Cui Dongmei. Atlas of Histology: with functional and clinical correlations. 1st ed

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Figure 20-14A and B. A representation of rods and cones.

Rods and cones are photoreceptor cells in the retina. They are similar in structure and both have (1) a synaptic region (2) a nuclear

region (3) inner segments, and (4) outer segments. There are numerous synaptic vesicles in the synaptic regions, where the photorecep-

tors synapse with the dendrites of bipolar cells. The nuclear regions of rods and cones lie in the outer nuclear layer. The inner and outer

segments form the photoreceptor layer of the retina. The inner segments contain mitochondria, rough endoplasmic reticulum, Golgi

apparati, and other organelles that support the synthesis of proteins. The outer segments are in contact with the apical region of the

pigmented epithelium cells. Outer segments of rods are composed of a series of superimposed disks, which have individual membranes,

are stacked on each other, and are enclosed within the plasma membrane. Outer segments of cones have disks that are formed by invagi-

nations of the plasma membrane. The membranes of the disks are continuous with the plasma membrane of the cell. The inner region is

much wider than the outer region, which gives a cone appearance. Other differences between rods and cones are listed in Table 20-1.

D. Cui

Inner rod fiber

Synaptic vesicles

Synaptic

region

Nucleus

Nuclear

region

Outer rod fiber

Mitochondrion

Basal body

Disk

Plasma membrane

Modified cilium

Inner

segment

Outer

segment

D. Cui

Synaptic

region

Inner cone fiber

Nuclear

region

Nucleus

Mitochondrion

Basal body

Modified cilium

Outer cone fiber

Inner segment

Outer segment

Types of

Photoreceptor

Cells

Synaptic

Region

Nuclear

Region

Inner Segment Outer Segment Distribution in

the Retina

Main

Function

Rods Spherule

synapses with

dendrites of

one bipolar

neuron

Small,

round

nucleus

Fewer

mitochondria than

cones; synthesized

proteins passed

only to newly

forming disks

Cylindrical in shape; disk

membranes are separate

and are not connected

to the outside plasma

membrane; disks contain

rhodopsin protein

Numerous in the

peripheral retina;

none in fovea

Vision in

dim light;

motion

detection

Cones Pedicle

synapses with

dendrites of

several bipolar

neurons

Large,

ovoid

nucleus

mitochondria than

rods; synthesized

proteins passed

to entire outer

segment

Cone shaped; disk

membranes are connected

to the plasma membrane

and form invaginated

membranes; disks contain

iodopsin protein

Highly

concentrated in

the fovea and

macula lutea;

less numerous in

periphery

Color

vision;

perception

of ﬁ ne detail

TABLE 20-1 Comparison of Rods and Cones

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Organ Systems

Figure 20-15A. Retinal layers. H&E, 463

There are 10 histologic layers in the retina; see Figure

20-13B and Synopsis 20-1 for details of the compo-

nents in each layer.

Ganglion cell

Cone

Rod

10 μm

Nerve fiber layer (9)

Ganglion cell layer (8)

Inner plexiform layer (7)

Inner nuclear layer (6)

Outer plexiform layer (5)

Outer limiting layer (3)

Photoreceptor layer (2)

Bruch membrane

Outer nuclear layer (4)

Pigment epithelium

layer (1)

Inner limiting membrane (10)

Ganglion cell

Cone

Rod

Figure 20-15B. Scanning electron micrograph of retinal layers.

Freeze-fracture preparation, 1,200

This freeze-fracture preparation of the peripheral retina shows the three-

dimensional shapes of the various cells in the retina. This section from

the peripheral retina has fewer ganglion cells than the macular retina

illustrated at left. Compare Figure 20-13B and C.

T. Yang

Neural

outflow

Light

Nerve fiber layer

Inner plexiform layer

Inner nuclear layer

Outer nuclear layer

Outer plexiform layer

Outer limiting layer

Photoreceptor layer

Pigment

epithelium layer

Ganglion cell layer

Inner limiting

membrane

Ganglion

cell

Müller cell

Amacrine

cell

Bipolar cell

Horizontal

cell

Rod cell

Cone cell

Pigment cell

Figure 20-15C. A representation of the functional retinal

layers.

