Eye And Retina

August 3, 2012 by Dr. Tangen Leave a Comment

Eye Chart

Eye: a white pressurized ball lined with a net of vision processors

Retina literally means a net. And it’s a good description of the multi-layers system of cells that lines the back of the eye. There are receptors for color (3 types of cones) and shades of gray (rods). And lots of other neural structures that support the process. Vision has been studied extensively. So there is much to learn about how the eyes operate.

Here’s what is included in this lesson:

horizontal cells for sharp images
target detection vs. target identification
2 separate but cooperative systems of vision
why the eye is wired backwards

Here are the resources you need:

cluster
slides
terms
notes
quiz
videos
- Eyes 01
- Eyes 02
- Eyes 03
- Eyes 04
- Eyes 05

SLIDES

TERMS

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4:1 ratio
900 free-floating lamellae
amacrine cells
amplitude
aqueous humor
astigmatism
astrocytes
blind spot
blood vessels
cataract
choroid
color
cones
cornea
crystalline
cytoplasm
drusen
dry macular degeneration (nonexudative)
electromagnetic energy
epithelial
eye
feet of Müller cells
floaters
fovea
fovea centralis
frequencies
ganglion cells
glaucoma
horizontal cells
hyperpolarization
inner limiting membrane
intensity
iris
l cones
lamellae
lateral inhibition
lazy eye (amblyopia)
lens
light
light collectors
m cones
macula
macular degeneration
midget ganglion cells
Müller cells (glia)
night vision
nystagmus
optic nerve (optic tract)
parafovea
perceived color
periforvea
photopic system
photopigments
pigment
posterior vitreous detachmentor (PVD)
pupil
radial = open pupil
resting potential
retina
retinal circulatory system
rhodopsin (visual purple)
rod spherule
rods
s cones
satrabismus
Schlemm’s canal
sclera
sclerosis
scotopic system
spatial summation
squinting
steady state information
strabismic amblyopia
target detection
target identification
transparent
visual purple (rhodopsin)
vitreous humor
wavelength
wet macular degeneration (exudative)
white of the eye
zonules

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NOTES

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Electromagnetic Energy
Light
Frequency
- How often it comes
- Wave length
  - Peak to peak
  - Long length is slow
  - Color
Amplitude
- How tall wave
- Intensity: brightness
Absorb & Reflect
- Light sources:
  - sun, light bulb, candle, moon
- Reflection based on:
- Relative distance
- Object color & smoothness
- Perceived color is reflected
- Everything but purple absorbed
Eye
- Transducer of light into neural signal
- Sclera
  - Greek for hard
  - 1 mm thick
  - Fibrous strands in parallel; like fiber strapping tape
  - White of the eye
  - Covers entire ball
  - Not cornea & optic nerve exit
  - Fibers resist internal pressure
    - twice the atmosphere
- Muscles
  - Held-moved by 6 tiny muscles
  - Nystagmus = can’t hold eyes still
  - Strabismu (strabismic amblyopia)
    - Lazy Eye or Amblyopia
    - Eyes don’t point same direction
    - Two don’t help perceive depth
    - Treatment
      - Patch over active eye
      - Play action video games
- Cornea
  - Bulges out from sclera
  - Smooth, neatly organized
  - Transparent (no blood vessels)
  - Very sensitive to touch (close lid)
  - Nourished by tears (on outside)
    - aqueous humor (on inside)
  - 2/3 of focus of eye
  - Dome-shaped
    - Irregularity of surface
      - Astigmatism = blurry lines
      - Astigmatism Symptoms
        
        squinting & blurred vision
        
        headaches, eye strain
      - Cornea warping
        
        Blurred vision for lines in one direction
      - Treatment
        
        Glasses before age 3-4 years
- Aqueous Humor
  - Spongy tissue
  - Keeps eye inflated
  - Removes waste
  - Mostly water
    - Also an antioxidant
    - Protects from UV rays
  - Provides oxygen, nourishment to cornea & lens
  - Continuously refreshed
    - In from ciliary body
    - Drained into Schlemm’s canal
  - Glaucoma
    - Blockage of aqueous humor
    - Damage to iris
    - Blindness
- Iris
  - 2 layers
  - Outer layer of pigment
  - Color part of eye
  - Can be translucent (albinos)
  - Inner layer of blood vessels
- Pupil of the Iris
  - Hole in middle of iris
  - 2 sets of muscles
    - circular = close pupil
    - radial = open pupil
  - Varies in size (4:1 ratio)
    - Allows 16: 1 ratio of light
      - actual ratio changes with age
      - in dim light, 80 yr old has half as wide opening as 20 yr old
    - Advantages of small opening = depth of field
- Lens
  - Held in place by strings (zonules)
  - Suspended
  - Crystalline (clear crystal-like proteins)
  - Bean shaped
    - Diameter & thick of large aspirin
  - Has no blood vessels
    - Mostly water & protein
  - 3 parts
    - elastic covering
      - changes shape of lens
      - controls flow of aqueous humor
    - epithelial
      - toward edge of lens
      - synthesizes proteins
    - lens
      - Never stops growing
      - Adds fibers to edge
        
