Mechanism of Vision

1. PRESENTED BY-ANUPAM DUTTA
ROLL NO-245

2. THE WALL OF EYEBALL IS
COMPOSED OF THREE
CONCENTRIC LAYERS,
DESCRIBED BELOW
1.SCLERA- COMPOSED OF
DENSE CONNETIVE TISSUE.AT
THE FRONT IT FORMS THE
CORNEA WHICH ACTS AS A
REFRACTING STRUTCTURE.
2.CHOROID- IT IS THE MIDDLE
COAT.ITS ROLE IS TO PREVENT
THE REFLECTIONS OF LIGHT
RAYS WITHIN THE EYES.
3.RETINA-IT IS THE INNER
COAT.IT CONTAINS ACTUAL LIGHT
RECEPTORS,THE RODS AND THE
CONES

3. RETINA CONTAINS THREE LAYERS OF CELL
FROM INSIDE TO OUTSIDE-GANGLION
CELLS,BIPOLAR CELLS AND
PHOTORECEPTOR CELLS.
1.RODS AND CONES- TWO TYPE OF
PHOTORECEPTOR CELLS.THEY CONTAIN
THE LIGHT-SENSITIVE PROTEINS CALLED
THE PHOTOPIGMENTS.
2.BIPOLAR CELLS- NEXT TO
PHOTORECEPTOR CELLS IS THE
INTERMEDIATE LAYER OF SHORT SENSORY
BIPOLAR CELLS.BIPOLAR CELLS ARE
ACTUALLY BIPOLAR NEURONS WITH A
AXON AND A DENDRITE.
3.GANGLION CELLS- INNER TO LAYER OF
BIPOLAR CELLS,IS THE LAYER OF RETINAL
GANGLION CELLS.BIPOLAR CELLS
SYNAPSE WITH THE RETINAL GANGLION
CELLS.THE AXONS OF THE RETINAL

4. THE HUMAN EYE HAS TWO FUNCTIONAL PARTS- 1.DIOPTRIC PART
2.RECEPTOR PART
1.DIOPTRIC OR FOCUSING PART- IT CONSISTS OF
CONJUNCTIVA,CORNEA,AQUEOUS HUMOUR,LENS AND VITROUS
HUMOUR.THEY REFRACT THE LIGHT RAYS PASSING THROUGH
THE EYE TO BRING THEM TO A FOCUS ON THE RETINA.MAXIMUM
REFRACTION IS CAUSED BY THE CORNEA,WHICH PLACES THE
IMAGE APPROXIMATELY ON THE RETINA.
2.RECEPTOR PART- IT COMPRISES THE RETINA.THE IMAGE
FORMED ON THE RETINA IS INVERTED AND SMALLER.IT
CONVERTS THE SPECIFIC WAVELENGTHS OF LIGHT INTO
RECEPTOR POTENTIALS OF NERVE FIBRES.THE NERVE IMPIULSES
ARE CARRIED BY THE OPTIC NERVE TO THE VISUAL AREAS OF
CEREBRAL HEMISPHERES WHERE THE REAL PERCEPTION OF
SIGHT ARISES AND SEES THE OBJECT UPRIGHT.

5. ACCOMMODATION IS THE REFLEX MECHANISM BY WHICH THE
FOCUS OF THE EYE CHANGES TO MAKE THE IMAGES OF DISTANT
AND NEAR OBJECTS SHARP ON THE RETINA.HUMAN EYE HAS A
GOOD POWER OF ACCOMMODATION.IT REQUIRES REFRACTION OF
LIGHT RAYS TO FOCUS THEM ON THE RETINA.IN THE
EYE,REFRACTION TAKES PLACE AT THE AIR-CORNEAL SURFACE
AND AT THE LENS.THE DEGREE OF REFRACTION AT THE CORNEAL
SURFACE CANNOT BE VARIED AS IT DEPENDS ON THE ANGLE AT
WHICH THE LIGHT STRIKES THE CORNEA AND THIS,IN
TURN,DEPENDS UPON THE DISTANCE OF THE OBJECT FROM THE
CORNEA.THEREFOR THE DEGREE OF REFRACTION IS CHANGED BY
CHANGING THE CONVEXITY OF THE LENS.THIS IS DONE WITH THE
HELP OF CILIARY MUSCLES AND SUSPENSORY LIGAMENT.

