by the
following:


**FIGURE 25.20.** **Photomicrograph of the vestibular duct and spiral organ**
**of Corti.** This higher magnification photomicrograph of the cochlear duct
shows the structure of the spiral organ of Corti. Relate this structure to the
_inset_, which labels the structural features of the spiral organ. ×180. **Inset.**
Diagram of the sensory and supporting cells of the spiral organ of Corti. The
sensory cells are divided into an inner row of sensory hair cells and three
rows of outer sensory hair cells. The supporting cells are the inner and outer
pillar cells, inner and outer (Deiters) phalangeal cells, outer border cells
(Hensen cells), inner border cells, Claudius cells, and Böttcher cells.


**Inner hair cells** (close to the spiral lamina) and **outer hair cells** (farther
from the spiral lamina)
**Inner phalangeal (supporting) cells** and **outer phalangeal cells**
**Pillar cells**


Several other named cell types of unknown function are also present in
the spiral organ.


**The hair cells are arranged in inner and outer rows of cells.**


The **inner hair cells** form a single row of cells throughout all 2.75 turns of
the cochlear duct. The number of cells forming the width of the continuous
row of **outer hair cells** is variable. Three ranks of hair cells are found in
the basal part of the coil (Fig. 25.21). The width of the row gradually
increases to five ranks of cells at the apex of the cochlea.


**FIGURE 25.21.** **Scanning electron micrograph of the spiral organ of**
**Corti.** This electron micrograph illustrates the configuration of stereocilia on
the apical surfaces of the inner row and three outer rows of the cochlear
sensory hair cells. ×3,250.


**The phalangeal and pillar cells provide support for the hair cells.**


**Phalangeal cells** are supporting cells for both rows of hair cells. The
phalangeal cells associated with the inner hair cells surround the cells
completely (Fig. 25.22a). The phalangeal cells associated with the outer hair
cells surround only the basal portion of the hair cell completely and send
apical processes toward the endolymphatic space (Fig. 25.22b). These
processes flatten near the apical ends of the hair cells and collectively form a
complete plate surrounding each hair cell (Fig. 25.23).


**FIGURE 25.22.** **Electron micrograph of an inner and outer hair cell. a.**
Observe the rounded base and constricted neck of the inner hair cell. Nerve
endings ( _NE_ ) from afferent nerve fibers ( _AF_ ) to the inner hair cells are seen
basally. _IP_, inner pillar cell; _IPH_, inner phalangeal cell. ×6,300. **b.** Afferent
( _AF_ ) and efferent ( _EF_ ) nerve fiber endings on the base of an outer sensory
hair cell are evident. Outer phalangeal cells ( _OPH_ ) surround the outer hair
cells basally. Their apical projections form the apical cuticular plate ( _ACP_ ).
Note that the lateral domains in the middle third of the outer hair cells are not
surrounded by supporting cells. ×6,300. (Reprinted with permission from
Kimura RS. Sensory and accessory epithelia of the cochlea. In: Friedmann I,
Ballantyne J, eds. _Ultrastructural Atlas of the Inner Ear_ . Butterworth; 1984.)


**FIGURE 25.23.** **Structure of the outer phalangeal cell. a.** This scanning
electron micrograph illustrates the architecture of the outer phalangeal
(Deiters) cells. Each phalangeal cell cups the basal surface of an outer hair
cell and extends its phalangeal process apically to form an apical cuticular
plate that supports the outer sensory hair cells. ×2,400. **b.** Schematic
drawing showing the relationship of an outer phalangeal cell to an outer hair
cell.


The apical ends of the phalangeal cells are tightly bound to one another
and to the hair cells by elaborate tight junctions. These junctions form the
**reticular lamina** that separates the endolymphatic compartment from the
true intercellular spaces of the organ of Corti (see Figs. 25.20 and 25.22b).
The extracellular fluid in this intercellular space is **cortilymph** . Its
composition is similar to that of other extracellular fluids and to perilymph.

