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Figure 17-4a External Features and Accessory Structures of the Eye

Gross and superficialanatomy of the accessory structures

Sclera

Lateral canthus

Eyelashes

Pupil

Palpebra

Palpebral fissure

Medial canthus

Lacrimal caruncle

Corneal limbus

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Figure 17-5b The Sectional Anatomy of the Eye

Cornea

Sclera

Neural part

Pigmented part

Fibrouslayer

Neural layer(retina)

Anteriorcavity

Posteriorcavity

Vascular layer(uvea)

Iris

Ciliary body

Choroid

Horizontal section of right eye

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Figure 17-5c The Sectional Anatomy of the Eye

Lacrimal punctum

Nose

Lens

Edge ofpupil

Visual axis

Anterior cavity

Posteriorchamber

Anteriorchamber

Lacrimal caruncle

Medial canthus

Ciliaryprocesses

Ciliary body

Ora serrata

Ethmoidallabyrinth

Medial rectusmuscle

Optic disc

Optic nerve

Central arteryand vein

Horizontal dissection of right eye

Orbital fat

Fovea

Lateral rectusmuscle

Posteriorcavity

Retina

Choroid

Sclera

Lateralcanthus

Lower eyelid

Conjunctiva

Corneal limbus

Suspensory ligament of lens

Iris

Cornea

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Figure 17-6 The Pupillary Muscles

Pupillary constrictor(sphincter)

Pupil

The pupillary dilatormuscles extend radially awayfrom the edge of the pupil.Contraction of these musclesenlarges the pupil.

Pupillary dilator(radial)

Decreased light intensityIncreased sympathetic stimulation

Increased light intensityIncreased parasympathetic stimulation

The pupillary constrictormuscles form a series ofconcentric circles around thepupil. When these sphinctermuscles contract, the diameterof the pupil decreases.

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Figure 17-5c The Sectional Anatomy of the Eye

Lacrimal punctum

Nose

Lens

Edge ofpupil

Visual axis

Anterior cavity

Posteriorchamber

Anteriorchamber

Lacrimal caruncle

Medial canthus

Ciliaryprocesses

Ciliary body

Ora serrata

Ethmoidallabyrinth

Medial rectusmuscle

Optic disc

Optic nerve

Central arteryand vein

Horizontal dissection of right eye

Orbital fat

Fovea

Lateral rectusmuscle

Posteriorcavity

Retina

Choroid

Sclera

Lateralcanthus

Lower eyelid

Conjunctiva

Corneal limbus

Suspensory ligament of lens

Iris

Cornea

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Figure 17-7a The Organization of the Retina

Amacrine cell

Horizontal cell Cone Rod

Pigmentedpart of retina

Rods andcones

Bipolar cells

Ganglion cells

LIGHT

The cellular organization of the retina. The photoreceptors are closest to the choroid, rather than near the posterior cavity (vitreous chamber).

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Figure 17-7a The Organization of the Retina

Choroid

Pigmentedpart of retina

Rods andcones

Bipolar cells

Ganglion cells

The cellular organization of the retina. The photoreceptors are closest to the choroid, rather than near the posterior cavity (vitreous chamber).

Retina

Nuclei ofganglion cells

Nuclei of rodsand cones

Nuclei ofbipolar cells

LM 350

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Figure 17-7b The Organization of the Retina

Central retinal vein

Central retinal artery

Sclera

ChoroidOptic nerve

Optic disc

The optic disc in diagrammatic sagittal section.

Pigmentedpart of retina

Neural part of retina

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Figure 17-7c The Organization of the Retina

Fovea

Macula

A photograph of the retina as seen through the pupil.

Central retinal artery and veinemerging from center of optic disc

Optic disc(blind spot)

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Figure 17-8 A Demonstration of the Presence of a Blind Spot

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Figure 17-9 The Circulation of Aqueous Humor

Cornea

Pupil

Lens

Scleral venous sinus

Body of iris

Conjunctiva

Ciliary body

Sclera

Choroid

Retina

Posterior cavity(vitreous chamber)

Anterior cavity

Anterior chamber

Posterior chamber

Ciliary process

Suspensoryligaments

Pigmentedepithelium

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Figure 17-12a Image Formation

Light from a point at the top of anobject is focused on the lowerretinal surface.

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Figure 17-12b Image Formation

Light from a point at the bottom ofan object is focused on the upperretinal surface.

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Figure 17-12c Image Formation

Light rays projected from a verticalobject show why the image arrivesupside down. (Note that the image isalso reversed.)

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Figure 17-12d Image Formation

Light rays projected from a horizontalobject show why the image arriveswith a left and right reversal. Theimage also arrives upside down. (Asnoted in the text, these representa-tions are not drawn to scale.)

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Figure 17-11 Accommodation

For Close Vision: Ciliary Muscle Contracted, Lens Rounded

Lens rounded

Ciliary musclecontracted

Focal pointon fovea

Lens flattened

Ciliary musclerelaxed

For Distant Vision: Ciliary Muscle Relaxed, Lens Flattened

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Figure 17-11a Accommodation

For Close Vision: Ciliary Muscle Contracted, Lens Rounded

Lens rounded

Ciliary musclecontracted

Focal pointon fovea

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Figure 17-11b Accommodation

Lens flattened

Ciliary musclerelaxed

For Distant Vision: Ciliary Muscle Relaxed, Lens Flattened

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Figure 17-13 Refractive Problems (Part 5 of 5).