6. 1.DISTANT OBJECTS- WHEN
FOCCUSED FOR SEEING DISTANT
OBJECTS,THE EYE IS SAID TO BE AT
REST.LIGHT RAYS FROM DISTANT
OBJECTS ARE PARALLEL WHEN THEY
STRIKE THE EYE.THE CILIARY
MUSCLES ARE FULLY
RELEXED,SUSPENSORY LIGAMENT IS
UNDER MAXIMUM TENSION AND THE
LENS IS FLATTENED.
2.NEAR OBJECTS- LIGHT RAYS FROM
NEAR OBJECTS ARE DIVERGING WHEN
THEY STRIKE THE
EYE.THEREFORE,GREATER
REFRACTION OF LIGHT IS NEEDED FOR
FOCUSING THE NEAR OBJECTS.THE
CILIARY MUSCLES ARE
FULLYCONTRACTED,SUSPENSORY
LIGAMENT IS RELAXED AND THE
HOW THE EYE FOCUSES THE LIGHT

7. THE RECEPTOR CELLS OF EYE ARE CALLED PHOTORECEPTORS,OR VISUAL CEL
THESE CELLS CONTAIN THE LIGHT SENSITIVE PROTEINS CALLED THE
PHOTOPIGMENTS.THE PHOTORECEPTOR CELLS ARE OF TWO TYPES-
1.ROD CELLS
2.CONE CELLS
1.ROD CELLS- THEY CONTAIN
A PURPLISH PIGMENT CALLED
VISUAL PURPLE OR
RHODOPSIN.THEY FUNCTION IN DIM
LIGHT AND AT NIGHT.BRIGHT LIGHT
SPLITS RHODOPSIN INTO A
CAROTENOID PIGMENT CALLED
RETININE OR RETINAL AND
SCOTOPSIN,IN A PROCESS CALLED
BLEACHING.IN THE DARK
RHODOPSIN IS RESYNTHESIZED
FROM SCOTOPSIN AND
RETINAL.THE PROCESS IS CALLED
DARK ADAPTATION.IT MAKES THE

8. 2.CONE CELLS –
THE CONES CONTAIN A PIGMENT CALLED VISUAL VIOLET,OR
IODOPSIN.THEY FUNCTION IN DAYLIGHT AND ARTIFICIAL BRIGHT LIGHT,
PRODUCE DETAILED IMAGES AND GIVE COLOUR VISION.THEY ARE NOT AS
SENSITIVE AS THE ROD CELLS AND DO NOT RESPOND TO DIM LIGHT.THE
CONE CELLS GIVE COLOUR VISION BECAUSE THEY CONTAIN THREE
DIFFERENT PIGMENTS,EACH ABSORBING LIGHT OF DIFFERENT
WAVELENGTHS.
THEY ARE ERYTHROPSIN-SENSITIVE TO RED LIGHT,CHLOROPSIN-SENSITIVE
TO
GREEN LIGHT AND CYANOPSIN-SENSITIVE TO BLUE LIGHT.THE COLOUR
OTHERS
THAN THE ABOVE THREE PERCEIVED BY THE SIMULTANEOUS STIMULATION
OF 2
OR ALL THE 3 TYPES OF CONES.IN MOONLIGHT WE CANNOT SEE COLORS
BECAUSE ONLY THE RODS ARE FUNCTIONING.DUE TO LOW LIGHT LEVEL
CONES ARE NOT FUNCTIONING.

9. THE PHOTOSENSITIVE PIGMENT OF RODS AND CONES ARE CONCERNED
WITH
CHEMICAL BASIS OF VISUAL PROCESS.CHEMICAL REACTIONS INVOLVED
IN THESE PIGMENTS LEAD TO THE DEVELOPMENT OF ELECTRICAL
ACTIVITY
IN RETINA AND GENERATION OF IMPULSES WHICH ARE TRANSMITTED
THROUGH
OPTIC NERVE.
RHODOPSIN- IT IS THE PHOTOSENSITIVE PIGMENT OF ROD CELLS.
CHEMICALLY IS A CONJUGATED PROTEIN MADE UP OF A PROTEIN PART
CALLED OPSIN AND A CHROMOPHORE CALLED SCOTOPSIN.THEY ARE
RESPONSIBLE FOR DEVELOPING COLOUR IN CELLS.CHROMOPHORE
PRESENT IN ROD CELLS ARE CALLED RETINAL.IT IS THE ALDEHYDE OF
VITAMIN A OR RETINOL.RETINAL IS PRESENT IN THE FORM OF 11-CIS
RETINAL KNOWN AS RETININE-1.IT IS PRESENT IN HUMAN
EYES.SIGNIFICANCE OF 11-CIS FORM OF RETINAL IS THAT,ONLY IN THIS

