2. Agenda
• Eyes and its components.
• Types of cells and how they work.
• Vision in dim light.
• Vision in coloured light.
• Coloured Blindness.
• Night Blindness.
4. Eyes(Photoreceptor organs)
• Vision is such an everyday occurrence that we seldom stop to think
and wonder how we are able to see the objects that surround us.
Yet the vision process is a fascinating example of how light can
produce molecular changes.
• The retina contain the molecules that undergo a chemical change
upon absorbing light, but it is the brain that actually makes sense of
the visual information to create an image.
• The light image is mapped on the surface of the retina by activating
a series of light-sensitive cells known as RODS and CONES or
photoreceptors. The rods and cones convert the light into electrical
impulses which are transmitted to the brain via nerve fibers. The
brain then determines, which nerve fibers carried the electrical
impulse activate by light at certain photoreceptors, and then creates
an image.
• Single eye gathers light and focuses it to many receptor cells.
Extraordinary sensitive instrument
• Wavelength range (400-800nm)
10. Rod cells contain Rhodopsin
• First discovered by German physiologist Franz
Boll.
• Known as visual purple because of absorption
maxima=500nm
• Human rhodopsin has mol. wt.=41000 and has
348 amino residues.
• This works at night time.
• Cannot distinguish colour.
11. • Rhodopsin chromophore is polyunsaturated
aldehyde, 11-cis retinal (A).
• Rhodopsin is the product of the reaction of this
aldehyde and protein called Opsin (B).
• Reaction is between the aldehyde group of A
and alpha amino group of the chain of the
protein by the loss of a water molecule.
• Other interactions of the –SH bonds of protein
keeps the aldehyde on place.
12.
13. Irradiation of rhodopsin leads to a series of
conformational changes
When rhodopsin absorbs a photon of light the 11-cis retinal isomerizes to all trans form
16. Photochemical changes
• Absorption of one quantum does not result in vision.
• Several quanta(2-6) should reach the rod in short time.
• The absorption of light leads to chain reaction and it
takes energy from the metabolism.
• From the retina, amplification of the visual signal takes
place with high gain and low noise ratio.
• Photoreceptor act as photomultiplier.
• Visual pigments are integral membrane proteins that
reside in photoreceptors outer segment.
• Photoisomerisation of retinal triggers many
conformational changes in attached proteins that creates
an enzyme site.
• Which ultimately produces signal.
19. Colour Vision
• Occurs due to cone cells.
• Photosensitive pigment is Iodopsin.
• The chromophore of Iodopsin is also 11-cis retinal.
• 3 types of Iodopsin in cones.
Blue (440nm)
Green (535nm)
Red (565nm)
• Less sensitive to light than rod cells, so in dim light
objects appear in shade of grey.
21. Color Blindness
• Missing or defective pigment proteins for certain
cone cell types
– ~8% of men, rare in women.
• Types of Color Blindness
– Achromatopsia - black and white vision.
– Dichromacy - 2 functioning cone types.
– Anomalous Trichromacy - shifted cone absorption.
• Non-Genetic Causes
– Disease, Accidents, Medication.
22. Colour Blindness
Everyone should see number 12 ‘normal’ see number 8
Red-green deficiency see number 3
Total colour blind see no number
Take care of your eyes
23. Night Blindness (Nyctalopia)
• The most common cause of nyctalopia is retinitis
pigmentosa, a disorder in which the rod cells in
the retina gradually lose their ability to respond
to the light.
• Patients suffering from this genetic condition
have progressive nyctalopia and eventually their
daytime vision may also be affected.
