Retina structure and function

retina its detailed anatomy on its structure and physiological functions of different cells in retina structure with respect to vision.

1. RETINA STRUCTURE AND FUNCTION

2. EMBRYOLOGY

3. A SCHEMATIC FLOWCHART SHOWING THE ORIGIN OF THE VARIOUS LAYERS OF THE RETINA DURING EMBRYOGENESIS

4. ANATOMY Innermost tunic of eyeball Thin,delicate and transparent membrane Most highly developed tissue Extend:optic disc to ora serata. Surface area: 266mm 2 Thickness: posterior pole 0.56mm,equator 0.18 to 0.2mm,ora serata 0.1mm Colour:purplish red (visual purple of rod) Regions: 3distinct regions - Optic disc - Macula lutea - Peripheral retina

5. Optic disc Colour- pale pink,diameter-1.5mm The central depressed part of this is called cup. All retinal layers terminate at optic disc,except nerve fibre layer which pass through lamina cribrosa to run into optic nerve. Macula lutea/Macular area/Area centralis Yellow spot,dark area,diameter- 5.5mm Location:posterior pole of eyeball,temporal to optic disc 15 o of visual field Primary Functions - photopic vision and colour vision 3 main areas : fovea,parafovea,perifovea Fovea -1.50 mm diameter,1.55mm thickness,5 of visual field -foveola: 0.35mm diameter,highest visual acuity, 1 of visual field -umbo

6. Parafovea: 0.5mm width,surrounds foveal margin Perifovea: 1.5mm width,surrounds parafoveal region Peripheral retina Divided into 4 regions Near periphery Mid periphery Far periphery Ora serata

7. Microscopic Structure It consists of 10 layers Retinal pigment epithelium Layer of rods and cones External limiting membrane Outer nuclear layer Outer plexiform layer Inner n u c l ear layer Inner plexiform layer Ganglion cell l ayer Nerve fibre layer 10. Internal limiting membrane

8. 1.RETINAL PIGMENT EPITHELIUM (RPE) Outermost layer Single layer of hexagonal cells Functions -photoreceptor renewal and recycling of Vit-A -maintain integrity of sub retinal space,mechanical support -transport of nutrients and metabolites Glucose,lipids,amino acids,vitamins,minerals,adequate O 2 ,proteins etc are supplied to retinal tissues from the blood stream. Production of energy GLYCOLYSIS – Carbohydrate -> pyruvate and lactate(occurs in sufficient supply of O 2 unlike other tissues) KERB’S CYCLE –> carbonic acid and H 2 O Glucogen is stored in muller fibres(glial cells) of retina and serves as buffer against conc of glucose in tissues.

9. 2.LAYER OF RODS AND CONES ROD CELL CONE CELL Rhodopsin iodopsin scotopic(night) vision photopic(colour) vision 120 million 6.5 million absent at fovea Highest density at fovea 40-60 µm long 40-80 µm long OUTER SEGMENT Cylinderical,highly refractile, transversely striated Conical,shorter than rods INNER SEGMENT Thicker than outer segment Bulged and directly continuous with nucleus

10. Function of Rods (rhodopsin) VISUAL CYCLE

11. PHOTOTRANSDUCTION

12. 3.EXTERNAL LIMITING MEMBRANE Fenestrated membrane which extends from ora serata to edge of optic disc,through which process of rods and cones pass by. Formation: junctions b/w cell membrane of photoreceptors and muller’s cells. Not a basement membrane. 4.OUTER NUCLEAR L A Y E R Formed by nuclei of rods and cones. Rod nuclei form the bulk of this multilayered layer except cone dominated in foveal region. Cone nuclei larger and lie in a single layer. Number of rows of nuclei and thickness varies region to region Nasal to disc - 8 - 9 layers of nuclei,thickness-45 µm. Temporal to disc – 4 rows of nuclei,thickness-22µm. Foveal region – 10 rows of nuclei,thickness-50µm. Rest of retina except ora serata – 1 row of cone nuclei and 4 rows of rod nuclei,thickness-27µm.

