biochemistry, phototransduction and light synthesis in human retina is explained.

1. BIOCHEMISTRY OF RETINA

2. Functions of retina
• Is to absorbs photons of light
• To translate light into a biochemical message
• Translate biochemcial message into electrical impulse
• Transmit electrical impulse into the brain via ganglion
cells

3. Retinal pigment epithelium
• The retinal pigment epithelium (RPE) is a
monolayer of pigmented cells.
• 1 RPE cell = 23 photoreceptors (functional unit)
choriocapillaries

4. Functions of RPE
• Absorption of light
• Transepithelial transport
• Spatial buffering of ions
• Visual cycle
• Phagocytosis
• Secretions
• Immune privilege of the eye

5. Absorption of light
• RPE absorb scattered light to improve the quality of the optical system.
• Absorbed by RPE
• Increase in temperature
• Heat is transported by the blood
vessel into the choriocapillaries
The relative blood perfusion of the choriocapillaries is higher than that of kidney

6. Defense
• Melanin of melanosomes
• Carotenoids lutein and zeaxanthin Absorb light energy
• Ascorbate
• Alpha tocopherol
• Beta carotene
• Glutathione
• melanin
Antioxidants
Cells natural ability to repair damaged DNA, lipids and proteins

7. Transepithelial transport:
• Tight epithelium-
paracellular
resistance>transcellular
resistance
The RPE Transports Nutrients and Ions Between
Photoreceptors and the Choriocapillaries which relies
on transepithelial transport.

8. Water and lactic acid
An increase in transport of lactic
acid
Intracellular
acidication
Inhibits the transport
activity of Cl-/HCO3-
exchanger
Reduces transport of Cl- in
opposite direction to the
Cl- channels
Increases the transepithelial transport of Cl-
and water
Increased by activation of
C1C2 channels
Decrease in
extracellular pH

9. Phagocytosis
• Destruction of photoreceptor outer segments due to large density of
light energy and reactive oxygen species
• Destroyed tips of photoreceptor outer segment are shed and
phagocytosed by RPE
• Every 11 days a whole length of photoreceptor outer segment is
renewed from the inner segments
Alpha5-integrins
• binding occurs
• Activation of focal adhesion
kinase
 Phosphorylation of c-mer
tyrosine kinase
 Circadian regulation of
phagocytosis
C-mer tyrosine kinase(MerTK)
• Triggers Internalization of
photoreceptor outer segment
CD36
• Important for process of
internalization

10. Secretion
• Secretion of large variety of growth
factors, cytokines and immune
modulators
 Maintain the structural integrity of
photoreceptors
 Maintain the fenestrated structure of
endothelium of the chorio-capillaries
 Actively interact with the immune
system
• Intracellular regulation of the secretion
involves voltage-dependent L-type Ca2+
channels of neuro-endocrine subtype
• Regulation is by tyrosine kinase

11. Phototransduction
• What is it?
series of biochemical
events
• What does it do?
a photon of light is
changed to an electrical
signal
• Where does it take place?
in the photoreceptors.
• Activation phase and
recovery phase

12. Phototransduction - General
• Photons hit the pigment
of a photoreceptor 
  enzymes are
activated in the cell
which modify its state
of polarization  the
signals are sent to visual
area of the occipital
lobe of the brain
through the optic nerve

13. Dark- adapted rods
• Resting dark adapted state
• The activation phase of light response
• The recovery phase

14. Resting dark adapted state
• The membrane potential
• The dark current
• Ca2+ and the exchanger
• Control of cGMP
• Rhodopsin
• Lipid mileu

16. Activation phase of light response
• Photoisomerization of rhodopsin
• G- protein activation
• PDE6 activation
• Channel closing
• Slowing of neurotransmitter release

17. Photoreception - In More Detail

18. Photoreception

19. Recovery phase
• Rhodopsin phosphorylation, retinoid cycling
and regeneration
• Arrestin binding
• cGMP restoration by guanylate cyclase
activation
• G-protein and PDE6 inactivation by RGS9

20. Bleaching and Regeneration of Visual Pigments