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