Download MBBS Anatomy PPT 58 Visual Pathway Notes

Visual pathway

Visual pathway consists of a series of cells & synapses that

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carry visual information from environment to brain for

processing.

Components : Retina Optic nerve Optic chiasma

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Optic tract

Lateral geniculate body

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Geniculostriate tract

Optic radiation

to visual sensory area

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occipital lobe 17,18 &19


Visual field & retinal quadrant:

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

eye.

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

eye of that side.

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? For convenience visual field is divided into right & left

halves.

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Optic nerve, optic chiasma & Optic tract

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

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retina

? Fibers of optic nerve arising from four quadrants of

retina maintain same relative position with in nerve.

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

opposite side after crossing in chiasma.

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

corresponding side.

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

cortex.

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Lateral geniculate body

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

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end in lamina 2, 3, & 5.

? Fibers from opposite side

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of eye end in 1, 4 & 6.

? Macular fiber end in central

& posterior part of body &

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this area is relatively large


Geniculocalcarine tract & visual cortex

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? Fibers arising from

lateral geniculate body
form geniculocalcarine
tract or optic radiation.

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? These fiber pass through

retrolentiform part of
internal capsule.

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? Radiation end in visual

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

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? Cortex Occipital ?

17,18 & 19
receives impulses

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from retinal
halves of same
side ( from
opposite halves of
field of vision)

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? Cortical area of

macula is much
larger than that for

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peripheral area.
Visual Functional areas

? Primary visual area: 17 occipital pole ? visual

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perception

? Visual association area- 18 & 19 ? parastriate

cortex ,

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? Area -18 ? linear stimuli &

? Area-19 ?angular stimuli.

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? Higher visual association area- 39 ? angular

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

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Visual area....

? Visual Association area- 18 & 19 - correlation of

past and present visual experiences , assess distance

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,speed, and orientation in 3d space.

? Lesion- Visual agnosia ? person is unable to identify

an object or a person seen in past.

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Eyes & retina:

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Fovea: central fixation
point of each eye -
region of retina with
highest visual acuity.

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Macula: oval region
approximately 3-5 mm
that surrounds fovea,
also has high visual
acuity.

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Eyes & retina:


Eyes & retina:

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

Photoreceptors are absent over optic disc >>

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creates small blind spot located 15 lateral
and inferior to central fixation point of each
eye.

Photoreceptors:

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Rods: more numerous than
cons-20:1, have poor spatial
& temporal resolution of
visual stimuli, do not detect

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colors >> vision in low level
lighting conditions.

Cons: less numerous, much
more highly represented in

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fovea >> have high spatial &
temporal resolution >> they
detect colors.

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Optic nerve, chiasma and tract:

Visual processing pathways:

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Dorsal Pathway:
Project to parieto-
occipital ass.
Cortex.

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Ventral Pathway:
Project to occipito-
temporal ass.
Cortex.

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Positive phenomenon:

? Light flashes >> retinal detachment.

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? Rainbow-colored halos around objects >> acute

glaucoma.

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? Migraine: visual blurring, scotoma that have

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

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? Pulsating colored lights/moving geometric shapes

>> occipital seizures.

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Describe the visual field defect ?

Junctional scotoma: lesion at junction of

optic nerve and chiasm

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Describe visual field defect ?

Bitemporal Homonymous Hemianopia

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Describe visual field defect ?


Describe visual field defect ?

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Left sector sparing homonymous hemianopia >> lesion at

LGN.

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Describe visual field defect ?

Right superior quadrantanopia >>

temoporal lobe lesion

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Describe visual field defect ?

Left inferior quadrantanopia >> parietal

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lobe lesion

Describe visual field defect ?

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Left homonymous hemianopia with

macular sparing

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Macular sparing:

Watershed area with respect to blood
supply.

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The `macular' visual cortex is supplied
by terminal branches of posterior &
middle cerebral arteries.

Visual cortex subserving the

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

supplied by a more proximal `not

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terminal' vessel.

Optic disc drusen: globules of

Retinitis Pigmentosa

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mucoproteins and
mucopolysaccharides that
progressively calcify in the optic
disc.

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Describe the visual field defect ?

Left incongruous homonymous hemianopia

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Describe visual field defect ?

Right congruous homonymous hemianopia

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Describe visual field defect ?

Enlarged Blind Spot