Specific learning objectives Functional anatomy of eye Photoreceptor

• • Specific learning objectives: Functional anatomy of eye Photoreceptor mechanism Image-forming mechanism Visual pathway Visual Acuity, light and dark adaptations Color Vision

eye

• EYE has two major parts: • Optical system Helps to focus and form an image on the receptor cells-light rays fall. • Neural system. Transmits optical signal in the form of Aps along the optic nerve to visual cortex.

• Eye ball-hollow spherical organ • Optical system –Anterior part of the eye ball • Visual receptors –posterior surface-optic nerve arises and proceeds towards occipital cortex.

• • Interior of eyeball is divided into three spaces: Anterior chamber Posterior cavity

• Sclera • White avascular fibrous coat-composed of collagen fibers. • Shape to eyeball • Protective function • Extraocular muscles-regulates the eyeball movement. • Central portion of eye-transparent cornea

• Lacrimal gland • • • Lateral corner of eye. Secretes tear Keeps cornea moistened. Prevent infection Drained through naso-lacrimal duct.

Choroid-posterior 2/3 rd of eyeball Numerous blood vessels Nourishes structure of eyeball Front thickened portion-ciliary body-absorb extra amount of light. • Ciliary body • Attached to suspensory ligament at one end. • Other end-crystalline lens • •

• Two types of smooth muscle: • Circular and longitudinal. • Accommodation for near vision. • • Iris Pigmented and opaque muscular structure. Colour to eye. Center-aperture-pupil-light enters the eye.

• • Two types of muscles: Sphincter and dilator pupillae Determines size of pupil. Functions Regulates intensity of light. Absorbs extra amount of light. Prevents entry of light through periphery of lens. Increases the depth of focus by constriction of pupil.

• Retina • Outer pigmented layer-attached to inner surface of choroid. • Inner layer-nerve cells and nerve fibersphotoreceptors.

• • Crystalline lens Circular biconvex Formation of image on retina. Has no blood supply.

• • All the nerve fibers-retina-optic nerve-brain Optic disc-optic nerve leaves eye. Blind spot Macula lutea-posterior pole of eye. Marks the location of fovea centralis. Absence of rods and densely packed cones. Greatest visual acuity.

• Line joining the anterior pole-posterior pole of the eyeball-optical axis • Line joining the fixation point to fovea centralis-visual axis.

Fluids in the eye: • Aqueous Humor-anterior cavity • Vitreous Humor-posterior cavity.

• Aqueous humor-protein –free clear fluid • Formed- Ciliary processes-posterior chamberpupil-anterior chamber. • Aqueous pressure-15 -18 mm Hg higher than intracranial pressure. • Helps to maintain shape of eye.

• Composition • High content of Na. Cl, vitamin C, lactic acid and hyaluronic acid.

Outflow of Aqueous Humor: Aqueous Humor is formed in ciliary processes Through the pupil In to the anterior chamber Anterior to the lens The angle b/w cornea and the iris Canal of Schlemm Extra ocular veins

• Functions • It provides nutrition to all avascular structures of eye. • It maintains intraocular pressure. • It maintains shape of the eye.

• Vitreous Humour • Interior between the lens and retina is filled with albumin and hyaluronic acid.

Functions of vitreous humor: • Prevent the wall of eyeball from collapsing. • It maintain the intraocular pressure and keep the intraocular structures in position. • Acts as a refractive medium.

Glucoma/Ocular Hypertension: Pressure increase to 60 -70 mm of Hg. Normal : 15 mm of Hg(12 -20 mm of Hg) Pressure above 25 -30 mm of Hg can cause blindness if maintained for long duration. Causes: • Blockage of canal of Schlemm • Excessive production of the fluid

2 types: Ø Primary: • Open angle: after 40 yrs • Closed angle: after 60 yrs. Ø Secondary : Cataracts, Trauma, Intraocular haemorrage.

Retina

LAYERS OF RETINA: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Pigmented epithelium Layers of rods and cones External limiting membrane Outer nuclear layer Outer synaptic layer Inner nuclear layer Inner synaptic layer ganglion cell layer Optic nerve Internal limiting membrane.