Light passes through all retinal layers to activate the photore-

ceptor cells (rods and cones), and excess light is absorbed by

pigmented epithelial cells. The photoreceptors transduce the

light into electrochemical signals, which pass to the conduct-

ing neurons (bipolar cells, then ganglion cells). Horizontal and

amacrine cells are association neurons that have long dendrites.

The dendrites of the horizontal cells synapse with photorecep-

tors and contribute to the elaborate neuronal connections in the

outer plexiform layer. The dendrites of amacrine cells are in con-

tact with bipolar and ganglion cells and modulate signals in the

inner plexiform layer. The ganglion cells collect all visual infor-

mation and send it along their axons in the optic nerve. Müller

cells are large supporting neuroglial cells and extend from the

inner limiting membrane to the outer limiting membrane. Their

processes surround all the neuronal elements of the retina.

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The ganglion cells and their axons are part of the central nervous system. They cannot regenerate following severe damage.

Embryologically, the pigmented epithelium layer and the neural layers of the retina originate from different layers of the optic

vesicle. Therefore, retinal detachment may occur between the pigmented epithelium layer and the rest of the neural retina.

SYNOPSIS 20-1 Retinal Layers

(1) Pigment epithelium layer, a layer of cuboidal cells which are rich in melanin granules. This layer is important in absorbing

excess light and in esterifying vitamin A.

(2) Photoreceptor (rods and cones) layer, consists of inner and outer segments of the photoreceptor cells.

(3) Outer limiting layer, a plexus of junction complexes that joins the membranes of the photoreceptor cells and Müller

cells (retinal glial cells).

(4) Outer nuclear layer, contains nuclei of the photoreceptor cells (rods and cones).

(5) Outer plexiform layer, consists of axodendritic synapses between the axons of the photoreceptor cells and dendrites of

the bipolar and horizontal cells.

(6) Inner nuclear layer, contains nuclei of bipolar cells, horizontal cells, amacrine cells, and Müller cells.

(7) Inner plexiform layer, composed of axodendritic synapses between the bipolar cell axons, ganglion cell dendrites, and

processes of the amacrine cells.

(8) Ganglion cell layer, contains nuclei of the ganglion cells.

(9) Nerve ﬁ ber layer, contains axons of the ganglion cells.

(10) Inner limiting membrane, the basement membrane of the Müller cells.

CLINICAL CORRELATIONS

Figure 20-16A.

Age-Related Macular Degeneration.

Age-related macular degeneration (ARMD) is a degenerative eye

disease affecting the central portion of the retina. Symptoms include

blurred vision, distortion of straight lines, and worsening of color

vision. There are two forms: exudative (wet) and nonexudative (dry).

Wet ARMD is characterized by the growth of new vessels from the

choroidal circulation and serous ﬂ uid leakage from the new vessels.

This causes detachment of the retina and macula, producing a rapid

and irreversible loss of central vision. Treatment options include

laser photocoagulation, surgery, and drugs. Dry ARMD is char-

acterized by subretinal drusen deposits (focal eosinophilic material

arising from the Bruch membrane and lying between the pigment

epithelium and the Bruch membrane), and atrophy and degeneration

of the outer retina, including the retinal pigment epithelium, Bruch

membrane, and choriocapillaris. The causes of ARMD are not well

understood, but risk factors include age, smoking, family history,

hypertension, and ethnicity (higher in non-Hispanic Caucasians).

D. Cui

Pigment epithelium

Vitreous fluid-

filled space

Detached

retina

Photoreceptor

layer

Retinal

hole

Vitreous fluid

Figure 20-16B.

