Download phototransduction

By
Prof Dr.
Soheir helmy
 OUTER layer (protective)
 Middle layer (nutritive)

iris
cilliary body
choroid
 Inner layer (retina-photosensetive)
It is the innermost layer of the eyeball.
Histologically it is formed of 10 layers.
Physiological:layer of pigmented cells
layer of rods & cones {photoreceptors}
layer of bipolar cells.
layer of ganglion cells
.
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1)the retinal pigmented epithelium :Functions:1- contain pigments which absorb light and
prevent its reflection inside the eye.
2-store large amount of vit. A
3- phagocytosis of old rods and cones.
4-produce sticky extracellular matrix
 A photoreceptor cell is a specialized type of neuron



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found in the retina that is capable of photo
transduction.
they convert light (visible electromagnetic radiation)
into signals that can stimulate biological processes
.there are two types of photoreceptors:Rods
cones
Rods
cones
Number
120 million
8 million
Site
Peripheral part of the
retina
Central part of the retina
Pigment
Rhodopsin
Iodopesin
Time of vision
Dark
light
Connection
Function
 It is composed of : 1-Outer segment
 2-Inner segment
 3- synaptic part
The outer segment consists of a stack of discs embedded in the cell membrane.
The photoreceptor's light-sensitive pigments are located on these discs.
(rhodopsin)
rods have a long, cylindrical, outer segment with many discs
 while cones have a short, tapering outer segment with relatively few discs.
 Each disk contains: 1- photopigment (rhodopsin)
 2- G protien transducin
 3-CGMP phpspho diesterase enzyme
 In the dark: receptor potential equals -40 mv
 Dark current
 Unstimulated (in the dark), cyclic-nucleotide gated
channels in the outer segment are open because cyclic
GMP (cGMP) is bound to them.
 positively charged ions sodium ions) enter the
photoreceptor, depolarizing it to about −40 mV
(resting )
 in other nerve cells is usually −70 mV). This
depolarizing current is often known as dark current.
1-When light hits a photoreceptive pigment within the
photoreceptor cell
.2- The pigment, called iodopsin or rhodopsin, consists
of large proteins called opsin and retinal (a derivative of
vitamin A).
3-The retinal. activate a regulatory protein called
transducin which leads to the activation of cGMP
phosphodiesterase,
which breaks cGMP allows the ion channels to close,
preventing the influx of positive ions, hyperpolarizing
the cell, and stopping the release of neurotransmitters
 1- optic disc ( blind spot )
 2-fovea centralis
 3- extra foveal area
it is slight medial to posterior of the globe.
No rods or cons >>>not sensitive to light.
It is the optic nerve head
Blood vessels enter & leave the eye at this point.
Overlap of two visual field>>cannot notice it
It is the area of acute
vision.
It contains only cones.
All retinal layers are pulled
aside to allow light to fall
directly on the receptors.
 Light reflects on an object ,and if one is looking at the object-
it enters the eye.
 Light rays pass through the cornea, aqueous humor, lens, and
vitreous humor. All these structure reflect the light that it
falls on the retina. This is called focusing.
 Maximum focusing is done by the cornea and the lens.