Phototransduction & Image perception Dr. Nisreen Abo-elmaaty Physiology Department
The Visual Pathway
The Visual Pathway
Perception of a Visible Object <ul><li>I- Phototransduction: </li></ul><ul><li>-  How light stimulation of photoreceptors ...
1 st  , u have to know what is the retina & its major elements; <ul><li>It is the innermost photosensitive layer. </li></u...
Special Parts of the Retina <ul><li>Optic Disc (Blind Spot) </li></ul><ul><li>3 mm medial to post. Pole of eye ball. </li>...
Fovea Centralis (yellow spot) <ul><li>An oval depression lateral to optic disc, ~ 0.4 mm diameter </li></ul><ul><li>Presen...
10 Histological Layers of the Retina
4 retinal layers of physiological significance
Why Fovea is the most sensitive spot in retina? <ul><li>All layers are shifted aside leaving outer segments of photosensor...
Why Fovea is the most sensitive spot in retina?
What is the Role of the 4 retinal layers of physiological significance?
1)- Retinal Pigmented Epithelium <ul><li>Absorption of light (due to presence of black pigment melanin)  ->  reduction of ...
2- Photoreceptors; Rods & Cones The 1 st  order neuron in visual pathway
Convergence vs. No Convergence
<ul><li>↓   light sensitivity </li></ul><ul><li>↑   visual acuity </li></ul><ul><li>+ colour vision  </li></ul><ul><li>Day...
<ul><li>In darkness, RMP of photoreceptors is ~ - 40mV (Dark Na current) </li></ul><ul><li>Light  ->  certain reactions  -...
3)- Bipolar Cells <ul><li>The 2 nd  order neuron in visual pathway </li></ul><ul><li>All bipolar cells secrete excitatory ...
4)- Ganglion cells <ul><li>The 3 nd  order neuron in visual pathway. </li></ul><ul><li>~ 1.6 million in number. </li></ul>...
<ul><li>There are 3 Types of Ganglion cells: </li></ul><ul><li>1-  Magnocellular (M)   </li></ul><ul><li>Less common (10%)...
♥   The Receptive Field (RF) of Ganglion Cells <ul><li>Is the part of the retina it sees through its photoreceptor input; ...
♥   The Receptive Field (RF) of Ganglion Cells
<ul><li>2 types of ganglion cells </li></ul><ul><li>: According to the effect exerted by the receptive field on them: </li...
<ul><li>Thus, wide illumination of the retina has weaker effect than pinpoint illumination </li></ul><ul><li>Because wide ...
The Receptive Field (RF) of Ganglion Cells Response of ON-centre Ganglion cell to Light
What is the role of Lateral Cells? <ul><li>Horizontal Cells </li></ul><ul><li>Lateral connection between rods & cones, bet...
Phototransduction    = Ionic Basis of photoreceptor Potential = Genesis of electric responses in retina
Phototransduction   <ul><li>Light falling on photoreceptors </li></ul><ul><li>Causes a change in photoreceptor potential (...
Effect of Light on Retinal Neurons
<ul><li>1- Decomposition of photosensitive pigment (Visual purple) in rods & cones by light </li></ul><ul><li>Rhodopsin (V...
<ul><li>2- Generation of receptor potential by activated rhodopsin </li></ul><ul><li>Activated rhodopsin  ->  activates Tr...
Ionic Basis of Light-evoked Hyperpolarization in Photoreceptors
<ul><li>Termination of excitation </li></ul><ul><li>Occurs when the activated rhodopsin is inactivated by the enzyme rhodo...
Retinal on pathway
The Visual Pathway
Neural Processing of Visual Information <ul><li>On, Off retinal pathways </li></ul><ul><li>Lateral inhibition serving to s...
Lateral Geniculate Body; LGB <ul><li>The Subcortical thalamic relay nucleus of vision, starting the process of co-ordinati...
LGB <ul><li>Each layer receives input from one eye only </li></ul><ul><li>- Layers 1,4,6 from contralateral eye </li></ul>...
Primary Visual area (Area 17, V1) <ul><li>On medial aspect of each occipital lobe </li></ul><ul><li>Its neurons arranged i...
