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- VIII. Retina:
-
- A. Macular area of the fundus: (See diagram 4)
- B. Function of the retina:
- 1. Converts photons into nerve impulses.
- 2. Transmit signal to brain via optic nerve.
- 3. Sensory system: Vision occurs in the brain.
- C. Histology of the retina: 10 layers. (See diagram 5)
- 1. Cones: Daytime vision, photopic, color vision, 6 million cones in retina.
- 2. Rods: Low light vision, scotopic, 120 million rods in retina.
- 3. Function of rods and cones: Transduction of light into electrochemical energy, no action
- potential at this point.
-
- 10 Retinal layers:
-
- 1. Retinal pigmented epithelium (RPE):
- a. Description:
- 1. Single layer of hexagonal cells.
- 2. Tightly cemented to choroid at inner layer of Bruch's membrane.
- 3. Cells joined by ZO, outer half of blood/retinal barrier.
- 4. Contains granules:
- a. Melanin: Inner 1/3 to 1/2 of cell (away from choroid).
- b. Lipofuscin: in residual bodies, undigested material, increases with age, throughout the
- cell.
- 5. Microvilli on inner side: Project closely around rods and cones, space between microvilli is
- filed with MPS.
- b. Function:
- 1. Phagocytosis and destruction of rod and cone outer segments, undigested part
- becomes lipofuscin granules, process is part of rod/cone renewal system.
- 2. Vitamin A metabolism: Vitamin A in segments of rods and cones change chemically as
- photo response.
- 3. Filters and controls metabolites to outer retina: ZO junctions between RPE cells.
- 4. Suppress light reflection: Pigment absorbs stray light.
-
- 2. Rod/cone layer (photoreceptor layer):
- a. Outer segment of rods and cones:
- 1. Stack of true membrane discs, (2 layers), lamellae.
- a. Disc contain visual pigment, rods (rhodopsin), protein, cones (red, blue, or green sensing)
- pigments are a piece of protein plus a piece of Vitamin A molecule.
- b. The pigment changes photochemically with light.
- 2. Rod renewal system:
- a. Discs constantly produced at inner end of outer segment.
- b. Discs migrate from inner end to outer end.
- c. Discs phagocytosed by PE cells at outer end when light is low.
- d. Found process through sequential dissection of animals provided with radioactive tracer
- amino-acids.
- b. Cilium: Connects outer and inner segment, area of flow from inner to outer.
- c. Inner segment:
- 1. Ellipsoid area, outer, contains mitochondria which provide the energy for disc formation.
- 2. Myoid area: Inner, Golgi vesicles filled with MPS.
- 3. Around myoid area are villons, processes of Muller's cells (fiber baskets of Schultze,these
- are Muller's cells that are pointing inward).
-
- 3. External limiting membrane (ELM):
- a. Line of attachment between cell types.
- 1. Photoreceptor cells to Muller's cells.
- 2. Muller's cells to Muller's cells.
- 3. Rarely, photoreceptor cells to each other.
- b. Desmosome and ZA attachments between cells, very strong and tight.
- c. Function:
- 1. Support retina.
- 2. Keeps cells in alignment.
-
- 4. Outer nuclear layer:
- a. Contains cell bodies of rods and cones, cytoplasm and nucleus.
- b. Outer fiber: From ELM to nucleus, short.
- c. Inner fiber: From nucleus to end of cell in outer plexiform layer, at inner end is cone pedicule or
- rod spherule which are specialized synaptic junctions.
- d. Cone nuclei are fewer and outer to rod nuclei.
-
- 5. Outer plexiform layer: (Outer molecular)
- a. Contains cone pedicules and rod spherules.
- b. Layer of synaptic connections: not one to one, very complex.
- 1. Rods/cones to bipolar cells.
- 2. Rods/cones to horizontal cells.
- 3. Horizontal cells to horizontal cells.
- 4. Horizontal cells to bipolar cells.
-
- 6. Inner nuclear layer: (See diagram 1)
- a. Characteristics of Muller's cells:
- 1. Cell processes extend from ILM past ELM (Fiber baskets of Schultze).
- 2. Contain glycogen: Metabolic and physical support.
-
- 7. Inner plexiform layer: Layer of synaptic connections.
- a. Bipolar to amacrine.
- b. Bipolar to ganglion.
- c. Amacrine to amacrine.
- d. Amacrine to ganglion.
- e. Amacrine to bipolar.
- (See diagram 2)
- 8. Ganglion cell layer: Cell bodies.
