Physiology of the Eye (1)

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Physiology of The Eye

By

Mila CitrawatiJakarta, May 23rd 2006


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Anatomy of The Eye

Rongga orbita : atas [os frontalis, os sfenoidalis],

bawah [os maxilaris,os zigomatikum, os palatinum],

medial [os maxilaris, os lakrimalis, os sfenoidalis],

lateral [os zigomatikum, os sfenoidalis, os frontalis] Bulbus okuli

N optikus

Otot penggerak bola mata [m. rektuslateral/medial/superior/inferior, m.oblikus

superior/inferior]


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Bulbus okuli

Terdiri dari 3 tunika/lapisan :

- Lap. Fibrosa [sklera, kornea]

- Lap. Vaskulosa [khoroid, badan siliar, iris]

- Lap. Nervosa [retina]1. Lapisan fibrosa bfungsi :

- provide mechanical support

- attachment site for extrinsic eye muscles

- assist in the focusing process Sclera consist of collagen, elastin, small blood

vessels, nerves. Known as white of the eye


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2. Vascular layer [uvea] consist of blood vessels,

lymph vessels, intrinsic occulomotor muscles

Functions :- provide route for blood vessels and lymph that

supply tissue of the eye

- regulate amount of light entering the eye

- secreting and reabsorbing aqueous humor

- control the shape of lens

Iris : m.pupillary constrictor, m. pupillary

dilatorpupil in bright 1,5-2 mm in dark 7-8 mm

[control amount of light, path of light] light reflex

and consensual light reflex. Contain different

amount of pigmen [color of the eye]


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Ciliary body function :

hold lens posterior to the iris and centered onthe pupil.

Choroid function :

suply oksigen and nutrient to rods and cones Uvea filled with aqueous humor similar to

CSF, circulate from COP [space between

vitreus and iris] to COA [space between irisand cornea] absorb to canal of Schlemm


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3. Neural layer

Consist of 2 distinct layers :

- outer layer of pigment [function : absorb

light, absorb damaged photoreceptors

biochemical interaction to the photoreceptors]

- inner layer of retina consist of photoreceptors

and blood vessels


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Visual sensoris

Consist of : retina, optic nerve, optic chiasm,optic tract, lateral geniculate nucleus,

geniculocalcarina tract, calcarine cortex,

visual cortex

Optic disc : 3-4 mm medial [nasal] to fovea

centralis. Site where retina nerve fibers,arteries, and veins leave the eye contain

no photoreceptors


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External structure

Palpebrae [superior/inferior] consist of m. riolani, m.

orbicularis oculi, m. levator palpebrae [function :

protect eye from light, dust, foreign body],

Meibomian gland [function : prevent evaporation oflubricating fluids]

Eye brows [function : protect eye from sweat, dust,

sunlight]

Eye lashes [function : protect eye from dust, sunlight,

dirt]


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Lacrimal apparatus

Secretoir : lacrimal glands and lacrimal ducts Excretoir : lacrimal punctum sup/inf, lacrimal canal,

nasolacrimal duct, meatus inf.

Lacrimal glands :

- Os frontalis [lacrimal fosa]- Below superior fornix conjunctive

- Secrete fluid since 3 weeks old neonatus

- Lacrimal gland + Meibomian gland + Zeis gland oil

slick 1ml/day [function : lubricating, preventevaporation, reduce friction, remove debris, preventinfection, provide nutrient and oksigen]


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Conjunctive [palpebrae and bulbi]: mucous membranesurface palpebrae and bulbi, contain a. conjunctiveposterior, a. cilliar anterior, n.V, lymph vessels.

Cornea: anterior part of the eye crossed by light fromoutside to retina. Transparent, contain lymph vessels,nerves and no blood vessels. Diameter approximately12 mm. Consist of 5 layers [epithel, Bowman

membrane, stroma, descemet, endothel]. Function asrefractive surface.

Aqueous humor [function: hold pressure]

Iris : heavily pigmented muscles

Lens : refractive media lies behind the cornea, held inplace by suspensory ligaments

Vitreous body [function : stabilize shape of the eye,

physical support to the retina]


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Development of the eye

Birth : 8-18 inch, poor coordination, night blind [lessphotopigment]

3-8 weeks : binocular vision

2-3 months : iris become darker, detail of facialfeatures

3-4 months : accomodation, lens flat, binocularvision

5-7 months : visual acuity 20/200, night vision,hand-eye coordination

8-9 months : maculae becomes mature, depthperception, point with finger


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1 year : visual acuity 20/100, geometric shape,

facial expression

2 years : visual acuity 20/40, see small objects anddetail

3 years : visual acuity 20/30

4 years : visual acuity 20/20, reading 5 years : depth perception, color

6 years : visual acuity 20/20, shades of color

8 years : eyeball adult 40 years : presbyopia

+60 year : lenses become rigid, floaters [because of

vitreus shrinkage], reduce tears, cataract


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Refraction

Definition : bending of light rays as they travel fromone transparent media to another

Light : a form of energy known as electro magnetic

radiation Refraction media : aqueous humor, lens, vitreous

humor

Refraction surface : cornea, anterior and posterior

surface of the lens Correction lens : spheris (-), spheris (+), cylindris

(-/+)


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Emetrop : parallel light that comes to the resting eyewill be refracted right at the maculae lutea / retina

Ametrop : parallel light that comes to the resting eyeisnt refracted right at the retina

