Retina preliminary

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Retina preliminary

  1. 1. <ul><li>RETINA - PRELIMINARY </li></ul><ul><li>RAJVIN SAMUEL PONRAJ </li></ul>
  2. 2. <ul><li>Gross anatomy : </li></ul><ul><li>Inner most ,cup shaped , thin , transparent tunic of eyeball appears purplish red - underlying choroid </li></ul><ul><li>Divided into posterior pole and peripheral retina </li></ul>
  3. 3. <ul><li>Embryology: Neuroectoderm of optic cup </li></ul><ul><li>Outer layer – pigment layer </li></ul><ul><li>Inner layer – neural layer </li></ul><ul><li>[mantle,matrix, </li></ul><ul><li>marginal ] </li></ul><ul><li>Location : sandwitched neuroepithelium </li></ul><ul><li>b/w mesenchyme of vitreous and </li></ul><ul><li>choroid </li></ul>
  4. 4. <ul><li>Extent : anteriorly to ora serrata </li></ul><ul><li>posteriorly to optic disc </li></ul><ul><li>Distance from limbus : Equator=13.7 mm </li></ul><ul><li>Ora serrata = 8 mm </li></ul><ul><li>Circumference : </li></ul><ul><li>Equatorial = 72 mm </li></ul><ul><li>Ora serrata = 60 mm </li></ul>
  5. 5. <ul><li>Optic nerve head: measures 1.8 mm vertical </li></ul><ul><li>1.5 mm horizontally </li></ul><ul><li>- pinkish pale colour </li></ul><ul><li>- terminaton of all layers except </li></ul><ul><li>nerve fibre layer </li></ul><ul><li>- cup and disc </li></ul><ul><li>Thickness of retina : optic disc =0.56 mm </li></ul><ul><li>equator = 0.1 mm </li></ul>
  6. 6. <ul><li>MACULA : Round yellow area at posterior </li></ul><ul><li>pole 5.5 mm size – 3 mm temporal </li></ul><ul><li>1 mm inferior to disc </li></ul>
  7. 7. <ul><li>Fovea - 1.5 mm wide , thin bottom- 22” clivity </li></ul><ul><li>thick basement margin </li></ul><ul><li>- prone for macular holes </li></ul><ul><li>-Henle’s layer-oblique cones </li></ul><ul><li>Foveola - 0.35 mm wide , thin pit , Densely cones </li></ul><ul><li>Bowing vitreally- fovea externa </li></ul><ul><li>Umbo - Tiny depression - Foveal light reflex </li></ul><ul><li>0.15 mm - bouquet of cones - narrowed </li></ul><ul><li>gateau nucleaire </li></ul>
  8. 9. RETINA FOVEA vs
  9. 10. <ul><li>Blood -retinal barrier Outer =Rpe,choroid vessel </li></ul><ul><li>bruch membrane </li></ul><ul><li>Inner= Retinal vascular </li></ul><ul><li>endothelium </li></ul><ul><li>Water shed Zones 1.At outer plexiform,equator </li></ul><ul><li>2.Btw optic disc & macula </li></ul><ul><li>Capillary free zones 1.Foveal avascular zone </li></ul><ul><li>2. Periarterial zone </li></ul>
  10. 11. <ul><li>Retinal vasculature – supply prelaminar optic nerve, </li></ul><ul><li>inner retinal quadrants </li></ul><ul><li>- No anastamosis/ end arteries </li></ul><ul><li>- Interdigitations among venules </li></ul><ul><li>arterioles[common sheet] </li></ul><ul><li>--adequate perfusion </li></ul><ul><li>- Laminar meshwork of capillaries </li></ul><ul><li>- Blood flow under Autoregulation-- </li></ul><ul><li>pericytes,O2,CO2,Ph,… </li></ul>
