Anatomy of Retina and Visual pathway.pptx

Anatomy of retina

1. ANATOMY OF RETINA AND VISUAL PATHWAY

2. CONTENTS Gross anatomy of retina Microanatomy of retina Visual Pathway Arrangement of nerve fibres in the Visual pathway Blood supply Lesions of Visual Pathway

3. Gross anatomy Extent: From optic disc to ora serrata. Surface area: about 266 mm² Thickness: At the posterior pole in peripapillary region is approx. 0.56mm; at the equator 0.18 to 0.2 mm; and at ora serrata approx. 0.1 mm. Colour: Purplish-Red due to visual purple of rods.

4. REGIONS Optic disc Macula Lutea Peripheral retina

5. OPTIC DISC Pale-pink, Well defined circular area of About 1.5mm diameter Physiological cup of the optic disc is a depression seen in it. The central retinal vessels emerge from the centre of this cup.

6. MACULA LUTEA 5.5 mm in diameter lies temporal to optic disc. Also called area centralis Corresponds to approx. 15 ͦ of visual field. Primary functions are photopic vision and colour vision. Oxygenated carotenoids , in particular lutein and zeaxanthine , accumulate within the central macula and cause yellow colour. 3 main areas: Fovea, Parafovea , Perifovea .

7. FOVEA CENTRALIS Central depressed part of macula 1.5mm in diameter, 0.15±0.02 mm in thickness Corresponds to 5 ͦ of visual field. Most sensitive part of Retina.

8. FOVEOLA 0.35 mm in diameter, 0.15mm thick Central floor of the fovea 2 Disc Diameter(3mm) away from temporal edge of optic disc. Umbo is tiny depression in the centre of foveola . Greatest concentration of cones is found in umbo, thus, referred to as Central bouquet of cones. FAZ: Foveal Avascular zone: central avascular region is known as the foveal avascular zone (FAZ).

9. PARAFOVEA 0.5 mm wide belt that surrounds the foveal margin. Ganglion cell layer, inner nuclear layer and henle’s layer are thickest (i.e. the retina is the thickest) PERIFOVEA 1.5mm wide belt surrounding parafoveal region

10. PERIPHERAL RETINA Near Periphery : 1.5mm wide area around macula Mid Periphery : 3mm wide zone around the near periphery. Its outer limit corresponds to the equator. Far Periphery : Extends from equator to ora serrata. Width is 6mm. The peripheral retinal pathologies are measured in clock hours. 1 clock hour corresponds to 5-6mm. So the peripheral retinal belt can be divided into 12 squares of 6X6mm. Extreme periphery : Area of pars plana and ora serrata .

11. Microscopic structure of the retina 1. Retinal pigment epithelium 2. Layer of rods and cones 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 fibre layer 10. Internal limiting membrane

12. Retinal pigment epithelium (RPE) Outermost layer of retina Made up of single layer of hexagonal cells containing pigments Firmly adherent to the underlying Bruch's membrane & loosely attached to layer of rods and cones Sub-retinal space: Potential space between RPE & sensory retina. Contains Sub-retinal fluid. Melanin and Lipofuscin are major RPE pigments. RPE aligned alongside CC, Choriocapillaris; BM, Bruch’s membrane; RPE. retinal pigment epithelium; ap, apical processes; os, outer segments; C, cones. R, rods.

13. Functions of RPE Renewal of photoreceptor & recycling of Vit. A i.e. visual pigment Provides mechanical support to processes of photoreceptors Manufacturing of pigments which absorb light Phagocytosis and digestion of photoreceptors Maintains Subretinal space by forming outer Blood-retinal barrier and pumping ions and water out of this space. Transport of nutrients across blood retinal barrier. Regenerative and reparative function after surgery and injury.

14. Retinal detachment The neural retina is detached from the retinal pigment epithelium. The loss of vision results because the neural retina is dependent on the retinal pigment epithelium for 11-cis retinal, nutrients and photoreceptor integrity.

15. Layer of Rods & Cones Rods & cones transform light energy into visual (nerve) impulses Rods contain photosensitive substance rhodopsin whereas cones contain photosensitive substance iodopsin Cone cells – Central vision and photoptic vision Rod cells – Peripheral vision and scotopic vision

16. Structure of Rod cell Length :- 40-60 um Outer segment is cylindrical composed of numerous lipid protein lamellar discs which contain visual purple 600- 1000 discs/rod Inner segment :- consist of ellipsoid & myoid region. Ellipsoid is rich in mitochondria and myoid is rich in golgi bodies and other cell organelles. Outer rod fibre arises from inner segment of rod and further swells into densely stained nucleus . The nucleus terminates further into inner rod fibre The inner rod fibre ends as a bulb called rod spherule.