Visual Projection to Area 17
Role of Area 17 (V1) <ul><li>Perception of visible objects  without knowing the meaning of these objects: </li></ul><ul><l...
<ul><li>For fusion of the two images (perception of two images as one),  the 2 images must fall on corresponding points on...
Secondary Visual Processing: Association Areas (18 &19; V2,3 & V4,5)  <ul><li>In parietal & temporal lobes </li></ul><ul><...
Retinotopic Organization & Processing of visual information
COLOUR VISION
<ul><li>Colour vision is the ability to discriminate different wavelengths that constitute the visible spectrum </li></ul>
COLOUR VISION <ul><li>Our perception of light is related to wavelengths of light that are </li></ul><ul><li>transmitted, r...
<ul><li>Colour vision is cone-mediated function </li></ul><ul><li>explained by  ( Trichromatic Theory ): </li></ul><ul><li...
Colour-sensitive Cones
<ul><li>Although each type of cones is stimulated maximally by a certain wavelength but still can be stimulated by other b...
Colour Blindness <ul><li>Sex-linked recessive disorder </li></ul><ul><li>Defect in colour gene  ->  deficiency of colour p...
THANK YOU
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Phototransduction & Visual Pathway Mmp March 10

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Phototransduction & Visual Pathway Mmp March 10

  1. 1. Phototransduction & Image perception Dr. Nisreen Abo-elmaaty Physiology Department
  2. 2. The Visual Pathway
  3. 3. The Visual Pathway
  4. 4. Perception of a Visible Object <ul><li>I- Phototransduction: </li></ul><ul><li>- How light stimulation of photoreceptors is converted into nerve impulses? </li></ul><ul><li>II- Visual Pathway from Retina </li></ul><ul><li>- How nerve impulses are carried from retina to higher centres? </li></ul><ul><li>III- Processing of Visual Information for perception of an image </li></ul>
  5. 5. 1 st , u have to know what is the retina & its major elements; <ul><li>It is the innermost photosensitive layer. </li></ul><ul><li>Blood supply of retina; </li></ul><ul><li>- receptors (rods & cones) supplied by the choroid blood vessels (retinal detachment is so damaging to receptor layer), </li></ul><ul><li>- inner layers of retina supplied by central retinal artery. </li></ul>
  6. 6. Special Parts of the Retina <ul><li>Optic Disc (Blind Spot) </li></ul><ul><li>3 mm medial to post. Pole of eye ball. </li></ul><ul><li>exit of optic nerve, (+ retinal bl. vessels) </li></ul><ul><li>no photoreceptors -> blind spot. </li></ul>
  7. 7. Fovea Centralis (yellow spot) <ul><li>An oval depression lateral to optic disc, ~ 0.4 mm diameter </li></ul><ul><li>Present in the centre of a small yellow pit (Macula Lutea; 4.5 mm diameter). </li></ul><ul><li>Fovea contains cones only -> highest visual acuity </li></ul>
  8. 8. 10 Histological Layers of the Retina
  9. 9. 4 retinal layers of physiological significance
  10. 10. Why Fovea is the most sensitive spot in retina? <ul><li>All layers are shifted aside leaving outer segments of photosensors to be hit directly by light </li></ul><ul><li>High density of small diameter Cones with long outer segments </li></ul><ul><li>1:1 convergence (cone-BC-GC) </li></ul><ul><li>Small RF of foveal ganglion cells </li></ul><ul><li>Wide presentation in occipital primary visual area </li></ul>
  11. 11. Why Fovea is the most sensitive spot in retina?
  12. 12. What is the Role of the 4 retinal layers of physiological significance?