  11. 12. <ul><li>Choroidal Vasculature : supply directly optic disc </li></ul><ul><li>Circle of Zinn,major & minor </li></ul><ul><li>arterial iridis </li></ul><ul><li>peripapillary & sub macular </li></ul><ul><li>Segmental supply-choriocapillary </li></ul><ul><li>[functionally] </li></ul><ul><li>Honey comb pattern[interconnect </li></ul><ul><li>Blood flow regulation - </li></ul><ul><li>sympathetic innervation </li></ul>
  12. 13. <ul><li>Reinal pigment epithelium : </li></ul><ul><li>4 to 6 million </li></ul><ul><li>Single / cuboidal / hexagonal </li></ul><ul><li>Tight junctions – Zonulae occludens </li></ul><ul><li>Apical, mid,basal portions </li></ul><ul><li>melanosomes </li></ul><ul><li>Functions : </li></ul><ul><li>Vitamin A synthesis, phagocytosis </li></ul>
  13. 14. Number : Rods- 110 to 125 million Cones- 6.3 to 6.8 million Distribution :Rods 0 1,60,000 30,000 Cones 1.47,000 to 5000
  14. 16. <ul><li>Outer segment- Discs </li></ul><ul><li>Cilium </li></ul><ul><li>Inner segment- Inner myeloid </li></ul><ul><li>outer ellipsoid </li></ul><ul><li>[ golgi,ER, </li></ul><ul><li>mitochondria] </li></ul><ul><li>Nucleus </li></ul><ul><li>Synaptic process :Cone pedicle </li></ul><ul><li>Rod spherule </li></ul>
  15. 17. Intracellular disk Disk membrane Connecting cilium outer segment outer segment Disk membrane Intracellular space Extracellular space Visual pigment Visual pigment Extracellular space Plasma membrane Intracellular space Connecting cilium ROD CELL CONE CELL PHOTORECEPTORS IN THE EYE
  16. 18. VISUAL PIGMENTS : Rhodopsin : Opsin [glycoprotein] & Retinal Two isomeric states of retinal – All trans form --11 cis form Unbleached rhodopsin – reddish purple [due to visual purple] Peak absorption at 500 nm wave length -coresponds to deep green,.. Photopsin : absorption peak at 565 nm blue sensitive - 440 nm green sensitive – 540 nm red sensitive - 570 nm
  17. 20. In Dark :- Depolarisation - Na extruded from inner to outer [-40 mv] segment – open channel In light :- Hyperpolarization – Closure of Na channel [-70 mv]
  18. 21. <ul><li>Rhodopsin + transducin, </li></ul><ul><li>GDP[complex] </li></ul><ul><li>GDP exchanged with GTP </li></ul><ul><li>Transducin released </li></ul><ul><li>Activates Phosphodiesterase </li></ul>
  19. 22. Photo receptor cell renewal 1-2 Weeks - turnover , Daily 100 discs shed Balance maintained between-shedding and resynthesis of disc membranes. Discs engulfed by RPE into phagosome vesicles then digested by lysosomes and enter bruch membrane,..
