1. The Eye: I. Optics of Vision
Faisal I. Mohammed , MD, PhD
University of Jordan

2. 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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3. 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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4. 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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5. Light Refraction
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8. Refractive Principles of a Lens
Convex lens focuses light rays (converging lens)
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9. Refractive Principles of a Lens
Concave lens diverges light rays (diverging lens)
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10. Spherical Lens (Focal points) Cylindrical Lens (Focal line)
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12. The Refractive Power of a Lens
Figure 49-8; Guyton and Hall
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13. 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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14. 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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15. 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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16. 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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17. Mechanism of Accommodation
Contraction pulls
ligament forward
relaxing tension on
suspensory ligament
making the lens fatter
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20. Presbyopia; The Inability to Accommodate
Caused by progressive denaturation of the proteins of the lens.
Makes the lens less elastic.
Begins about years of age.
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21. Errors of Refraction Normal vision Far sightedness Near sightedness
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22. Correction of Vision Myopia corrected with concave lens
Hyperopia corrected
with convex lens
Myopia corrected with
concave lens
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23. Errors of Refraction
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25. 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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26. Visual Acuity Test The diameter of the cones in the fovea is  1.5 m
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27. 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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29. 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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30. Formation and Flow of Fluid in the Eye
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32. 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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33. 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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34. Thank You
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35. The Eye: II. Receptor and Neural Function of the Retina
Faisal I. Mohammed, MD,PhD
University of Jordan

36. 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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37. 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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42. 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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43. 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 1:1.
May explain the high degree of visual acuity in the central retina
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44. Rods Cones lower sensitivity; specialized for day vision
less photopigment
less amplification (less divergence 1:1 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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45. 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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46. Structure of the Rods and Cones
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48. 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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49. 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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50. 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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52. Rhodopsin Cycle
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54. 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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56. The Dark Current
In the dark an inward current (the dark current) carried by the Na+ ions flows into the outer segment of the rod.
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57. 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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59. Rod Receptor Potential (Cont’d)
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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60. 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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61. 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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62. 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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63. 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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64. Dark Adaptation and Rods and Cones
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65. 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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66. 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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67. 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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68. Each Cone is Receptive to a Particular Wavelength of Light
Rods
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69. 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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70. 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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71. Neural Organization of the Retina
Direction of
light
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73. 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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74. 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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75. Lateral inhibition, the
function of horizontal
cells

76. 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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77. 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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78. 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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