The Eye: I. Optics of Vision

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The Eye I. Optics of Vision

• Faisal I. Mohammed , MD, PhD

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Objectives

• Describe the visual receptors
• List the types of lenses and recognize how they
  work
• Determine the power of lenses
• Describe accommodation for near vision and far
  vision
• Recognize nearsightedness and farsightedness and
  determine its correction
• Describe visual acuity and its abnormalities
• Determine intraocular pressure and glaucoma

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Refractive Index

• Speed of light in air 300,000 km/sec.
• Light speed decreases when it passes through a
  transparent substance.
• The refractive index is the ratio of the speed of
  light in air to the speed of light in the
  substance.
• e.g., speed of light in substance 200,000
  km/sec, R.I. 300,000/200,000 1.5.

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Refraction of Light

• Bending of light rays by an angulated interface
  with different refractive indices.
• The degree of refraction increases as the
  difference in R.I. increases and the degree of
  angulation increases.
• The features of the eye have different R.I. and
  cause light rays to bend.
• These light rays are eventually focused on the
  retina.

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Light Refraction
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Refractive Principles of a Lens

• Convex lens focuses light rays (converging lens)

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Refractive Principles of a Lens

• Concave lens diverges light rays (diverging lens)

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Spherical Lens (Focal points)
Cylindrical Lens (Focal line)
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The Refractive Power of a Lens
Figure 49-8 Guyton and Hall
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Focusing Power of the Eye

• Most of the refractive power of the eye results
  from the surface of the cornea.
• a diopter is a measure of the power of a lens
• 1 diopter is the ability to focus parallel light
  rays at a distance of 1 meter, it is a measure of
  power of lenses
• Diopter 1/ focal length in meters i.e the
  power of a lens with focal length 0.5 meter is 2
  (more convex)
• the retina is considered to be 17 mm behind the
  refractive center of the eye
• therefore, the eye has a total refractive power
  of 59 diopters (1000/17)

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Image formation on the retina-requirements

• Light refraction or bending the light by the
  refractive media Cornea, Aqueous humor, Lens
  and Vitreous humor
• Accommodation An increase in the curvature of
  the lens for near vision,
• The near point of vision is the minimum distance
  from the eye an object can be clearly focused
  with maximum accommodation
• Constriction (meiosis) and dilation (Mydriasis)
  of the pupil
• Convergence and divergence of the eyes for
  binocular vision

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Accommodation

• Refractive power of the lens is 20 diopters.
• Refractive power can be increased to 34 diopters
  by changing shape of the lens - making it fatter
  (more convex).
• This is called accommodation.
• Accommodation is necessary to focus the image on
  the retina.
• Normal image on the retina is upside down.

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Mechanism of Accommodation

• A relaxed lens is almost spherical in shape.
• Lens is held in place by suspensory ligament
  which under normal resting conditions causes the
  lens to be almost flat.
• Contraction of an eye muscle attached to the
  ligament pulls the ligament forward and causes
  the lens to become fatter (more convex) which
  increases the refractive power of the lens.
• Under control of the parasympathetic nervous
  system.

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Mechanism of Accommodation
Contraction pulls ligament forward relaxing
tension on suspensory ligament making the lens
fatter
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Presbyopia The Inability to Accommodate

• Caused by progressive denaturation of the
  proteins of the lens.
• Makes the lens less elastic.
• Begins about 40-50 years of age.

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Errors of Refraction
Normal vision
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Correction of Vision
Myopia corrected with concave lens
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Errors of Refraction
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Other Errors of Vision

• Astigmatism
• unequal focusing of light rays due to an oblong
  shape of the cornea
• Cataracts
• cloudy or opaque area of the lens
• caused by coagulation of lens proteins

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Visual Acuity Test
The diameter of the cones in the fovea is ? 1.5
?m
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Visual Acuity depends on the density of
receptors (primarily Cones)

• 6/6
• ability to see letters of a given size at 6
  meters
• 6/12
• what a normal person can see at 12 meters, this
  person must be at 6 meters to see.
• 6/60
• what a normal person can see at 60 meters, this
  person must be at 6 meters to see.

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Fluid System of the Eye

• Intraocular fluid keeps the eyeball round and
  distended.
• 2 fluid chambers
• aqueous humor which is in front of the lens
• freely flowing fluid
• vitreous humor which is behind the lens
• gelatinous mass with little flow of fluid

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Formation and Flow of Fluid in the Eye
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Formation of Aqueous Humor

• Produced by the ciliary processes of the ciliary
  body at a rate of 2-3 microliters/min.
• Flows between the ligaments of the lens, through
  the pupil into the anterior chamber, goes between
  the cornea and the iris, through a meshwork of
  trabeculae to enter the canal of schlemm which
  empties into aqueous veins and then into
  extraocular veins.