**Pillar cells** have broad apical and basal surfaces that form plates and a
narrowed cytoplasm. The inner pillar cells rest on the tympanic lip of the
spiral lamina; the outer pillar cells rest on the basilar membrane. Between
them, they form a triangular tunnel, the **inner spiral tunnel** (see Fig.
25.20).


**The tectorial membrane extends from the spiral limbus over the**
**cells of the spiral organ of Corti.**


The **tectorial membrane** is attached medially to the modiolus. Its lateral
free edge projects over and attaches to the organ of Corti by the stereocilia
of the hair cells. It is formed from the radially oriented bundles of collagen
types II, V, and IX embedded in a dense amorphous ground substance.
Glycoproteins unique to the internal ear, called **otogelin** and **tectorin**, are
associated with the collagen bundles. These proteins are also present in the


otolithic membranes overlying the maculae of the utricle and saccule as well
as in the cupulae of the cristae in the semicircular canals.

### **Sound Perception**


As described on pages 1018-1019, sound waves striking the tympanic
membrane are translated into simple mechanical vibrations. The ossicles of
the middle ear convey these vibrations to the cochlea.


**In the internal ear, the vibrations of the ossicles are transformed**
**into waves in the perilymph.**


Movement of the stapes in the oval window of the vestibule sets up
vibrations or traveling waves in the perilymph of the scala vestibuli. The
vibrations are transmitted through the vestibular membrane to the scala
media (cochlear duct), which contains endolymph, and are also propagated
to the perilymph of the scala tympani. Pressure changes in this closed
perilymphatic–endolymphatic system are reflected in movements of the
membrane that covers the round window in the base of the cochlea.

As a result of **sound vibrations** entering the internal ear, a traveling
wave is set up in the basilar membrane (Fig. 25.24). A sound of a specified
frequency causes displacement of a relatively long segment of the basilar
membrane, but the region of maximal displacement is narrow. The point of
maximal displacement of the basilar membrane is specified for a given
frequency of sound and is the morphologic basis of frequency
discrimination. High-frequency sounds cause maximal vibration of the
basilar membrane near the base of the cochlea; low-frequency sounds cause
maximal displacement nearer the apex. Amplitude discrimination (i.e.,
perception of sound intensity or loudness) depends on the degree of
displacement of the basilar membrane at any given frequency range. Thus,
coding acoustic information into nerve impulses depends on the vibratory
pattern of the basilar membrane.


**FIGURE 25.24.** **Schematic diagram illustrating the dynamics of the three**
**divisions of the ear.** The cochlear duct is shown here as if straightened.
Sound waves are collected and transmitted from the external ear to the
middle ear, where they are converted into mechanical vibrations. The
mechanical vibrations are then converted at the oval window into fluid
vibrations within the internal ear. Fluid vibrations cause displacement of the
basilar membrane (traveling wave) on which rest the auditory sensory hair
cells. Such displacement leads to stimulation of the hair cells and a
discharge of neural impulses from them. Note that high-frequency sounds
cause vibrations of the narrow, thick portion of the basilar membrane at the
base of the cochlea, whereas low-frequency sounds displace basilar
membrane toward the apex of the cochlea near its helicotrema.


**Movement of the stereocilia of the hair cells in the cochlea initiates**

**neuronal transduction.**


**Hair cells** are attached through the **phalangeal cells** to the basilar
membrane, which vibrates during sound reception. The **stereocilia** of these
hair cells are, in turn, attached to the tectorial membrane, which also
vibrates. However, the **tectorial membrane** and the **basilar membrane**
are hinged at different points. Thus, a shearing effect occurs between the
basilar membrane (and the cells attached to it) and the tectorial membrane
when sound vibrations impinge on the internal ear.

Because they are inserted into the tectorial membrane, the stereocilia of
the hair cells are the only structures that connect the basilar membrane and
its complex epithelial layer to the tectorial membrane. The shearing effect
between the basilar membrane and the tectorial membrane deflects the
stereocilia and thus the apical portion of the hair cells. This deflection
activates **MET channels** located at the tips of stereocilia and generates
action potentials that are conveyed to the brain via the **cochlear nerve**
(cochlear division of the vestibulocochlear nerve, cranial nerve VIII).