17. Structure of Cone cells Length :- 40-80um At periphery :- 40um ( shortest) At fovea :- 80um (longest) Outer segment is conical, shorter than rod and contains iodopsin pigment packed in lamellar discs 1000-1200 discs/cone Inner segment is similar to rods Inner segment is directly continuous with nucleus A stout cone inner fibre runs from the nucleus & has lateral processes at the end called cone foot or cone pedicle

18. Distribution of cones Highest at fovea 1-3lakh/mm² at fovea Rapidly decrease from fovea 6000/mm² at 3mm away from fovea Rods and cone density in retina Distribution of rods Lowest at fovea 0.35mm rod free zone Maximum below the Optic Nerve- 1,70,000/mm² Number reduced towards periphery

19. External limiting membrane extending from the ora serrata upto the edge of optic disc Formed by the junction between the cell membrane of photoreceptors & Muller’s cell

20. Outer nuclear layer Made up of the nuclei of rods & cones OUTER PLEXIFORM LAYER Made of synapses between the rod spherules & cone pedicles with the dendrites of bipolar cells and processes of horizontal cells.

21. Inner nuclear layer It consists of: Bipolar cells Horizontal cells Amacrine cells Muller’s cells Capillaries of the Central retinal vessels

22. BIPOLAR CELLS Neurons of first order of vision. Their dendrites arborize with the rod spherules and cone pedicles in outer plexiform layer. Under light microscopy nine types a. Rod bipolar cells– arborise only with rod spherules b. Invaginating midget bipolar c. Flat midget bipolar d. Invaginating diffuse bipolar e. Flat diffuse bipolar f. On-centre blue cone bipolar g. Off-centre blue cone bipolar h. Giant bistratified bipolar i . Giant diffuse invaginating bipolar Make connections only with the triads of cone pedicle Make connections with cone pedicle only but not with their triad. Innervate more than 1 cone pedicle

23. AMACRINE CELLS Flat cells having numerous horizontal associative and neuronal interconnections between photo receptors and bipolar cells in the outer plexiform layer. Type A: Have contact with cone cells only Type B: Have contact with rod cells only Lateral inhibition of surrounding cells Contrast enhancement Spatial information processing HORIZONTAL NEURONS Form connections with the axons of Bipolar cells and the dendrites of ganglion cells in the Inner Plexiform Layer. Temporal processing Negative feedback arrangement Initial analysis of visual signals

24. MULLER’S CELLS Provide structural support and contribute to metabolism of sensory retina. Take part in formation of external and internal limiting membrane. Form horizontal extending reticulum in outer and inner plexiform layer. Act as ion reservoir express voltage-gated ion channels, neurotransmitter receptors and various uptake carrier systems. By these properties Muller cells regulate the extracellular concentration of neuroactive substances such as K+, GABA and glutamate. provide trophic and anti-oxidative support of photoreceptors and neurons regulate the tightness of the blood-retinal barrier.

25. Inner Plexiform Layer Synapses between Axons of Bipolar cells (1 st order neurons), dendrites of Ganglion cells (2 nd order neurons) and the processes of Amacrine cells. Also contains processes of Muller cells which form horizontal extending reticulum.

26. GANGLION CELL LAYER Cell bodies and nuclei of ganglion cells lie in this layer. Composed of single row of cells except in macula where it is multi layer and on temporal side of disc it has two layers. It is absent at foveola . Classification of ganglion cells: a. w, x, y ganglion cells b. P & M ganglion cells c. Off centre & on centre d. Mono & Polysynaptic

27. W-Ganglion cells <10um 40% Receive excitation from Rods Rod vision under dark Directional movements X- Ganglion cells 10-15um 55% Input from cones Colour vision Visual image mainly transmitted through these cells Y-Ganglion cells Upto 35um (Largest cells) 5% (fewest) Rapid changes in visual image- movement or light intensity

28. NERVE FIBRE LAYER Consists of unmyelinated axons of the ganglion cells which converge at the optic nerve head, pass through the lamina cribrosa and become ensheathed by myelin posterior to the lamina cribrosa. Neuroglial cells are also present. They can be macroglia , which have a structural role, or microglia , which play a role during tissue injury and phagocytose the debris. Retinal vessels lie in this layer. A rich bed of superficial capillary network is present in this layer.