  13. 13. 1)- Retinal Pigmented Epithelium <ul><li>Absorption of light (due to presence of black pigment melanin) -> reduction of glare (Albinos!!) </li></ul><ul><li>Production of extracellular matrix -> keeping outer segments of photoreceptors straight </li></ul><ul><li>Storage of vitamin A (precursor of 11-cis retinal) for regeneration of photosensitive pigments </li></ul><ul><li>Phagocytosis of the tips of outer segments after their shedding off by the photoreceptors -> continual renewal of outer segments </li></ul>
  14. 14. 2- Photoreceptors; Rods & Cones The 1 st order neuron in visual pathway
  15. 15. Convergence vs. No Convergence
  16. 16. <ul><li>↓ light sensitivity </li></ul><ul><li>↑ visual acuity </li></ul><ul><li>+ colour vision </li></ul><ul><li>Day vision </li></ul><ul><li>↑ light sensitivity </li></ul><ul><li>↓ visual acuity </li></ul><ul><li>- colour vision </li></ul><ul><li>Night vision </li></ul>Function No convergence (1:1; direct private line) Convergence (300:1 connection) Connection 3 types (iodopsin) Rhodopsin Photosensitive pigment ~ 6 millions in each retina More in centre Present ~ 120 millions in each retina More in periphery Non in Fovea Number Distribution Cones Rods
  17. 17. <ul><li>In darkness, RMP of photoreceptors is ~ - 40mV (Dark Na current) </li></ul><ul><li>Light -> certain reactions -> blockage of Dark current -> hyperpolarization (MP of -60:-70 mV) </li></ul>
  18. 18. 3)- Bipolar Cells <ul><li>The 2 nd order neuron in visual pathway </li></ul><ul><li>All bipolar cells secrete excitatory transmitter (glutamate) </li></ul><ul><li>2 types: </li></ul><ul><li>On-bipolar cells ; Light -> ↓ release of inhibitory transmitter from the ends of photoreceptors-> disinhibition of BC [depolarized] -> ↑release of excitatory transmitter -> facilitation of GC (↑ ↑fR). </li></ul><ul><li>Off-bipolar cells ; darkness ->↑ release of glutamate from ends of photoreceptors -> inhibition of BC [hyperpolarized] -> ↓ release of excitatory transmitter -> disfacilitation of GC ( ↓ ↓ fR) </li></ul>
  19. 19. 4)- Ganglion cells <ul><li>The 3 nd order neuron in visual pathway. </li></ul><ul><li>~ 1.6 million in number. </li></ul><ul><li>The only retinal neuron that respond to stimulation by a full regenerative action potential; depolarization </li></ul><ul><li>(all others transmit signals by electrotonic currents) </li></ul><ul><li>Ganglion cells are always active; when unstimulated they keep sending signals at a rate of 5-40/sec, the visual signals are superimposed on this background. </li></ul>
  20. 20. <ul><li>There are 3 Types of Ganglion cells: </li></ul><ul><li>1- Magnocellular (M) </li></ul><ul><li>Less common (10%) </li></ul><ul><li>of large receptive field corresponding to 100s of BC </li></ul><ul><li>gross analysis of visual image & location of objects in visual field </li></ul><ul><li>2- Parvocellular (P) </li></ul><ul><li>Numerous (80%) </li></ul><ul><li>of small receptive field corresponding to 1 or few BC </li></ul><ul><li>Responsible for fine detailed vision (shape & texture) </li></ul><ul><li>3- Coniocellular </li></ul><ul><li>Few (10%) </li></ul><ul><li>Medium in size </li></ul><ul><li>Controlling pupillary reflexes. </li></ul>4- Ganglion cells
  21. 21. ♥ The Receptive Field (RF) of Ganglion Cells <ul><li>Is the part of the retina it sees through its photoreceptor input; </li></ul><ul><li>The retinal region from which stimulatory or inhibitory effect originates -> modulating the rate of firing of ganglion cell </li></ul><ul><li>In fovea, the RF of ganglion cell is ~ 2 μ m; corresponding to width of 1 cone (one line connection) </li></ul><ul><li>In peripheral retina, the RF of ganglion cell is ~ 1mm; (connected to hundreds of photoreceptors). </li></ul>
  22. 22. ♥ The Receptive Field (RF) of Ganglion Cells
  23. 23. <ul><li>2 types of ganglion cells </li></ul><ul><li>: According to the effect exerted by the receptive field on them: </li></ul><ul><li>On-centre GC </li></ul><ul><li>Off-centre GC </li></ul>