1. Outer pigmented epithelial layer. • • Rich in melanocytes. Prevents scattering of light. Phagocytosis. Storage of Vit A.

• Rods and cones-Photoreceptors • Each rod and cone is divided into outer segment, inner segment and a synaptic zone.

• Outer and inner segments form the layer of rods and cones. outer segment Modified cilia-piles of flattened discs. Discs - Rhodopsin and Iodopsin

• Rods-thin , rod like appearance • Old discs-shed and removed by pigmented epithelium. • New discs-inner edge of outer segment. • Synapse with several rod bipolar cells

• Cones • Conical outer segment • Renewal –diffused-multiple sites at outer segment.

• Inner segment: Rich in mitochondria In cones-thick, oval and is larger • Synaptic Zone. Synaptic vesicles –glutamate.

• External limiting membrane. Glial tissues. • Outer nuclear layer Nucleus of rods and cones • Outer synaptic layer synapse between ends of rods and cones with dendrites of bipolar cells and horizontal cell (Horizontal cells–interneuron-connects receptor cells ).

• Inner nuclear layer o Bipolar cells o Amacrine cells -interneurons-synaptic contacts with adjacent ganglion cells. • Inner synaptic layer • Bipolar cells-synaptic contacts with ganglion cells. Site of processing of visual image.

• Ganglion cell single layer of cell containing round cells • Optic nerve axons of ganglion cells • Internal limiting membrane-processes muller cells –supporting glial cells-nutritive function. Separates retina from vitreous humour

• How light rays reach photoreceptors Light rays Ganglion cell Bipolar cell photoreceptors

• Functions of Rods • sensitive to light. • dim light vision or night vision or scotopic vision. • Low threshold

• Functions of cones. • High threshold. • bright light vision or daylight vision or photopic vision. • Cones are also responsible for acuity of vision and the color vision.

Rods : 1. 120 million. 2. Mainly in peripheral retina. 3. Slender , Elongated, Rod like. 4. More pigments/rod. 5. Only one type of pigment-Rhodopsin. 6. Cannot detect color. 7. Functions better in dim light. 8. Loss- Night blindness Cones: 1. 6 million 2. Mainly in the central retina. 3. Conical shape. 4. Less pigments/cone. 5. Three types: blue colour(cyanolabe) Green (chlorolabe) red (erythrolabe) 6. Can detect color. 7. Functions better in day light 8. Loss-functional blindness.

• • • Chemistry of Rhodopsin is a conjugated protein. Protein called opsin and a chromophore. Opsin -scotopsin. Chromophore - retinal. Retinal is the aldehyde of vitamin A or retinol. Retinal is present in the form of 11 -cis retinal known as retinine 1

Rhodopsin –Retinal visual cycle

RHODOPSIN LIGHT ENERGY BATHORODOPSIN LUMIRHODOPSIN METARHODOPSIN 1 SCOTOPSIN 11 -CIS RETINAL 11 -CIS RETINOL ISOMERASE METARHODOPSIN 2 ALL-TRANS RETINAL ALL-TRANS RETINOL (Vit A)

• Rhodopsin --metarhodopsin II. Resynthesis of Rhodopsin • all-trans retinal - 11 -cis retinal ( enzyme retinal isomerase). • 11 -cis retinal immediately combines with scotopsin to form rhodopsin. • all-trans retinal-all-trans retinol(dehydrogenase)-11 -cis retinol

PHOTOTRANSDUCTION The process by which light energy is converted into receptor potential in visual receptor.

• In darkness-visual receptors -resting membrane potential is about – 40 m. V. • sodium ions leak back into the rod cellsreduce the electronegativity inside rod cell • Dark current.

Maintenance of dark current in outer segment of rod cell

isomerase all –trans (Conformational change) retinal

Functions of retina: 1. Visual functions: • Light sense: Help to perceive light. • Form sense: Appreciate the shape. • Color sense: Perceive and recognize different color and different intensities of the same color

2. Reflexes: • Light reflex • Accommodation reflex. 3. Help to maintain the Tone , Posture and Equlilibrium.

IMAGE FORMING MECHANISMS

• Light rays-falls on eye-image formation on retina: • Refraction of light by cornea and lens • Ciliary muscle activity-accommodation of lens • Change in pupil size –iris muscles.

