Physiology of visual cycle

for new ophthalmic residents

1. PHYSIOLOGY OF VISUAL
CYCLE
Othman Al-Abbadi, M.D

2. INDEX
Vitamin A
Dietary sources
Absorption & storage
Transport
Utilization
Visual pigments
Rhodopsin
Cone pigments
Light-induced changes
Rhodopsin bleaching
Rhodopsin regeneration
Visual cycle

3. DIETARY SOURCES

4. DIETARY SOURCES
Dietary sources of vitamin A include animal food
& plant food.
Animal food contain vitamin A as Retnol.
The liver is always the best source
Plant foods contain vitamin A as Carotenes
which is a precursor.
Carotenes must be converted to Retinol by
metabolic activity in the wall of the small
intestine.
Three types of carotenes (alpha, beta, & gamma)
are present in plant food
Beta Carotenes yield 2 molecules of vitamin A,
while Alpha & Gamma yield one molecule.

5. Absorption
Storage
Transport
Attach to receptors
in RPE
Transported inside
RPE to
photoreceptors

6. UTILIZATION OF VITAMIN A
Retinol passes through RPE to phtoreceptors
unchanged.
Oxidation of Retinol to Retinene (11-cis Retinal)
by Retinene Reductase in photoreceptor outer
segment.
Retinene combines with Opsin to form
Rhodopsin.
The freshly formed Rhodopsin is incorporated
into the newly forming double discs at the
innermost portion of outer segment of
photoreceptors.

7. VISUAL PIGMENTS
Substances that have the property of absorbing
light.
The peak of each pigment’s absorption curve is
called “absorption maximum.”

8. RHODOPSIN
Membrane-bound glycolipid.
Consists of Scotopsin & Retinal.
Molecular weight of 40,000
Fat soluble
Sensitive to heat & chemical agents which
denature the protein.
Scotopsin is a 348 amino acid protein & crosses
the disc membrane 7 times.

9. The light absorbing form is 11-cis retinal.
The absorption spectrum for rhodopsin has a
peak of 493-505 nm; absorbing yellow wavelength
primarily & transmitting violet & red to appear
purple (visual purple).

10. CONE PIGMENTS
NOT intensively studied.
Three kinds of cones.
Cone pigments are different than rhodopsin in
that they respond to specific wavelengths of
light… giving us the ability for colour vision.
Amino acid sequence for all human cone opsins is
almost identical except for few different amino
acids that makes the difference in spectral
absorbance.
11-cis retinal remains the same.
Peak absorbance of blue, green & red is 435, 535
& 580 nm respectively.
Blue-sensitive cones are the least prevalent.

11. The light falling on the retina is absorbed by the
photosensitive pigments & initiates
phtotchemical changes that initiate electrical
changes.
Photochemical changes take place in the outer
segments.
Bleaching (light dependent)
Regeneration (light independent)
Visual cycle

12. RHODOPSIN –RETINAL VISUAL CYCLE IN
THE RODS.
Rhodopsin and its decomposition by light energy:
•The outer segment of the rod that projects into the
pigment layer of retina has a concentration of about
40% of light sensitive pigments called Rhodopsin or
visual purple.
•Rhodopsin = scotopsin(protein) + retinal(carotenoid
protein).
•Retinal is present in the form of 11-cis retinal known
as retinene.
•cis form of retinal is important because only this form
can bind with scotopsin to synthesize rhodopsin.

13. CHANGES OCCURING IN
RHODOPSIN
Rhodopsin Light barthorhodopsin

lumirhodopsin
BLEACHING
metarhodopsin I

metarhodopsin II
Scotopsin
11-cis retinal isomerase all-trans retinal
REGENERATION
11-cis retinol isomerase all trans retinol

16. PHOTOTRANSDUCTION
The transduction of light into a neural signal
takes place in the outer segment of a retinal rod
or cone photoreceptor

20. VISUAL CYCLE
Equilibrium state
between bleaching &
regeneration.
Another portion of the
adaptive mechanism
occurs under the
influence of neural
elements at the level of
the synapses of
photoreceptors with
Bipolar cells & may
involve Horizontal
cells.

21. DIFFERENCES THAN OTHER
RECEPTORS
Receptor potential of photoreceptors is local
graded potential. (not all or non)
Hyperpolarization rather than depolarization.

22. THANK YOU