vision 300

Copyright © 2006 by Elsevier, Inc.

1

• The Eye: II. Receptor and Neural Function of the Retina


2

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


3

Layers of the retina



4


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


6

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

7



• 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.

8



• 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 acuity less 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 amacrine Horizontal and amacrine cells form local circuits with the bipolar cells.

• E.Ganglion cells Ganglion cells are the output cells of the retina.• Axons of ganglion cells form the optic nerve Axons of ganglion cells form the optic nerve

10


11

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 comes Most of the visual input that reaches the brain comes from foveafrom fovea.

12


13

Inner layers of the retina pulled away at the fovea

centralis allowing greater penetration of light


Functions of the rods and cones

14

Function Rods Cones

Sensitivity to light Sensitive to low-intensity light; night vision

Sensitive to high intensity light; day vision

Acuity Lower visual acuityNot present in fovea

Higher visual acuityPresent in fovea

Dark adaptation Rods adapt later Cones adapt first

Color vision No Yes


15

Albino eye

en.wikipedia.org/wiki/Albinism


16

Figure 50-3; Guyton & Hall Figure 50-4; Guyton & Hall

Structure of the Rods and Cones


17

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

18



• 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.

19



• D. Decreased levels of cGMP cause closure of Na+ channels, decreased inward Na+ current, and , as a result, hyperpolarization hyperpolarization of the receptor cell membrane. Increased light activity increases the degree of hyperpolarization.hyperpolarization.

20

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 that opposite 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.

21

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

22

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

23

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

24


25



• 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.

26


27

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


28

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)


29

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.


30

Red Green Color Blindness

Normal reads 74Red-Green reads 21

Normal reads 42Red blind reads 2Green blind reads 4


31

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.


32

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


33

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


34

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

35

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-absorbingEach 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.


36

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.


37

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


38

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


39

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


40

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


41

Medial and lateral recti move eyes side to side

Superior and inferiorrecti move eyes upand down

Superior and inferiorobliques rotate the eyes

Eye Movements are Controlled by 3 Separate Pairs of Muscles.

Figure 51-7; Guyton & Hall

LR6 SO4/3


42

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

43


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.

44

vision 300

Documents

redgreen reads

red cone

red blind

hyperpolarization of

excitatory neurotransmitter

inhibitory neurotransmitter

green blind reads

outer segment