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What layer of the eye is the Retina?
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Visual Pathway of light
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LECTURE 20: RETINA
SGIII
| Question | Answer |
|---|---|
| What layer of the eye is the Retina? | Innermost shell of the eye |
| Visual Pathway of light | light (phototransduction)->neural signal (modified throughout retina by interneurons)->subsequent structures in visual pathway |
| 3 cells of the visual pathway | 1. photoreceptors2. bipolar cells3. ganglion cells |
| others cells of the visual pathway | 1. interneurons2. neuroglial cells |
| what are interneurons? | modify signal as it goes through the retina |
| types of interneurons | amacrine cellshorizontal cellsinterplexiform cells |
| types of neuroglial cells? | Muller cells, Microglia, Astrocytes |
| What are Muller cells? | -supportive, maintains physiologic environemnt of retina |
| what are microglia? | -wandering phagocytic cells, found anywhere in the retina |
| what are astrocytes? | -star-shaped cells, located in the nerve fiver layer (ganglion cell layer), help support blood vessels and nerve fibers |
| what are neuroglial cells? | -supportive in nature,keep retina intact |
| what are the 10 layers of the retina? | 10. RPE 9. Photoreceptor Layer 8. External Limiting Membrane 7. Outer Nuclear Layer 6. Outer Plexiform Layer 5. Inner Nuclear Layer 4. Inner Plexiform Layer 3. Ganglion Cell Layer 2. Nerve Fiber Layer 1. Internal Limiting Membrane |
| What is the cell layer of RPE? | monolayer of cuboidal cells |
| What does the RPE do? | extends delicate, microvillous processes to fold/lock together with the photoreceptors for phagocytosis of shed photoreceptor discs. |
| What does the PR layer contain? | contains the outermost portions of th ephtooreceptors (specifically photopigment portion) |
| PR cells are specialized for? | highly specialized to concert light energy into neural messages (phototransduction) |
| Outermost vs inner part of the PR | OUTER: capture light INNER: transmit neural signal |
| Two types of receptors are? | Rods and Cones |
| Common parts to both rods and cones? | Outer Segment (OS), cilium( connecting stalk), inner segment (IS), outer fiber, cell body, inner fiber, and a synaptic terminal |
| Outer Segment (OS) is made up of? | stack of membranous discs enclosed by plasma membrane of the cell |
| Location of the photopigment | located in the membrane of the disc and the plasma membrane of the photoreceptor |
| intradisc space vs extradisc space | areas inside the disc vs area between the disc |
| orientation of the OS | APEX: faces the RPE BASE: inner segment |
| Microvilli in the OS | project from the RPE and surround the distal 1/3 of the OS. Then the RPE will phagocytize the discs that are shed |
| What does the PR layer contain? | contains the outermost portions of the photoreceptors (the photopigment part. |
| What is the PR cells specialized for? | highly specialized for the process of converting light energy into a neural message (phototransduction) |
| Adaptation of the outermost vs inner part of the PR | Outermost: captures lightInner: transmits the neural signal |
| 2 types of receptors | Rods and Cones |
| Common features of rods and cones | outer segment, cilium, inner segment, the outer fiber, cell body, inner fiver, and synaptic terminal |
| What is the OS made of? | stack of membranous discs enclosed by plasma membrane of the cell |
| Where is the photopigment located? | membrane of the disc and the plasma membrane of the photoreceptors |
| extradisc space vs intradisc space | extradisc space: area between the discintradisc space: inside the disc |
| Orientation of the OS | APEX: faces the RPE; BASE: toward the inner segment |
| How are the discs shed? | microvilli project from the RPE and develop the distal 1/3 of the OS. Then the RPE phagozytizes the shed disc |
| What is the cilium? | bridge between the inner and outer segment |
| Inner segment consists of 2 regions | Ellipsoid- closer to the OS; contains mitochondria making the ellipsoid the powerhouse of the cell; myoid- closer to cell body and contains organelles involved in syntehsizing protein (ER and Golgi apparatus) |
| what is the location of the Outer Fiber? | from the inner segment to the cell body |
| what does the cell body consists of? | contains the nucles of the photoreceptor |
| What is the inner fiber? | axon of the photoreceptor. from the cell body, the inner fiber goes inward and forms a specialized ending containig synaptic vesicles. |
| What photopigment does all rods contain? | rhodopsin |
| Location of the rods? | mostly outside the fovea (density peak in a ring ~4.5mm from the fovea); No rods IN THE FOVEA!!! |
| How many rods during scotopic vision? | scotopic vision= vision in dim light; about 92-120 million rods |
| Why are the rod OS called rods? | because of its long slender shape. |
| Describe the disc membrane | rod outer segment is made up of 600-1000 discs piled on top of each other. ecah disc membrane is seaparte from the plasma membrane of the cell (excep the base). |
