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“VISUAL PATHWAY AND ITS LESIONS”
DR.TASNEEM.
VISUAL PATHWAY
• is the part of the central nervous system which enables organisms
to process visual detail.
• As well as enabling several non-image forming photoresponse
functions.
“VISUAL SYSTEM”
The visual system accomplishes a number of complex tasks,
including,
• Reception of light and the formation of monocular representations;
• The construction of a binocular perception from a pair of two
dimensional projections;
• The identification and categorization of visual objects;
• Assessing distances to and between objects;
• Guiding body movements in relation to visual objects.
“VISUAL SYSTEM”
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the visual system consists of:
The eye, especially the retina
The optic nerve
The optic chiasma
The optic tract
The lateral geniculate body
The optic radiation
The visual cortex
The visual association cortex
“VISUAL SYSTEM”
the principal visual pathways
the principal visual pathways from the two retinas to the visual cortex.
• The visual nerve signals leave the retinas through the optic nerves.
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At the optic chiasm, the optic nerve fibers from the nasal halves of
the retinas cross to the opposite sides, where they join the fibers
from the opposite temporal retinas to form the optic tracts.
• The fibers of each optic tract then synapse in the dorsal lateral
geniculate nucleus of the thalamus,
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From there, geniculocalcarine fibers pass by way of the optic
radiation (also called the geniculocalcarine tract) to the primary
visual cortex in the calcarine fissure area of the medial occipital
lobe.
HOW EYE WORK
• Light rays bounce off all objects.
• If a person is looking at a particular object, such as a tree, light is
reflected off the tree to the person's eye and enters the eye through
the cornea (clear, transparent portion of the coating that surrounds
the eyeball).
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Next, light rays pass through an opening in the iris (colored part of
the eye), called the pupil.
The iris controls the amount of light entering the eye by dilating or
constricting the pupil.
In bright light, for example, the pupils shrink to the size of a
pinhead to prevent too much light from entering.
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In dim light, the pupil enlarges to allow more light to enter the eye.
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Light then reaches the crystalline lens.
The lens focuses light rays onto the retina by bending (refracting)
them.
The cornea does most of the refraction and the crystalline lens finetunes the focus.
In a healthy eye, the lens can change its shape (accommodate) to
provide clear vision at various distances.
If an object is close, the ciliary muscles of the eye contract and
the lens becomes rounder.
To see a distant object, the same muscles relax and the lens
flattens.
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Behind the lens and in front of the retina is a chamber called the
vitreous body, which contains a clear, gelatinous fluid called
vitreous humor.
Light rays pass through the vitreous before reaching the retina.
The retina lines the back two-thirds of the eye and is responsible
for the wide field of vision that most people experience.
For clear vision, light rays must focus directly on the retina.
When light focuses in front of or behind the retina, the result is
blurry vision.
The retina contains millions of specialized photoreceptor cells
called rods and cones that convert light rays into electrical signals
that transmitted to the brain through the optic nerve.
Rods and cones provide the ability to see in dim light and to see in
color, respectively
The macula, located in the center of the retina, is where most of
the cone cells are located.
The fovea, a small depression in the center of the macula, has the
highest concentration of cone cells.
The macula is responsible for central vision, seeing color, and
distinguishing fine detail.
The outer portion (peripheral retina) is the primary location of rod
cells and allows for night vision and seeing movement and objects
to the side (i.e., peripheral vision).
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The optic nerve, located behind the retina, transmits signals from
the photoreceptor cells to the brain.
Each eye transmits signals of a slightly different image, and the
images are inverted.
Once they reach the brain, they are corrected and combined into
one image.
This complex process of analyzing data transmitted through the
optic nerve is called visual processing.
• The visual nerve signals leave the retinas through the optic nerves.
• At the optic chiasm, the optic nerve fibers from the nasal halves of
the retinas cross to the opposite sides, where they join the fibers
from the opposite temporal retinas to form the optic tracts.
• The fibers of each optic tract then synapse in the dorsal lateral
geniculate nucleus of the thalamus,
• From there, geniculocalcarine fibers pass by way of the optic
radiation (also called the geniculocalcarine tract) to the primary
visual cortex in the calcarine fissure area of the medial occipital
lobe.
“TARGETS OF VISUAL FIBERS TO OTHER AREAS OF BRAIN”
Visual fibers also pass to several older areas of the brain:
(1) from the optic tracts to the suprachiasmatic nucleus of the
hypothalamus, presumably to control circadian rhythms that
synchronize various physiologic changes of the body with night and
day.
(2) into the pretectal nuclei in the midbrain, to elicit reflex movements
of the eyes to focus on objects of importance and to activate the
pupillary light reflex.
(3) into the superior colliculus, to control rapid directional movements
of the two eyes.
(4) into the ventral lateral geniculate nucleus of the thalamus and
surrounding basal regions of the brain, presumably help control
some of the body’s behavioral functions.
“OPTIC PATHWAYS AND LESIONS”
• Axons of the ganglion cells form the optic nerve and optic
tract,ending in the lateral geniculate body of the thalamus.
• The fibers from each nasal hemiretina cross at the optic
chiasm,whereas the fibers from each temporal hemiretina remains
ipsilateral,therefore fibers from the left nasal hemiretina and fibers
from the right temporal hemiretina form the right optic tract and
synapse on the right lateral geniculate body.
• Fibers from the lateral geniculate body form the geniculocalcarine
tract and pass to the occipital lobe of the cortex.
• a,)cutting the optic nerve causes blindness in the ipsilateral eye.
• b)cutting the optic chiasm causes heteronymous bitemporal
hemianopia.
• c)cutting the optic tract causes homonymous contralateral
hemianopia.
• d)cutting the geniculocalcrine tract causes homonymous
hemianopia with macular sparing.