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 Schematic eye. Determine intraocular pressure and glaucoma

1. M. Pandian
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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
 Schematic eye.
 Determine intraocular pressure and glaucoma
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5. PHYSICAL PRINCIPLES OF OPTICS
 optical system of the eye requires familiarity
with the basic principles of optics,
 including the physics of light refraction,
focusing, depth of focus, and so forth.
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6. 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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7. Refraction of Light - A
 The only effect that occurs is decreased
velocity of transmission and
 shorter wavelength, by the shorter distances
between wave fronts.
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8. Refraction of Light - B
 The rays bend if the refractive indices of the
two media are different from each other
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
angulations 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. 8

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Light Refraction

11. Refractive Principles of a Lens
 Convex lens focuses light rays (converging lens)
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Focal point

12. Refractive Principles of a Lens
Concave lens diverges light rays (diverging lens)
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13. Refraction by spherical lenses
 Light rays are (bent)refracted when they pass
from one medium into medium of different
density.
 But except when they strike perpendicular to the
interface.
 i.e. rays get refracted towards the centre, when
they enter low density medium to high density
medium.
 The rays away from the centre when they move
from high to low density medium.
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14. 14
Spherical Lens
(Focal points)
Cylindrical Lens
(Focal line)

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The Refractive Power of a Lens
Figure 49-8; Guyton and Hall

17. Refractive index of transparent
substances
Velocity of light in air
=
Velocity of light in that substances
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18. 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)
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19. 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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22. 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
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23. Constriction (meiosis) and dilation (Mydriasis)
of the pupil
Convergence and divergence of the eyes for
binocular vision
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24. 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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25. 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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29. 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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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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35. Faisal I. Mohammed, MD,PhD
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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
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
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
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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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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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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
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In the dark an inward current
(the dark current) carried by
the Na+ ions flows into the
outer segment of the rod.

57. The Dark Current
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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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Each Cone is Receptive to a Particular
Wavelength of Light
Rods

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
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Normal read 74, Red-Green read
it 21
Normal read it 42, Red blind
read 2, Green blind read it 4

71. Neural Organization of the Retina
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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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