Transcript Vision-lecture-2 Photoreceptors- 1430

The Special Senses
Vision - 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
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
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
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
Normal Fundus
Photoreceptors
are not
distributed
uniformly across
the retina
Optic disc
Macula
5000um
650,000
cones
Fovea
1500um
100,000 cones
Foveola
350um
25,000 cones
Human foveal pit
INL
Light
ONL
Foveola
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 pigments
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
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
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
From light reception to receptor potential
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:
Increased sensitivity of the
photoreceptors when vision shifts
from bright to dim light
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)