Photo transduction and image formation

1. Photo transductionand image formation By Prof Dr. Soheirhelmy

2. Layers of the eye ball • OUTER layer (protective) • Middle layer (nutritive) • iris cilliary body choroid • Inner layer (retina-photosensetive)

3. Physiological anatomy ofthe eye

4. The retina 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.

5. Retina and Photoreceptor /

6. The retina 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

7. photoreceptors • 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

8. Comparison between Rods & Cones

9. photoreceptors • 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)rodshave a long, cylindrical, outer segment with many discs • while cones have a short, tapering outer segment with relatively few discs.

10. photo receptor

11. Each disk contains:- • 1- photopigment (rhodopsin) • 2- G protien transducin • 3-CGMP phpspho diesterase enzyme

13. Photoreceptor potential • 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 sodiumions) 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.

14. Exposure to light

16. 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

17. Special areas of the retina • 1- optic disc ( blind spot) • 2-fovea centralis • 3- extra foveal area

18. 1- optic disc ( blind spot ) 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

19. Blind spot

21. 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. The fovea centralis

22. Extrafoveal part of the retina

23. Blood supply of the retina

24. The Organization of the Retina

25. Retina and optic nerve.

27. Image formation • 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.

28. Visual axis.

30. Visual pathway