The Special Senses Vision - 2

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The Special SensesVision - 2

• Professor A.M.A Abdel Gader
• MD, PhD, FRCP (London Edinburgh), FRSH (London)
• Professor of Physiology, College of Medicine
• King Khalid University Hospital
• Riyadh, Saudi Arabia

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The Physiology of Vision

• Objectives
• At the end of this lecture the student should be
  able to
• Understand the optical bases of image formation
  on the retina
• Understand and explain the optical bases of
  common refractive errors
• Understand the electrical bases of the
  photoreceptor function
• Understand the nature and function visual
  pigments
• Understand color vision

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The Physiology of Vision

• Objectives
• At the end of this lecture the student should be
  able to
• Understand the optical bases of image formation
  on the retina
• Understand and explain the optical bases of
  common refractive errors
• Understand the electrical bases of the
  photoreceptor function
• Understand the nature and function visual
  pigments
• Understand color vision

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Physiology of Vision

• Stimulus Light
• Receptor Retina (Photoreceptors)

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Light

• Definition
• elctromagnetic radiation that is capable of
  exciting the human eye
• Extremely fast

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Which travels faster light or sound?
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Electromagnetic spectrum The visible light
spectrum
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The Electromagnetic Spectrum
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Visible light Duplicity Theory of vision

• Visible light Spectrum
• Extends from 397 to 723nm
• Eye functions under two 2 conditions of
  illumination
• Bright light (Photopic vision)Cones
• Dim light (Scotopic vision) ..Rods
• Duplicity theory
• of vision

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Duplicity theory

• Photopic visibilty curve peaks at 505nm
• Scotopic 550nm

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PhotoreceptorsRods Cones

• Morphology Distribution

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Retina
Back of retina, pigment epithelium (Choroid)
Light
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Rods and Cones
Figure 17.13
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Photoreceptors
Figure 16.11
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Retina distribution photoreceptors
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Distribution of photoreceptors
Receptor density (cells x 103 / mm2)
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Normal Fundus
Photoreceptors are not distributed uniformly
across the retina
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Human foveal pit
INL
ONL
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Convergence rod/cone cells
Low Convergence Cone-Fed Circuits
Bipolar cell
Cone
Retinal ganglion cell
High Convergence Rod-Fed Circuits
Retina ganglion cell
Bipolar cell
Rod
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Retina photoreceptors

• 100,000,000 rods
• 5,000,000 cones

Cones Rods
Fovea Periphery
High light levels Low light levels
Color Monochromatic
Good acuity Poor acuity
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Electrophysiology of VisionGenesis of electrical
responses
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Retinal photoreceptors mechanism

• Light
• Absorption by photosensitive substances
• Structural change in photosensitive substances
• Phototransduction
• Action potential in the optic nerve

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Action Potential Propagated and
All-or-None
Receptor PotentialLocal Graded
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Retina Neural Circuitry
Light hits photoreceptors, sends signal to the
bipolar cells
Bipolar cells send signal to ganglion cells
Ganglion cells send signal to the brain
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In Darkness
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Photoreception-cont.
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Retina
Light
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Electrophysiology of Vision

• Electric recording in Retinal cells
• Rods Cones Hyperpolarization
• Bipolar cells Hyper- Depolarization
• Horizental cells Hyperpolarization
• Amacrine cells Depolarizing potential
• Ganglion cellsDepolarizing potential

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Rods and Cones
outer segment
outer segment
Disk membrane
Intracellular disk
Extracellular space
Visual pigment
Intracellular space
Disk membrane
Extracellular space
Plasma membrane
Intracellular space
Visual pigment
Connecting cilium
Connecting cilium
ROD CELL
CONE CELL
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Comparison Scotopic and Photopic systems
Rods Cones
Light Environment Dim light - scotopic Bright light - photopic
Spectral sensitivity 1 pigment 3 pigments
Color discrimination No Yes
Absolute sensitivity High Low
Speed of response Slow Fast
Rate of dark adaptation Fast Slow
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Photoreceptor pigments
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Photoreceptor pigments

• Composition
• Retinine1 (Aldehyde of vitamin A)
• Same in all pigments
• Opsin (protein)
• Different amino acid sequence in different
  pigments
• Rhodopsin (Rod pigment)
• Retinine scotopsin

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Photoreceptor compounds-cont Rhodopsin (visual
purple, scotopsin)

• Activation of rhodopsin
• In the dark
• retinine1 in the 11-cis configuration
• All-trans isomer
• Metarhodopsin II
• Closure of Na channels

Light
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Visual cycle

• Rhodopsin
• Prelumirhdopsin
• Inermediates including
• Metarhodopsin II
• Vitamin A Retinine
  Scotopsin
• Scotopsin

Light
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• Light
• Change in photopigment
• Metarhodopsin II
• Activation of transducin
• Activation of phophodiesterase
• Decrease IC cyclic GMP
• Closure of Na channels
• Hyperpolarization of receptor
• Decrease release of synaptic tramitter
• Action potential in optic nerve fibres

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From light reception to receptor potential
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Retina Neural Circuitry
Light hits photoreceptors, sends signal to the
bipolar cells
Bipolar cells send signal to ganglion cells
Ganglion cells send signal to the brain
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Photoreception
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Photoreception- cont.
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Retina

• 100,000,000 rods
• 5,000,000 cones
• 1,000,000 ganglion cells
• Convergence

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Convergence

• Cones
  Rods
• Photoreceptors
• Ganglion cells

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Convergence and Ganglion Cell Function
Figure 17.18
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• Dark adaptation

Dark adaptation
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• Dark adaptation
• Increased sensitivity of the
• photoreceptors when vision shifts from bright to
  dim light

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Dark adaptation

• Reaches max in 20 minutes
• First 5 minutes threshold of cones
• 5 to 20 mins . Sensitvity of rods
• Mechanism of dark adaptation
• Regeneration of rhodopsin

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Dark adaptation-cont.

• In vitamin A deficiency
• What happens to Dark adaptation?
• Night blindness
• (Nyctalopia)