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Intraocular Pressure

• Normally 15 mm Hg with a range of 12-20 mm Hg.
• The level of pressure is determined by the
  resistance to outflow of aqueous humor in the
  canal of schlemm.
• increase in intraocular pressure caused by an
  increase in resistance to outflow of aqueous
  humor through a network of trabeculae in the
  canal of schlemm (Glaucoma)
• can cause blindness due to compression of the
  axons of the optic nerve

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Thank You
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The Eye II. Receptor and Neural Function of the
Retina

• Faisal I. Mohammed, MD,PhD

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Objectives

• Describe visual receptors and characterize them
• List the layers of the retina and its cellular
  makeup
• Explain visual transduction mechanism
• Outline light and dark adaptation
• Describe vitamin A importance for vision
• Explain color blindness

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Retina

• light sensitive portion of the eye
• contains cones for day and color vision
• contains rods for night vision
• contains neural architecture
• light must pass through the neural elements to
  strike the light sensitive rods and cones

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The Fovea

• A small area at the center of the retina about 1
  sq millimeter
• The center of this area, the central fovea,
  contains only cones
• these cones have a special structure
• aid in detecting detail
• In the central fovea the neuronal cells and blood
  vessels are displaced to each side so that the
  light can strike the cones directly.
• This is the area of greatest visual acuity

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Rods, Cones and Ganglion Cells

• Each retina has 100 million rods and 3 million
  cones and 1.6 million ganglion cells.
• 60 rods and 2 cones for each ganglion cell
• At the central fovea there are no rods and the
  ratio of cones to ganglion cells is 11.
• May explain the high degree of visual acuity in
  the central retina

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Rods Cones

• lower sensitivity specialized for day vision
• less photopigment
• less amplification (less divergence 11 is more)
• saturate with intense light
• fast response, short integration time
• more sensitive to direct axial rays

• high sensitivity specialized for night vision
• more photopigment
• high amplification single photon detection
• saturate in daylight
• slow response, long integration time
• more sensitive to scattered light

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Rods Cones

• low acuity highly convergent retinal pathways,
  not present in central fovea
• achromatic one type of rod pigment

• high acuity less convergent retinal pathways,
  concentrated in central fovea
• trichromatic three types of cones, each with a
  different pigment that is sensitive to a
  different part of the visible spectrum, Red,
  Green and Blue

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Structure of the Rods and Cones
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Pigment Layer of Retina

• Pigment layer of the retina is very important
• Contains the black pigment melanin
• Prevents light reflection in the globe of the eye
• Without the pigment there would be diffuse
  scattering of light rather than the normal
  contrast between dark and light.
• This is what happens in albinos (genetic absence
  of melanocyte activity)
• poor visual acuity because of the scattering of
  light

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Photochemistry of Vision

• Rods and cones contain chemicals that decompose
  on exposure to light.
• This excites the nerve fibers leading from the
  eye.
• The membranes of the outer-segment of the rods
  contain rhodopsin or visual purple.
• Rhodopsin is a combination of a protein called
  scotopsin and a pigment, retinal (Vitamin A
  derivative)
• The retinal is in the cis configuration.
• Only the cis configuration can bind with
  scotopsin to form rhodopsin.

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Light and Rhodopsin

• When light is absorbed by rhodopsin it
  immediately begins to decompose.
• Decomposition is the result of photoactivation of
  electrons in the retinal portion of rhodopsin
  which leads to a change from the cis form of the
  retinal to the trans form of the molecule.
• Trans retinal has the same chemical structure but
  is a straight molecule rather than an angulated
  molecule.
• This configuration does not fit with the binding
  site on the scotopsin and the retinal begins to
  split away.
• In the process of splitting away a number of
  intermediary compounds are formed.

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Rhodopsin Cycle
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Mechanism for Light to Decrease Sodium Conductance

• cGMP is responsible for keeping Na channel in
  the outer segment of the rods open.
• Light activated rhodopsin (metarhodopsin II)
  activates a G-protein, transducin.
• Transducin activates cGMP phosphodiesterase which
  destroys cGMP.
• Rhodopsin kinase deactivates the activated
  rhodopsin (which began the cascade) and cGMP is
  regenerated re-opening the Na channels.

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The Dark Current
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The Dark Current
When rhodopsin decomposes in response to light it
causes a hyperpolarization of the rod by
decreasing Na permeability of the outer segment.
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Rod Receptor Potential (Contd)

• When rhodopsin decomposes it causes a
  hyperpolarization of the rod by decreasing Na
  permeability of the outer segment.
• The Na pump in the inner segment keeps pumping
  Na out of the cell causing the membrane
  potential to become more negative
  (hyperpolarization).
• The greater the amount of light the greater the
  electronegativity.