- a. Nucleus in ganglion cell layer, thickest at macular area.
- b. Dendrites to inner plexiform layer.
- c. Single axon in nerve fiber layer, transmits information from retina to lateral geniculate nucleus
- (LGN) of the brain.
-
- 9. Nerve fiber layer (NFL): Stratum Opticum.
- a. Ganglion cell axons: Nerve fibers.
- 1. Parallel to retinal surface.
- 2. Collect at optic disc.
- 3. Pass through lamina cribrosa.
- 4. Forms optic nerve.
- b. Papillo-macular bundle: Acuity, from fovea to temporal side of optic disc.
- c. Fibers from temporal periphery enter optic disc superior and inferior.
- d. Fibers from nasal periphery enter nasal side of optic disc.
- e. Cell types:
- 1. Astrocyte: In ganglion cell layer and NFL.
- 2. Microglia: Found in ganglion cell layer and NFL, could also be central nervous system
- macrophages (WBC's).
- 3. Retinal blood vessels.
-
- 10. Internal limiting membrane (ILM): MPS
- a. Basement membrane of glial cells (astrocytes, microglia, Muller's).
- b. Collagen fibers from vitreous cortex, very loose attachment.
-
-
- Summary:
-
- Convergent: 125 million photoreceptors converge into 1 million ganglion cells.
- Divergent: Each photoreceptor spreads out to several ganglion cells.
- (See diagram 3)
-
- D. Retinal blood supply:
- 1. Outer retinal blood supply is choriocapillaris: Supplies RPE to outer plexiform layer by diffusion.
- 2. Inner retinal blood supply:
- a. Central retinal artery (CRA): A branch of the ophthalmic artery.
- 1. Pierces optic nerve 10-12 mm. behind the eyeball.
- 2. Goes to center of optic nerve, enters eye at center of optic disc.
- 3. Divides near surface of optic disc into superior and inferior branches.
- 4. Divides at edge of optic disc into nasal and temporal branches.
- 5. Divides dichotomously proceeding to ora serrata.
- 6. Ends in capillaries called end arteries at ora serrata, loops around and joins to form the
- central retinal vein. (the end arteries themselves do not join).
- 7. Central retinal vein exits at center of optic disc.
- b. Characteristics:
- 1. Vessels travel in nerve fiber layer.
- 2. Capillaries are sent out to and including inner nuclear layer.
- 3. Plexus: Usually at random.
- a. Superficial: Ganglion cell and nerve fiber layers.
- b. Deep: Inner nuclear layer.
- 4. Capillary structure:
- a. Endothelial cells and basement membrane.
- b. ZO between endothelial cells, inner half of blood retinal barrier (not fenestrated).
- c. Outside basement membrane is interupted, layer of pericytes.
- 5. No retinal capillaries in center of anatomical fovea (just choriocapillaris).
- c. Cilio-retinal artery: From choroid at edge of optic disc to fovea, doesn't supply center of
- fovea (15-20% of the population have this).
- d. Outer plexiform layer blood supply is by diffusion from choriocapillaris and inner nuclear
- layer. This is where the two blood supplies meet (retinal and choriocapillaris).
-
- E. Regional organization: Maplike.
- 1. Foveola:
- a. .35 mm. diameter/.13 mm. thick (peripheral retina is .37 mm. thick).
- b. Contains 2500 closely packed cones resembling rods in shape. There are no rods present.
- c. Muller's cells separate to cones.
- d. Layers present are from RPE to OPL (Henle's fiber layer)and ILM. (six layers compared to ten).
- e. Inner fibers (axons) from cone cells leave foveola horizontally to form Henle's fiber layer.
- 2. Fovea: 100,000 cones.
- a. 1.5 mm. diameter.
- b. Depression in posterior pole.
- c. Mostly cones, rod free area is .57 mm. in diameter.
- d. Blood vessel free area is .50 mm. in diameter.
- e. Under the fovea, the choriocapillaris is thicker and the RPE cells are taller.
- f. Layers are picked up on the clivus.
- 3. Parafoveal region:
- a. .5 mm. band around the fovea.
- b. Mostly cones.
- c. Largest accumulation of nerve cells in the retina. Thick ganglion cell layer and inner nuclear
- cells pushed out of foveal area.
- d. Henle's fiber layer present and makes synapses.
- 4. Perifoveal region:
- a. 1.5 mm. band around parafoveal region.
- b. Ganglion cell layer is reduced from very thick to one cell thick.
- 5. Area centralis: The total area of parts 1 to 4 above (macula).
