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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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