EYE REVIEW

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EYE REVIEW!
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Cornea and sclera are continuous, but have
different morphology function
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Light enters eye through cornea, through anterior
chamber, passes through pupil, lens, vitreous
body, image falls on the retina. Between
cornea iris is the anterior chamber (filled
with aqueous humor, continuously produced).
Vitreous body (NOT continuously produced) is a
chamber between lens retina.
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Accommodation

• The lens changes shape to focus light on the back
  of the eye regardless of the distance of the
  object
• Cornea curvature is fixed, so focus comes from
  changes in the lens curvature through the ciliary
  muscles
• Lens with no tension would be curved/round
• normal state of lens flattened by the tension of
  the zonules/suspensatory ligaments.
• To curve lens ciliary muscles contract and
  ciliary body moves closer to the lens. Zonules go
  slack.
• To flatten the lens ciliary muscles relax,
  ciliary body moves away from the lens. Stretches
  the zonules.
• IMPORTANT POINTS 1. Natural state of the lens is
  round. 2. Ciliary muscle is a sphincter

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Lens

• 2-3000 lens fibers extremely long cells with no
  nuclei, stretch anterior to posterior. Cytoplasm
  filled with crystallin, arranged in a regular
  lattice
• Anterior surface layer of cuboidal cells with
  nuclei, merges with nucleated cells at the lens
  margin (proliferate throughout life to become
  lens fibers)
• Lens capsule thick basement membrane surrounding
  lens. Attachment site for the zonules.
• Posterior chamber small space between zonules
  and lens, filled with aqueous humor.
• Transparent because of anucleate nature and
  fibers containing crystalline proteins.

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Cornea

• Transparent part of the outer/scleral coat of the
  eye.
• Highly innervated (protects from harm),
  avascular, gets O2 nutrients from aqueous humor
  and outside surface. Kept moist by tears.
• 5 layers
• Corneal epithelium 3-7 or 4-6 cells thick,
  non-keratinized stratified squamous
• Bowmans Membrane thick, retractile CT layer
  that separates stromaepithelium
• Corneal stroma fibroblasts 200 thin,
  precisely org. lamellae of type I collagen
• Descemets membrane thick elastic basement
  membrane
• Corneal endothelium highly metabolically active
  (mitochondria pumps for fluid-h20proteoglycans
  in stroma would destroy org.), no stem cell
  population
• Limbus transition between cornea sclera. Stem
  cells for proliferation of corneal epithelium.
• Conjunctiva epithelium at limbus. stratified
  columnar epithelium with goblet cells. Extends
  from limbus to cover interior eyelids.
• Sclera Deeper layers of the cornea become opaque
  sclera. Covers eye with a tough coat, pierced at
  lamina cribrosa at posterior pole for optic nerve
  exit
• Transparency from regular and highly organized
  ECM, tight junctions in corneal epithelium, and
  fluid pumping by corneal endothelium (removes h20)

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Corneas optical properties

• Cornea transparent surface that bends/refracts
  the light and focuses it on the back of the eye
• Responsible for most of the eyes ability to
  refract and focus light
• Refraction of the cornea is fixed.

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Iris

• Colored structure surrounding the pupil.
• Controls amount of light entering the eye
• Controls the size of the pupil through the
  sphincter pupillae (shrinks pupil) and a diffuse
  dilator pupillae (enlarges pupil)
• Spongy stroma with melanocytes faces anterior
  chamber
• Pigmented epithelium faces posterior chamber.
  This epithelium becomes the ciliary body
  laterally.

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Ciliary Body

• 2 roles
• 1. Smooth muscle allows changing in lens shape
• 2. Epithelium produces aqueous humor
• Aqueous humor low-protein plasma-like substance
  made continuously by the epithelium of ciliary
  body. Nourishes cornea, lens, iris, corneal
  endothelium, stroma. Secreted into posterior
  chamber, flows around iris through pupil to the
  angle
• the angle angle between cornea and iris.drains
  through trabeculae in edge of cornea and through
  CT until it enters Canal of Schlemm. Then to
  episcleral venous system.
• Posteriorly, ciliary body is in continuity with
  the choroid.
• Choroid vascular pigmented tissue supplying
  nutrients to the outer retina. (inner retina
  nourished by central retinal artery).
• Ciliary body joins modified retina at the ora
  serrata.
• Bruchs membrane a thick common basement
  membrane between choroid RPE. photoreceptors
  RPE receive O2 and nutrients by diffusion from
  choroidal circulation through this membrane.