13. 5.OUTER PLEXIFORM LAYER Contains synapses b/w rod sperules and cone pedicles with dendrites of bipolar cells and processes of horizontal cells. Thickest at macula Consists predominantly of oblique fibres deviated from the fovea-Henles’s layer 6. INNER NUCLEAR LAYER Disappears at fovea and in rest of the retina it consists of : 1. Bipolar cell’s Neurons of first order of vision Depolarizing cell’s respond to excitatory neurotransmitter ie,glutamate where hyperpolarizing cells do not. Some receives direct excitation from photoreceptors whereas others receive signal indirectly through the horizontal cell’s The depolarizing and hyperpolarizing cell’s provides a second mechanism for lateral inhibition(spatial information processing) Receptive field – circular in configuration,centre-surround antagonism.

14. 2.Horizontal cell’s Flat cells,neuronal interconnections between photoreceptors and bipolar cells in the outer plexiform layer. Transmit signals horizonta l ly in the outer plexiform layer from R&C ->B Main function- to enhance the visual contrast by causing lateral inhibitions Receptive field-very large in contrast to the photoreceptor cell

15. 3. Amacrine cell’s Connections with axons of the bipolar cell’s and dendrites and soma of the ganglion cells. Recieve information at the synapse of these cell’s and use these information for temporal processing at the other end of bipolar cell,negative feedback arrangement Electrically,it produce depolarizing potentials Functions: Some are direct pathway for rod vision (impulse from B->A->G) Some responds very strongly at the onset of a visual signal,but the response dies out rapidly and vice versa. Some responds to both when a light is turned on or off signalling. Some are direction sensitive and respond to movement of a spot across the retina in a specific direction.

16. 4. Muller cell’s Provide structural support 5. Capillaries of cent r al retinal vessels 7.INNER PLEXIFORM LAYER Consists of synapses b/w the axons of bipolar cells,dendrites of ganglion cells and processes of amacrine cells. Absent in foveola 8.GANGLION CELL LAYER The cell bodies and the nuclei of the ganglion cells lie in this layer. 2 nd order of neurons in the visual pathway 3 rd order neurons of visual pathway in the LGB Absent in foveola Electrical response of bipolar cell after modification by the amacrine cells is transmitted to the ganglion cells which in turn transmit their signals by means of action potential. Produce 2 types of propogated spikes : “on-centre” and “off-centre”

17. 3 types of ganglion cells W – GANGLION CELLS Small 40% of total ganglion cells Important for rod vision under dark conditions Responsible for directional movements X – GANGLION CELLS Medium 55% of total cells Visual image is mainly transmitted through these cells Responsible for colour vision Y – GANGLION CELLS Fewest and largest 5% of total cells Very broad dendritic field- able to pick up signals from widespread retinal areas Respond to rapid changes in visual image

18. 9. NERVE FIBRE LAYER Consists of the unmyelinated axons of the ganglion cells which converge at the optic head, pass through lamina cribrosa and become ensheathed by myelin posterior to lamina. ARRANGEMENT OF NERVE FIBRES IN RETINA

19. 10. INTERNAL LIMITING MEMBRANE True basement membrane Consists of 4 elements: Collagen fibrils Proteoglycans Basement membrane Plasma membrane of muller cells and glial cells of retina Neurotransmitters in Retina Glutamate – excitatory transmitter Amarcrine cells – produce 5 types of inhibitory transmitters gammaaminobutyric acid(GABA) Glycine Dopamine Acetylcholine Indolamine Transmitters of bipolar and horizontal cells Cholinenesterase – dominant synaptic neurotransmitter in human Carbonic anhydrase

20. BLOOD SUPPLY OF RETINA

21. BLOOD–RETINAL BARRIER

22. APPLIED ANATOMY

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