ByProf Dr .

Soheir helmy

OUTER layer (protective)Middle layer (nutritive) iris cilliary body

choroidInner layer (retina-

photosensetive)

It is the innermost layer of the eyeball.HistologicallyHistologically it is formed of 10 layers.PhysiologicalPhysiological-:-:

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 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

Connection

Function

Pigment Rhodopsin Iodopesin

Time of vision Dark light

It is composed of :-1-Outer segment2-Inner segment3- synaptic partThe 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 transducin3-CGMP phpspho diesterase enzyme

In the dark: receptor potential equals -40 mvDark currentUnstimulated (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 spot1- 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 headBlood 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.