Retinal Detachment. H&E, 62

Retinal detachment is an eye condition in which the sensory retina

separates from the underlying retinal pigment epithelium and

choroid (see red arrows). It is categorized into three major types:

(1) rhegmatogenous retinal detachment ([RRD] illustrated) is the

most common; it occurs when vitreous ﬂ uid leaks into the subreti-

nal space through a break in the retina; (2) exudative or serous

detachment occurs in association with inﬂ ammation or a tumor;

(3) traction detachment occurs when scar tissue on the retina’s sur-

face contracts and causes the retina to separate from the retinal pig-

ment epithelium. Retinal detachment is a medical emergency that

can lead to permanent vision loss. Time is critical for surgical reat-

tachment of the retina, which will usually preserve vision. Symptoms

include ﬂ ashes, ﬂ oaters, and visual ﬁ eld distortion or loss. Treatment

options include laser photocoagulation, cryoretinopexy, vitrectomy,

and silicone oil repair.

J. Lynch

Choroid

Abnormal blood

vessels, leaking fluid

Bruch membrane

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Organ Systems

Lam

Lamina

Subarachnoid

space

(filled with CSF)

Retinal nerve

fibers

(nonmyelinated)

Sclera

Retina nerve

fibers

Neuroglial cells

Optic disk

(optic papilla)

Subarachnoid

space

(filled with CSF)

Pia mater

Optic nerve

(myelinated)

Optic nerve

(myelinated)

Sclera

cribrosa

Optic Nerve

Figure 20-17A. Optic nerve and optic disk. H&E, 22; inset, 83

The optic nerve is a nerve ﬁ ber trunk formed by the convergence of

retinal ganglion cell axons at the posterior pole of the eye. From there,

they leave the globe on their way to the brain. Each optic nerve con-

tains about one million myelinated axons and even more neuroglial

cells. The surface of the optic nerve is covered by pia mater, which is

continuous with that on the surface of the brain. The optic disk (optic

nerve papilla) is the small circular site in the retina where the retinal

nerve ﬁ ber layer (nonmyelinated nerve ﬁ bers) continues into the optic

nerve. The nonmyelinated nerve ﬁ bers begin to acquire myelin at the

level of the lamina cribrosa (thin dotted line), a perforated, sievelike

region of the sclera through which optic nerve ﬁ bers and blood ves-

sels pass. The myelinated segments of ganglion cell axons, therefore,

form the optic nerve. The neuroglial cells include oligodendrocytes,

which produce myelin for axons in the CNS, and astrocytes, which

perform several nutritive and supportive functions.

CLINICAL CORRELATION

Figure 20-17B.

Papilledema.

Papilledema is a noninﬂ

ammatory swelling of the optic disk. It is

produced by increased intracranial pressure that is transmitted

along the optic nerve sheath as elevated cerebrospinal ﬂ uid pres-

sure, and it is, therefore, a sign of a potentially life-threatening

condition. The increased pressure disrupts blood circulation

within the optic nerve, causing leakage of water, protein, and

other contents into the extracellular spaces of the optic disk. The

most noticeable symptoms of increased intracranial pressure are

headache and vomiting. Clinical ﬁ ndings include a swollen and

elevated optic disk (papilledema), engorged and tortuous retinal

veins, and focal hemorrhages. Cerebral tumors, subdural hema-

toma, malignant hypertension, and hydrocephalus are the most

common causes of increased intracranial pressure and papille-

dema. Because papilledema is a sign of many systemic intracra-

nial and spinal diseases, the correct diagnosis of the underlying

disease is essential for proper treatment.

D. Cui

Swollen and elevated

optic disk

Normal

CSF

Optic

disk

Elevated

CSF

pressure

Engorged and

tortuous central vein

Blepharitis ■ : Inﬂ ammation of the eyelids and especially of their margins (Fig. 20-3C).

Ptosis

■ : Drooping or falling down, usually of the upper eyelid, because of muscle weakness or paralysis (Fig. 20-4C).

Open angle (glaucoma)

■ : Raised intraocular pressure in which obstruction to aqueous ﬂ ow occurs at the ultrastructural

level, within the walls of the tiniest interstices of the trabecular meshwork (Fig. 20-12C).