### **Innervation of the Internal Ear**

**The vestibular nerve originates from the sensory receptors**
**associated with the vestibular labyrinth.**


The **vestibulocochlear nerve (cranial nerve VIII)** is a special sensory
nerve and is composed of two divisions: a vestibular division called the
_vestibular nerve_ and a cochlear division called the _cochlear nerve_ . The
**vestibular nerve** is associated with equilibrium and carries impulses from
the sensory receptors located within the vestibular labyrinth. The **cochlear**
**nerve** is associated with hearing and conveys impulses from the sensory
receptors within the cochlear labyrinth (Fig. 25.25).


**FIGURE 25.25.** **Diagram illustrating the innervation of the sensory**
**regions of the membranous labyrinth.** Note that cochlear and vestibular
nerves form the vestibulocochlear nerve (cranial nerve VIII). The cochlear
nerve carries the sound impulses from the spiral organ of Corti located within
the cochlear duct; the vestibular nerve carries balance information from the
three cristae ampullares of the semicircular canals, utricle, and saccule. The
cell bodies of these sensory fibers are located in the spiral ganglion (for
hearing) and vestibular ganglion (for equilibrium).


The cell bodies of the bipolar neurons of the **vestibular nerve** are
located in the **vestibular ganglion (of Scarpa)** in the internal acoustic


meatus. Dendritic processes of the vestibular ganglion cells originate in the
cristae ampullares of the three semicircular ducts, the macula of the utricle,
and the macula of the saccule. They synapse at the base of the vestibular
sensory hair cells, either as a chalice around a type I hair cell or as a bouton
associated with a type II hair cell. The axons of the vestibular nerve
originate from the vestibular ganglion, enter the brainstem, and terminate in
four vestibular nuclei. Some secondary neuronal fibers travel to the
cerebellum and to the nuclei of cranial nerves III, IV, and VI, which
innervate the muscles of the eye.


**The cochlear nerve originates from the sensory receptors of the**
**spiral organ of Corti.**


Neurons of the **cochlear nerve** are also bipolar, and their cell bodies are
located in the **spiral ganglion of Corti** within the modiolus. Dendritic
processes of spiral ganglion cells exit the modiolus through the small
openings in the bony spiral lamina and enter the spiral organ.
Approximately 90% of dendrites originating from the spiral ganglion cells
synapse with the inner hair cells; the remaining 10% of dendrites synapse
with the outer hair cells of the spiral ganglion. The axons of the spiral
ganglion cells form the cochlear nerve, which enters the bony cochlea
through the modiolus to appear in the internal acoustic meatus (see Fig.
25.25). From the internal acoustic meatus, the cochlear nerve enters the
brainstem and terminates in the cochlear nuclei of the medulla. Nerve fibers
from these nuclei pass to the geniculate nucleus of the thalamus and then to
the auditory cortex of the temporal lobe.

The organ of Corti also receives a small number of efferent fibers
conveying impulses from the brain that pass parallel to the afferent nerve
fibers of the vestibulocochlear nerve (olivocochlear tract, cochlear efferents
of Rasmussen). Efferent nerve fibers from the brainstem pass through the
vestibular nerve. They synapse either on afferent endings of the inner hair
cell or on the basal aspect of an outer hair cell. Efferent fibers are thought to
affect the control of auditory and vestibular input to the central nervous
system, presumably by enhancing some afferent signals while suppressing
other signals. Damage to the organ of Corti, cochlear nerve, nerve
pathways, or auditory cortex is responsible for **sensorineural**
**hearing loss** (see Folder 25.2).


The sensation of rotation without equilibrium ( **dizziness**, **vertigo** )
signifies dysfunction of the vestibular system. Causes of vertigo include


viral infections, certain drugs, and tumors such as **acoustic neuroma** .
Acoustic neuromas develop in or near the internal acoustic meatus and
exert pressure on the vestibular division of cranial nerve VIII or branches
of the labyrinthine artery. Vertigo can also be produced normally in
individuals by excessively stimulating the semicircular ducts. Similarly,
excessive stimulation of the utricle can produce motion sickness
(seasickness, carsickness, or airsickness) in some individuals.