10. DURING EXPOSURE
TO
LIGHT,RHODOPSIN IS
BLEACHED AND THE
COLOUR
BECOMES YELLOW
FROM RED.WHEN
RHODOPSIN
ABSORBS THE
LIGHT
THAT FALLS ON THE
RETINA,IT IS SPLIT
INTO RETININE AND
A PROTEIN CALLED
OPSIN THROUGH
VARIOUS
INTERMEDIATE
CHEMICAL

11. 1.RHODOPSIN IS DECOMPOSED INTO BATHORHODOPSIN WHICH IS VERY
UNSTABLE.
2.BATHORHODOPSIN IS CONVERTED INTO LUMIRHODOPSIN.
3.LUMIRHODOPSIN IS DECAYS INTO METARHODOPSIN I.
4.METARHODOPSIN I IS CHANGED TO METARHODOPSIN II.
5.METARHODOPSIN II SPLITS INTO SCOTOPSIN AND ALL TRANS-RETINAL.
6.ALL TRANS-RETINAL IS CONVERTED INTO ALL TRANS-RETINOL IN THE
PRESENCE OF THE ENZYME NICOTINAMIDE ADENINE
DINUCLEOTIDE(NADH2).
7.ALL-TRANS RETINOL IS CONVERTED INTO 11-CIS RETINOL AND AGAIN
CHANES INTIO 11-CIS RETINAL.
8.11-CIS RETINAL AGAIN SYNTHESISE RHODOPSIN.

12. VISUAL OR PHOTOTRANSDUCTION IS THE PROCESS
BY WHICH LIGHT ENERGY IS CONVERTED INTO
RECEPTOR POTENTIAL IN VISUAL
RECEPTORS.RESTING MEMBRANE POTENTIAL IN
OTHER SENSORY RECEPTOR CELLS IS USUALLY
BETWEEN-70 mV TO -90 mV.HOWEVER IN THE
VISUAL RECEPTORS DURING
DARKNESS,NEGATIVITY IS REDUCED AND RESTING
MEMBRANE POTENTIAL IS -40 mV.IT ISBECAUSE OF
INFLUX OF SODIUM IONS.NORMALLY IN DARK
SODIUM IONS ARE PUMPED OUT OF INNER
SEGMENTS OF ROD CELLS TO ECF.THESE SODIUM
IONS LEAD BACK INTO ROD CELLS THROUGH
MEMBRANE OF OUTER SEGMENT AND REDUCE THE
ELECTRONEGATIVITY INSIDE THE ROD CELLS.THUS
SODIUM INFLUX MAINTAINS A DECREASED
NEGATIVE POTENTIAL UPTO -40mV.INFLUX
ON SODIUM IONS INTO OUTER SEGMENT OF ROD
CELLS OCCUR MAINLY BECAUSE OF cGMP(CYCLIC
GUANOSINE MONOPHOSPHATE) PRESENT IN THE
CYTOPLASM OF THE CELL.THE cGMP ALWAYS
KEEPS THE SODIUM CHANNELS OPEN.CLOSURE OF
SODIUM CHANNELS OCCUR DUE TO REDUCTION IN
cGMP.WHEN LIGHT FALLS ON THE
Fig-Maintenance of dark current
In outer segment of rod cell

13. THE ROD CELLS ARE COMPOSED OF RETINAL(AN ALDEHYDE OF VITAMIN
A)
AND OPSIN(A PROTEIN).AND THE CONE CELLS ARE COMPOSED OF
RETINAL
AND THREE DIFFERENT TYPES OF PROTEINS TO WHICH RETINALS ARE
ATTACHED.
WHEN LIGHT FALLS ON RODS AND CONES,IT CAUSES DISSOCIATION OF
RETINAL FROM OPSIN.THIS CAUSES CHANGE IN THE STRUCTURE OF THE
OPSIN.THIS CHANGE IN THE STRUCTURE OF OPSIN,CAUSES MEMBRANE
PERMEABILITY CHANGES.AS A RESULT,POTENTIAL DIFFERENCE ARE
GENERATED IN THE PHOTORECEPTOR CELLS,i.e.,RODS AND CONES.AS
THE PHOTORECEPTOR CELLS SYNAPSE WITH THE BIPOLAR
CELLS,PRESENT IN NEXT LAYER,A SIGNAL IS PRODUCED
THATGENERATES ACTION POTENTIALS IN THE BIPOLAR CELLS ALSO.

