Vision 300

Copyright © 2006 by Elsevier, Inc.
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• The Eye: II. Receptor and
Neural Function of the Retina

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Retina
• light sensitive portion of the eye
– contains cones for color vision
– contains rods for night vision
• Layers of the retina
– Pigmented layer
– Layer of rods and cones projecting into pigment
– Outer nuclear layer
– Outer plexiform layer
– Inner nuclear layer
– Inner plexiform layer
– Ganglion cell layer
– Layer of optic nerve fiber
– Stratum opticum

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Layers of the retina

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Pigmented epithelial cells
• The phoreceptor outer segments interdigitate with the
melanin-filled processes of pigment epithelial cells. These
processes are mobile, and they elogate into the pigmented
layer when the light is bright(photopic conditions) and
retract when the light is dim(scotopic conditions). This
mechanism combines with contraction of the iris to protect
the retina from bright conditions that would otherwise
damage the photoreceptors.
• The iris, pigment epithelium, and circuitry of the retinal
cells all contribute to the eye’s ability to resolve the visual
world over a wide range of light conditions 5

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Pigment Layer of Retina
• Contains pigment melanin (black)
• Prevents light reflection into the eye globe
• Without there would be diffuse scattering of light
• Normal contrast would be lost.
• Albinos
– poor visual acuity because of the scattering of light
– Light reflects off the underlying sclera
– Receptors are continually stimulated
– 20/100 to 20/200 best case scenario

Pigment layer of the retina
• The pigment layer stores large quantities of
vitamin A. This vitamin A is exchanged back and
forth through the cell membrane of the outer
segment of the rods and cones, which themselves
are embedded in the pigment.
• Vitamin A is an important precursor of the
photosensitive chemicals of the rods and cones
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• A. Pigment epithelium absorb stray light
and prevent scatter of light.
• Convert 11-cis retinal to all-trans retinal.
• B. Receptor cells and rods and cones
• Rods and cones are not present on the optic
disc; the result is a blind spot.
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• C. Bipolar cells.
• The receptor cells(i.e., rods and cones synapse on
bipolar cells, which synapse on the ganglion cells.
• 1. Few cones synapse on a single bipolar cell,
which synapses on a single ganglion cell. This
arrangement is the basis for the high acuity and
low sensitivity of the cones. In the fovea, where
acuity is highest, the ratio of cones to bipolar cells
is 1:1. 9

• 2. Many rods synapse on a single bipolar cell. As
a result, there is less acuityless acuity in rods than in the
cones. There is also greater sensitivity in the
rods because light striking any one of the rods
will activate the bipolar cell.
• D. Horizontal and amacrineHorizontal and amacrine cells form local
circuits with the bipolar cells.
• E.Ganglion cellsGanglion cells are the output cells of the retina.
• Axons of ganglion cells form the optic nerveAxons of ganglion cells form the optic nerve10

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The Fovea
• A small area at the center of the retina about 1 sq
millimeter (.3mm diameter)
• The center of this area, “the central fovea,”
– contains only cones
– these are specialized cones
– especially long and slender
– aid in detecting detail
– Regular thicker cones are at the perifery of the the fovea
• Neuronal cells and blood vessels are displaced so that
the light can strike the cones directly.
• This is the area of greatest visual acuity

Macula and Fovea
• At the posterior pole of the eye is a yellowish spot, the
macula lutea, the center of which is a depression called the
fovea centralis.Near the fovea, the inner retinal layers
become thinner and eventually disappear so that, at the
bottom of the foveal pit, only the outer nuclear layer and
photoreceptor outer segments remain. This allows a
maximum amount of light to reach thephotoreceptors with
optimal fidelity.
• Most of the visual input that reaches the brain comesMost of the visual input that reaches the brain comes
from foveafrom fovea.
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Inner layers of the retina pulled away at the fovea
centralis allowing greater penetration of light

