6. FOVIA CENTRALIS
Structural characteristics
Cones – maximum, rods
absent
Cones – long with thin
outer segment.
Reduced distortation as
layers push aside.
No capillaries
No convergence of cones so
one to one relay of cones
with bipolar cells.
10. VISUAL PIGMENTS
Rhodopsin (visual purple)
Protein opsin (scotopsin) &
carotenoid (Retinal –
aldehyde of vitamin A)
Mole wt – 40,000.
Absorbs primarily yellow
wavelength light,
transmitting violet & red to
appear purple so called
Visual purple.
Cone pigments
3 types.
Respond to specific
wavelength of light.
Chromophore 11 cis-
retinal is same .
Difference in only opsin
portion of molecule.
15. PHOTOTRANSDUCTION
Activation of Rhodopsin
Activation of Transducin
Conversion of cGMP to
GMP
Production of receptor
potential
Cone versus rod receptor
potential
Rhodopsin
Metarhodopsin II
Activation of transducin
Activation of
Photodiestarase.
CGMP.
Closure of Na channels
Hyper polarization
Synaptic transmitter.
Bipolar cells & other
neurons
17. Processing & transmission of the
visual impulse in retina
Concept of receptive field
It is influence area of a sensory neuron.
Circular in configuration.
Receptive field of individual
photoreceptor
In dark –Photoreceptors Depolarized – NT
released.
In light – Photoreceptors Hyperpolarized – Less
NT is released
18. Receptive field of individual
Photoreceptor
Horizontal cells – very large receptive field.
Function – enhance visual contrast by lateral
inhibition. (Processing of spatial information)
Bipolar cells – 2 types (one stimulated & one
inhibited by photoreceptors)
Provide 2nd
mechanism for lateral inhibition.
Amacrine cells -- Processing of spatial
information
Ganglion cells – centre surround antagonism.
Wednesday, June 8, 2016
19. Ganglion cells – centre
surround antagonism.
Wednesday, June 8, 2016
20. Processing of Image
Concept of parallel processing pathway
Concept of serial processing of image in retina
Synaptic mediators in the retina
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23. Concept of Serial processing of image
in retina First image.
Action of light on photoreceptors.
Breakup image into small spots of light & darkness.
Second image.
By Bipolar cells.
Spatial summation by lateral inhibition by horizontal
cells
Third image.
By Ganglion cells
Temporal summation by lateral inhibition by amacrine
cells.
Wednesday, June 8, 2016