• Principles of optics • Refraction-Change in the direction of light rays • Lens-transparent glass-two spherical surfaces 3 types: Convex Concave Cylindrical

• Optical centre/nodal point-centre point of lens • Principal axis-line joining centre of two spherical part lens surfaces. • Principal focus vconvex lens-point on principal axis-light rays –converges v. Concave lens-point on principal axis-light rays –diverges

• Focal length-distance between optic centre and principal focus of the lens. • Light rays-distant object(>6 m)-parallel and object closer(<6 m)-diverging.

• Power • Reciprocal of focal length. • S. I-dioptre. • Convex lens-positive focal length and concave lens-negative.

• Reduced /schematic Eye. • Differences in refractive indices of eye structure • Reduced eye –single spherical surface –single principal and nodal point. • Nodal point-7 mm anterior surface of cornea.

• Human eye-24 mm in length-focal length 17 mm. • Refractive power of reduced eye-59 D • Normal eye(59 D)-behaves as the reduced eye.

ACCOMMODATION: The ability of the eye ball by which the near objects are clearly focused on the retina. • • • Changing the curvature of lens. Ciliary muscle contracts Suspensory ligaments relax Tension on lens decreases. Lens becomes convex.

Near point: The nearest point to the eye at which an object can be brought in to clear focus by accommodation. Far point: The farthest point which can be brought to focus.

• Near vision • Nearest point to the eye, at which object seen clearly. • Accommodation is maximum.

Changes associated with near vision: • Change in the anterior curvature of the lens. • Constriction of pupil. • Convergence of the eye ball.

Far vision: Near vision: • Ciliary muscles relax. • Ciliary muscles contract. • Lens is held under tension by the lens ligament. • Lens ligaments are relaxed. • Lens flattens. • Lens convex shape.

• Defects of image forming mechanisms

• Emmetropia= Normal eye • Ametropia-refractive errors of eye ü Axial length of eye ü Refractive power -curvature of surface of cornea/lens -refractive indices of media - position of lens

Emmetropia= Normal eye. Myopia/Near sightedness: Objects are focused infront of retina. Cause: • Too long an eye ball. • Stronger curvature of the eye. Seen in: • 5 % of newborns and infants. Correction : • Biconcave lens • Surgical correction. Refractive power +ive

Hypermetropia/Farsightedness. Object is focused beyond the retina. Cause: • Too short an eye ball. • Weak curvature of the eye. Seen in: • New born and infants 80% • Rare in young adults. Correction : • Biconvex lens. Refractive power : -ive

Astigmatism: Failure to focus light at one point- blurring of the image. -unequal refraction at different meridians. Cause: due to the faulty curvature of the cornea. Correction: • Cyclindrical lens

Presbyopia: Lens looses its elastic nature and becomes relatively solid mass( Denaturation of proteins). • Accommodation power decreases. • Far point not affected. • Eye remains focused at a constant distance. Correction: Bifocal lens

Normal aberrations of vision:

Spherical Aberration: • Crystalline lens of the eye is not regularly formed. • Non uniform refractive index of lens. • Light rays-periphery refracted more. • Light of single wavelength gets focused at different points on retina

Chromatic aberration: Light –longer wavelength-refracted more Light rays passing through the periphery of the lens is affected most- color fringes appear. Color fringes are ignored by the brain. Light of different colors get focused at different points on retina.

Adaptation: Getting accustomed or used to a new condition. Dark adaptation: the decrease in the visual threshold or increase in the sensitivity of the eye to light.

• Causes for Dark Adaptation • Increased sensitivity of rods as a result of resynthesis of rhodopsin. • Dilatation of pupil

• Dark Adaptation Curve • Demonstrates the relationship between threshold of light stimulus and time spent in dark.