Closed angle (glaucoma)

■ : The ﬂ ow of aqueous ﬂ uid is blocked by the root of the iris. This can be a generalized closure

(hyperopic eyes) or in multiple focal adhesions caused by inﬂ ammation (peripheral anterior synechiae [Fig. 20-12C]).

Cupping of the optic disk

■ : Glaucoma destroys retinal axons as they traverse the optic disk in the walls of a central, cup-

shaped conduit. As the walls disappear, the cup enlarges (Fig. 20-12C).

Dry macular degeneration

■ : Scattered drusen damage the pigment epithelium of the macula, gradually decreasing visual

acuity (Fig. 20-16A).

Wet macular degeneration

■ : Abnormal capillaries break through the retinal pigment epithelium and leak underneath the

retina. When close to the fovea, they can decrease visual acuity (Fig. 20-16A).

Limbu

■ s: An anatomic transition zone between cornea and sclera (corneoscleral junction), which is an important landmark

for eye surgery procedures (Fig. 20-11A).

Ora serrata

■ : A denticulate border (junction) between the ciliary body and the retina; this is an important anatomic landmark

for the ophthalmologist (Fig. 20-12B).

SYNOPSIS 20-2 Clinical Terms for the Eye

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Ear

Introduction and Key Concepts for the Ear

Figure 21-1A Overview of the Ear: Outer, Middle, and Inner Ear

Figure 21-1B Middle Ear Structures

Figure 21-2 Major Structures of the Inner Ear

Figure 21-3A Membranous Labyrinth

Figure 21-3B Cochlear Duct and Modiolus

Auditory System

Figure 21-4A Cross section of Cochlea

Figure 21-4B Scala Media and Organ of Corti

Figure 21-5 Organ of Corti and Associated Structures

Figure 21-6A Sound Transduction

Figure 21-6B Stereocilia Displacement

Figure 21-6C Auditory Hair Cells

Figure 21-7A Inner and Outer Hair Cells

Figure 21-7B Stereocilia of an Outer Hair Cell

Figure 21-7C Clinical Correlation: Sensorineural Hearing Loss

Vestibular System

Figure 21-8A Sensory Receptors

Figure 21-8B Crista Ampullaris and Macula Utriculi

Figure 21-9A Ampulla of the Semicircular Canal

Figure 21-9B Crista Ampullaris

Figure 21-9C Clinical Correlation: Ménière Disease

Figure 21-10A Macula of the Utricle

Figure 21-10B Macula of the Utricle with Otoconia

Figure 21-10C Otoconia from the Macula of the Utricle

Figure 21-11A Vestibular Hair Cells

Figure 21-11B Excitation and Inhibition in Hair Cells

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Figure 21-11C Clinical Correlation: Otitis Media

Figure 21-12A Clinical Correlation: Vestibular Schwannoma

Figure 21-12B Clinical Correlation: CT and MRI of Inner Ear Structures

Synopsis 21-1 Pathological and Clinical Terms for the Ear

Introduction and Key Concepts

for the Ear

The ear is a complex structure that serves two important sen-

sory functions, hearing (through the auditory system) and bal-

ance (through the vestibular system). Sensory receptor organs

that serve the two functions are supplied by two distinct

branches of cranial nerve (CN) VIII, the acoustic branch and

the vestibular branch. The ear can be divided into three general

regions, the outer ear, middle ear, and inner ear (Fig. 21-1A).