  20. 23. <ul><li>Complex molecules of Gag’s surrounding pigment cells and photoreceptors. </li></ul><ul><li>Serve to attach Neural retina to RPE for physical support. </li></ul><ul><li>Other factors contributing for strength: </li></ul><ul><li>-vitreous gel </li></ul><ul><li>-Intraocular fluid pressure </li></ul><ul><li>-RPE water transport mechanism. </li></ul>
  21. 24. <ul><li>Visual threshold : The minimal light intensity </li></ul><ul><li>evoking a visual sensation </li></ul><ul><li>Intensity of illumination From 0 to 9 log units </li></ul><ul><li>Sensitivity adjustment by: </li></ul><ul><li>-pupil size </li></ul><ul><li>-level of neural activity </li></ul><ul><li>-steady state concentration </li></ul><ul><li>of photosensitive pigments </li></ul><ul><li>Average level of sensitivity allows capture of image </li></ul><ul><li>of max clarity & contrast detail </li></ul>
  22. 25. <ul><li>Rods – more light sensitive – Low illumination </li></ul><ul><li>Cones – less light sensitive – High illumination </li></ul><ul><li>Scotopic vision lack in spatial and </li></ul><ul><li>Temporal acuity and colour perception. </li></ul><ul><li>Purkinje shift : Change in spectral </li></ul><ul><li>sensitivity with intensity of </li></ul><ul><li>illumination.Wavelength of light that </li></ul><ul><li>Is maximally sensitive </li></ul><ul><li>Is longer for photopic vision than scotopic vision. </li></ul>
  23. 26. <ul><li>When one enters a dark room from bright light ,sensitivity of rods gradually increase and adaptation reached after 20-30 mins ,.. </li></ul>
  24. 27. <ul><li>- Ability to discriminate </li></ul><ul><li>btw shapes of objects </li></ul><ul><li>- Both retinal function and psychological </li></ul><ul><li>- Cones play a major role /high foveal activity </li></ul><ul><li>Visual acuity : measure of spatial resolution </li></ul><ul><li>/ability to discriminate btw </li></ul><ul><li>points. </li></ul>
  25. 28. <ul><li>To achieve resolution of 2 targets/edges ,one unstimulated cone btw 2 cones needed… </li></ul><ul><li>Two cones separated by 2 um space subtends 25 sec arc </li></ul><ul><li>Minimal resolvable –The minimum separation btw 2 </li></ul><ul><li>points to discriminate between. </li></ul><ul><li>The resolving power of the eye to produce a minimum </li></ul><ul><li>image size of an object 0.004 mm and the object </li></ul><ul><li>subtends 1 minute angle at the nodal point </li></ul><ul><li>- Minimal angle of resolution </li></ul>
  26. 29. <ul><ul><li>Resolution of eye due to Cones-- </li></ul></ul><ul><li>- finite size [1-2 um] </li></ul><ul><li>- tight arrangement </li></ul><ul><li>at fovea </li></ul><ul><li>The orientation as light pipes / fibre optic directed to second nodal point on lens. </li></ul><ul><li>The configuration of foveal pit with small concavity </li></ul><ul><li>- walls which prevents glare striking the centre </li></ul><ul><li>Yellow macular filters blue light [short wavelenth] </li></ul><ul><li>-prevents chromatic aberration </li></ul>
  27. 30. <ul><li>Ability of eye to perceive slight changes in luminance between regions not separated by definite borders. </li></ul><ul><li>Types : </li></ul><ul><li>Letter contrast sensitivity </li></ul><ul><li>contrast sensitivity gratings </li></ul><ul><li>Also distances between repeats of patterns taken into account </li></ul>
  28. 31. <ul><li>Letter contrast Contrast sensitivity </li></ul><ul><li>sensitivity grating </li></ul>
  29. 33. <ul><li>Spherules & pedicles in outer plexiform layer </li></ul><ul><li>A typical synaptic triad – 2 axon terminal </li></ul><ul><li>[horizontal cells] </li></ul><ul><li>1 dendritic terminal </li></ul><ul><li>[bipolar cells] </li></ul><ul><li>The dendrites of flat midget , flat top , and giant bistratified bipolar cells - basal junctions with cone pedicles. </li></ul><ul><li>Rod bipolar cell dendrites end as central elements at ribbon synaptic complex of rod spherules. </li></ul><ul><li>Basal processes from cone pedicles contact the cone pedicles, and rod spherules forming gap junctions. </li></ul>
  30. 34. <ul><li>Receptive field of visual neuron </li></ul><ul><li>Locus of all points within Visual field [Spatial/ temporal/colour </li></ul><ul><li>in which light stimuli effective in eliciting response from the cell… </li></ul><ul><li>CONNECTIONS </li></ul><ul><li>1. The photoreceptors themselves—the rods and cones —outer plexiform layer, where they synapse with bipolar cells [inner nuclear layer] </li></ul>
  31. 35. <ul><li>2. The horizontal cells, in the [outer plexiform layer] from the rods and cones to bipolar cells </li></ul><ul><li>3. The bipolar cells vertically from the rods, cones, and horizontal cells to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells </li></ul>
  32. 36. <ul><li>4. The amacrine cells, which transmit signals in two directions, either directly from bipolar cells to ganglion cells or horizontally within the [inner plexiform layer] from axons of the bipolar cells to dendrites of the ganglion cells or to other amacrine cells </li></ul><ul><li>  </li></ul><ul><li>5. The ganglion cells, which transmit output signals from the retina through the optic nerve into the brain </li></ul>
  33. 37. <ul><li>Provide opposing excitatory and inhibitory signals in the visual pathway: </li></ul><ul><li>- That is, some bipolar cells depolarize when the rods and cones are excited, and others hyperpolarize when they are inhibited.[Level of polarisation] </li></ul><ul><li>- Response by slow electric potentials by Centre – surround behaviour [contrast-sensitivity] </li></ul><ul><li>Centre - Hyperpolarisation </li></ul><ul><li>Surround - Depolarisation </li></ul>
  34. 38. They connect laterally between the synaptic bodies of the rods and cones, as well as connecting with the dendrites of the bipolar cells. The outputs of the horizontal cells are always inhibitory. This is called Lateral inhibition .