Surgical Correction

Variable success at correcting myopia andhyperopia has been achieved by surgery thatreshapes the cornea. In photorefractivekeratectomy (PRK) a computer-guidedlaser shapes the cornea to exact specifications.The entire procedure can be done in less thana minute. A variation on PRK is called LASIK(Laser-Assisted in-Situ Keratomileusis). In thisprocedure the interior layers of the cornea arereshaped and then recovered by the flap of originalouter corneal epithelium. Roughly 70 percent of LASIKpatients achieve normal vision, and LASIK has become the mostcommon form of refractive surgery. Even after surgery, many patients still need reading glasses,and both immediate and long-term visual problems can occur.

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Figure 17-14a Structure of Rods, Cones, and Rhodopsin Molecule.

In a cone, the discs are infoldings of

the plasma membrane, and the outer

segment tapers to a blunt point.

In a rod, each disc is an independent

entity, and the outer segment forms

an elongated cylinder.

Discs

Connecting

stalks

Mitochondria

Golgi

apparatus

Nuclei

Pigment Epithelium

The pigment epithelium

absorbs photons that are

not absorbed by visual

pigments. It also

phagocytizes old discs

shed from the tip of the

outer segment.

Melanin granules

Outer Segment

The outer segment of a

photoreceptor contains

flattened membranous

plates, or discs, that

contain the visual pigments.

Cone Rods

Inner Segment

The inner segment contains

the photoreceptor’s major

organelles and is responsible

for all cell functions other

than photoreception. It also

releases neurotransmitters.

Each photoreceptor

synapses with a bipolar cell.

Bipolar cell

LIGHT

Structure of rods and conesa

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Figure 17-14b Structure of Rods, Cones, and Rhodopsin Molecule

In a rod, each disc is an independententity, and the outer segment formsan elongated cylinder.

Rhodopsinmolecule

OpsinRetinal

Structure ofrhodospin molecule.

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Figure 17-16 Photoreception (Part 8 of 8).

IN LIGHT

ACTIVE STATE

−70 mV

The reduction in the rate

of Na+ entry reduces the dark

current. At the same time, active

transport continues to export

Na+

from the cytoplasm.

When the sodium

channels close, the membrane

potential

drops toward –70 mV. As the

plasma membrane

hyperpolarizes, the rate

of neurotransmitter

release decreases. This decrease

signals the adjacent bipolar cell

that

the photoreceptor has absorbed

a photon. After absorbing a

photon, retinal does not

spontaneously revert to

he 11-cis form. Instead,

the entire rhodopsin molecule

must be broken down into retinal

and

opsin, in a process called

bleaching. It is then

reassembled.

Na +

Dark current isreduced and rate ofneurotransmitterrelease declines

4

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Figure 17-17 Bleaching and Regeneration of Visual Pigments.

Na+

Na+

2Opsin activation changesthe Na+ permeability of theouter segment, and thischanges the rate ofneurotransmitter releaseby the inner segment atits synapse with a bipolar cell.

3

1On absorbing light, retinal changesto a more linear shape. This changeactivates the opsin molecule.

11-trans retinal

Photon

ADP ATP

6

11-cis retinal andopsin are reassembled

to form rhodopsin.

Opsin

Opsinenzyme

11-cisretinal

11-transretinal

Once the retinal has beenconverted, it can recombinewith opsin. The rhodopsinmolecule is now readyto repeat the cycle. Theregeneration processtakes time. After exposureto very bright light,photoreceptors areinactivated while pigmentregeneration is under way.

4

5

Neuro-transmitterrelease

Bipolarcell

Ganglioncell

Changes inbipolar cellactivity aredetected byone or moreganglion cells.The location ofthe stimulatedganglion cellindicates thespecific portionof the retinastimulated bythe arrivingphotons.

After absorbing aphoton, the rhodopsinmolecule begins tobreak down into retinaland opsin. This isknown as bleaching.

The retinal is converted toits original shape. Thisconversion requires energyin the form of ATP.

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Figure 17-15 Cone Types and Sensitivity to Color.

RodsRedcones

Bluecones

Violet Blue Green Yellow Orange Red400 450 500 550 600 650 700

WAVELENGTH (nm)

Greencones

100

75

50

25

0

Lig

ht

abso

rpti

on

(per

cen

t o

f m

axim

um

)

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Figure 17-20 The Visual Pathways

Combined

Left side Right side

onlyRight eye

onlyBinocular visionLeft eye

Retina

Optic disc

SuprachiasmaticnucleusDiencephalon

andbrain stem

The VisualPathway

Photoreceptorsin retina

Optic nerve(N II)

Optic chiasm

Optic tract

Lateralgeniculate

nucleus

Superiorcolliculus

Right cerebralhemisphere

Left cerebralhemisphere

Projection fibers(optic radiation)

Visual cortexof cerebral

hemispheres

Visual Field

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