(1) The outer ear consists of a pinna (auricle), an irregularly

shaped structure with a core of cartilage covered on both sides

by thin skin, and an external auditory meatus that conducts

sound to the middle ear. (2) The middle ear includes the tym-

panic membrane, the tympanic cavity containing the ossicles,

and the auditory tube (Fig. 21-1A,B). The tympanic membrane,

the landmark between the outer ear and middle ear, covers the

medial end of the external auditory meatus and converts sound

waves in the air to mechanical vibrations. The tympanic cavity

is an air-ﬁ lled space that contains the ossicles, three tiny bones

that conduct the mechanical vibrations of the tympanic mem-

brane to the oval window of the cochlea. The tympanic cavity is

connected to the nasopharynx by the auditory tube (eustachian

tube), thereby allowing the air pressure on each side of the tym-

panic membrane to be equalized when the ambient air pressure

changes (e.g., by changes in altitude). (3) The inner ear consists

of structures contained within the bony labyrinth, a system of

tunnels and cavities in the petrous portion of the temporal bone,

the hardest bone in the body (Figs. 21-2 and 21-3A). The struc-

tures include the cochlear labyrinth or cochlea (Latin for “snail

shell”), which subserves hearing. It contains a spiral, ﬂ uid-ﬁ lled

tunnel within which a membranous tube, the cochlear duct, is

suspended (Fig. 21-3A,B). The sensory receptors that detect

sound are located in a strip of specialized epithelium, the organ

of Corti (spiral organ), in the cochlear duct. The vestibular lab-

yrinth consists of a complex group of ﬂ uid-ﬁ lled tunnels and

cavities in the temporal bone that contain a group of intercon-

nected membranous structures, the semicircular ducts, utricle

and saccule (Figs. 21-2 and 21-3A). The sensory receptors that

are involved in balance are located in specialized regions of

the semicircular ducts (rotation) and in the utricle and saccule

(static head position and acceleration).

Auditory System

Sound is normally produced by compression and rarefaction

waves in air, of various frequencies, that impinge upon the

tympanic membrane where they are converted into mechani-

cal vibrations in the ossicles. The mechanical vibrations, in

turn, are transferred to the ﬂ uid of the vestibule at the oval

window (Fig. 21-2). This ﬂ uid is perilymph, a sodium-rich ﬂ uid

that is similar in composition to cerebrospinal ﬂ uid and extra-

cellular ﬂ uid. The resulting vibrations, or pressure waves, in

the perilymph spread into the scala vestibuli of the cochlear

labyrinth and act upon mechanoreceptors in the cochlear duct

to produce the sensation of hearing (Fig. 21-6A). The cochlear

duct is a membranous tube, triangular in cross section, that

coils inside the spiral tunnel in the cochlea (Fig. 21-3A,B). It is

suspended within the cochlear labyrinth so that it divides the

labyrinth into two tunnels, the scala vestibuli (above) and the

scala tympani (below), which are connected with each other by

a small opening, the helicotrema (Figs. 21-4A,B and 21-6A).

The cochlear duct encloses the scala media, a space contain-

ing endolymph, a potassium-rich ﬂ uid similar in composition

to intracellular ﬂ uid. The scala media is bounded above by the

vestibular membrane, below by the basilar membrane, and

externally by the spiral ligament and stria vascularis (Fig. 21-5).

The sensory receptors for hearing are specialized epithelial cells

(hair cells) in the organ of Corti. The hair cells get their name

from clumps of stereocilia that project from their apical sur-

faces and contact an overlying gelatinous structure, the tectorial

membrane. The organ of Corti sits on the basilar membrane

and extends its entire length, from the base of the cochlea to

the apex. It contains a single row of inner hair cells and three or

four rows of outer hair cells (Figs. 21-5 and 21-7A). In humans,

there are about 3,500 inner hair cells and 12,000 outer hair

cells, yet 95% of the afferent axons in the auditory nerve con-

tact only inner hair cells. The primary function of the inner hair

cells appears to be basic frequency and loudness discrimination,

whereas the outer hair cells appear to be primarily concerned

with the ﬁ ne-tuning of frequency discrimination in the cochlea

(Figs. 21-6C and 21-7A,B). When sound waves cause pressure

waves to occur in the scala vestibuli and scala media, the basilar

membrane vibrates up and down, and a shearing force is gener-

ated between the surface of the organ of Corti and the tectorial

membrane. The shearing force bends the stereocilia of the hair

cells, leading to the release of neurotransmitters by the hair cells

and the initiation of action potentials in auditory nerve axons

(Fig. 21-6A,B).