• Dark adaptation curve is biphasic. • First part of the curve represents threshold of photopic vision-cone adaptation. • Second part of the curve represents threshold of scotopic vision-rod adaptation

Cone adaptation: • This first phase is rapid and it is completed in 8 to 10 minutes. • During this period the threshold decreases by 2 to 3 log units

Rod-cone break • After the first phase, there is a sudden change in slope of the curve -rod-cone break. • rod sensitivity begins to exceed cone sensitivity. continuing adaptation of rods. • During this phase threshold decreases further by 5 to 6 log units.

Rod adaptation • Second phase of the curve is slow. • gradual decrease in the threshold. • completed in 20 to 30 minutes.

• LIGHT ADAPTATION • Light adaptation is the process in which eyes get adapted to increased illumination. • Reduced sensitivity of rods • Constriction of pupil • Rods - deactivated • Cones-stimulated. • Duration – 5 mins

Changes occurring during dark adaptation: • Dilation of pupil. • Photoreceptor function, changes from Cones to Rods. • Resynthesis of Rhodopsin. Changes occurring during light adaptation. • Pupil constrict. • Photoreceptor function , Rods-> Cones. • Photo pigments are bleached and their concentration decrease.

• • • Night blindness: Blindness in dim light Vitamin A deficiency Degeneration of neural layers of retina Scotopic vision

Color vision

• • Visibility range of vision-400 nm-750 nm Wavelength of spectral colours(VIBGYOR) Violet-400 nm Blue-450 nm Bluish –green-500 nm Green-550 nm Red -650 -750 nm

Achromatic vision: It is the sensation of white color and no color has been assigned to it. Chromatic vision: Spectral colors vision and extra spectral color vision (carbon blue). Primary colors: Red Green blue. Complementary color: when two colors are mixed an appropriate amounts= white.

• Maximum sensitivity –scotopic vision-500 nmbluish Green • Photopic -560 nm- greenish yellow • Purkinje shift-photopic to scotopic vision. • Shifting of sensitivity. • Seen towards evening.

Theories of color vision; 1. Thomas young and Von –Helmholtz's theory( Trichromatic color theory). Three different types of cones each containing a different photosensitive pigment and maximum sensitivity to one type of primary color.

2. Mullers doctrine of specific nerve energy theory: There are specific nerve fibers with specific ganglion cells responding to three primary colors. After image: After one stops looking at a color he may continue to see it for a short time(+ive after image) Or he may see its complementary color(-ive after image)-RED+BLUISH GREEN-WHITE

Granites dominator and modulator theory. Two types of ganglion cells: • Dominators: they respond to the whole visual spectrum

Modulators: respond maximum to a narrow wave length of light. Blue: 450 -470 nm(Peak=445 nm )-cyanolabe Green: 520 -540 nm(Peak=535 nm)-cholorolabe Red: 500 -600 nm(Peak=570 nm) –erythrolabe In dark adapted eye-sensitivity-scotopic vision In light adapted eye-senstivity-photopic vision.

Hering ‘s opponent color theory • Extension of first theory. • 4 primary color: Red , Blue, Green, Yellow • Rand G oppose each other. • B and Y oppose each other

Single opponent theory • Ganglion cells and cells of LGB Double-opponent theory • Cells of visual cortex

Color blindness: This is based on the young Helmholtz theory of color vision: 8%of males, 0. 4% females Inherited as x-linked recessive disorder Females-carriers. males –sufferers.

Types of color blindness:

Monochromatism • Total inability to perceive color. • It is also called total color blindness. • Very rare. • Monochromats • They can see black , white and shades of grey.

Dichromatism 2%-affected males • Appreciate only two colors. • Dichromats.

• • Trichromatism -6% Have three cone system. One cone system is defective. trichromats

• • • Anomaly-colour weakness Anopia- colour blindness. Prot-red Deuter-blue defective cone Trit-green

Deficiency of color vision. Causes • Chronic diseases • Drugs • Toxins • Alcohol • aging

Test for color vision. • Ishihara chart • Wool sorting test. • Edridge green lantern method