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Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
Phototransduction & Visual Pathway  Mmp  March 10
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Phototransduction & Visual Pathway Mmp March 10

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

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