Download MBBS Anatomy PPT 42 Visual Pathway Notes

Visual pathway

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Visual pathway consists of a series of cells & synapses that

carry visual information from environment to brain for

processing.

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Components : Retina Optic nerve Optic chiasma

Optic tract

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Lateral geniculate body

Geniculostriate tract

Optic radiation

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to visual sensory area

occipital lobe 17,18 &19

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Visual field & retinal quadrant:

? One eye is closed.
? Area seen by open eye constitutes visual field of that

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eye.

? Visual field of the two eye overlap to a great extent.
? On either side there is a small area which is seen only by

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eye of that side.

? For convenience visual field is divided into right & left

halves.

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Optic nerve, optic chiasma & Optic tract

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? Optic nerve is made up of axons from ganglion cells of

retina

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? Fibers of optic nerve arising from four quadrants of

retina maintain same relative position with in nerve.

? Fibers of nasal half of each retina enter optic tract of

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opposite side after crossing in chiasma.

? Fibers from temporal half enter optic tract of same side.
? Optic tract carries these fibers to lateral geniculate body of

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corresponding side.

? Finally they are relayed into area 17, 18 & 19 of occipital

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cortex.

Lateral geniculate body

? Part of metathalamus

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? Grey matter in 6 layers
? Fibers from same side of

end in lamina 2, 3, & 5.

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? Fibers from opposite side

of eye end in 1, 4 & 6.

? Macular fiber end in central

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& posterior part of body &
this area is relatively large

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Geniculocalcarine tract & visual cortex

? Fibers arising from

lateral geniculate body

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form geniculocalcarine
tract or optic radiation.

? These fiber pass through

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retrolentiform part of
internal capsule.

? Radiation end in visual

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areas of cerebral cortex
(Area 17, 18 & 19)

? Cortex Occipital ?

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17,18 & 19
receives impulses
from retinal
halves of same
side ( from

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opposite halves of
field of vision)

? Cortical area of

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macula is much
larger than that for
peripheral area.

Visual Functional areas

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? Primary visual area: 17 occipital pole ? visual

perception

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? Visual association area- 18 & 19 ? parastriate

cortex ,

? Area -18 ? linear stimuli &

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? Area-19 ?angular stimuli.

? Higher visual association area- 39 ? angular

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gyrus of parietal lobe ? comprehension of
various signs & symbols of language by vision.

Visual area....

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? Visual Association area- 18 & 19 - correlation of

past and present visual experiences , assess distance
,speed, and orientation in 3d space.

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? Lesion- Visual agnosia ? person is unable to identify

an object or a person seen in past.

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Eyes & retina:

Fovea: central fixation

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point of each eye -
region of retina with
highest visual acuity.

Macula: oval region

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approximately 3-5 mm
that surrounds fovea,
also has high visual
acuity.

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Eyes & retina:

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Eyes & retina:

Optic disc: region where axons leaving retina
gather to form Optic nerve.

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Photoreceptors are absent over optic disc >>
creates small blind spot located 15 lateral
and inferior to central fixation point of each
eye.

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Photoreceptors:

Rods: more numerous than
cons-20:1, have poor spatial
& temporal resolution of

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visual stimuli, do not detect
colors >> vision in low level
lighting conditions.

Cons: less numerous, much

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more highly represented in
fovea >> have high spatial &
temporal resolution >> they
detect colors.

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Optic nerve, chiasma and tract:

Visual processing pathways:

Dorsal Pathway:

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Project to parieto-
occipital ass.
Cortex.

Ventral Pathway:

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Project to occipito-
temporal ass.
Cortex.

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Positive phenomenon:

? Light flashes >> retinal detachment.

? Rainbow-colored halos around objects >> acute

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glaucoma.

? Migraine: visual blurring, scotoma that have

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scintillating appearance or consist of jagged
alternating light and dark zigzag lines (fortification
scotoma).

? Pulsating colored lights/moving geometric shapes

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>> occipital seizures.

Describe the visual field defect ?

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Junctional scotoma: lesion at junction of

optic nerve and chiasm

Describe visual field defect ?

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Bitemporal Homonymous Hemianopia

Describe visual field defect ?

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Describe visual field defect ?

Left sector sparing homonymous hemianopia >> lesion at

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LGN.

Describe visual field defect ?

Right superior quadrantanopia >>

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temoporal lobe lesion

Describe visual field defect ?

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Left inferior quadrantanopia >> parietal

lobe lesion

Describe visual field defect ?

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Left homonymous hemianopia with

macular sparing

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Macular sparing:

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Watershed area with respect to blood
supply.

The `macular' visual cortex is supplied
by terminal branches of posterior &

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middle cerebral arteries.

Visual cortex subserving the
midperipheral & peripheral field is
supplied only by the PCA. The area is

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supplied by a more proximal `not

terminal' vessel.

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Optic disc drusen: globules of

Retinitis Pigmentosa

mucoproteins and

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mucopolysaccharides that
progressively calcify in the optic
disc.

Describe the visual field defect ?

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Left incongruous homonymous hemianopia

Describe visual field defect ?

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Right congruous homonymous hemianopia


Describe visual field defect ?

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Enlarged Blind Spot