1. BY SREEJITH T

2. Introduction Retina is a multilayered sensory tissue that lines the back of the eye. It contains millions of light receptors that captures light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned to images. 3. Gross Anatomy Retina extends from the optic disc to the ora serrata. Ora serrata is the last region where the retina ends and ciliary body starts; it consists of tooth like projections. Retina is divided into two distinct regions: posterior pole and peripheral retina separated by retinal equator. 4. Posterior pole It refers to the area of the retina posterior to the retinal equator. The posterior pole of the retina includes 2 distinct regions: Optic disc Macula lutea 5. Optic Disc and Macula Lutea 6. Optic Disc It is pink coloured, well defined circular area of 1.5 mm diameter. Photoreceptors are absent here; hence known as blind spot. At the optic disc all the retinal layers terminate except the nerve fibers, which pass through the lamina cribrosa to run into the optic nerve. A depression seen on the disc is called physiological cup. The central retinal artery and vein emerge through the centre of this cup. 7. Macula lutea It is also called yellow spot Fovea centralis is the central depressed part of macula. It is about 1.5 mm in diameter. An area about 0.8 mm in diameter(including foveola and some surrounding area) does not contain any retinal capillaries and is called foveal avascular zone(FAZ) 8. Structure of Fovea Centralis In this area, there are no rods. Cones are tightly packed and it is the most sensitive part of retina. Its central part is called foveola. All other retinal layers are absent in this region. 9. Fovea Centralis 10. Fovea Centralis 11. Peripheral Retina It refers to the area bounded posteriorly by the retinal equator and anteriorly by the ora serrata. 12. Layers of Retina 1) Pigment Epithelium 2) Photoreceptor layer 3) External Limiting Membrane 4) Outer Nuclear layer 5) Outer Plexiform Layer 6) Inner Nuclear Layer 7) Inner Plexiform Layer 8) Ganglion Cell Layer 9) Nerve Fiber Layer 10) Internal Limiting Membrane 13. Mnemonics for Layers of Retina In New Generation It Is Only Ophthalmologists Examining Patients Retina RPE, 2 Outer, 2 Inner, GNI 14. 1) Pigment Epithelium It is the outermost layer of the retina. It consist of a single layer of cells containing the pigment melanin. Around the optic disc, they are heaped up as choroidal ring. It is firmly adherent to the underlying basal lamina (Bruchs membrane) of the choroid. 15. Choroidal Ring 16. 2) Photoreceptor Layer Rods and Cones are the end organs of vision and are also known as photoreceptors. Rods(120 million) contain a photosensitive substance rhodopsin(visual purple) and helps in peripheral vision and vision of low illumination (scotopic vision) Cones (6.5 million) also contain a photosensitive substance and helps in highly discriminatory central vision(photopic vision) and colour vision. 17. Rods and Cones cones rods 18. 3) External Limiting Membrane It is a fenestrated membrane, on which rods and cones rest and their processes pierce. 19. 4) Outer Nuclear Layer It consists of nuclei of the rods and cones. Cone nuclei are larger and more oval and carry a layer of cytoplasm. 20. 5) Outer Plexiform Layer The innermost portion of each rod and cone cell is swollen with lateral processes known as spherules and pedicles respectively. This layer consist of connections of rod spherules and cone pedicles with the dendrites of bipolar cells and horizontal cells. 21. 6) Inner Nuclear Layer It mainly consists of nuclei of bipolar cells. It also contains nuclei of Amacrine and Mullers cells. The bipolar cells constitute the first order neurons in visual pathway. 22. 7) Inner Plexiform Layer It essentially consists of connections of bipolar cells with the ganglion cells and amacrine cells. 23. 8) Ganglion Cell Layer It mainly contains cell bodies of ganglion cells(the second order neurons of visual pathway) There are 2 types of ganglion cells The midget ganglion-cells present in the macular region, each such cell synapse with single bipolar cell The polysynaptic ganglion-cells lie predominantly in the peripheral retina, each such cell may synapse with up to a hundred bipolar cells. 24. 9) Nerve Fiber Layer (Stratum Opticum) It consists of axons of ganglion cells, running parallel to the retinal surface. The layer increases in depth as it converges to optic disc. It passes through the lamina cribrosa to form the optic nerve. 25. 10) Internal Limiting Membrane It is the innermost layer and separates the retina from the vitreous. It is formed by the union of terminal fibers of the Mullers fibers. It is essentially a basement membrane. 26. Blood Supply Outer 4 layers of the retina- Choroidal vessels Inner 6 layers- Central retinal artery which is a branch of Ophthalmic artery. Fovea is avascular but partially gets blood supply from choroidal vessels Macula- Central retinal artery and cilioretinal artery. Central retinal artery emerges from the centre of the physiological cup of optic disc and divides into 4 branches These are end arteries i.e., they do not anastomose with each other. 27. Photochemistry of Vision 28. What Happens in Retina The light rays are focused directly onto the retina, the light sensitive tissue lining the back of the eye. Light energy is converted into neural signal Through visual pathway, these signals reach brain. 29. Physiology of vision The main mechanisms are [ ]Initiation of vision (Phototransduction) [ ]Processing and transmission of visual sensations [ ]Visual perception 30. Phototransduction (Initiation of Vision) The whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction. Light falling upon retina cause photochemical changes( ) which trigger a cascade of biochemical reactions that result in generation of electrical changes( ). 31. ( )Photochemical Changes RHODOPSIN BLEACHING Rhodopsin refers to the visual pigment present in the rods-the receptors for night(scotopic) vision. Its maximum absorption spectrum is around 500 nm. Rhodopsin consists of a colourless protein called opsin coupled with a carotenoid called retinine(Vit A or 11-cis- retinal) 32. Light falling on the rods converts 11-cis-retinal into all-trans-retinal through various stages. The all-trans-retinal so formed is soon separated from the opsin. This process of separation is called photodecomposition. Rhodopsin is said to be bleached by the action of light. 33. Visual Cycle 34. RHODOPSIN REGENERATION The 11-cis-retinal is regenerated from the all-trans- retinal and Vit A supplied from blood. The 11-cis-retinal then reunites with opsin in the rod outer segment to form rhodopsin. This whole process is called rhodopsin regeneration. The bleaching occurs under the influence of light, whereas the regeneration process is independent of light. 35. VISUAL CYCLE In the retina of living animals , under constant lght stimulation, a steady state must exist under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated. This equilibrium between the photo-decomposition and regeneration of visual pigments is referred to as visual cycle. 36. ( )Electrical Changes Activated rhodopsin cascade of biochemical reactions generation of receptor potential Thus light energy is converted to electrical energy 37. Dark Adaptation Ability of the eye to adapt to decreasing illumination. When one goes from bright sunshine into a dimly-lit room, one cannot perceive the objects in the room until some time has elapsed. This is called dark adaptation time. It is the time taken for regeneration of rhodopsin pigment which was bleached by the bright light.