Lecture on Basic of visual transduction. Dr Shamshad loni Majamaah university

1. Lecture:
Mechanism of Visual Transduction
Dr Shamshad
Majamaah University

2. 1. Describe the photochemistry of vision in rods & cones.
2. Explain the genesis of electrical activity of retina.
3. Describe the duplicity theory of vision.
4. Define and describe the process of retinal adaptation.
Objectives

3. Definition :
Phototransduction is the process where light energy
convert into electrical signals.
• When light hit the retina of eye it causes the conformational
changes in the photoreceptors.

4.  Light energy is part of electromagnetic spectrum,
 Our brain perceives only a small portion of this broad energy
spectrum.
 Electromagnetic energy is measured in photons
 Waves of visibility/sensitivity of range of motion in human is
with
• Frequency : 4.0-7.5X1014 cycles/second (hertz) And
• Wavelength: 400-750 nm.

5. Layers of Retina

7. No. Layers Of Retina Functions
1 Pigmented layer  Absorbs light
 Prevents reflection of light back
from retina
 Stores Vitamin A
 Phagocytic action: Removes debris
from rods & cones.
2 Layer of rods &
cones
 Light sensitive portions of visual
receptor cells

8. 3 Outer nuclear
layer
 Fibers & granules of rods & cones
 Granules contain nucleus
4 Outer
plexiform layer
 Terminal fibers of rods & cones &
dendrites from bipolar cells form
Reticular meshwork
5 Inner nuclear
layer
 Polar cells
 Nuclei of Muller supporting fibers ,
horizontal cells & amacrine cells

9. 6 Inner plexiform
layer
 Processes from amacrine cells
7 Ganglion cell
layer
 Multipolar cells (Giant & midget cells)
 Also has Retinal blood vessels
8 Optic nerve fibre
layer
 Non- myelinated axons ganglionic cells
 Muller cells
 Blood vessels
9 Inner limiting
membrane
 Separates retina from vitreous body
 Hyaline membrane formed Muller fibers

10. No. Cells in retina Functions
1 Pigment
epithelial cells
 Absorb stray light
 Prevent scattering of light,
 Convert 11-cis retinal to all-trans
retinal.
2 Photoreceptors
Rods & Cones
 Convert Light energy to electrical
signals
 Transmit signals to the outer plexiform
layer,
 Synapse with bipolar & horizontal
cells.

11. 3 Bipolar
cells
 Transmit signals vertically from the rods,
cones, &
 Horizontally with Horizontal cells to the inner
plexiform layer,
 Synapse with ganglion cells & Amacrine cells.
4 Ganglion
cells
 Axon of these cells forms optic nerve fibers,
 Transmit output signals from the retina
through the optic nerve into the brain.

12. 5 Horizontal
cells
 Transmit signals horizontally in the outer
plexiform layer from the rods & cones to
bipolar cells.
6 Amacrine
cells
 Transmit signals in two directions,
i. Directly from bipolar cells to ganglion cells
ii. Horizontally within the inner plexiform layer
from axons of the bipolar cells to dendrites
of the ganglion cells or to other amacrine
cells

13. 7 Interplexiform
cells
 Not prominent,
 Transmits signals in the retrograde
direction from the inner plexiform layer to
the outer plexiform layer,
 Inhibitory
 Believed to:
• Control lateral spread of visual signals by
the horizontal cells in the outer plexiform
layer,
• Help control the degree of contrast in the
visual image.

14. Photoreceptors
Rods & Cones

16. Outer
segment
Photosensitive pigments in the discs constitute about
40 % of the entire mass
Each of the discs is an infolded shelf of cell
membrane.
Rhodopsin & color pigments are conjugated proteins.
Inner
segment
Contains cytoplasmic organelles:
Mitochondria: provides energy photoreceptors.
Cell body Nucleus.
Synaptic
body
Connects with subsequent neuronal cells, the
horizontal & bipolar cells.

17. Neurotransmitters Released by Retinal Neurons
Rods & cones Glutamate at their synapses with the
bipolar cells.
Amacrine cells
(at least 8 types)
GABA, glycine, dopamine, Ach
Indolamine (inhibitory)
Horizontal cells Inhibitory transmitters
Bipolar &
Interplexiform cells
Unclear

18. Rods Cones
Location Around periphery Around center :Fovea
Ratio: bipolar cells Many : 1 bipolar cells 1: 1 bipolar cells
Color sensitivity All wavelengths Red, Green, & Blue
Number & types One
with only rhodopsin
three with different
Iodopsin pigments
Relative
abundance
Many Fewer

19. Rods Cones
Functions Sensitive to low
intensity light ,
Sensitive to high
intensity light ,
Night /Scotopic vision Day/ Photopic vision
Types of vision Achromatic
( black and white )
Color
Visual acuity Lower
Not present in fovea
Higher
present in fovea
Dark adaptation Rods adapt later Cones adapt first

20. Wald visual Cycle

21. Hyperpolarization Inhibition
Glutamate

24. Depolarizing and Hyperpolarizing Bipolar Cells
Two types of bipolar cells
opposing excitatory and inhibitory signals in the visual pathway:
(1) the depolarizing bipolar cell and
(2) the hyperpolarizing bipolar cell.