| Where is the photopigment of the rod (rhodopsin) located? | disc membrane |
| What is the protein synthesized in the inner segment used for? | to build the disc in the OS; |
| Process to make disc | Disc gradually moves outward to be shed. Then the RPE phagocytizes the shed disc of the rodOS. |
| What time of day are rods discs shed? | early morning |
| spherule | |
| what is the cilium? | bridge between the inner and outer segments |
| What two regions make up the inner segment? | ellipsoid and myoid regions |
| What is the ellipsoid region? | closer to the OS; contains mitochondria, making the ellipsoid region the powerhouse of the cell |
| what is the myoid region? | closer to the cell body; contains organelles involved in synthesizing proteins (ie: ER and Golgi apparatus) |
| Where is the Outer Fiber located? | from the inner segment (IS) to the cell body |
| What does the Cell Body contain? | nucleus of the photoreceptor |
| What is the Inner Fiber? | axon of the photoreceptor |
| What does the Inner Fiber form? | goes inward from the cell body and forms specialized ending (containing synaptic vesicles) |
| How many rods are there during scotopic vision? | scotopic vision= dim light; ~92-120 million rods for vision |
| What photopigment does all rods contain? | rhodopsin |
| Where are rods located? | predominates OUTSIDE the fovea. NO RODS IN THE FOVEA! the peak density: ~4.5mm from the fovea |
| Why is the rod OS called a rod? | becuase of its long slender shape. rod OS is made up of 600-1000 discs piled on top of each other. Each disc membrane is seaparte from the plasma membrane of the cell(except at the base) |
| where is the photopigment of the rod (rhodopsin) located? | disc membrane |
| what is the protein synthesized in the inner segment used for? | to build the disc in the outer segment. The disc move outward and are shed. RPE phagocytizes the shed discs of the rod OS. |
| What time of day are most of the rod discs shed? | early morning |
| Describe the synaptic terminal of the RODS | rod inner fibers end at a specialized terminal (spherule); each rod spherule contains synapses with rod bipolar dendrites and horizontal processes |
| How many cones are there during photopic and color vision? | photopic vision= vision in bright illumination; 5 million cones |
| What are the 3 cones and their regions in the light spectrum? | 1 of 3 different opsins sensitive to different regions of the light spectrum. S cones(blue cones)peak absorption: 420nm; M cones (green cones) peak absorption: 531 nm; L cones (red cones) peak absorption: 588 nm |
| disc of the cones OS | wider at the base and taper towards the apex (gives cone appearance) |
| What time of day are the cones discs shed? | evening |
| Size of the cone nuclei | larger than rod nuclei. |
| where are one of the three photopigments stores? | disc membrane. THe plasma membrane of the photoreceptor is continuous with the disc membrane of the cones |
| What are the synaptic terminals of the cones? | inner fibers form a terminal structure= pedicle. the pedicle synapses with several interneurons |
| At high magnification, the thin dark line of the ELM is seen as dots, what do each ddots represent? | each dot represents zonula adherens. Zonula adherens holds photoreceptors to one another and to Muller cells. |
| between the photoreceptors and muller cells, the zonula adherens act as? | a barrier to the passage of large molecules into and out of the reinta. |
| what does the Outer Nuclear Layer contain? | nuclei of the photoreceptor cells (rods and cones). they lie closer to the outer edge near the ELM |
| The synaptic terminals (spherule(rods) and pedicle (cones)) of the photoreceptors will synapse with dendrites of what kind of cells? | bipolar and horizontal cells |
| why is the middle limiting membrane (a discontinuous membrane) is often seen as a series of dashed lines in a histological section of the retina? | the interweaving between the bipolar and horizontal processes in the outer plexiform layer is extensive with many desmosome- like attachments (synaptic densities) between the processes |
| What are synaptic densities? | numerous desmosome- like attachements between the processes |
| What is the middle limiting membrane? | a discontinious membrane. it is the demarcation between the inner retinal layers (supplied by the central retainl vasculature) and the outer retinal layers (supplied by choroidal vasculature |
| Cell bodies of what kind of cells are located in the Inner Nuclear Layer? | bipolar cells, horizontal cells, amacrine cells, interplexiform cells, and Muller cells |
| How do the bipolar cells and ganglion cells contact the photoreceptors? | bipolar cells- dendritic terminals and ganglion cells- axons |
| What makes the Inner Nuclear Layer so unique? | this layer marks the deepest penetration of blood vessels from the central retinal vasculature in the normal retina |
| What is the Inner Plexiform Layer made of? | nerve fibers( mainly the dendrities of the ganglion cells and the axons of the bipolar cells) |
