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The Retina © Wesner, M. F.. We know there is retinal heterogeneity.

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Presentation on theme: "The Retina © Wesner, M. F.. We know there is retinal heterogeneity."— Presentation transcript:

1 The Retina © Wesner, M. F.

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3 We know there is retinal heterogeneity.

4 visual axis fovea centralis foveal pit parafoveal area Outer plexiform layer Inner plexiform layer

5 The more eccentric from fovea, the greater the “rod intrusion”.. cones rods

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7 The retina: The outer nuclear layer structure Components of rod and cone photoreceptors. The outer segment is a stack of disks containing light-sensitive pigment molecules. The inner segment includes the cell nucleus, and synaptic terminals housed in pockets called clefts.

8 Stacked disks Membrane folds (invaginations)

9 hv hv What determines successful pigment absorption is (i.e. the photon  state).  = h  = h

10 3 different cone types

11 Four receptor Types Rods: Rhodopsin (visual purple)Rhodopsin (visual purple) Cones: Long-wavelength Sensitive (LWS, L-cones)Long-wavelength Sensitive (LWS, L-cones) Erythrolabe {“red catcher”} Middle-wavelength Sensitive (MWS, M-cones)Middle-wavelength Sensitive (MWS, M-cones) Chlorolabe {“green catcher”} Short-wavelength Sensitive (SWS, S-cones)Short-wavelength Sensitive (SWS, S-cones) Cyanolabe {“blue catcher”}

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13 The retina: Photoreceptor responses The probability that a photon will be absorbed depends on:   Direction—photons traveling through the center of the lens are more likely to be absorbed.   Frequency—photons with a frequency near the peak of a receptor’s spectral sensitivity are more likely to be absorbed.   Once a photon has been absorbed, the photoreceptor has no way of distinguishing its frequency.

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16 This is why the terms R, G & B are misnomers.

17 Four receptor Types Rods: Rhodopsin (visual purple)Rhodopsin (visual purple) Cones: Long-wavelength Sensitive (LWS, L-cones)Long-wavelength Sensitive (LWS, L-cones) Erythrolabe {“red catcher”} Middle-wavelength Sensitive (MWS, M-cones)Middle-wavelength Sensitive (MWS, M-cones) Chlorolabe {“green catcher”} Short-wavelength Sensitive (SWS, S-cones)Short-wavelength Sensitive (SWS, S-cones) Cyanolabe {“blue catcher”}

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19 Transduction - the conversion of photon energy into electrochemical (neural) energy.

20 What happens when the pigments get photolyzed?

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22 Chromophore (contains retinal) hv Photon aborption resulting in the conversion from 11-cis to all-trans retinal is known as photoisomerization. Note: Isomers are molecules that have the same number of atoms but different physical structures, thus different properties

23 DARK LIGHT Na + “dark current” Na + Light activated receptors are hyperpolarized! Turn OFF neuro- transmitter. Cyclic guanosine monophosphate (cGMP) is a 2nd messenger which is ACTIVE in dark (cis-retinal + opsin) trans- Phosphodiesterase cGMP GMP

24 dark current....thus NT is released in the dark.. + -

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26 ophthalmoscopic examination - Ophthalmologist views the retina through an ophthalmoscope. He or she views the fundus.

27 OD (oculus dexter)- right eye fundus

28 OS (oculus sinister)- left eye fundus Two common symptoms of disease found in the fundus: macular degeneration: heavy pigmentation around the fovea and parafovea. Results in degeneration of cones which affect central vision (scotomas). macular degeneration: heavy pigmentation around the fovea and parafovea. Results in degeneration of cones which affect central vision (scotomas). glaucoma: “cupping” or excavation of the optic disk (nerve head) due to increases in intraocular pressure (IOP).glaucoma: “cupping” or excavation of the optic disk (nerve head) due to increases in intraocular pressure (IOP).