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The Rod Receptor Potential

• Normally about -40 mV
• Normally the outer segment of the rod is very
  permeable to Na ions.
• In the dark an inward current (the dark current)
  carried by the Na ions flows into the outer
  segment of the rod.
• The current flows out of the cell, through the
  efflux of K, ions in the inner segment of the
  rod.

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Duration and Sensitivity of the Receptor Potential

• A single pulse of light causes activation of the
  rod receptor potential for more than a second.
• In the cones these changes occur 4 times faster.
• Receptor potential is proportional to the
  logarithm of the light intensity.
• very important for discrimination of the light
  intensity

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Role of Vitamin A

• Vitamin A is the precursor of all-trans-retinal,
  the pigment portion of rhodopsin.
• Lack of vitamin A causes a decrease in retinal.
• This results in a decreased production of
  rhodopsin and a lower sensitivity of the retina
  to light or night blindness.

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Dark and Light Adaptation

• In light conditions most of the rhodopsin has
  been reduced to retinal so the level of
  photosensitive chemicals is low.
• In dark conditions retinal is converted back to
  rhodopsin.
• Therefore, the sensitivity of the retinal
  automatically adjusts to the light level.
• Opening and closing of the pupil also contributes
  to adaptation because it can adjust the amount
  entering the eye.

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Dark Adaptation and Rods and Cones
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Importance of Dark and Light Adaptation

• The detection of images on the retina is a
  function of discriminating between dark and light
  spots.
• It is important that the sensitivity of the
  retina be adjusted to detect the dark and light
  spots on the image.
• Enter the sun from a movie theater, even the dark
  spots appear bright leaving little contrast.
• Enter darkness from light, the light spots are
  not light enough to register.

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Dark Adaptation

• Gradual increase in photoreceptor sensitivity
  when entering a dark room.
• Maximal sensitivity reached in 20 min.
• Increased amounts of visual pigments produced in
  the dark.
• Increased pigment in cones produces slight dark
  adaptation in 1st 5 min.
• Increased rhodopsin in rods produces greater
  increase in sensitivity.
• 100,000-fold increase in light sensitivity in
  rods.

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Color Vision

• Color vision is the result of activation of
  cones.
• 3 types of cones
• blue cone
• green cone
• red cone
• The pigment portion of the photosensitive
  molecule is the same as in the rods, the protein
  portion is different for the pigment molecule in
  each of the cones.
• Makes each cone receptive to a particular
  wavelength
• of light

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Each Cone is Receptive to a Particular Wavelength
of Light
Rods
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Color Blindness

• lack of a particular type of cone
• genetic disorder passed along on the X chromosome
• occurs almost exclusively in males (blue color
  blindness is usually autosomal recessive gene but
  it is rare)
• about 8 of women are color blindness carriers
• most color blindness results from lack of the red
  or green cones
• lack of a red cone, protanope.
• lack of a green cone, deuteranope.

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Color Blindness Charts
Normal read 74, Red-Green read it 21
Normal read it 42, Red blind read 2, Green blind
read it 4
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Neural Organization of the Retina
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Signal Transmission in the Retina

• Transmission of signals in the retina is by
  electrotonic conduction.
• Allows graded response proportional to light
  intensity.
• The only cells that have action potentials are
  ganglion cells and amacrine cells.
• send signals all the way to the brain

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Lateral Inhibition to Enhance Visual Contrast

• horizontal cells connect laterally between the
  rods and cones and the bipolar cells
• output of horizontal cells is always inhibitory
• prevents the lateral spread of light excitation
  on the retina
• have an excitatory center and an inhibitory
  surround
• essential for transmitting contrast borders in
  the visual image

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Function of Amacrine Cells

• About 30 different types
• Some involved in the direct pathway from rods to
  bipolar to amacrine to ganglion cells
• Some amacrine cells respond strongly to the onset
  of the visual signal, some to the extinguishment
  of the signal
• Some respond to movement of the light signal
  across the retina
• Amacrine cells are a type of interneuron that aid
  in the beginning of visual signal analysis.

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Three Types of Ganglion Cells

• W cells (40) receive most of their excitation
  from rod cells.
• sensitive to directional movement in the visual
  field
• X cells (55) small receptive field, discrete
  retinal locations, may be responsible for the
  transmission of the visual image itself, always
  receives input from at least one cone, may be
  responsible for color transmission.
• Y cells (5) large receptive field respond to
  instantaneous changes in the visual field.

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Excitation of Ganglion Cells

• spontaneously active with continuous action
  potentials
• visual signals are superimposed on this
  background
• many excited by changes in light intensity
• respond to contrast borders, this is the way the
  pattern of the scene is transmitted to the brain

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