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Flashcards in Visual System Deck (77)
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1

1/2 of Brain Mass Devoted to Visual Processing

-vast amount of usable information
-Perception: occipital cortex
-Visual memory: parietal & temporal lobes
-Brain stem/spinal cord: visual reflexes
-Circadian rhythm, general metabolic rate, hormonal function, mood- pineal gland & diencephalon

2

Visual Testing

-easily administered
-locates the lesion
-cognitive status: visual memory
-motor reflexes: postural reflex
-autonomic function: pupil dilation
-inner ear problem: eye movement

3

Incoming Light from Object of interest?

-focused on fovea
-refracted by cornea
-inverted through pupillary opening
-variably refracted by the lens (under control of suspensory ligaments/ciliary muscles)
-Projected on the fovea (pit) in center of the macula

4

Sclera

protection

5

Choroid

vascular layer, high blood flow

6

Fovea

devoid of blood vessels

7

Extraocular Muscles

-maintain visual axis on fovea

8

Retina

-external view of blood vessels & neural tissues
-more cell types than most organs
-retinal pigmented epithelial cells (pigment cells)
-provide barrier to retina from the choroid
-photoreceptors (cones & rods) come in contact with the RPE (cell bodies located in outer nuclear layer with projections lying in outer plexiform layer
-photoreceptor signal through the outer plexiform layer to the bipolar cells, whose cell bodies are in the inner nuclear layer, final layer of neurons are retinal ganglion cells

9

Regional Differences in the Retina

-photoreceptors line outermost surface
-pigmented epithelium support photoreceptors
-retinal pathway: photoreceptors to bipolars to ganglion cells
-Interneurons: horizontal cells (outer plexiform) amacrines (inner plexiform)
-ganglion cell axons coalesce to form optic nerve head; blind spot
-Fovea: all cellular elements except outer segments are displaced radially

10

Retinal Pigmented Epithelium

-layer that separates the choroid (vascular) from retina (neural)
-contains black pigment-melanin
-diseases of retina often affect tight junctions b/w the RPE and the retina-allows substances to leak in & out
-phagocytosis of outer rod segments, retinal nutrition, & protection of photoreceptors from light damage

11

Drusen

-autoflurescence from the dec. in effectiveness of RPE to eat rod outer segments every 10 days (more proteins left in RPE)
-key factor in dry macular degeneration

12

Rods

-named for shape of outer segment
-process Black & White (scotopic) info
-light is detected & transduced in outer segment
-synaptic expansion called the spherule
-rod outer segments undergo phagocytosis-with a new outer segment being formed approximately every 10 days

13

Cones

-named for shape of outer segment
-process color (photopic) info
-3 types of cones:
L-cones-long wavelengths (red)
M-cones-medium wavelengths (green)
S-cones-short wavelengths (blue)
-each of these responds to different color of light
-every color is represented by a unique combination of L,M, & S cones
-if one type of cone is absent b/c of genetic defects - person will be missing the corresponding opsin = color blind
-M & L

14

Function of Photoreceptors

-purpose is to absorb quanta of light & convert to an electrical signal
-both cones & rods undergo hyperpolarization in response to light
-only sensory system in which hyperpolariation is the response to stimulus

15

Visual Steps of Rods

1. light in & converst 11-cis-retinal to all-trans retinal which activates the rod opsin (rhodopsin)
2. active rhodopsin can activate GTP (activates cGMP phosphodiesterase)
3. Active PDE lowers cGMP levels to hyperpolarize the membrane & close sodium & calcium channels (visual response)
4. Na & Ca stuck in membrane, Ca begins to be pumped out through the Na/Ca exchanger, as Ca dec. cGMP levels rise & re-polarize the membrane to pre-stumulus levels (channel re-opens)
5. SLOWLY: rhodopsin is dephosphorlated and all-trans-retinal is converted back to 11-cis-retinal
-now rhodopsin kinase comes back to re-phosphorylate rhodopsin which is bound by arrestin to render it inactive REPEAT

16

Phototransduction in cones

-simular to rods except there are 3 opsins with each corresponding to retinal in a specific manner

17

Bipolar Cells

-High deg. of convergence - 100 million receptor cells synapsing onto 1 million ganglion cells
-at the bipolar-ganglion cell synapse, amacrine cell synapse, amacrine cells detect major changes in activity levels
-cones & rods have their own bipolar cells with cone bipolar cells in the central retina & rod in the periphery

18

Ganglion Cells

1. alpha: predominate in peripheral retina (most input from rods)
-extensive dendritic trees
-large axons
-participate little in color perception
-project to Magnocellular layer of lateral geniculate nucleus (location of object in space)
2. beta: primarily in central retina
-small receptive fields
-small dendritic arbors
-responsive to color stimuli
-project to parvocellular region in lateral geniculate nucleus - define color & texture of object

19

Fovea

-1 photoreceptor/1 ganglion cell
-allows for crisp image representation

20

Central Visual Pathways

-visual image is reversed & inverted due to pinhole effect of pupil
-180deg. visual field viewed by both eyes except 45deg. peripherally due to nose
-ganglion cell axons in optic nerve partially decussate in optic chiasm (only nasal fibers cross)
-optic tract projects to lateral geniculate
-optic radiations carry geniculate fibers to primary visual cortex in occipital lobe (all fibers are retinotopically organized)
-Left visual field is processed by right visual cortex

21

Visual Cortex Information

-cells that respond perferentially to input from one eye(ocular dominant) are segregated into layers in the lateral geniculate & the visual cortex
-the M pathway originates from magnocellular ganglion cells, projects to lateral geniculate layers 1 and 2, to layer 4C-alpha in cortex (space info)
-the P pathway originates from parvocellular ganglion cells, projects to lateral geniculate layers 3-6, to layer 4C-beta in cortex (form info)

22

Vision Related Diseases

1. age related macular degeneration
2. inherited retinal degeneration
3. glaucoma
4. retinal detachment
5. diabetic retinopathy
6. vascular insults
7. multiple sclerosis
8. pituitary tumor
9. cortical infarctions/tumor
10. visual migraines

23

What causes DR?

1. Changes in vasculature
-angiogenic growth factors
-loss of angiostatic growth factors
2. Changes in neurons
-loss of photoreceptors
-loss of ganglion cell

Don't Know!

24

Macular Degeneration VS Diabetic Retinopathy

AMD: DR:
Choroid NV retina NV
age induced no age effect
some genetic some genetic
lose central vision lose all vision

25

Dry AMD VS wet AMD

Dry AMD: Wet AMD:
Drusen Sometimes drusen
little vision loss vision loss
often in 1 eye progresses to both eyes
can progress to wet

26

Causes of AMD?
Treatments?

age
smoking
genetics
changes in angiogenic & angiostatic factors
Treatments:
laser coagulation
anti-angiogenic therapies: Avastin & Macugen

27

Scotomas

-blind spot that moves exactly as the eye moves

28

Floaters

-float around eye even when eye movement stops

29

Amblyopia

-"lazy eye"

30

Causes of Vision Loss

-Refractive Error
-Vitreous
-Cornea/Tear Film Anterior Chamber
-Optic Nerve or Optic Tract or Macula Chiasm
-Optic Radiations
-Occipital Cortex