Vestibular System

The sense of balance is critical to our ability to walk, run, jump,

or even just stand still with eyes closed. One important source

of neural signals that aid in controlling such behaviors is the

peripheral vestibular apparatus. This includes the vestibular lab-

yrinth, consisting of the vestibule, a cavity within the temporal

bone, and three semicircular canals, curved tunnels that connect

with the vestibule (Fig. 21-2). One canal is in approximately the

horizontal plane; the other two are in approximately the ver-

tical plane and at right angles to each other. These cavities in

the bone are ﬁ lled with perilymph. Floating within the vestibule

are two membranous saclike structures, the utricle and saccule.

Within each of the semicircular canals is a membranous tube

called a semicircular duct, which joins the utricle at each of its

ends (Figs. 21-2 and 21-3A). The utricle, saccule, and semicir-

cular ducts contain endolymph. The semicircular ducts detect

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rotational movements of the head. Each duct has an enlargement

at one end where it joins the utricle. This swelling is called the

ampulla and contains the sensory receptors that are stimulated

by rotational movements (Fig. 21-9A). A short wall of connec-

tive tissue, the crista ampullaris, extends across part of each

ampulla. Hair cells, similar to those of the organ of Corti, cover

the upper surface of the crista. Their stereocilia and kinocilia

are embedded in a gel-like structure, the cupula, which blocks

the ampulla. When the head turns, the inertia of the endolymph

in the semicircular ducts causes the ﬂ uid to push against the

cupula and deﬂ ect the cilia of the hair cells, thereby initiating

action potentials in axons in the vestibular nerve (Figs. 21-8B

and 21-9A,B). Vestibular hair cells are also clustered in a small

region of the utricle, the macula utriculi (Figs. 21-8B and

21-10A,B). The stereocilia and kinocilia of these hair cells are

embedded in a gelatinous structure, the otolithic membrane (Fig.

21-10A). Thousands of tiny calcium carbonate crystals, otoco-

nia, are clustered on the surface of the otolithic membrane (Fig.

21-10A–C). These crystals are heavier than the surrounding

endolymph. Gravity or linear acceleration, therefore, exerts a

force on the otoconia, causing the underlying cilia to be deﬂ ected

and consequently sending a neural signal to the central nervous

system (CNS) related to head position or acceleration. The sac-

cule contains a similar region, the macula sacculi.

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Organ Systems

Tympanic membrane

Stapes

(in oval

window)

Round

window

Tympanic

cavity

Stapes

Stapedius

muscle

Tympanic

membrane

Auditory tube

Cochlea

External auditory meatus

Semicircular canals

Temporal bone

Malleus

Anterior malleal

ligament

Posterior incudal

ligament

Vestibular nerve

Facial nerve

Cochlear nerve

Vestibule

Incus

Cartilage

Incus

Pinna

Outer

ear

Middle

ear

Inner

ear

Insertion

of tensor

tympani

muscle

Figure 21-1A. Overview of the ear: Outer, middle, and inner ear.

The ear is divided into three regions: the outer ear, middle ear, and

inner ear. The outer ear includes the pinna and the external auditory

meatus. The pinna (or auricle) is an irregularly shaped structure of elas-

tic cartilage covered by a layer of perichondrium (connective tissue)

and thin skin. One important function of the pinna is to selectively

ﬁ lter higher frequency sound waves and, therefore, aid in spatial local-

ization of sounds in the environment. The external auditory meatus is

a tunnel that carries sound waves to the tympanic membrane, where

the compressions and rarefactions of air are converted into mechanical

vibrations. The outer third of the meatus is lined with a continuation of

the cartilage of the pinna and a continuation of the perichondrium and

thin skin that covers the pinna. In the inner two thirds of the meatus,

the skin adheres directly to the periosteum of the temporal bone. The

middle ear is an air-ﬁ lled cavity (tympanic cavity) that is separated from

the outer ear by the tympanic membrane. It contains three tiny bones,

the ossicles, which are the smallest bones in the body. These ossicles

transfer sound-induced movement of the tympanic membrane to the

ﬂ uid contained in the cochlea, where the vibrations are transduced into

nerve impulses. The middle ear is connected to the posterior region of

the nasopharynx by the auditory tube (eustachian tube), which allows

the equalization of air pressure on each side of the tympanic mem-

brane. The inner ear contains the cochlea and the vestibular apparatus.

