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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