This document summarizes the physiology of the visual cycle. It discusses that vitamin A is obtained through diet and converted to retinal in the body. Retinal binds to opsins in the photoreceptors to form visual pigments like rhodopsin. Upon exposure to light, rhodopsin undergoes changes to bleach the retinal, after which it is regenerated through a series of reactions to restore the visual cycle. The process maintains an equilibrium between bleaching and regeneration of visual pigments to facilitate continuous vision.

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