Functions of the rods and cones
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Function Rods Cones
Sensitivity to light Sensitive to low-intensity
light; night vision
Sensitive to high intensity
light; day vision
Acuity Lower visual acuity
Not present in fovea
Higher visual acuity
Present in fovea
Dark adaptation Rods adapt later Cones adapt first
Color vision No Yes

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Albino eye
en.wikipedia.org/wiki/Albinism

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Figure 50-3; Guyton & Hall Figure 50-4; Guyton & Hall
Structure of the Rods and Cones

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Blood supply to the Retina
• 1.Central Retinal
Artery
– Enter via center of
the optic nerve
– Divides and
supplies inner
retinal surface
2. Ciliary arteries

Steps in photoreception in the rods
• The photosensitive element is rhodopsin, which is
composed of opsin( a protein) belonging to the
superfamily of G-protein-coupled receptors and retinal.
• A. Light on the retina converts 11-cis retinal to all-trans
retinal, a process called photoisomerization. A series of
intermediates is then formed, one of which is
metarhodopsin II.
• Vitamin A is necessary for the regeneration of cis retinal.
Deficiency of vitamin A causes night blindness
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• B. Metarhodopsin II activates G protein
called transducin, which in turn activates a
phosphodiesterase
• C. Phosphodiesterase catalyzes the
conversion of cyclic guanosine
monophosphate(cGMP) to 5 -GMP, and΄
cGMP levels decrease.
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• D. Decreased levels of cGMP cause closure
of Na+ channels, decreased inward Na+
current, and , as a result, hyperpolarizationhyperpolarization
of the receptor cell membrane. Increased
light activity increases the degree of
hyperpolarization.hyperpolarization.
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Excitation of the photo receptors
• Excitation of the rod causes increased negativity
of the intrarod membrane potential, which is a
state of hyperpolarization, there are more
negativity than normal inside the rod membrane.
• This is opposite to the decreased negativity thatopposite to the decreased negativity that
occurs in other sensory receptors.occurs in other sensory receptors.
• When rhodopsin(visual purple) at the outer
segment of the rod decomposes by light, the
conductance of sodium decreases at the outer
segment.
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Excitation of the photo receptors
• Sodium ions continued to be pumped
outward through the membrane of the inner
segment.
• More sodium ion leave than leave back, there
will be more negativity inside causing greater
hyperpolarization
• At maximum light intensity, the membrane
potential approaches -70 to-80 millivolts
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Rod under dark condition
• The inner segment of the rod continually
pump sodium from inside the rod to the
outside, creating negative potential on the
inside of the entire cell.
• The outer segment of the rod is very leaky to
the sodium ion. Positively charged sodium
ions continually leak back. There is reduced
electronegativity inside measuring about -40
millivolts
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Hyperpolarization receptor potential
• Both rods and cones go
through hyperpolarization.
• The bipolar and horizontal
cells become depolarized by
inhibitory neurotransmitter;
and they are hyperpolarized
by excitatory
neurotransmitter.
• Decreased c-GMP causes
hyperpolarization of rods and
cones
• Neurotransmitters are
released by hyperpolarization
of rods and cones
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• E. When the receptor cell is hyperpolarized, there is
decreased release of either an excitatory
neurotransmitter or an inhibitory neurotransmitter.
• 1. If the neurotransmitter is excitatory, then the response of
the bipolar or horizontal cells to light is hyperpolarization.
• 2. If the neurotransmitter is inhibitory, then the response to
the bipolar or horizontal cell to light is depolarization. In
other words, inhibition of inhibition is excitation.
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Color photochemistry
• The chemical event are almost exactly the
same in R&C.
– Opsins instead of scotopsin
• Proteins differ slightly leading to:
– Red-sensitive pigment
– Green-sensitive pigment
– Blue-sensitive pigment

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This is how we perceive color
• Our brain interprets a ratio of the 3 cone
types for each given wavelength of light.
• The ratio is presented as:
Red:Green:Blue
– Ex 1. 83:83:0 (yellow)
– Ex 2. 31:67:36 (green)