25. Two possible explanations for this difference.
1: The two bipolar cells are of entirely different types—one
responding by depolarizing in response to the glutamate
neurotransmitter released by the rods and cones, and the
other responding by hyperpolarizing.
2: One of the bipolar cells receives direct excitation from the
rods and cones, whereas the other receives its signal
indirectly through a horizontal cell.

26. Because the horizontal cell is an inhibitory cell, this would
reverse the polarity of the electrical response.
Regardless of the mechanism for the two types of bipolar
responses, it allows half the bipolar cells to transmit

28. If a person has been in bright light for hours,
 large portions of the photochemicals in both the rods & the
cones reduced to retinal & opsins.
 Retinal is converted to retinol (Vitamin A).
 Depletion of retinal causes two effects
1. Reduced concentrations of the photosensitive chemicals
2. Reduced sensitivity of the eye to light.
This is phenomenon is called light adaptation.
Light adaptation

29. Sensitivity of the retina in darkness
 Immediately when a person enters in darkens it is very low,
 Within 1 minute, increases to 10-fold,
 End of 20 minutes, increases 6000-fold, and
 At end of 40 minutes, 25,000-fold.

30. Two parts of adaptation curve
1: Early part of the curve:
 Caused by adaptation of the cones, four times as rapidly than rods.
 But the cones do not achieve anywhere near the same degree of
sensitivity change in darkness as the rods do.
 After few minutes, the cones cease adapting slowly.
2: Later part of the curve:
 Adapting rods continue to adapt for many minutes to hours,
 Their sensitivity increasing tremendously.
 Neuronal signal convergence of >100 rods onto a single ganglion cell
in the retina; summate to increase their sensitivity,

31. Mechanisms of Light and Dark Adaptation
1: Change in pupillary size
2: Neural adaptation: the neurons in the successive stages of
the visual chain in the retina and in the brain.
 When light intensity first increases, the signals transmitted
by the bipolar cells, horizontal cells, amacrine cells, &
ganglion cells are intense.
 Later most of these signals decrease rapidly at different
stages of transmission in the neural circuit.

32. Purkinje effect / shift
Definition : The shifting of sensitivity of eye from photopic to
scotopic vision.
Or
The tendency for the peak luminance sensitivity of the human eye to
shift toward the blue end of the color spectrum at low illumination
levels.
 Seen towards evening.

33. In bright sunlight, flowers appear bright red against the dull green
of their leaves,
In Dark red petals appear dark red or black, and the leaves
relatively bright.
The sensitivity to light in scotopic vision varies with wavelength.
The Purkinje shift is the relation between the absorption maximum
of rhodopsin, reaching a maximum at about 500 nm, and that of the
opsins in the long-wavelength and medium-wavelength cones that
dominate in photopic vision, about 555 nm.

34. • Occurs at the transition between photopic (cone-based) and
scotopic (rod-based) systems,
• Mesopic state: as intensity dims, the rods take over, and
before color disappears completely, it shifts towards the rods’
top sensitivity.
• Rod sensitivity after 5–10 minutes in the dark, take about 30
minutes of darkness to regenerate photoreceptors and reach
full sensitivity.

35. Application of the effect:
Due to insensitivity of rods to long-wavelength light
Red lights are used :
Ex,
 Control rooms of submarines
 Research laboratory
 Aircraft,
 During naked-eye astronomy.

36. Duplicity theory of vision
 Explains single type of receptor cells are incapable of providing high
sensitivity & high resolution.
 Hence two kinds of receptors (rods & cones) exist that work
maximally under different conditions of illumination.
1. Photopic system: Day vision : mainly cones.
2. Scotopic system : Night vision :mainly rod.
3. Transition zone of vision: In B/w the two is with function
of cones overlap the rods.

37. Photopic vision Scotopic vision
Intensity Daylight vision Dim light vision
photoreceptor Cones Rods
Threshold for
Light
Higher for intensity
(1millilambert)
but greater visual acuity
Lower for Intensity
(0.01 millilambert)
Vision Color vision &
Vision in bright light
Black and white
Vision in dark
Maximum
Sensitivity
About 560nm ie for
greenish yellow light
About 500nm ie
bluish green light.

38. References:
1. Ganong's Review of Medical Physiology, Twenty sixth Edition by Kim
Barrett, Susan Barman, et al.
2. Guyton and Hall Textbook of Medical Physiology (Guyton Physiology)
by John E. Hall PhD. |
3. Text book Physiology 6th Edition by Linda S. Costanzo PhD .
4. Human Physiology: An Integrated Approach by Dee Silverthorn.