  35. 39. <ul><li>Response based on intensity of light & wavelength of different colour stimuli </li></ul>
  36. 40. <ul><li>Amacrine cell responds strongly at the onset of a continuing visual signal, but the response dies rapidly. </li></ul><ul><li>Respond strongly at the offset of visual signals. </li></ul><ul><li>Respond when a light is turned either on or off, signalling simply a change in illumination. </li></ul><ul><li>Responds to movement of a spot across the retina in a specific direction; therefore, these amacrine cells are said to be directional sensitive . </li></ul>
  37. 41. <ul><li>Determines rate of firing rather </li></ul><ul><li>than level of polarisation </li></ul><ul><li>All or none spikes of electrical activity. </li></ul><ul><li>X cells[parvo/midget]- </li></ul><ul><li>small dendritic fields for </li></ul><ul><li>fine details image[sustained response] </li></ul><ul><li>Y CELLS-[Magno]- </li></ul><ul><li>broader dendritic fields for </li></ul><ul><li>spatial summation[transient response] </li></ul><ul><li>The above 2 are relatively brisk response,… </li></ul><ul><li>W CELLS - large receptive fields </li></ul><ul><li>Respond sluggishly-slow axon conduction </li></ul>
  38. 42. <ul><li>Sensation of colour is subjective </li></ul><ul><li>Colour perceived depends on the following: </li></ul><ul><li>-wavelengths from object ,other objects </li></ul><ul><li>and wavelengths before looking at the </li></ul><ul><li>object. </li></ul><ul><li>White light split into its component wavelengths –Property of Dispersion by prism </li></ul>
  39. 43. <ul><li>Retina contains 3 classes of cones with different but overlapping spectral sensitivity. </li></ul>
  40. 44. <ul><li>[cone classes] [spectral sensitivity peaks] </li></ul><ul><li>Blue receptors – 440 to 450 nm </li></ul><ul><li>Green receptors - 535 to 555 nm </li></ul><ul><li>Red receptors - 570 to 590 nm </li></ul>
  41. 45. <ul><li>There are three opponent channels that exist. </li></ul><ul><li>1.White vs black </li></ul><ul><li>2.Red vs green </li></ul><ul><li>3. Yellow vs blue </li></ul><ul><li>Response to one colour in the opponent channel </li></ul><ul><li>Is antagonistic to the other, that is excitatory by one and inhibitory by the other. </li></ul><ul><li>These mechanisms to receive and process information at a complex level. </li></ul>
  42. 46. <ul><li>Double opponent cell is a cell which has a centre and a surround to its receptive field and both are colour coded. </li></ul><ul><li>They are concerned with simultaneous colour contrast </li></ul><ul><li>Opponent colour cells are found among ganglion cells of retina and lateral geniculate </li></ul><ul><li>nucleus. </li></ul><ul><li>Double opponent cells are found in the striate cortex area 17. </li></ul>
  43. 47. <ul><li>THANK YOU </li></ul>

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