Transcript 07.Vision2

The Special Senses
Vision - 2
Professor A.M.A Abdel Gader
MD, PhD, FRCP (Lond., Edin), FRSH (London)
Professor of Physiology, College of Medicine &
King Khalid University Hospital
Riyadh, Saudi Arabia
The Physiology of Vision
Objectives:
At the end of these 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
Physiology of Vision
Light
Receptor: Retina (Photoreceptors)
• Stimulus:
•
Light
• Definition:
‘elctromagnetic’ radiation that is
capable of exciting the human
eye’
• Extremely fast
Which travels faster: light or sound?
Electromagnetic spectrum &
The visible light spectrum
The Electromagnetic Spectrum
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
Duplicity theory
• Photopic visibilty curve peaks at 505nm
• Scotopic “”
”
“
“ 550nm
Photoreceptors
Rods & Cones
Morphology & Distribution
Retina
Back of
retina,
pigment
epithelium
(Choroid)
Light
Rods and Cones
Figure 17.13
Photoreceptors
Figure 16.11
Retina: distribution photoreceptors
Receptor density (cells x 103 / mm2)
Distribution of
photoreceptors
Fundus
Photoreceptors are not distributed uniformly acrossNormal
the retina
Optic disc
Macula
5000um
650,000
cones
Fovea
1500um
100,000 cones
Foveola
350um
25,000 cones
INL
Light
ONL
Foveola
Human foveal pit
Low Convergence Cone-Fed Circuits
Retinal
ganglion
cell
Bipolar
cell
Cone
High Convergence Rod-Fed Circuits
Retina
ganglion cell
Bipolar
cell
Rod
Convergence rod/cone cells
Retina: photoreceptors
• 100,000,000 rods
• 5,000,000 cones
Cones
Fovea
High light levels
Color
Good acuity
Rods
Periphery
Low light levels
Monochromatic
Poor acuity
Electrophysiology of Vision
Genesis of electrical responses
Retinal photoreceptors mechanism
Light
Absorption by photosensitive substances
Structural change in photosensitive
substances
Phototransduction
Action potential in the optic nerve
Action Potential
Propagated and
“All-or-None”
Receptor Potential
Local & Graded
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
In Darkness
Photoreception-cont.
Retina
Light
Electrophysiology of Vision
Electric recording in Retinal cells:
• Rods & Cones: Hyperpolarization
• Bipolar cells: Hyper- & Depolarization
• Horizental cells: Hyperpolarization
• Amacrine cells: Depolarizing potential
• Ganglion cells:Depolarizing potential
outer segment
outer segment
Disk membrane
Intracellular disk
Intracellular
space
Disk membrane
Extracellular
space
Visual
pigment
Extracellular
space
Intracellular
space
Visual
pigment
Plasma
membrane
Connecting
cilium
Connecting
cilium
ROD CELL
CONE CELL
Rods and Cones
Rods
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
Starlight
Moonlight
No color vision
Poor acuity
Scotopic
Absolute
threshold
Cones
Indoor lighting
Good color vision
Best acuity
Mesopic
Cone
threshold
Sunlight
Photopic
Rod
Saturation
begins
Best
acuity
Indirect
Ophthalmoscope
Damage
Possible
Comparison Scotopic
and Photopic systems
Photoreceptor compounds
• Composition:
– Retinine1 (Aldehyde of vitamin A)
• Same in all pigments
– Opsin (protein)
• Different amino acid sequence in
different pigments
Rhodopsin (Rod pigment):
Retinine + scotopsin
Photoreceptor compounds
-cont
Rhodopsin (visual purple, scotopsin):
Activation of rhodopsin:
• In the dark:
retinine1 in the 11-cis configuration
Light
All-trans isomer
Metarhodopsin II
Closure of Na channels
Visual cycle
Rhodopsin
Light
Prelumirhdopsin
Inermediates including
Metarhodopsin II
Vitamin A +
Scotopsin
Retinine & Scotopsin
From light reception to receptor potential
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
Retina: Neural Circuitry
Light hits
photoreceptor
s, sends signal
to the bipolar
cells
Bipolar cells
send signal to
ganglion cells
Ganglion
cells send
signal to
the brain
Photoreception
Photoreception- cont.
Retina
• 100,000,000 rods
• 5,000,000 cones
• 1,000,000 ganglion cells
Convergence
Convergence
Cones
• Photoreceptors
• Ganglion cells
Rods
Convergence and Ganglion Cell Function
Figure 17.18
Dark adaptation
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
Dark adaptation-cont.
In vitamin A deficiency
What happens to Dark adaptation?
Night blindness
(Nyctalopia)
Color
(Photopic)
Vision
Photopic vision
(CONES)
Primary colors:
723-647
575-492
492-450
When mixed >>> white or any other color
Trichromatic/dichrom
atic color vision
Visual pigment protein
Relative Absorbance
419
496 531 599
ROD
CONES
11-cis retinal
400
450
500
550 600 650
Wavelength (nm)
Visual pigment and sensitivity/wavelength
Photopic vision
(CONES)
Cone pigments: three kinds
565
535
440
Photopic vision
Young Helmholtz theory
Three types of cones containing a
photoreceptor pigment most sensitive
to one primary color
1. Cones (contain red-sensitive pigment)
2. Cones (contain green-senstive pigment)
3. Cones (contain blue-sensitive pigment)
in the fovea centralis
Colour Blindness
Weakness or total blindness in
detecting a primary color:
Humanbeings are:
1. Trichromats: see the 3 1ry colors
2. Dichromats: blind to one 1ry color
3. Monochromats: have color pigment
Dichromat – missing one whole group of photopigments
• Protanopia
• Loss of the L pigment
• Sex-linked, L-pigment gene on X chromosome
• Frequency about 1%
• Deuteranopia
• Loss of the M pigment
• Sex-linked, M-pigment gene on X chromosome
• Frequency about 1%
• Tritanopia
• Loss of the S pigment
• S-pigment gene on chromosome 7
• Frequency about 1 in 10,000
Protoanomaly and deuteranomaly – milder forms of “color blindness”
Anamolous trichromacy in which one pigment is replaced
by another with different spectral qualities – 7-8% males
Color Blindness
types