INTRODUCTION Retina is multi-layered sensory tissue that lines the back of eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned to images.
HOW EYE WORKS• Light rays enter the eye through the clear cornea, pupil and lens.• These light rays are focused directly onto the retina, the light sensitive tissue lining the back of the eye.• The retina converts light rays into impulses; sent through the optic nerve to your brain, where they are recognized as images.
WHAT HAPPENS IN RETINA• Converts light energy into neural signal.• Contains first three cells of visual pathway3. photoreceptor-converts photon to neural signal through phototransduction and pass signal to bipolar cell.4. Bipolar cells – it synapse with ganglion cells.5. ganglion cells – it transmits signal from eye.• Other cells like horizontal cells, amacrine cells, and interplexiform neurons modify and integrate the signal before it leaves the eye.
DIMENSIONS OF RETINA 2.1mm temporally , 0.7-0.8mm nasally. The total surface area of retina is 266mm square.
OPTIC DISC pale pink in colour, circular in shape. photo receptors are absent. known as Blind spot.
MACULAknown as yellow spot.elliptical in shape.diameter is 5.5mm.responsible for central vision
FOVEA It is the most sensitive part of the Retina. Its diameter is 1.85mm . It has 5 degree of visual field. thickness is 0.25mm.
ORA SERRATA It is the last region where the retina ends and ciliary body starts. consist of tooth like projection . Retina is attached both to the vitreous &retinal pigmented epithelium.
LAYERS OF RETINA RETINAL PIGMENTED EPITHELIUM• Outermost layer• Consist of Single layer of hexagonal• It is Loosely attached to the layers of rods &cones• The potential space between RPE &the sensory retina is called subretinal space filled with subretinal fluia
LAYERS OF RETINA LAYERS OF ROD AND CONESROD contains rhodopsin as a photosensitive substances. helps in vision of low illumination. 120 million in number,absent in foveaCONES helps in phototopic vision 6.5 million in number,highest in fovea
LAYERS OF RETINA EXTERNAL LIMITING MEMBRANE• layer that separate the inner segment portions of the photoreceptors from their cell nucleus. OUTER NUCLEAR LAYER• this layer contains the rod and cell bodies.• the cone cell body and nucleus are larger.
LAYERS OF RETINA OUTER PLEXIFORM LAYER• projections of rod and cones ending in the rod spherule and cone pedicle respectively.• these make synapses with dendrites of bipolar in the macular region also known as fiber layer of henle.
LAYERS OF RETINA INNER NUCLEAR LAYER• consist of the cell bodies of horizontal cells,bipolar cells,amacrine cells,interplexiform neurons. INNER PLEXIFORM LAYER• Cointains the synapse between the bipolar cell axon and the dendrites of the ganglion and amacrine cells.
LAYERS OF RETINA GANGLION CELL LAYER• contains nuclei of ganglion cells, the axons of which become the optic nerve fibres for messages. NERVE FIBRE LAYER• Consist of ganglion cell axon. INNER LIMITING MEMBRANE• Innermost boundary• Composed of muller cells.
BLOOD SUPPLY OF RETINA Four layers RPE,layers of rod and cones,external limiting membrane,outer nuclear layer gets blood supply from choriocapillaries. The rest six layers gets from central retinal artery. Fovea is avascular but partially gets blood supply from choriocapillaries. Macular area gets from central retinal artery and cilioretinal artery.
PHOTOCHEMISTRY OF VISION VITAMIN A AND VISUAL PIGMENTS• dietary sources of retinol for human is animal and plant food.• animal food contains retinol but plant do not.• carotenes in plant food is converted to retinol by metabolic activity of small intestine.
PHOTOCHEMISTRY OF VISIONABSORPTION AND STORAGE In intestine, Vit A is esterified and reaches the blood stream. Retinol from the blood stream is transported to the liver. Retinol becomes bound with retinol binding protein. The retinol protein complex enters the circulation and reaches the target tissues In retina it becomes attached to the basal surfaces of the RPE cells.
PHOTOCHEMISTRY OF VISION VISUAL PIGMENTS• RHODOPSIN photosensitive pigment,present in outer segment of disc of rod, contain protein called opsin,insoluble in water,sensitive to heat and chemical agents.• PHOTOPSIN different from rod,responsible for central and colour vision.
PHYSIOLOGY OF VISION INITIATION OF VISION PROCESSING AND TRANSMISSION OF VISUAL SENSTAION VISUAL PERCEPTION
INITIATION OF VISION Also known as phototransduction. The whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction. Photochemical changes take place .
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
RHODOPSIN BLEACHING Light falling on the rods converts 11-cis- retinal componentof rhodopsin 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.
RHODOPSIN REGENERATION The 11-cis-retinal is regenerated from the all- trans-retinal separated from the opsin and vitamin-A supplied from the blood. The 11-cis-retinal then reunits with opsin in the rod outer segment to form the rhodopsin. This whole process is called rhodopsin regeneration The bleaching occurs under the influence of light,whereas the regeneration process is independent of light.
VISUAL CYCLE In the retina of living animals, under constant light stimulation, a steady state must exist under which the rate at which are bleached is equal to the rate at which they are regenerated. This equilibrium between the photodecomposition and regeneration of visual pigmentsis referred to VISUAL CYCLE.