29 OD (oculus dexter)- right eye fundus blood vessels maculamacula foveafovea optic disk (blind spot) temporal hemiretinanasal hemiretina centralis parafovea

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31 Outer plexiform layer Inner plexiform layer

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35 Lateral interacting cells (lateral antagonism)

36 Inner nuclear layer Outer plexiform layer Outer nuclear layer Inner plexiform layer Ganglion layer

37 * Roman numerals indicate which of von Graef’s IX layers are shown.

38 This is why the terms R, G & B are misnomers.

39 Trichromacy based on physiological response..

40 Based on psychophysically-derived equations from color matching of known congenital dichromats & heterochromatic flicker photometry (HFP). These curves were later corroborated by physiological monkey recordings. Trichromacy is revealed based on behavioral response..

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42 Spectral sensitivity curves are very similar to what can be derived physically.. & physiologically..

43 How do these photoreceptor events relate to bipolar activity? Properties of the first synaptic layer in the retina - the outer plexiform layer. Remember: Light “turns off” photo-receptor neurotransmitter release.

44 Two types of bipolars: 1. Flat bipolars (sign conserving) 2. Invaginating bipolars (sign inverting) Note: In the outer plexiform layer, you have synaptic triads. Photoreceptors synapsing with two horizontal cells and either a flat or invaginating bipolar cell.

45 Outer plexiform layer Inner plexiform layer

46 Lateral interacting cells (lateral antagonism)

47 LUMINANCE CONTRAST -INTENSITY DIFFERENCES Without these contrasts, the brain shuts down. CHROMATIC CONTRAST -WAVELENGTH DIFFERENCES TEMPORAL CONTRAST -TIME (EVENT) DIFFERENCES

48 flat bipolars invaginating bipolars PR HH BP sign conserving Excitatory NT (+) PR H H BP sign inverting Hyperpolarizations (-)

49 flat bipolars sign conserving hv PR HH BP NOTE: The flat bipolar hyperpolarizes because of the turning OFF of excitatory NT. ALWAYS EXCITATORY

50 invaginating bipolars sign inverting PR H H BP NOTE: The invaginating bipolar depolarizes be- cause special membrane properties hyperpolarize with presence of glutamate. hv ALWAYS EXCITATORY

51 flat bipolars sign conserving hv PR HH BP NOT (EXCITATORY) - Produces “OFF” center ganglion cell G

52 invaginating bipolars sign inverting PR H H BP hv G (EXCITATORY) - Produces “ON” center ganglion cell

53 flat bipolars invaginating bipolars PR HH BP sign conserving Depolarizations (+) PR H H BP sign inverting Hyperpolarizations (-)

54 Lateral inhibition (lateral antagonism)

55 How does lateral antagonism relate to human retina?

56 NOT (+) means not Horizontal NOT (-): Thus, depolarization (+) - +

57 (+) means yes, Horizontal response (-): Thus, hyperpolarization (-) -+

58 Spatial antagonism in the retina (i.e., the creation of ganglion cells that are either “On” center; “OFF” surround or “OFF” center; “ON” surround) allows the retina to begin processing for LUMINANCE CONTRAST.

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60 Synaptic Dyad - inner plexiform layer BP A G Steady response from horizontal and amacrine cell integration-produces a steady-state, tonic ganglion cell response (X-cells).

61 Synaptic Dyad - inner plexiform layer BP A G * *possible mechanism for transient response (self-inhibiting? delay response?) Transient response from amacrine input and feedback-produces a phasic ganglion response (Y-cells).

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63 NOTE: More neuronal convergence with eccentricity

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67 Neuronal convergence (spatial pooling) lowers spatial acuity. However, convergence also increases overall light sensitivity.

68 ..to optic nerve eccentricity Eccentric monosynaptic 1:1

69 ..to optic nerve eccentricity More eccentric polynomosynaptic coupling....means larger receptive fields.

70 ..to optic nerve eccentricity Eccentric monosynaptic 1:1

71 ..to optic nerve


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