Myop : refraction disorder which withoutaccomodation light that comes parallel will be focused

in front of the retina. Myop axial and myop refractive.Mild [0-3D], moderate [3-6D] severe [>6D].Complication ; retina ablation, strabismus, maculaebleeding. Cocrreted by spheric (-) lens

Hypermetrop : focused behind the retina.Hypermetrop axial and hypermetrop refractive.Corrected by spheric (+) lens


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Astigmat : refraction disorder caused by differentdegree of refraction at different meredian.Astigmat reguler [with the rule and against the

rule] and astigmat irreguler. Corrected by cylindriclens

Presbyop : reduced accomodation capability atlate age. Corrected by spheric (+) lens

Aphakia : condition of no lens [cataract op,congenital, dislocation of the lens]. Corrected byspheric (+) lens

Visual acuity : 0 [no visual image], 1/~ [visual oflight only in 1 m], 1/300 [visual of hand movementin 1 m], 1/60 [visual of finger in 1m], 6/66/10[emetrop]


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Refractive disorder examination :

1. SubjectiveSnellen chart and lenses

2. Objective Ophthalmoscope, retinoscope,

refractometer, ceratometer

3. Jaeger test

Near response : accomodation, convergance

of visual axis, miosis


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Close object viewed lens

thickening light

refraction better

focusing in retina

Distant object viewedlens flattening light

refraction better

focusing in retina


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Light and photoreception

Retina consist of pigment, rod and cone cells,

horizontal cells, bipolar cells, amacrine cells,ganglion cells

Human eyes are sensitive to light with 400-700 nmwavelength [visible light]. ROY G BIV [red, orange,

yellow, green, blue, indigo, violet] Photon : single energy packet of visible light.

Reflected photon by surface seen as color, absorbedphoton by surface seen as black

Photoreceptors detect photon. Rods respond to anyphoton, more sensitive. Cones respond to red, green,blue photon, less sensitive

Comparison of photoreceptors


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Comparison of photoreceptors

Rod:

Photopigment: 1rodhopsin

Acuity + threshold: low

Type of vision : black &

white Most concentrated

location: peripheral

Wavelength peak

sensitivity : 505 nm Estimated total number :

20 million

Cone :

3 [blue, green, redsensitive pigment]

High

Color

Central [fovea centralis]

Blue445 nm, Green

535, red 570 nm

7 million


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Maculae has highest density of photoreceptors. Foveacentralis contain only cones, therefore has the sharpestvision

Look directly to target means focusing the image righton the fovea [color vision]

In the dark photoreceptors release neurotransmitteracross synaps at inner segment continuously

Light activate opsin from cis form to trans form[has the ability as enzyme]activate 2nd enzyme[transducin and G protein] Transducin activatePDEcGMP breakdown Sodium channel closedmembrane hyperpolarizedNT release reducedmembrane potential decreasesignal of light struckthe retina


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Color Vision

Blue cones 16%, green cones 10%, red cones 74%.

The other colors are combination of two type cones

Anomaly shows color weakness, anopia shows color

blind

Protanomaly, deuteranomaly, tritanomaly, normalcolor vision grouped as trichromat

Dichromat has only two type of cone function well

Monochromat has only one type of cone function well

[can only see black, white, shade of grey]

Color blind is inhereted. Linked to X chromosom,

resesive.


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Eye movement

M. rectus lateralisabduction N.VI

M. rectus medialis adduction N.III

M. rectus superior elevation N.III

M. rectus inferior depressionN.III

M. oblique superior oblique depression

N.IV

M. oblique inferior oblique elevation

N.III

Disorder of occulomotor muscles can cause

strabismus


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Visual Fields and Binocular Vision

Visual field of each eye is the portion of the externalworld visible out of that eye.

Binocular vision central parts of visual fields of the 2eyes that coincide. Impulses set up in the 2 retinas by

light rays from an object are fuse at the cortical levelinto a single image

Perimetry is an examination of visual field usingdevice. Another simple examination is direct

confrontation Many pathologic conditions show narrow visual

fields


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Humans uses single binocular vision inviewing an object. Both eyes focus on the

same object. The light rays that enter the eyes

are focused onto corresponding points of thetwo retinas. In viewing near objects requires

the eyes to rotate medially in order for the

light rays to hit the same point of the two

retinas [convergance]


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Neural Pathways

Axons of ganglion cells pass caudally in optic

nerve and optic tract to end in the lateralgeniculate body.

Fibers from each nasal hemiretina decussate in

the optic chiasm.

Geniculocarcarine tract : fibers from nasal half

of one retina and temporal half of the other

Visual cortex : Brodmanns area 17


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Dark adaptation : decline of visual threshold. If

a person spends a considerable length of time

in brightly lighted surrounding and then movesto dimly lighted environment, the retinasslowly become more sensitive to light as theindividual becomes accustomed to the dark[20minutes]. It took times because most of therodhopsin has been broken down in the brightlight and the cones are inactive in the dim light.

With the time, rodhopsin molecules reform.This caused the sensitivity of the retinaincrease and visual threshold decreasevision


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Light adaptation : if one passes suddenly from a dim

to a brightly lighted environment, the light seems

intensely and even uncomfortably bright until the

eyes adapt to the increased illumination and the visual

threshold rises [5 minutes]

The presence of bright light causes the breakdown ofthe visual pigments of the rods and the cones into

retinal and opsin. The retinal is converted to vit A.

Prolonged exposure to bright light decrease the

consentration of visual pigments in rods and cones.The result is a reduced sensitivity of the eye to light


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Physiology of the Eye (1)

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