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BiomechanicsofEye Anatomyofeye AnatomyoftheEye Func3onsVs.Physiology ImportantlayersofCornea Cataract Cataract Glaucoma Glaucoma DevelopmentofGlaucoma DevelopmentofGlaucoma DevelopmentofGlaucoma GlaucomaDamage OpenVs.ClosedAngleGlaucoma GlaucomaEffect Glaucoma BlackVs.Whiteblindness OCULARMUSCLEFUNCTIONS Musclesoftheeye–responsiblefor ocularmovement Musclesoftheeye–responsiblefor ocularmovement Musclesoftheeye–responsiblefor ocularmovement IndividualMuscleFunc3ons IndividualMuscleFunc3ons Eachofthesixextraocularmusclesplaysaroleinposi3oningtheeye aboutthreeaxesofrota3on.Theprimaryac3onofamuscleisthe principaleffectithasoneyerota3on.Lessereffectsarecalledsecondary orter3aryac3ons.Theexactac3onofanymuscledependsonthe direc3onoftheeyeinspace. SurgicalCorrec3onofOcularmuscles FOCUSING © 2013 Pearson Education, Inc. Point sources Focal points Focusing of two points of light. The image is inverted—upside down and reversed. © 2013 Pearson Education, Inc. Focusing Light on The Retina • Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors • Light refracted at boundaries along pathway – Air to cornea/aqueous humor – Aqueous humor to lens – Lens to vitreous humor • Most bending happens at air-cornea boundary • Lens curvature is the “fine adjustment” © 2013 Pearson Education, Inc. Focusing For Distant Vision • Eyes best adapted for distant vision • Far point of vision – Distance beyond which no change in lens shape needed for focusing • 20 feet for emmetropic (normal) eye • Cornea and lens focus light precisely on retina • Ciliary muscles relaxed • Lens stretched flat by tension in ciliary zonule © 2013 Pearson Education, Inc. Sympathetic activation Nearly parallel rays from distant object Lens Ciliary zonule Ciliary muscle Inverted image Lens flattens for distant vision. Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens. © 2013 Pearson Education, Inc. Focusing For Close Vision • Light from close objects (<6 m) diverges as approaches eye – Requires eye to make active adjustments using three simultaneous processes • Accommodation of lenses • Constriction of pupils • Convergence of eyeballs © 2013 Pearson Education, Inc. Focusing For Close Vision • Accommodation – Changing lens shape to increase refraction – Near point of vision • Closest point on which the eye can focus – Presbyopia—loss of accommodation over age 50 • Constriction – Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye • Convergence – Medial rotation of eyeballs toward object being viewed © 2013 Pearson Education, Inc. Parasympathetic activation Divergent rays Inverted from close object image Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge. © 2013 Pearson Education, Inc. Problems Of Refraction • Myopia (nearsightedness) – Focal point in front of retina, e.g., eyeball too long – Corrected with a concave lens • Hyperopia (farsightedness) – Focal point behind retina, e.g., eyeball too short – Corrected with a convex lens • Astigmatism – Unequal curvatures in different parts of cornea or lens – Corrected with cylindrically ground lenses or laser procedures © 2013 Pearson Education, Inc. Emmetropic eye (normal) Focal plane Focal point is on retina. © 2013 Pearson Education, Inc. Myopic eye (nearsighted) Eyeball too long Uncorrected Focal point is in front of retina. Corrected © 2013 Pearson Education, Inc. Concave lens moves focal point further back. Hyperopic eye (farsighted) Eyeball too short Uncorrected Focal point is behind retina. Corrected © 2013 Pearson Education, Inc. Convex lens moves focal point forward. Functional Anatomy Of Photoreceptors • Rods and cones – Modified neurons – Receptive regions called outer segments • Contain visual pigments (photopigments) – Molecules change shape as absorb light – Inner segment of each joins cell body © 2013 Pearson Education, Inc. PHOTORECEPTION Photorecep3on ConeandRodcelldistribu3on ConeandRodcelldistribu3on Conearesensi3vetocolor(i.e.wavelength) Rodsarenotsensi3vetocolor Rodsaremoresensi3vetolightthancones (intensity) Absorp3onspectraofHumanVisual Pigments ConecellsandRodCells ChipinSynthe3cvisualsensor Re3naldisease • Largenumberofprogressivere3nal degenera3onsthatpredominately affectthephotoreceptorlayeror “lightsensing”cellularlayerofthe re3na. • Danbehereditaryordevelopasa newcondi3on • Willworsenwith3me. AMDprimarilyaffectsthehighestresolu3on centralpor3onofthere3naknowntoasthe macula Producesa“dry”orgeographicformof damageresul3nginblindspots. DryAMDmayprogressintolargerareasof visionlosswiththedevelopmentofa“wet” formthatoccursfrombloodvessel,withinthe affectedre3nalareasleakingorbleeding. Scarringofthesere3nalareasmayalsooccur. MEASUREMENTOFINTRAOCULAR PRESSURES(EYEPRESSURE) IntraocularPressure • Normal–10to21mmofHg • Higherthan21mmofHgisocular hypertension • Lowerthan5mmofHgistermedashypotony • Higherthan21mmofHgdoesnotmeana personhasglaucomabuthastherisktobe affectedwithhighpressure. Tonometers Indenta3ontype Tonometers Applana3ontype(Contact) NoncontactTonometers Applana3ontype(Non-Contact) IOP LASIKSurgery