29. INTERNAL LIMITING MEMBRANE Consists of PAS positive true basement membrane that forms the interface between retina and vitreous. It consists of Collagen fibrils, Proteoglycans, Basement Membrane, Plasma Membrane of the Muller cells and possibly other glial cells of the retina.

30. Retinal layers, imaged with OCT

31. Blood–Retinal Barrier The BRB consists of inner and outer components (inner BRB [ iBRB ] and outer BRB [ oBRB ]) It regulates fluids and molecular movement between the ocular vascular beds and retinal tissues and prevents leakage into the retina of macromolecules and other potentially harmful agents. Inner BRB Tight junctions (zonulae occludentes ) between neighboring retinal capillary endothelial cells. Continuous endothelial cell layer, which forms the main structure of the iBRB , rests on a basal lamina that is covered by the processes of astrocytes & Müller cells & Pericytes Outer BRB Tight junctions (zonulae occludentes) between neighbouring retinal pigment epithelial (RPE) cells. Separates the neural retina from the fenestrated choriocapillaris Regulating access of nutrients from the blood to the photoreceptors, as well as eliminating waste products and maintaining retinal adhesion.

32. VISUAL PATHWAY Retina optic nerve optic chiasma lateral geniculate bodies optic tracts optic radiations the visual cortical areas

33. Optic nerve The optic nerve is about 50 mm long and extends from the eye to the optic chiasma . It is often described as consisting of four portions: Intraocular portion (the optic disc, 1 mm in anterior–posterior length). Intraorbital portion (about 25 mm long). Intracanalicular portion within the optic canal (about 9 mm long). Intracranial portion (about 16 mm long).

34. INTRAOCULAR PART Four anatomical zones occur within the 1-mm long intraocular optic nerve (optic disc): Surface Nerve fibre layer Prelaminar zone Laminar zone Retrolaminar zone. First 3 layers are 1.5 mm in diameter, but the retrolaminar zone is 3mm in diameter due to myelin sheath.

35. INTRAORBITAL PART 25 mm, Extends back from eyeball to optic foramina Covered by dura , arachnoid and pia . The subarachnoid space is continuous with the intracranial subarachnoid space and carries cerebrospinal fluid. Central retinal artery enters nerve on its inferomedial aspect about 10mm behind eyeball Some fibers of superior rectus and medial rectus are adherant to its sheath so painful ocular movements in retrobulbar neuritis.

36. INTRACANALICULAR PART 6-9mm length closely related to ophthalmic artery INTRACRANIAL PART 16mm length Lies above cavernous sinus and converges with its fellow to form chiasma covered by pia only

37. Myelination Anterior to the Lamina Cribrosa Myelination of the anterior part of the optic pathway begins during fetal life, starting at the lateral geniculate body and progressing forward through the optic tracts, the optic chiasma , and finally the optic nerves. Myelination reaches the posterior aspect of the lamina cribrosa shortly after birth. Myelination does not occur anterior to the lamina cribrosa . Occasionally, oligodendrocytes extend forward into the retina during development and bring about abnormal myelination in the axons of the ganglion cells around the optic disc. Because myelin is opaque to light, this area of the retina is nonfunctional and the individual has an enlarged blind spot.

38. OPTIC CHIASMA Flattened structure Measuring 12mm Horizontally, 8mm Anterio -posteriorly Situated about 10 mm above the pituitary gland, which rests in the sella turcica of the sphenoid bone. Continues posteriorly as optic tracts Forms anterior wall of third ventricle Nerve fibers arising from nasal halves of the two retinae decussate at the chiasma

39. Variation in location of Chiasma Central 80% of normal cases Lies directly above sella Expanding pituitary tumours involve chiasma first Prefixed 10% of normal cases Located anteriorly over tuberculum sellae Pituitary tumours involve the Optic tracts first. Postfixed 10% of normal cases Located posteriorly over dorsum sellae Pituitary tumours involve the Optic nerve first.