14. NOW BIPOLAR CELLS SYNAPSE WITH
THE GANGLION CELLS,SO ACTION
POTENTIALS GENERATED IN THE
GANGLION CELLS THROUGH BIPOLAR
CELLS.SO WE CAN ASSUME THAT
ACTION POTENTIALS GENERATED IN
PHOTORECEPTOR CELLS ARE
TRANSMITTED TO THE GANGLION
CELLS THROUGH BIPOLAR CELLS.
WE KNOW THAT THE AXONS OF
RETINAL GANGLIOM CELLLS ARE
BUNDLE
TOGETHER TO FORM OPTIC NERVE.SO
FINALLY THE ACTION POTENTIALS
(IMPULSES)GENERATED IN THE
GANGLION CELLS ARE TRANSMITTED
THROUGH THEIR AXONS,i.e,OPTIC
NERVE TO THE VISUAL CORTEX AREA
OF THE BRAIN.
THE VISUAL CORTEX AREA IS A PART
OF OCCIPITA; LOBES OF
CEREBRUM.AFTER
REACHING THE VISUAL CORTEX
AREA,THE NERVE IMPULSES ARE
ANALYSED AND
IMAGE FORMED ON THE RETINA IS
RECOGNISED.THE RECOGNITION IS
BASED
ON EARLIER MEMORY AND
EXPERIENCE STORED IN THE
BRAIN.THIS SHOULD BE
NOTED THAT THE IMAGE FORMED BY
THE LENS OF EYE ON THE RETINA IS
INVERTED AND
REVERSED.HOWEVER,OBJECTS ARE
PERCEIVED THE RIGHT WAY
UP BECAUSE OF THE WAY IN WHICH
THE BRAIN INTERPRETS THE IMAGES.

15. VISUAL RECEPTORS
IN RETINA
FIRST ORDER NEURONS-
BIPOLAR CELLS
SECOND ORDER NEURONS-
GANGLIONIC CELLS
OPTIC NERVE
OPTIC CHIASMA
OPTIC TRACT
THIRD ORDER NEURONS-
LATERAL GENICULATE
BODY
OPTIC RADIATION
VISUAL CORTEX

16. IMAGE FORMED ON THE RETINA IS RECOGNISED BASED ON EARLIER MEMORY
AND EXPERIENCE.
ACTION POTENTIALS ARE TRANSMITTED BY THE OPTIC NERVE TO THE VISUAL
CORTEX OF BRAIN.
IT GENERATES ACTION POTENTIAL IN THE GANGLIONIC CELLS THROUGH THE
BIPOLAR CELLS.
MEMBRANE PERMEABILITY OF RODS AND CONES CHANGES.
LIGHT INDUCES DISSOCIATION OF THE RETINAL FROM OPSIN RESULTING IN THE
CHANGES IN THE STRUCTURE OF THE OPSIN.
ACTIVATION OF PHOTOPIGMENT OF RODS AND CONES.
LIGHT RAYS IN VISIBLE WAVELENGTH FOCCUSED ON THE RETINA.

17. 1.MYOPIA OR
NEARSIGHTEDNESS-THE
EYEBALL IS ELONGATED SO THAT
THE IMAGE OF DISTANT OBJECTS
IS FORMED A LIITLE IN FRONT OF
RETINA.CAN BE CORRECTED BY
CONCAVE LENS.
2.HYPERMETROPIA OR LONG
SIGHTEDNESS-CAN SEE DISTANT
OBJECTS BUT NOT THOSE WHICH
ARE CLOSER.IMAGE IS FORMED
BEHIND RETINA.CAN BE
CORRECTED BY USING CONVEX
LENS.
3.PRESBYOPIA-A COMMON
DEFECT IN OLD AGE PEOPLE DUE
TO LOSS OF ELASTICITY OF
LENS.CAN BE CORRECTED BY

18. 4.ASTIGMATISM-THE DISORDER DUE TO ROUGH CURVATURE
OF CRNEA OR LENS WHICH CAN BE CORRECTED BY THE USE OF
CYLINDRICAL GLASSES.
5.CATARACT-THE SIGHT IS IMPAIRED DUE TO THR LENS
BECOMING
OPAQUE.THE DEFECT CAN BE CORRECTED BY SURGICAL
REMOVAL OF THE DEFECTIVE LENS.
6.GLAUCOMA-IT OCCURS DUE TO INCREASE IN INTRA-
OCCULAR
PRESSURE AS MAY DDEVELOP DUE TO BLOCKAGE OF CANAL OF
SCHLEMN.IT EXERTS PRESSURE ON OPTIC NERVE CAUSING ITS
DAMAGE.IT LEADS TO PERMANENT BLINDNESS.

19. THANK
YOU