Mechanism of Vision in Human Beings with Diagram

Mechanism of Vision

1. TOPIC -MECHANISM OF VISION PRESENTED BY-ANUPAM DUTTA ROLL NO- 245

2. BRIEF INTRODUCTION OF THE STRUCTURE OF HUMAN EYE 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. STRUCTURE OF RETINA 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 GANGLION CELLS BUNDLE TOGETHER AS THE OPTIC NERVE.

4. WORKING OF THE EYE 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 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 LENS IS THICK. HOW THE EYE FOCUSES THE LIGHT

7. PHOTORECEPTORS THE RECEPTOR CELLS OF EYE ARE CALLED PHOTORECEPTORS,OR VISUAL CELLS. 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 RODS FUNCTIONAL.

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 CONE S . IN MOONLIGHT WE CANNOT SEE COLORS BECAUSE ONLY THE RODS ARE FUNCTIONING.DUE TO LOW LIGHT LEVEL CONES ARE NOT FUNCTIONING.

9. CHEMICAL REACTIONS INVOLVED IN VISUAL PROCESS 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 FORM IT CAN COMBINE WITH SCOTOPSIN TO SYSNTHESISE RHODOPSIN .

10. PHOTOCHEMICAL CHANGES IN RHODOPSIN-WALDS VISUAL CYCLE 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 REACTION-

11. CHANGES DUE TO ABSORPTION OF LIGHT ENERGY BY RHODOPSIN 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. PHOTOTRANSDUCTION 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 RETINA,RHODOPSIN IS EXCITED LEADINGTO DEVELOPMENT OF RECEPTOR POTENTIAL IN THE ROD CELLS. Fig-Maintenance of dark current In outer segment of rod cell

13. THE VISUAL MECHANISM 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 REPRESENTATION OF VISUAL PATHWAY

16. SUMMARY OF VISUAL PROCESS

17. SOME DISORDERS OF EYE 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 CONVEX LENS.

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.

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