4th Lecture on Physiology of Eye by Dr. Roomi

7/28/2019 4th Lecture on Physiology of Eye by Dr. Roomi

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PHYSIOLOGY OF EYE

BY

DR. MUDASSAR ALI ROOMI (MBBS, M.Phil.)


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Determination of Distance of an Object from theEyeDepth Perception

The ability to determine distance iscalled depth perception.

A person normally perceives distanceby three major means:

(1) the sizes of the images of known

objects on the retina (2) the phenomenon of moving parallax,

(3) the phenomenon of stereopsis,binocular Vision.


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by using this mechanism ofmoving parallax, one cantell the relative distances of different objects eventhough only one eye is used.

It is almost entirely this moving or binocular parallax

(or stereopsis)that gives a person with two eyes fargreater ability to judge relative distances when objectsare nearbythan a person who has only one eye.

However, stereopsisis virtually useless for depth

perception at distances beyond 50 to 200 feet.


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Opthalmoscope

An instrument to examinethe inside of eye, especiallythe retina and optic disc

It has a light source on the

end. Eye can be magnified.

We can look directlythrough the pupil to theback of eye.


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Opthalmoscopy


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visual acuity

Ability of the eye todetermine theprecise shape and

details of the objectis called visualacuity

For humans, it is 30

seconds of an arc.


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Snellens chart is use for distantvision


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METHOD FOR STATING VISUALACUITY

Chart for testing eyes consistsof letters of different sizesplaced 20 feet ( 6 meters) awayfrom the person being tested.

Person is said to have normal

vision if he can see the lettersfrom the distance of 20 feet or6 meters. so he have vision of20/20 or 6/6


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COLOR VISION BY CONES

PHOTOCHEMICALS in cones havealmost the same composition as

Rhodopsin in Rods. Protein portion (opsins) in cones are

called Photopsins.

The color sensitive pigments of thecones, are the combinations ofretinal and photopsins.


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Cones are selectively sensitive to differentcolours:

Blue

Green

Red

Only one of three types of color pigments is

present in each of the different cones

These color pigments are respectively called:

Blue-sensitive pigment

Green sensitive pigmentRed-sensitive pigment

YOUNG-HELMHOLTZ THEORY(theory of trichromatic colorvision)


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ABSORPTION CHARACTERSTICS OFPIGMENTS

BLUE SENSITIVE PIGMENTS

Peak absorbance of lightwavelength 445nm

GREEN SENSITIVE PIGMENTS


RED SENSITIVE PIGMENTS


RHODOPSINPeak absorbance of light

wavelength 505nm


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How retina detects the different gradations ofcolor in the visual spectrum?


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TRICOLOR MECHANISM OF COLORDETECTION

Human eye can detect all gradationsof colors.

Red, green & blue monochromatic

light mixed in different combinations.


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INTERPRETATION OF COLORS INCNS.

Orange monochromaticlight stimulates:

Red cones-------99%.

Green cones -----42%.

Blue cones---------0%.

Ratio of stimulation-------99:42:0.

CNS interprets this ratio assensation of orange color.


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INTERPRETATION OF COLORS IN CNS

Blue monochromaticlight stimulates

Red cones-------0%.

Green cones -----0%.

Blue cones---------97%.

Ratio of stimulation-------0:0:97

CNS interprets this ratioas sensation of blue color.


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INTERPRETATION OF COLORS IN CNS

Ratio of83:83:0-------------yellow.

Ratio of31:67:36-----------green.


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PERCEPTION OF WHITE LIGHT.

When there is Equal stimulation ofall cones there is perception of whitelight.

White is combination of all thewavelengths of the spectrum.


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COLOR BLINDNESS

CAUSE: Due to congenital absence of asingle group of color receptive cones fromthe eyes

Person is unable to distinguish somecolors from others.

Usually..absence of either L (Red) cones

or M (Green) cones. People with two functional cones are

called Dichromate.


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RED-GREEN COLOR BLINDNESS

Person is unable to distinguish red from green dueto missing of either of these cones.

Absence of M (Green)cones :Deuteranopia

Absence of L (Red) cones:Protanopia

Green, orange, red & yellow colors havewavelength 525 to 675nm.

These colors are normally distinguished from oneanother by red & green cones.


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RED-GREEN COLOR BLINDNESS

Genetic disorder only in males

Photopsins are coded on X chromosomes.

It never occurs in females ,because one of the

two X Chromosomes has normal gene for eachtype of Cone.

about 8% of women are color blindness carriers

Females are only color blindness carrier.


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Blue weakness

Rarely blue cones are missing.

Genetically inherited state.


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COLOR TESTS CHARTS(IshiharaCharts )

Rapid method to determine colorblindness.

Charts are arranged with a confusion ofspots of several different colors.

These charts observe spectral sensitivitycurves of the different cones at same time.

Ideally a collection of 38 plates filled with

colored dots build the base of this test.The dots are colored in different shadesand a number is hidden inside with shadesof another color.


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IshiharaCharts were made by aJapanese ophthalmologist ShinobuIshihara (1879-1963).

He was working at the MilitaryMedical School

He was asked to devise a test toscreen military recruits for

abnormalities of colour vision. His assistant was a colourblind

physician who helped him test theplates.


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ISHIHARA CHARTS.

The person withnormal colorvision reads 74,

where as thered green blindperson reads21.


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Person withnormalvision reads

42, redblindpersonreads2,andgreen blindpersonreads 4.

ISHIHARA CHARTS.

E M C ll d b


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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 byNeural Pathways for Control of EyeMovementairs of Muscles.

Figure 51-7; Guyton & Hall


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Fixation Movement:

Movement of eyes to bringa discrete portion of visualfield into focus on the fovea.

Controlled by

Frontal eye fields

Brodmanns area 8 and 19


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Neural Pathways for Controlof Eye Movement

Fixation movements of the eyes controlled by twoneuronal mechanisms, voluntaryand involuntary.

Voluntary fixation movements controlled by an area in the premotor

cortex.

Involuntary fixation mechanism causes eyes to lock on object of

attention found with the voluntary fixation mechanism. Controlled by

secondary visual areas of the occipital cortex.

Results from negative feedback mechanism controlled at the level of the

superior colliculus that prevents objects of attention from leaving thefoveal portion of the retina.


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Saccadic Movement

Jumping of eyes from one object toanother. each jump is called a Saccade and

the movement is called Opticokineticmovements.

Pursuit MovementFixation of eyes to a moving object


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Saccadic Eye Movements

When the visual scene is moving (turning thehead), the eyes fix on one highlight afteranother in the visual field jumping at a rate of2 to 3 jumps/sec. These jumps are called

saccades, and the movements are calledopticokinetic movements.

Saccades occur very rapidly (only 10% of thetime is spent making saccades).

Vision is suppressed during a saccadicmovement.


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TheSuperiorColliculi aremainly

responsiblefor

orientingthe eyesand headtowards a

visual orauditorystimulus


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4th Lecture on Physiology of Eye by Dr. Roomi

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