The most common vestibular disorder is **benign paroxysmal**
**positional vertigo (BPPV)** . In this condition, otoconia become
detached from the macula of the utricle and lodge in one of the three
cristae ampullares. The anatomic position of the posterior semicircular
canal (it has an opening inferior to the macula) makes it the most
common site for the detached otoconia to enter (81%–90%). The
otoconia remain either free floating within the canal ( **canalithiasis** ) or
are attached to the cupula ( **cupulolithiasis** ), causing inappropriate
movement of the stereocilia at the apical surface of the receptor hair
cells. Individuals with BPPV report episodes of an erroneous sensation
of spinning evoked by certain movements of the head. Otoconia may
detach following trauma or viral infections, but in many instances, it
occurs idiopathically.

Some diseases of the internal ear affect both hearing and
equilibrium. For example, people with **Ménière disease** initially
complain of episodes of dizziness and tinnitus (ringing in the ears) and
later develop low-frequency hearing loss. The causes of Ménière
disease are related to blockage of the cochlear aqueduct, which drains
excess endolymph from the membranous labyrinth. Blockage of this
duct causes an increase in endolymphatic pressure and distension of
the membranous labyrinth (endolymphatic hydrops).

### **Blood Vessels of the Membranous Labyrinth**


**Arterial blood is supplied to the membranous labyrinth by the**
**labyrinthine artery; venous blood drainage is to the venous dural**
**sinuses.**


The blood supply to the external ear, middle ear, and bony labyrinth of the
internal ear is from vessels associated with the external carotid arteries. The
**arterial blood supply** to tissues of the membranous labyrinth of the
internal ear is from the intracranial **labyrinthine artery**, a common branch
of the anterior inferior cerebellar or basilar artery. The labyrinthine artery is
a terminal artery: It has no anastomoses with other surrounding arteries.
Branches of this artery are exactly parallel to the distribution of the superior
and inferior parts of the vestibular nerve.


**Venous drainage** from the cochlear labyrinth is via the posterior and
anterior spiral modiolar veins that form the **common modiolar vein** . The
common modiolar vein and the vestibulocochlear vein form the vein of the
cochlear aqueduct, which empties into the inferior petrosal sinus. Venous
drainage from the vestibular labyrinth is via **vestibular veins** that join the
vein of the cochlear aqueduct and by the vein of vestibular aqueduct, which
drains into the sigmoid sinus.

## EAR


**OVERVIEW OF THE EAR**


The **ear** is a paired specialized sensory organ that is responsible for
sound perception and balance.
Tissues of the ear are derived from **surface ectoderm** (epithelia lining
of the membranous labyrinth) and components of the **first pharyngeal**
**pouch** (auditory tube and middle ear cavity), **first pharyngeal**
**groove** (external acoustic meatus), **first pharyngeal arch** (malleus,
incus, and anterior part of the auricle), and **second pharyngeal arch**
(stapes and posterior part of the auricle).


**EXTERNAL EAR**


The **auricle** is the external component of the ear that collects and
amplifies sound.
The **external acoustic meatus** extends from the auricle to the

tympanic membrane. It is lined by skin that contains hair follicles as
well as sebaceous and ceruminous glands (which produce **cerumen**, or
**earwax** ).


**MIDDLE EAR**


The **middle ear** is an air-filled space lined by a mucous membrane that
contains three **auditory ossicles** (malleus, incus, and stapes). It is
separated from the external acoustic meatus by the tympanic membrane
and is connected by the **auditory (Eustachian) tube** to the
nasopharynx.
The middle ear **amplifies mechanical forces** generated by the
vibration of the tympanic membrane.
The **tympanic membrane** is composed of skin of the external
auditory meatus, a thin core of connective tissue, and mucous
membrane of the middle ear.

The auditory ossicles ( **malleus**, **incus**, and **stapes** ) cross the space of
the middle ear in series and connect the tympanic membrane to the oval
window. Movement of the ossicles is modulated by the **tensor**
**tympani muscle** that inserts to the malleus and the **stapedius**
**muscle** that inserts to the stapes.