| What are Retinal ganglion cells? | large cells that form a line separating the NFL from another fibrous layer. They are the output cells of the retina that communicate with central visual system structures |
| What are the ganglion cell layers? | usually one cell thick, but near the macula, they pile up and become thicker |
| What is the Nerve Fiber Layer made of? | unmyelinated nerve fibers (axons of the ganglion cells) |
| why does the NFL increase in thickness when moving towards the optic nerve head? | all the nerve fibers from the retinal ganglion cells combine to form the optic nerve at the optic disc |
| where are the central retinal vessels located? | located mainly in the NFL, but may also run deeper in GCL |
| WHere is the ILM located? | closest to the vitreous cavity of ALL the sensory retinal layers |
| what is the ILM is formed by? | footplates of Muller cells (covered by BM). ANTERIORLY: ILM is continuous with ILM covering ciliary body; POSTERIORLY: at the optic disc, footplates from Muller cells are replaced by processes from astrocytes that form the ILM of the disc. |
| What is the central retina designed for? | high visual acuity. central retina has certain structural modifications compared to peripheral retina |
| The central retina contains what 3 structures? | 1. macula 2. fovea 3. foveola |
| What are the dimensions of the macula lutea (macula)? | DIAMETER: 5.5 mm; CENTER: 3.5mm LATERAL to the edge of the optic disc 1mm BELOW the center of the disc |
| Why does the macula appear as a darker region in the central retina? | change in morphology of the RPE. RPE is taller with more pigments making it appear darker. |
| WHy is there a yellow hue observed in the macula? | increased amount of lutein and zexanthin in ganglion and bipolar cells |
| What does the fovea of the retina provide? | best visual acuity due (ojnly cones are found here) |
| What kind of photoreceptors are found in the fovea? | ONLY CONES |
| What is the diameter of the fovea? | 1.5mm |
| What is the clivus? | when the walls of the fovea slope downward toward the foveola. It is avascular. shallow pit where ONLY the outer retinal layers are present. |
| What causes the shallow depression in the fovea? | displacement of the inner retinal layers causes the shallow depression so that ONLY photoreceptors are found in the fovea |
| What is the Nerve Fiber Layer of Henle? | horizontally oriented (not vertical as everywhere else in the retina)outer plexiform layer that have the inner fivers of the photoreceptrs |
| Where is the avascular capillary- free zone located? | 0.4 to 0.5 mm zone located in the central fovea so taht the light directed toward the OS is not impeded |
| Where is the foveola located? | center of the fovea where cones are most concentrated. DIAMETER .35mm |
| WHat is present/absent in the foveola? | PRESENT: internal limiting membrane, Muller cell processesABSENT: inner retinal layers |
| when is umbo observed? | Umbo is a reflex. during a clinical exam, the umbo is observed in the center of the foveola |
| Location of rods/cones in the peripheral retina | rods dominate here but disappear in the extreme preiphery and replaced by malformed cones near the ora serrata |
| What occurs at the ora seratta? | peripheral retina terminate.neural retina continuous with non-pigmented ciliary epithelium. RPE continuous with pigmented ciliary epithelium |
| How did the ora seratta get its name? | scalloped oral bays and dentate |
| What is the ILM of the retina continuous with? | ILM of the ciliary body |
| What are the two layers of the retain suppled by? | INNER: central retinal vasculature; OUTER: choriocapillaris |
| Pathway of the CENTRAL RETINAL VASCULATURE (CRV) | 1. enter retina through optic disc (slighlty nasal to central retinal vein) 2. CRA branches 3a. superior retinal artery 3b. inferior retinal artery 4. both branch into nasal and temporal branches to feed their respective quadrants by many smaller branche |
| Where are most of the blood vessels located? | In the NFL beneath the ILM |
| What are 2 capillary beds in the retina? | 1. superficial capillary plexus in the NFL 2. deep capillary plexus in the INL; knowing the location of the capillaries will help determine the location of a hemorrhage in the retina |
| capillaries of the retina | non-fenestrated. part of the blood-retina-barrier becuase the endothelial cells are joined by zonular occludens |
| Where is the cilioretinal artery? | 15-20% of the population has this. 1. derived from the short posterior ciliary arteries 2. enters the retina via temporal crescent of the disc 3. projects to the macula |
| What happens if there is a blockage to the cilioretinal artery? | the macula will still be viable |
| Regulation of retina vs choroid | RETINA: AUTOREGULATED; CHOROID: NEUROREGULATED |
| What is the venous darinage of the retina? | follows the arterial supply then drains via central retinal vein |
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dyoungee
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