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Vitreal Cavity

• Filled with vitreous humor
• Hyaluronic acid
• Proteoglycans
• Type II collagen
• Water
• Produces pressure that gives eye its form

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Retinal Pigment Epithelium

• Separated from choroid by Bruchs membrane
• Simple cuboidal epithelium resting on thick,
  Bruchs membrane.
• Pigment in cells absorb light
• Sockets in which ends of photoreceptors nestle
• Transport nutrients to photoreceptors and
  transport waste from photoreceptors.
• 3 roles
• Isolates retinal neurons from blood
• Phagocytoses membrane that is shed by
  photoreceptors
• Reduces light scatter

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Retina

• Photoreceptors contain photopigment in discs
  located within outermost segment. When light
  interacts with the photopigment, conformational
  change and neural signal
• Blood supply central retinal artery enters
  through optic disk and ramifies inner surface of
  retina
• CRA sends branches to inner 2/3 of the retina,
  but not the photoreceptors
• Capillary network of the choroid, the
  choroicapillaris supplies photoreceptors through
  Bruchs membrane and the RPE
• 2 types of photoreceptors
• Rods sensitive in dim light, not wavelength
  sensitive
• Cones sensitive in bright light, differential
  sensitivity to wavelengths color!

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Retinal cells

• Inside out structure light must pass all cell
  layers to reach the photoreceptors
• Simplest circuit photoreceptor --gt bipolar cell
  --gt retinal ganglion cell
• Lateral connections horizontal cells (between
  photoreceptors and bipoolar cells in the outer
  plexiform layer) amacrine cells (between
  bipolar cells and ganglion cells in the inner
  plexiform layer)
• Muller cells neuroglia. have somata in the inner
  nuclear layer and cytoplasm through the whole
  retinal thickness.

GCL
Ganglion Cell
IPL
Amacrine Cells
INL
Bipolar Cell
Horizontal Cells
OPL
ONL
Photoreceptor
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Fovea

• Specialized for the highest acuity, high
  resolution vision
• Small area, temporal to the optic disk. Most
  cells other than cones are displaced laterally,
  allow tight packing of photoreceptors sensitive
  to wavelength in daylight. Blood vessels
  excluded.
• Center of gaze

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Optic Disk

• 1) RGC axons exit the eye
• 2) retinal blood vessels enter the eye
• RGCs travel across the retinal surface to become
  the optic nerve head/optic papilla/optic disk,
  where they become myelinated.
• Only axons.. No photoreceptors--blindspot in your
  visual field.
• Axons make a right angle, penetrate sclera
  through perforations called the lamina cribrosa.
  Central retinal artery travels to retina through
  the center of the nerve and the central retinal
  vein is alongside the artery.

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Eye Development

• The optic stalk grows out of the brain
  (diencephalon) and forms the optic cup
• Optic cup a primordial retina (an
  undifferentiated neuroepithelium) separated from
  primordial RPE by a fluid-filled space that is in
  continuity with the 3rd ventricle
• When optic cup comes into contact with ectoderm,
  it induces formation of the lens vesicle, which
  then invaginates and pinches off.
• Retina and RPE are derived from brain, optic
  nerve is part of the central nervous system.
• Retinal neurons neuroglia are generated from
  multipotent stem cells in the neuroepithelium
  along inner/outer walls of optic cup. They
  migrate through neuroepithelium to the vitreal
  margin. First cells generated are RGCs, last are
  photoreceptors. Photoreceptors are still
  generated in early postnatal life. Retina isnt
  in its final form until well after birth.

Moore Persaud, The Developing Human, 5th Ed
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What is glaucoma?

• Glaucoma elevated intraocular pressure from
  overproduction of aqueous humor or blockage in
  drainage. High pressure in the anterior chamber
  transduced through vitreous body, pressure on
  retina. Can damage neural retina by impeding
  blood flow in reitinal arterioles, pressure on
  RGC axons. Cupping of lamina cribrosa and optic
  disc.

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Clinical Correlations

• Glaucoma elevated intraocular pressure from
  overproduction of aqueous humor or blockage in
  drainage. High pressure in the anterior chamber
  transduced through vitreous body, pressure on
  retina. Can damage neural retina by impeding
  blood flow in reitinal arterioles, pressure on
  RGC axons. Cupping of lamina cribrosa and optic
  disc.
• Open angle glaucoma increased production by
  ciliary body epithelium
• Closed angle glaucoma iris closes the angle,
  blocking drainage
• Cataract opacity of the lens, comes from UV
  light changing the conformation of crystallin in
  the cytoplasm of lens fibers.
• Presbyopia lose ability to accommodate b/c of
  lens hardening, contractility of ciliary muscle
  cant change shape of lens. Increases with age.
• Retinal detachment separation of retina from
  pigment epithelium
• Papilledema swelling of optic nerve head from
  increased intracranial pressure (from tumor or
  hemorrhage)