**COMPARTMENTS OF THE INTERNAL EAR**


The **internal ear** consists of two compartments within the temporal
bone: the **bony labyrinth** and the **membranous labyrinth**, which is
contained within the bony labyrinth.
The internal ear has three fluid-filled spaces: the **endolymphatic**
**space** within the membranous labyrinth (which has a high K [+] and a
low Na [+] concentration), the **perilymphatic space** between the wall of
the bony and membranous labyrinth (which has a low K [+] and a high
Na [+] concentration), and the **cortilymphatic space** that lies within the
tunnels of the organ of Corti of the cochlea.
The **bony labyrinth** consists of three connected spaces: **semicircular**
**canals**, **vestibule**, and **cochlea**, each containing different parts of the
membranous labyrinth.
The **membranous labyrinth** consists of a series of communicating
sacs ( **utricle**, **saccule**, and endolymphatic sac) and ducts ( **three**
**semicircular** **ducts**, **cochlear** **duct**, utriculosaccular duct,
endolymphatic duct, and ductus reuniens) that contain **endolymph** .


**SENSORY RECEPTORS OF THE MEMBRANOUS**

**LABYRINTH**


Specialized sensory cells are located in six regions in the membranous
labyrinth: three **cristae ampullares** in the ampullae of the


semicircular ducts (receptors for angular acceleration of the head), two
**maculae** in the utricle and saccule (receptors for position of the head
and its linear movements), and the **spiral organ of Corti** (receptors
for sound).
Utricle and saccule maculae contain **hair cells** that are epithelial
mechanoreceptors. These hair cells contain **hair bundles** on their
apical surfaces (formed by rows of stereocilia with a single kinocilium)
and are overlaid with a gelatin-like **otolithic membrane** that contains
otoliths (otoconia).
Movement of the **otoliths** is detected by the hair bundles, which
activate **mechanically gated ion channels** to generate an action
potential.
Sensory receptors in the **crista ampullaris** are also covered by a
gelatin-like mass without otoliths called the **cupula** . The cupula is
deflected during the flow of endolymph through the semicircular canal.
Movement of the cupula stimulates **mechanically gated ion**
**channels** to generate an action potential.
The **cochlear canal** is divided into three parallel compartments: **scala**
**media** or **cochlear duct** (the middle compartment filled with
endolymph that contains the spiral organ of Corti), **scala vestibuli**,
and **scala tympani** (both containing perilymph).
The **scala media** is a triangular space with its lower wall forming the
**basilar membrane** on which the spiral organ of Corti resides. The
upper wall ( **vestibular membrane** ) separates the scala media from
scala vestibuli, and the lateral wall contains the **stria vascularis** that
produces endolymph.
The **spiral organ of Corti** is composed of **hair cells** (arranged in
inner and outer rows), supportive **phalangeal cells**, and **pillar cells** .
Movement of the stereocilia on hair cells during interaction with the
overlying **tectorial membrane** generates electrical impulses that are
transmitted to the cochlear nerve.
**Sound waves** are transmitted from the vibrating tympanic membrane
by the ossicles to the oval window, where they produce movement
(waves) of the perilymph in the scala vestibule. This movement deflects
the basilar membrane and spiral organ of Corti to generate electrical
nerve impulses, which are perceived by the brain as sounds.
Nerve impulses from the cristae ampullares and maculae travel with the
**vestibular nerve**, and the impulses from the spiral organ of Corti
travel with the **cochlear nerve** . These two nerves join together in the
internal acoustic meatus to form the **vestibulocochlear nerve**