  24. 24. <ul><li>Thus, wide illumination of the retina has weaker effect than pinpoint illumination </li></ul><ul><li>Because wide illumination does not give the ganglion cells clear orders </li></ul><ul><li>Whereas pinpoint illumination by its antagonistic nature sharpens the orders to ganglion cells </li></ul>
  25. 25. The Receptive Field (RF) of Ganglion Cells Response of ON-centre Ganglion cell to Light
  26. 26. What is the role of Lateral Cells? <ul><li>Horizontal Cells </li></ul><ul><li>Lateral connection between rods & cones, between bipolar cells </li></ul><ul><li>Responsible for lateral inhibition giving a stop to lateral spread of excitation -> ↑visual accuracy </li></ul><ul><li>Amacrine Cells </li></ul><ul><li>Lateral connection between BC & GC </li></ul><ul><li>~ 30 types </li></ul><ul><li>Helping to analyse visual signals before leaving retina </li></ul>
  27. 27. Phototransduction = Ionic Basis of photoreceptor Potential = Genesis of electric responses in retina
  28. 28. Phototransduction <ul><li>Light falling on photoreceptors </li></ul><ul><li>Causes a change in photoreceptor potential (what is the ionic basis for this change) </li></ul><ul><li>This releases BC from inhibition (stimulated) </li></ul><ul><li>Excite GC whose firing rate is increased & transmitted along optic nerve. </li></ul>
  29. 29. Effect of Light on Retinal Neurons
  30. 30. <ul><li>1- Decomposition of photosensitive pigment (Visual purple) in rods & cones by light </li></ul><ul><li>Rhodopsin (Visual purple) = protein (scotopsin + pigment (11-cis retinal). </li></ul><ul><li>Rhodopsin Light conversion of angulated 11-cis retinal to straight all-trans retinal </li></ul><ul><li>This change -> splitting between scotopsin & all-trans retinal, thus </li></ul><ul><li>Rhodopsin -> -> metarhodopsin II (Activated Rhodopsin). </li></ul>Ionic Basis for Phototransduction
  31. 31. <ul><li>2- Generation of receptor potential by activated rhodopsin </li></ul><ul><li>Activated rhodopsin -> activates Transducin, G t (in disc membrane) -> cascade activation of the enzyme phosphodiestrase -> cascade hydrolysis of cGMP(splinter of Na + Ch) -> cascade closure of Na + Ch -> ↓ Na entry to outer segment -> ↑ the intrarod negativity (Hyperpolarization) -> ↓ transmitter (glutamate) release in synaptic terminal -> disinhibition of bipolar cells -> excitation of B cells -> excitation of GC that respond by generation of depolarizing action potential -> optic nerve -> visual pathway. </li></ul>Ionic Basis for Phototransduction
  32. 32. Ionic Basis of Light-evoked Hyperpolarization in Photoreceptors
  33. 33. <ul><li>Termination of excitation </li></ul><ul><li>Occurs when the activated rhodopsin is inactivated by the enzyme rhodopsin kinase -> reversal of all reactions -> reopening of Na + Ch. -> termination of excitation. </li></ul><ul><li>Regeneration of photopigment </li></ul><ul><li>All-trans retinal is taken up by RPE which contain isomeraze enzyme that converts it again into 11-cis retinal sent back to rods to recombine with opsin </li></ul>
  34. 34. Retinal on pathway
  35. 35. The Visual Pathway
  36. 36. Neural Processing of Visual Information <ul><li>On, Off retinal pathways </li></ul><ul><li>Lateral inhibition serving to sharpen visual image & increases contrast </li></ul><ul><li>Thalamic LGB; </li></ul><ul><li>Cortical Visual areas; </li></ul><ul><li>- Primary (area 17; V1) </li></ul><ul><li>- Secondary (areas 18, 19; association area, V2,3) </li></ul>
  37. 37. Lateral Geniculate Body; LGB <ul><li>The Subcortical thalamic relay nucleus of vision, starting the process of co-ordinating vision from the two eyes </li></ul><ul><li>C-shaped 6 defined layers </li></ul><ul><li>Layers 1, 2 receive from large M ganglion cells -> Magnocellular division </li></ul><ul><li>Layers 3,4,5 & 6 from small P ganglion cells -> Parvocellular division </li></ul>
  38. 38. LGB <ul><li>Each layer receives input from one eye only </li></ul><ul><li>- Layers 1,4,6 from contralateral eye </li></ul><ul><li>- Layers 2, 3,5 from ipsilateral eye </li></ul><ul><li>Responses of neurons are similar to retinal GC (on-centre & off-centre organization) </li></ul><ul><li>LGB also receives input from brain stem, reticular formation & feedback from cerebral cortex </li></ul>
