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VisionFRESHMEN YEAR PROGRAM

MEDICAL FACULTYUNIVERSITAS ISLAM BANDUNG

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Objectives

When you have completed this section, you shouldbe able to

Explain how the optical system of the eye creates animage on the retina;

Explain how the retina converts this image to nerveimpulses;

Explain why different types of receptor cells andneuronal circuits are required for day and night

vision; and Trace the visual projection pathways in the brain.

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Formation of an image

The visual process begins when light raysenter the eye

The light rays is refracted by the optic system

of the eye (cornea, aqueous humor, lens, andvitreous humor)

The image falls on the central fovea of the

retina The produced image is a real tiny inverted

image

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Formation of an Image

The visual process begins when light rays enter theeye, focus on the retina, and produce a tiny

invertedimage

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Focusing the image

The image must fall exactly on yellow spot of

the retina to get the clearest figure. This

process is provided by:

Constriction of the pupil

Constriction of the pupil function to minimize the

aberration

Accommodation Accommodation function to fall the image exactly on

the fovea centralis

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Focusing the image

Constriction of the pupil

Lenses cannot refract light rays at their edges as well as

they can closer to the center.

The image produced by any lens is therefore somewhat

blurry around the edges; this spherical aberration is quite

evident in an inexpensive microscope.

It can be minimized by screening out these peripheral light

rays, and for this purpose, the pupil constricts as you focus

on nearby objects. The pupil thus has a dualpurposeto adjust the eye to

variations in brightnessand to reduce spherical aberration.

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ABERRATION

ABERRATIONGREAT TINY

OPENING

LARGE SMALL

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Focusing the image

Accommodationis a change in the curvature of the lens thatenables you to focus on a nearby object.

During accommodation the ciliary muscle surrounding the lenscontracts. This narrows the diameter of the ciliary body, relaxesthe fibers of the suspensory ligament, and allows the lens to

relax into a more convex shape. In emmetropia, the lens is about 3.6 mm thick at the center; in

accommodation, it thickens to about 4.5 mm. A more convex lensrefracts light more strongly and focuses the divergent light raysonto the retina. The closest an object can be and still come into

focus is called the near point of vision. It depends on theflexibility of the lens. The lens stiffens with age, so the near pointaverages about 9 cm at the age of 10 and 83 cm by the age of60.

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Pupillary miosisand lens

accommodation

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Near Vision

Firing of parasympathetic nerves

to ciliary muscle

Contraction of ciliary muscle

Relaxation of zonular fibers

Relaxataion of lens so that it

becomes more spherical

Near object brought into focus

Accommodation

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Accomodation

Zonula zinii

Corpus ciliaris

Lens

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Accomodation

Near objects brought into focus

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Nearsighted is corrected by concave lens

In nearsighted eye the image falls in front

of the retina

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Farsighted is corrected by convex lens

In farsighted eye the image falls behind

the retina

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PHOTORECEPTORS

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PHOTORECEPTORS

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Mechanism of

Generating VisualSignals.

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Photoreceptor Cells

The photoreceptor cells in the retina are called rods

and cones because of the shapes of their light-

sensitive tips.

A pigmented layer which lies behind the retina, absorbslight and prevents its reflection back to the rods and

cones, which would cause the visual image to be

blurred.

The rods are extremely sensitive and respond to verylow levels of illumination, whereas the cones are

considerably less sensitive and respond only when the

light is brighter than, for example, twilight.

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Photoreceptor Cells

The photoreceptors contain molecules calledphotopigments, which absorb light.

There are four different photopigments in the

retina, one (rhodopsin) in the rods and one ineach of the three cone types.

Each photopigment contains an opsin and achromophore.

Opsin is a collective term for a group of integralmembrane proteins, one of which surrounds and

binds a chromophore molecule.

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Photoreceptor Cells

The chromophore, which is the actual light-sensitive partof the photopigment, is the same in each of the fourphotopigments and is retinal, a derivative of vitamin A.

The opsin differs in each of the four photopigments.

Since each type of opsin binds to the chromophore in adifferent way and filters light differently, each of the fourphotopigments absorbs light most effectively at adifferent part of the visible spectrum. For example, onephotopigment absorbs wavelengths in the range of red

light best, whereas another absorbs green light best.

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Photoreceptor Cells

Within the photoreceptor cells, the photopigments lie inspecialized membranes that are arranged in highlyordered stacks, or discs, parallel to the surface of theretina.

Light activates retinal, causing it to change shape. Thischange triggers a cascade of biochemical events thatlead to hyperpolarization of the photoreceptor cellsplasma membrane and, thereby, decreased release ofneurotransmitter (glutamate) from the cell. The decrease

in neurotransmitter then causes the bipolar cells, whichsynapse with the photoreceptor cell, to undergo ahyperpolarization in membrane potential.

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Photoreceptor Cells

After its activation by light, retinal changes

back to its resting shape by several

mechanisms that do not depend on light

but are enzyme mediated. Thus, in thedark, retinal has its resting shape, the

photoreceptor cell is partially depolarized,

and more neurotransmitter is beingreleased.

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Photoreceptor Cells

When one steps back from a place of bright sunlight into a

darkened room, dark adaptation, a temporary blindness,

takes place. In the low levels of illumination of the darkened

room, vision can only be supplied by the rods, which have

greater sensitivity than the cones. During the exposure to

bright light, however, the rods rhodopsin has been

completely activated. It cannot respond fully again until it is

restored to its resting state, a process requiring some tens

of minutes. Dark adaptation occurs, in part, as enzymes

regenerate the initial form of rhodopsin, which can respond

to light.

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Color Vision

Most nocturnal vertebrates have only rod cells, but manydiurnal animals are endowed with cones and colorvision.

Color vision is especially well developed in primates. It is

based on three kinds of cones named for the absorptionpeaks of their photopsins: blue cones, with peaksensitivity at 420 nm; green cones, which peak at 531nm; and red cones, which peak at 558 nm. Red conesdo not peak in the red part of the spectrum (558 nm light

is perceived as orangeyellow), but they are the onlycones that respond at all to red light.

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Color Vision

Our perception of different colors is based on amixture of nerve signals representing coneswith different absorption peaks.

Light at 400 nm excites only the blue cones,but at 500 nm, all three types of cones arestimulated. The red cones respond at 60% oftheir maximum capacity, green cones at 82%

of their maximum, and blue cones at 20%. Thebrain interprets this mixture of signals as blue-green.

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Color Vision

Some individuals have a hereditary lack of onephotopsin or another and consequently exhibit colorblindness.

The most common form is red-green color blindness,

which results from a lack of either red or green conesand renders a person incapable of distinguishing theseand related shades from each other.

The normal person has trichromaticcolor vision Red-green color blindness is a sex-linked recessive

trait. It occurs in about 8% of males and 0.5% offemales.

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What do you see in the picture?

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Colorblindness charts

What do you see here?

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The Visual Projection Pathway

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