**(cranial nerve VIII)** .


##### Internal ear, ear, guinea pig, hematoxylin and
###### eosin (H&E) ×20.

In this section through the **internal ear**, bone surrounds the
entire internal ear cavity. Because of its labyrinthine character,
sections of the internal ear appear as a number of separate
chambers and ducts. However, these structures are all
interconnected (except for the perilymphatic and endolymphatic
spaces, which remain separate). The largest chamber is the **vestibule** ( _V_ ). The left
side of this chamber ( _black arrow_ ) leads into the **cochlea** ( _C_ ). Just below the _black_
_arrow_ and to the right is the oval ligament ( _OL_ ) surrounding the base of the stapes
( _S_ ). Both structures have been cut obliquely and are not seen in their entirety. The
facial nerve ( _FN_ ) is in an osseous tunnel to the left of the oval ligament. The
communication of the vestibule with one of the semicircular canals is marked by the
_white arrow_ . Note the crista ampullaris ( _CA_ ) that is projecting into the lumen of the
semicircular canal. At the _upper right_ are cross sections of the membranous labyrinth
passing through components of the semicircular duct system ( _DS_ ).


The cochlea is a spiral, cone-shaped structure. The specimen illustrated here
makes 3½ turns (in humans, there are 2¾ turns). The section goes through the central
axis of the cochlea. This consists of a bony stem called the **modiolus** ( _M_ ). It
contains the beginning of the cochlear nerve ( _CN_ ) and the spiral ganglion ( _SG_ ).
Because of the plane of section and the spiral arrangement of the cochlear tunnel, the
tunnel is cut crosswise in seven places (note 3½ turns). A more detailed examination
of the cochlea and the organ of Corti is provided in Plate 25.2 (page 1044).


##### Semicircular canal, ear, guinea pig, H&E ×85;
###### inset ×380.

A higher magnification of one of the semicircular canals and
of the **crista ampullaris** ( _CA_ ) within the canal seen in the
_lower right_ corner of the previous figure is provided here. The
receptor for movement, the crista ampullaris (note its relationships
in the previous figure), is present in each of the semicircular
canals. The epithelial ( _EP_ ) surface of the crista consists of two cell types, supporting
cells and receptor hair cells. (Two types of hair cells are distinguished with the
electron microscope.) It is difficult to identify the hair and supporting cells on the
basis of specific characteristics; they can, however, be distinguished on the basis of
location (see _inset_ ), as the **hair cells** ( _HC_ ) are situated in a more superficial
location than the supporting cells ( _SC_ ). A gelatinous mass, the cupula ( _Cu_ ),
surmounts the epithelium of the crista ampullaris. Each receptor cell sends a hair-like
projection deep into the substance of the cupula.


The epithelium rests on a loose, cellular connective tissue ( _CT_ ) that also contains
the nerve fibers associated with the receptor cells. The nerve fibers are difficult to
identify because they are not organized into a discrete bundle.


**C,** cochlea
**CA,** crista ampullaris
**CN,** cochlear nerve
**CT,** connective tissue
**Cu,** cupula
**DS,** duct system (of membranous labyrinth)
**EP,** epithelium
**FN,** facial nerve
**HC,** hair cell
**M,** modiolus
**OL,** oval ligament
**S,** stapes
**SC,** supporting cell
**SG,** spiral ganglion
**V,** vestibule
**black arrow,** entry to cochlea
**white arrow,** entry to semicircular canal


##### Cochlear canal, ear, guinea pig, hematoxylin and
###### eosin (H&E) ×65; inset ×380.

A section through one of the turns of the cochlea is shown
here. The most important functional component of the cochlea is
the organ of Corti, enclosed by the _rectangle_ and shown at higher
magnification in the next figure. Other structures are included in
this figure. The spiral ligament ( _SL_ ) is a thickening of the
periosteum on the outer part of the tunnel. Two membranes, the basilar membrane
( _BM_ ) and the vestibular membrane ( _VM_ ), join with the spiral ligament and divide the
cochlear tunnel into three parallel canals, namely, the **scala vestibuli** ( _SV_ ), the
**scala tympani** ( _ST_ ), and the **cochlear duct** ( _CD_ ). Both the scala vestibuli and
the scala tympani are perilymphatic spaces; these communicate at the apex of the
cochlea. The cochlear duct is the space of the membranous labyrinth and is filled
with endolymph. It is thought that the endolymph is formed by the portion of the
spiral ligament that faces the cochlear duct, the stria vascularis ( _StV_ ). This is highly
vascularized and contains specialized “secretory” cells.