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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
• 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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Red Green Color Blindness
Normal reads 74
Red-Green reads 21
Normal reads 42
Red blind reads 2
Green blind reads 4

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Dark and Light Adaptation
• In light conditions most of the photochemicals have
been reduced to retinal and opsins
– so the level of photosensitive chemicals is low.
• In dark conditions retinal is converted back to
rhodopsin.
• Therefore, the sensitivity of the retina automatically
adjusts to the light level.
• The longer we are in the dark, the more photo
sensitive we become.

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The longer we are in the dark, the more
photo sensitive we become
1. R&C adaption
2. Change in pupillary
size
3. Neural adaptation

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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.
• In periphery 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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In rods, the chemical rhodopsin
breaks down to form scotopsin and
retinal (a Vitamin A derivative).
This chemical reaction generates an
electrical impulse. Rhodopsin is
then resynthesized in a slower
reaction. A deficiency of Vitamin
A will decrease the sensitivity of
rods, resulting in night blindness.

Cones
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In cones, the chemical reactions are brought about by different
wavelengths of light. It is believed there are three types of
cones:
red-absorbing
blue-absorbing
green-absorbing
Each type absorbs wavelengths over about one-third of the
visible light spectrum, causing red cones, for example, to
absorb light from the red, orange, and yellow wavelengths.
The chemical reactions in cones also generate electrical
impulses. Absent or nonfunctional cones are the cause of
colorblindness, with the most common form being the
inability to distinguish between red and green.

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Impulses from the rods and cones
are transmitted to ganglion
neurons, which converge at the
optic disc to become the optic
nerve, passing posteriorly through
the wall of the eyeball.

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The Snellen eye chart and a variety of other eye charts are
routinely used to determine visual
acuity, or the sharpness of vision. In expressing visual acuity, the
eye being tested is always rated in comparison to a "normal" eye.
The charts are made so that the "normal" eye should be able to
resolve the characters on the line marked 20 feet from the actual
distance of 20 feet. If the eye being tested can resolve smaller
characters, the eye has better visual acuity than normal. If the
eye being tested can only resolve larger characters, the eye has
visual acuity poorer than normal.

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Visual acuity is expressed as a ratio (i.e., 20/20). The first
number is always 20 and represents where you were standing
when you were able to correctly read all the letters of a
particular
line on the eye chart. The second number represents the
distance at which the normal eye would be able to correctly read
all the letters of that same line.

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To test the visual acuity, stand on
the 20 foot line and look at the eye
chart. Test each eye
independently by closing or
covering the opposite eye. Read the
chart while a lab partner
checks your responses. Stop as
soon as you make an error. Your
visual acuity should be
expressed as 20/the number of the
last line you resolved entirely
correctly. Record your
results on the Data/Analysis Sheet.

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If the second number of the ratio is
smaller than 20, visual acuity is
better than normal (i.e., you
can resolve the same characters
from farther away than the normal
eye).
If the second number of the ratio is
greater than 20, visual acuity is
diminished (i.e., the normal
eye can resolve the same characters
from farther away than you can).

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Medial and lateral
recti move eyes side
to side
Superior and inferior
recti move eyes up
and down
Superior and inferior
obliques rotate the
eyes
Eye Movements are Controlled by
3 Separate Pairs of Muscles.
Figure 51-7; Guyton & Hall
LR6 SO4/3

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Autonomic Pathways to the Eye
Figure 51-7; Guyton & Hall

Acknowledgement
• The information and images in this
presentation have benn taken from multiple
resources for teaching purpose only
• Dr. Dewan S. Raja MD,Mphil,MPH
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Acknowledgement
• The informations’s and images in this
presentation have been taken from multiple
resources for teaching purpose only.
• Dr. Dewan S. Raja MD,MPhil,MPH,CHES.
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