The cochlea, the snail shell–shaped organ of hearing, includes a mem-

branous tube lying within a ﬂ uid-ﬁ lled tunnel in the temporal bone (see

Fig. 21-2). Auditory hair cells in the cochlea are excited by vibratory

movements of the ﬂ uid and generate action potentials in auditory nerve

ﬁ bers. The vestibular apparatus is the body’s sensory organ for bal-

ance and consists of membranous structures contained within the three

semicircular canals and the vestibule as well as some accessory struc-

tures (see Fig. 21-2). Vestibular hair cells within the semicircular ducts

(inside the semicircular canals) detect rotational movement of the head

in three dimensions. Vestibular hair cells within the utricle and saccule

(inside the vestibule) detect static head position and linear acceleration

in the horizontal and vertical planes, respectively. The auditory and

vestibular branches of CN VIII innervate these structures.

Figure 21-1B. Middle ear structures.

The tympanic membrane (viewed here from its medial

side) is a thin, semitransparent cone-shaped sheet of col-

lagenous ﬁ bers and ﬁ broblasts, covered on the outer side

by a very thin layer of skin and on the inner side by the

mucosa that lines the rest of the tympanic cavity. The ﬁ rst

of the ossicles, the malleus (hammer), is attached to the

upper half of the tympanic membrane, the incus (anvil) is

attached to the malleus by a saddle-shaped synovial joint,

and the stapes (stirrup) is attached to the incus by a ball-

and-socket synovial joint. The footplate of the stapes is

attached to the oval window of the vestibule. When sound

waves cause the tympanic membrane to vibrate, the chain

of ossicles rotates about the anterior malleal and posterior

incudal ligaments, causing the footplate of the stapes to

rock on the oval window and therefore producing waves of

compression in the ﬂ uid that ﬁ lls the bony labyrinth. This

lever arrangement increases the pressure on the oval win-

dow approximately 20-fold compared to the air pressure

on the tympanic membrane. The tensor tympani muscle

contracts during very loud sounds, reducing the movement

of the ossicles and the oval window.

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Figure 21-2. Major structures of the inner ear.

The bony labyrinth is a complex cavity in the petrous portion of the temporal bone, the hardest bone in the body. The cavity is lined

with endosteum and contains perilymph (blue shading), a clear ﬂ uid with a composition very similar to that of cerebrospinal ﬂ uid.

The bony labyrinth is divided into three portions, the vestibule, the cochlea (anteriorly), and the semicircular canals (posteriorly).

The membranous labyrinth, consisting of the three semicircular ducts, the utricle, and the saccule, lies within the bony labyrinth.

The walls of the membranous labyrinth are, in general, composed of a thin layer of ﬁ brous connective tissue; a thin layer of

more delicate, more vascularized connective tissue; and an internal lining of simple epithelium. Two saclike structures, the utricle and

saccule, are found in the vestibule. A semicircular duct lies within each of the semicircular canals. A similar but more complicated

structure, the cochlear duct, lies within the bony cochlea. These membranous structures are continuous with each other and contain

endolymph (yellow shading), a ﬂ uid with a high concentration of K

(potassium) ions that is unique in the body. Specialized regions

within the membranous labyrinth (indicated by thick orange lines) contain receptor cells innervated by branches of the vestibuloco-

chlear nerve (CN VIII). These include the cristae (singular, crista, from Latin for “crest”) in each of the ampullae (singular, ampulla,

from Latin for “ﬂ ask” or “bottle”) of the semicircular ducts, the maculae (singular, macula, from Latin for “spot”) of the utricle and

saccule, and the organ of Corti (spiral organ) in the cochlear duct. The receptor cells in all these regions are specialized epithelial cells

(hair cells) that transduce movements of the basilar membrane into nerve impulses (see Figs. 21-5C, 21-7, and 21-11A).