A shelf of bone, the osseous spiral lamina ( _OSL_ ), extends from the modiolus to
the basilar membrane. Branches of the cochlear nerve ( _CN_ ) travel along the spiral
lamina to the modiolus, where the main trunk of the nerve is formed. The
components of the cochlear nerve are bipolar neurons whose cell bodies constitute
the spiral ganglion ( _SG_ ). These cell bodies are shown at higher magnification in the
_inset_ ( _upper right_ ). The spiral lamina supports an elevation of cells, the limbus
spiralis ( _LS_ ). The surface of the limbus is composed of columnar cells.


##### Organ of Corti, ear, guinea pig, H&E ×180; inset
###### ×380.

The cross section of the **cochlear canal** ( _CD_ ) visible in this
image appears as a triangular space. The upper wall (roof) of this
canal is formed by the vestibular membrane ( _VM_ ) that separates it
from the scala vestibuli ( _SV_ ). The lateral (outer) wall of the
cochlear canal is bordered the stria vascularis ( _StV_ ) and underlying
spiral ligament ( _SL_ ). The lower wall (or floor) is formed by the basilar membrane, an
extension of the osseous spiral lamina with visible branches of the cochlear nerve
( _CN_ ). The basilar membrane supports the spiral **organ of Corti** . The components
of the organ of Corti, beginning at the limbus spiralis ( _LS_ ), are as follows: inner
border cells ( _IBC_ ), inner phalangeal and hair cells ( _IP&HC_ ), and inner pillar cells
( _IPC_ ). The sequence continues, repeating itself in reverse as follows: outer pillar
cells ( _OPC_ ), hair cells ( _HC_ ) and outer phalangeal cells ( _OP_ ), and outer border cells
or cells of Hensen ( _CH_ ). Hair cells are receptor cells; the other cells are collectively
referred to as _supporting cells_ . The hair and outer phalangeal cells can be
distinguished in this figure by their location (see _inset_ ) and because their nuclei are
well aligned. Because the hair cells rest on the phalangeal cells, it can be concluded
that the upper three nuclei belong to outer hair cells, whereas the lower three nuclei
belong to outer phalangeal cells.


The supporting cells extend from the basilar membrane ( _BM_ ) to the surface of
the organ of Corti (this is not evident here but can be seen in the _inset_ ), where they
form a reticular membrane ( _RM_ ). The free surface of the receptor cells fits into
openings in the reticular membrane, and the “hairs” of these cells project toward, and
make contact with, the tectorial membrane ( _TM_ ). The latter is a cuticular extension
from the columnar cells of the limbus spiralis. In ideal preparations, nerve fibers can
be traced from the hair cells to the cochlear nerve ( _CN_ ).


In their course from the basilar membrane to the reticular membrane, groups of
supporting cells are separated from other groups by spaces that form spiral tunnels.
These tunnels are named the inner tunnel ( _IT_ ), the outer tunnel ( _OT_ ), and the internal
spiral tunnel ( _IST_ ). Beyond the supporting cells are two additional groups of cells,
the cells of Claudius ( _CC_ ) and the cells of Böttcher ( _CB_ ).


**BM,** basilar membrane
**CB,** cells of Böttcher
**CC,** cells of Claudius
**CD,** cochlear duct
**CH,** cells of Hensen
**CN,** cochlear nerve
**HC,** hair cells


**IBC,** inner border cells
**IPC,** inner pillar cells
**IP&HC,** inner phalangeal and hair cells
**IST,** internal spiral tunnel
**IT,** inner tunnel
**LS,** limbus spiralis
**OP,** outer phalangeal cells
**OPC,** outer pillar cells
**OSL,** osseous spiral lamina
**OT,** outer tunnel
**RM,** reticular membrane
**SG,** spiral ganglion
**SL,** spiral ligament
**ST,** scala tympani
**StV,** stria vascularis
**SV,** scala vestibule
**TM,** tectorial membrane
**VM,** vestibular membrane