- a. 5.5 mm. in diameter.
- b. Center is foveola: .35 mm. temporal and 1 mm. inferior to optic disc.
- 6. Peripheral retina: Outside of macula.
- a. Rods are predominate.
-
- Summary:
-
- Cones: six million: 100,000 in fovea: acuity:photopic:color.
- Rods: 120 million: few in fovea: scotopic: black and white.
- (See diagram 4)
-
- F. Optic nerve:
- 1. Optic disc:
- a. Description:
- 1. No choroid, no retina, no scleral layers.
- 2. Composed of neuroglial cells and ganglion axons.
- 3. Color is white or yellow.
- 4. Surface is viewed by ophthalmoscope.
- 5. 1.5 mm. in diameter.
- b. Physiological cup:
- 1. Small round depression at center of optic disc.
- 2. Cup to disc ratio: Normal is .3, range from 0 to .9, larger cups (.7-.9) indicate pathology.
- 3. Cup depth.
- 4. Cup symmetry.
- 5. Scleral crescent: Temporal side of disc, white due to oblique scleral canal or edge of sclera
- not covered by retina or choroid.
- 6. Choroidal crescent: Accumulation of RPE or choroidal cells, dark ring, temporal side of disc.
- 7. Lamina cribrosa: Visible in deep cup.
- 2. Histology of optic disc and optic nerve:
- a. Internal limiting membrane of Elshnig:
- 1. Internal covering of disc.
- 2. Astrocytes and basement membrane.
- 3. Continuous with retinal internal limiting membrane, transition is at vitreal attachment(area
- of Martegiani).
- 4. If thickened at center of cup it is called the central meniscus of Kuhnt, astocytes are not
- always present.
- b. Intermediary tissue of Kuhnt:
- 1. Astocyte layer separating end of retina from optic nerve fibers.
- c. Border tissue of Jacoby:
- 1. Astrocyte layer separating marginal tissue of Elshnig from optic nerve fibers.
- 2. Continuous with intermediary tissue of Kuhnt.
- d. Marginal tissue of Elshnig:
- 1. Connective tissue separating sclera and choroid from border tissue of Jacoby.
- e. Lamina cribrosa:
- 1. Bundles of collagen connecting sclera across scleral canal.
- f. Optic nerve fibers:
- 1. Enter in pattern from retina.
- 2. Anterior to lamina cribrosa fibers join in 1000 fascicles of 1000 fibers each. Astrocytes
- surround fascicles.
- 3. At lamina cribrosa connective tissue separation is added.
- a. Maintain this conformation to optic chiasm.
- b. Between bundles are astrocytes and connective tissue, the astrocyte layer is called the
- septum.
- 4. At lamina cribrosa and posterior: Fibers are myelinated.
- a. Oligodendrocites: CNS myelinating cell.
- b. Each oligodendrocite myelinates several cells.
- c. If myelin continues anterior of lamina cribrosa a larger blind spot s formed (caused by
- developmental problems).
- 3. Covering (sheaths) of the optic nerve:
- a. Dura: Outer, tough and fibrous, collagen.
- b. Arachnoid: Middle, thin loose collagen and fibroblasts.
- c. Pia: Inner, thin, vascularized collagen and elastin, continuous with inner layers of sclera
- that surrounds fascicles.
- d. Subarachnoid space: Between arachnoid and pia, continuous with subarachnoid space of
- the brain, loose meshwork of collagen fibers filled with cerebrospinal fluid. Pathology:
- When intracranial pressure increase, pressure on optic nerve causes papilledema, edema
- at optic disc, blurred disc margins, swollen, shallow cup. Vision is usually normal. Blood
- flow to optic nerve and flow of nutrition from ganglion cell nucleus to dendrites
- restricted at bend into optic disc, resulting in the blurred margins and swollen, shallow
- cup, respectively.
- e. Between pia and nerve fibers is a layer of astrocytes.
- 4. Blood supply to optic nerve:
- a. Branches from pia pierce and supply most of optic nerve.
- b. Near optic disc, branches of the central retinal artery.
- c. Circle of Zinn/Haller: Outside of optic disc in sclera.
- 1. Branches into optic disc.
- 2. Circular blood vessel: Anastomoses of short posterior ciliary arteries. Circle of Zinn
- is a secondary function, primary is choroidal supply.
- d. Branches from choroid.
- e. Cilio-retinal artery: Choroidal supply to retina, usually circle of Zinn to retina,
- anastomoses of choroidal and retinal blood supply, not always present.
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