Maculae

Crista

Semicircular

canal

Semicircular

ducts

Endolymphatic sac

Endo-

lymphatic

duct

Ampulla

Vestibule

Cochlea

Round

window

Tympanic

membrane

Oval window

Bone

Utricle

Ampulla

Crista

Saccule

Vestibule

Maculae

Cochlea

Mastoid

cavities

Oval window

Semicircular

ducts

Semicircular

canal

Endolymphatic sac

Endo-

lymphatic

duct

Perilymphatic duct

(cochlear duct)

Dura mater

Scala vestibuli

Cochlear duct

(scala media inside)

Scala tympani

Subarachnoid space

Brain tissue

Tympanic

membrane

Round

window

Auditory

(Eustachian)

tube

Osseous

spiral

lamina

External auditory

meatus

Scala vestibuli

(perilymph)

Bone

Scala tympani

(perilymph)

Scala media

(endolymph)

Vestibular

(Reissner)

membrane

Organ of Corti

(spiral organ)

Basilar

membrane

Cross section of cochlea

CUI_Chap21.indd 413 6/2/2010 8:20:05 PM

414

UNIT 3

■

Organ Systems

Figure 21-3A. Membranous labyrinth.

The endolymph-ﬁ lled membranous labyrinth ﬂ oats within the perilymph-ﬁ lled osseous (bony) labyrinth (Fig. 21-2). This anatomically

correct drawing shows the position of the cochlea and semicircular canals within the head. The labyrinth is viewed from the direc-

tion indicated by the pointer in the small inset of the head. The position of the nerves that innervate the cochlea, cristae of the ampul-

lae, and maculae of the utricle and saccule are illustrated. The positions and planes of section of the photomicrographs that follow in

this chapter are indicated by blue lines, arrows, or boxes. The dashed oval indicates the position of the oval window in the osseous

labyrinth (Fig. 21-2).

Saccule

Facial nerve

(CN VII)

Cochlear nerve

(CN VIII)

Fig. 21-4B

Fig. 21-6

Fig. 21-9B

Ampulla

Fig. 21-3B

Fig. 21-4A

Fig. 21-8B

Fig. 21-10B,C

Vestibular nerve

(CN VIII)

Vestibular

ganglia

(of Scarpa)

Utricle

Endolymphatic sac

Semicircular

ducts

Cochlear

duct

Apex

of cochlea

Base

of cochlea

Position of

oval window

in osseous

labyrinth

Ductus reuniens

Cochlear

duct

Apex

of cochlea

Modiolus

Base

of cochlea

Bone

Cochlear

nerve

Osseous

spiral lamina

Spiral

ganglion

Spiral

ligament

Scala

tympani

Figure 21-3B. Cochlear duct and modiolus.

The membranous cochlear duct lies within a spiral-shaped tunnel

in the temporal bone. In this illustration, the cochlea has been cut

in half along the plane indicated by the single blue line in Figure

21-3A and in the inset. The spiral of the cochlear duct makes

about two and one-half turns from the base of the cochlea to its

apex. The outer bony surface of the cochlea is indicated in light

gray. A spiral, screw-shaped bony structure, the modiolus forms

the central core of the cochlea and is indicated in darker gray. A

spiral cavity within the modiolus contains the cell bodies of the spi-

ral ganglion (green) and the proximal axons of the cochlear nerve

(CN VIII). The osseous (bony) spiral lamina curves around the

modiolus like the threads of a screw (red dashed line and inset).

The central edge of the cochlear duct is attached to the spiral

lamina; the outer wall of the cochlear duct is attached to the spiral

ligament. Histological sections of these structures are shown in

Figures 21-4A and 21-4B.

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