        center becomes thin
        
        some center fibers there at birth
  - As ages
    - more dense & hard (sclerosis)
    - less transparent (cataract)
    - Can be irregularly shaped
    - Can cause astigmatism, but not common
  - Cataracts
    - Born with cloudy lens
    - If surgically repaired
      - at 2-6 months old
      - eventually nearly normal vision
    - Early cataract in left eye
      - limits visual info to right hem.
      - face recognition
- Vitreous Humor
  - Jelly-like, like raw egg whites
  - Not continuously renewed
  - Floaters
  - More liquid with age
  - Can become detached
    - posterior vitreous detachmentor (PVD)
- Retinal Circulatory System
  - 1 of 2 blood supplies
    - in front of the retina
    - leaves shadows on retina; brain ignores
    - steady state information
  - Supplies nourishment to non-receptor structures
    - (ganglion, horizontal cells, etc.)
- Retina
  - Inner limiting membrane
    - Vitreous humor & retina
    - Formed by astrocytes
  - Feet of Müller cells: inner limiting membrane
  - Müller cells (glia): support cells for retina
    - Act as light collectors
    - Like a fiber optic plate
    - Funnels light to rods & cones
- Macula
  - Depression in retina
  - Unobstructed
  - Near center
  - Off to side
- Macular Degeneration
  - Dry (nonexudative)
    - Cellular debris (drusen)
    - Yellow deposits grow between retina & choroid
    - Drusen deposits grow, retina becomes detached
    - Severity depends on size and # of drusen
  - Wet (exudative)
    - Choroid blood vessels grow
    - Retina becomes detached
    - Blood vessels from choroid (blood system)
    - More severe
    - Treatments
    - Laser coagulation and meds
  - Older adults (major cause of blindness)
    - Loss of vision in center
    - Can’t read or recognize faces
    - Lose most detail of images
- Fovea
  - Also called fovea centralis
  - In center of macula
  - Fine details & sharp images
  - In center of macula
  - Most cones are here
  - No S-cones
  - Fovea = L & M cones; v. sharp
  - Parafovea = S & rods; sharpish
  - Periforvea = Outer region, poor acuity, mostly rods
- Retina
  - Net of layers
  - Ganglion cells = to brain
  - Amacrine cells = interneurons
  - Bipolar cells = receptor output
  - Horizontal cells = sharp edges
  - Rod & Cones
    - Two separate system; work together & separately
    - Scotopic system (rods)
    - Photopic system (cones)
- Rods
  - Outside rods
    - narrow and cylindrical in shape
    - filled with rod disks
  - Inside rods
    - Rod disks
      - 900 free-floating lamellae
      - Floating in cytoplasm
    - Contain visual purple (rhodopsin)
      - Like ink in laser printer
    - Can’t process purple light
    - Cell nucleus
    - Rod spherule = fiber ending in a single end-bulb
  - Polarization
    - Normal neuron
    - -70mV resting potential
    - depolarizes to +40mV.
  - Rods resting potential is -30mV
    - Hit by light
    - Hyperpolarizes to -60mV
  - Connect to bipolar cells
    - Many rods to one ganglion
    - Spatial summation
- Rods & Cones Comparison
  - Rods
    - Rods are peripheral
    - Night vision (10k more sens.)
    - Target detection
    - Fast processing
    - Low quality images
    - Intensity & shades of gray
    - Sensitive to lots of wavelengths
  - Cones
    - Cones are centralized
    - Day vision
    - Target identification
    - Slow processing
    - High quality images
    - Color
    - Sensitive to specific wavelengths
- Cones
  - Structure
  - Shorter, broader, and more tapered than rods
  - Have no visual purple
  - Contains 1 of 3 photopigments
    - Long
    - Medium
    - Short
- 1 to 1 connection
  - 1 cone to 1 bipolar cell
  - 1 bipolar to 1 ganglion cell
  - 1 ganglion cell chain to brain
  - Midget ganglions
  - Each cone has corresponding spot in visual cortex
- Midget Ganglion Cells
  - Small
  - Each cone has one
  - Each fovea cone has direct line to brain
  - Exact location of point of light
  - Wiring
    - 1st route is direct to bipolar cell
    - 2nd route is to horizontal cell
    - horizontal then goes to bipolar
- Horizontal Cells
  - 120 million rods (20:1)
  - 6 million cones
  - Need pattern recognition cells
  - Lateral inhibition
  - Horizontal cells inhibit neighbor
  - Inhibit bipolar cells
  - Activate 1 cone, tells next to stop
  - Give very sharp lines & edges
- Amacrine cells
  - Get info from bipolars
  - Sent into to
  - Bipolars
  - Other amacrines
  - Ganglions
- Bipolar cells
  - Separate ones for rods and cones
  - 10+ types for cones
  - 1 type for rods
- Ganglion cells
  - Form the optic nerve (optic tract)
  - Leave eye through blind spot
  - Receptor output
  - Receptors signal bipolar cells
  - Neurons close to center of eye
  - Bipolars signal ganglion cells
  - Even closer to center of eye
  - Ganglion axons join together
  - loop around, then out to brain
Color Is Important

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QUIZ

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Eye And Retina

Eye: a white pressurized ball lined with a net of vision processors

SLIDES

TERMS

NOTES

QUIZ

VIDEO

DISCUSSION ITEM

Eye: a white pressurized ball lined with a net of vision processors

SLIDES

TERMS

NOTES

QUIZ

VIDEO

DISCUSSION ITEM

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