Eye physiology from guyton and halls physiology Part 4

BY Muhammad Ramzan Ul Rehman Nishtar Ken 1

BY Muhammad Ramzan Ul Rehman Nishtar Ken

2

PHYSIOLOGY OF EYE


DETERMINATION OF DISTANCE OF AN OBJECT FROM THE EYE—“DEPTH PERCEPTION” The ability to determine

distance is called depth perception.

A person normally perceives distance by 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.


by using this mechanism of moving parallax, one can tell the relative distances of different objects even though only one eye is used.

It is almost entirely this moving or binocular parallax (or stereopsis) that gives a person with two eyes far greater ability to judge relative distances when objects are nearby than a person who has only one eye.

However, stereopsisis virtually useless for depth perception at distances beyond 50 to 200 feet.


OPTHALMOSCOPE An instrument to examine the

inside of eye, especially the retina and optic disc

It has a light source on the end.

Eye can be magnified.

We can look directly through the pupil to the back of eye.




OPTHALMOSCOPY



VISUAL ACUITY Ability of the eye to

determine the precise shape and details of the object is called visual acuity

For humans, it is 30 seconds of an arc.


SNELLEN’S CHART IS USE FOR DISTANT VISION


METHOD FOR STATING VISUAL ACUITY Chart for testing eyes consists of

letters of different sizes placed 20 feet ( 6 meters) away from the person being tested.

Person is said to have normal vision if he can see the letters from the distance of 20 feet or 6 meters. so he have vision of 20/20 or 6/6


COLOR VISION BY CONES PHOTOCHEMICALS in cones have almost the same

composition as Rhodopsin in Rods.

Protein portion (opsins) in cones are called Photopsins.

The color sensitive pigments of the cones, are the combinations of retinal and photopsins.


Cones are selectively sensitive to different colours:

BlueGreenRed

Only one of three types of color pigments is present in each of the different cones

These color pigments are respectively called:Blue-sensitive pigmentGreen sensitive pigment Red-sensitive pigment

YOUNG-HELMHOLTZ THEORY(theory of trichromatic color vision)


ABSORPTION CHARACTERSTICS OF PIGMENTS

BLUE SENSITIVE PIGMENTSPeak absorbance of light wavelength 445nm

GREEN SENSITIVE PIGMENTSPeak absorbance of light wavelength 535nm

RED SENSITIVE PIGMENTSPeak absorbance of light wavelength 570nm

RHODOPSIN Peak absorbance of light wavelength 505nm


HOW RETINA DETECTS THE DIFFERENT GRADATIONS OFCOLOR IN THE VISUAL SPECTRUM?


TRICOLOR MECHANISM OF COLOR DETECTION

Human eye can detect all gradations of colors.

Red, green & blue monochromatic light mixed in different combinations.


INTERPRETATION OF COLORS IN CNS.

Orange monochromatic light stimulates:

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

Green cones -----42%.

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

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

CNS interprets this ratio as sensation of orange color.


INTERPRETATION OF COLORS IN CNS

Blue monochromatic light stimulates

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

Green cones -----0%.

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

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

CNS interprets this ratio as sensation of blue color.


INTERPRETATION OF COLORS IN CNS

Ratio of 83:83:0-------------yellow.

Ratio of 31:67:36-----------green.


PERCEPTION OF WHITE LIGHT.

• When there is Equal stimulation of all cones there is perception of white light.

• White is combination of all the wavelengths of the spectrum.


CLOR BLINDNESS The term Colour blindness is misleading if taken literally, because

colourblind people CAN see colors, but cannot make out the difference between some complementary colors.


COLOR BLINDNESS CAUSE: Due to congenital absence of a single group of color receptive

cones from the eyes

Person is unable to distinguish some colors from others.

Usually…..absence of either L (Red) cones or M (Green) cones.

People with two functional cones are called Dichromate.


RED-GREEN COLOR BLINDNESS

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

Absence of M (Green)cones :Deuteranopia

Absence of L (Red) cones:Protanopia

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

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


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 each type of Cone.

about 8% of women are color blindness carriers

Females are only color blindness carrier.


BLUE WEAKNESS Rarely blue cones are missing.

Genetically inherited state.


COLOR TESTS CHARTS (ISHIHARA CHARTS )

Rapid method to determine color blindness.Charts are arranged with a confusion of spots of several different colors.

These charts observe spectral sensitivity curves 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 shades and a number is hidden inside with shades of another color.


Ishihara Charts were made by a Japanese ophthalmologist Shinobu Ishihara (1879-1963).

He was working at the Military Medical School

He was asked to devise a test to screen military recruits for abnormalities of colour vision.

His assistant was a colourblind physician who helped him test the plates.


ISHIHARA CHARTS.

The person with normal color vision reads 74, where as the red green blind person reads 21.


ISHIHARA CHARTS.

Person with normal vision reads 42, red blind person reads2,and green blind person reads 4.


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 Neural Pathways for Control of Eye Movementairs of Muscles.

Figure 51-7; Guyton & Hall


Fixation Movement:Movement of eyes to bring a discrete portion of visual field into focus on the fovea.

Controlled byFrontal eye fieldsBrodmann’s area 8 and 19


NEURAL PATHWAYS FOR CONTROL OF EYE MOVEMENT

Fixation movements of the eyes controlled by two neuronal mechanisms, voluntary and 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 the foveal portion of the retina.


Saccadic Movement

Jumping of eyes from one object to another. each jump is called a Saccade and the movement is called Opticokinetic movements.

Pursuit Movement

Fixation of eyes to a moving object


SACCADIC EYE MOVEMENTS When the visual scene is moving (turning the head), the eyes fix on one highlight after another in the visual field jumping at a rate of 2 to 3 jumps/sec. These jumps are called saccades, and the movements are called opticokinetic movements.

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

Vision is suppressed during a saccadic movement.


THE SUPERIOR COLLICULI ARE MAINLY RESPONSIBLE FOR ORIENTING THE EYES AND HEAD TOWARDS A VISUAL OR AUDITORY STIMULUS


BY Muhammad Ramzan Ul Rehman Nishtar Ken

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By Muhammad Ramzan UL Rehman

Eye physiology from guyton and halls physiology Part 4

Education

red cones

green cones

blue cones

functional cones

red green

color sensitive pigments

green blue monochromatic

red yellow colors