  39. 39. Primary Visual area (Area 17, V1) <ul><li>On medial aspect of each occipital lobe </li></ul><ul><li>Its neurons arranged in the form of columns (1x1x2 mm) forming 6 distinct layers </li></ul><ul><li>The input from the two eyes remain separated ( 2 columns; Ocular dominance columns ) </li></ul><ul><li>Fovea has broad presentation </li></ul><ul><li>Its neurons (layers 2,3,4) project into areas 18 & 19 (association or 2ry visual areas) </li></ul>
  40. 40. Visual Projection to Area 17
  41. 41. Role of Area 17 (V1) <ul><li>Perception of visible objects without knowing the meaning of these objects: </li></ul><ul><li>Details of image; shape, borders & colours </li></ul><ul><li>Orientation of object in space </li></ul><ul><li>3D vision (stereoscopic) </li></ul><ul><li>Fusion of visual images perceived from the two eyes (conscious perception of a single image of visual field occurs only after signals reaching association parietal areas) </li></ul>
  42. 42. <ul><li>For fusion of the two images (perception of two images as one), the 2 images must fall on corresponding points on 2 retinas </li></ul><ul><li>This makes images in register (precisely fused) in cortical neurons </li></ul><ul><li>When the 2 images are not in register; cortical neurons -> excitation of interference cortical neurons -> signals to Oculomotor apparatus -> convergence or divergence or rotation to re-establish fusion </li></ul>
  43. 43. Secondary Visual Processing: Association Areas (18 &19; V2,3 & V4,5) <ul><li>In parietal & temporal lobes </li></ul><ul><li>Dealing with complex perception of patterns & forms responsible for object recognition </li></ul><ul><li>10% of cells in inferotemporal cortex have large complex receptive fields & selective for specific stimuli (familiar faces) </li></ul><ul><li>♣ Lesion -> visual agnosia </li></ul>
  44. 44. Retinotopic Organization & Processing of visual information
  45. 45. COLOUR VISION
  46. 46. <ul><li>Colour vision is the ability to discriminate different wavelengths that constitute the visible spectrum </li></ul>
  47. 47. COLOUR VISION <ul><li>Our perception of light is related to wavelengths of light that are </li></ul><ul><li>transmitted, reflected or absorbed by objects of our visual world. </li></ul><ul><li>An object appears red because this object absorbs short wavelengths of light (would perceived as blue) & reflect long wavelengths which excite retinal cones most sensitive to red </li></ul>
  48. 48. <ul><li>Colour vision is cone-mediated function </li></ul><ul><li>explained by ( Trichromatic Theory ): </li></ul><ul><li>There are 3 types of cones; according to their sensitivity to a certain light wavelength: </li></ul><ul><li>S-cones most sensitive (maximally absorb & respond optimally) to short wavelength (peak absorption at 420nm; perception of blue , absent in central fovea) </li></ul><ul><li>M-cones most sensitive to medium wavelength (530nm; perception of green, greatest in central fovea) </li></ul><ul><li>L-cones most sensitive to long wavelength ( 560nm , perception of red ) </li></ul>COLOUR VISION
  49. 49. Colour-sensitive Cones
  50. 50. <ul><li>Although each type of cones is stimulated maximally by a certain wavelength but still can be stimulated by other but at a less degree </li></ul><ul><li>E.g. in response to light of 531nm wavelength, the green cones respond maximally, red cones less & blue cones not at all </li></ul><ul><li>When red & green cones are stimulated equally, one sees yellow </li></ul><ul><li>When all 3 types are equally stimulated -> perception of white </li></ul>COLOUR VISION
  51. 51. Colour Blindness <ul><li>Sex-linked recessive disorder </li></ul><ul><li>Defect in colour gene -> deficiency of colour photosensitive pigment -> inability to see one or more colours </li></ul><ul><li>Common in males (9%) than females (0.4%) </li></ul><ul><li>Protanopia (red blindness) </li></ul><ul><li>Deuteranopia (green blindness) </li></ul><ul><li>Tritanopia (blue blindness) </li></ul>
  52. 52. THANK YOU
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