40. OPTIC TRACTS Bundles of nerve fibers running outwards and backwards from postero -lateral aspect of optic chiasma Consist of temporal fibers of retina of same eye and nasal half of opposite eye Each optic tract ends in LGB

41. Lateral Geniculate Body Oval structures situated at termination of optic tract Each LGB consists of six layers of neurons alternating with white matter (formed by optic fibres ) Each body is split into 6 laminae Fibers from contralateral nasal Retina End in Lamina 1,4,6. Fibers from Ipsilateral temporal retina end in Lamina 2,3,5. This 6 laminae divide LGB into 2 portions Functions Relay station to relay visual information from optic tract to visual cortex To gate transmission of signals to visual cortex Parvocellular Layer 3-6 Small cells Input from X- ganglion cells Colour vision, texture, shape, depth Magnocellular Layer 1-2 Large cells Input from Y- ganglion cells Movement and flicker

42. OPTIC RADIATIONS Extend from LGB to visual cortex Fibers of optic radiation spread out fanwise to form medullary optic lamina Superior fibers of radiation ( which subserve inferior field ) proceed directly posteriorly through parietal lobe to the visual cortex. Inferior fibers of radiation ( which subserve upper visual field ) first sweep anteriorly in meyers loop around anterior tip of temporal horn of lateral ventricle and then into temporal lobe.

43. VISUAL CORTEX Located on medial aspect of occipital lobe Subdivided into Primary Visual Cortex or Visuosensory area (striate area 17) Secondary Visual Cortex or Visuopsychic area ( peristriate area 18 and parastriate area 19)

44. ARRANGEMENT OF NERVE FIBRES IN THE RETINA From nasal half of the retina come directly to the optic disc as superior and inferior radiating fibres ( srf and irf ) From macular region pass straight in the temporal part of the disc as papillomacular bundle ( pmb ) From temporal retina arch above and below the macular and papillomacular bundle as superior and inferior arcuate fibres ( saf and iaf ) with a horizontal raphe in between.

45. ARRANGEMENT OF NERVE FIBRES OF THE OPTIC NERVE HEAD Most lateral quadrant (thinnest)  Upper temporal and lower temporal quadrant  Most medial quadrant  Upper nasal and lower nasal quadrant (thickest) THICKNESS OF NERVE FIBRE LAYER AT THE DISC Fibres from the peripheral part of the retina lie deep in the retina but occupy the most peripheral (superficial) part of the optic disc. Fibres originating closer to the optic nerve head lie superficially in the retina and occupy a more central (deep) portion of the disc.

46. CLINICAL SIGNIFICANCE OF DISTRIBUTION AND THICKNESS OF NERVE FIBRES AT THE OPTIC DISC MARGIN Papilloedema appears first of all in the thickest quadrant (upper nasal and lower nasal) and last of all in the thinnest quadrant (most lateral). Arcuate nerve fibres which occupy the superior temporal and inferior temporal quadrants of optic nerve head are most sensitive to glaucomatous damage , accounting for an early loss in corresponding regions of visual field. Macular fibres occupying the lateral quadrant are most resistant to glaucomatous damage and explain the retention of the central vision till end.

47. Optic nerve Optic nerve head arrangement of fiber exactly same as retina Distal region of optic nerve Proximal region of optic nerve (near the chiasma ) macular fibers present centraly Temporal fibers present temporaly and nasal fibers present nasally

48. Optic Chiasma Temporal fibers remain uncrossed and runs backward in lateral part of optic chiasma Nasal peripheral fibers- ¾ of fibers Cross over to enter medial part of opposite optic tract in following manner lower nasal fibers in optic tract traverse chiasma low and anteriorly Upper nasal fibers in optic tract trasverse chiasma high and posteriorly Macular fibers- Some fibers cross and run backwards in opposite optic tract Some fibers remain uncrossed and runs on same side in optic tract

49. Optic Tract Macular fibers – occupy dorso -lateral aspect of the optic tract Upper peripheral fibers – situated medially in the optic tract Lower peripheral fibers – situated laterally in the optic tract LGB The macula fibers coming in the optic tract occupy the posterior 2/3 rd of the LGB. Upper retinal fibres – medial half of anterior 1/3 rd of the LGB Lower retinal fibres – lateral half of anterior 1/3 rd of the LGB

50. Optic Radiations Upper retinal fibers – upper part of optic radiations Lower retinal fibers – lower part of optic radiations Macular fibers – central part of optic radiations Visual Cortex Point to point projection of retina Right visual cortex receives impulses from temporal half of right retina and nasal half of left retina. Left visual cortex receives impulses from temporal half of left retina and nasal half of right retina.

51. BLOOD SUPPLY RETINA Outer 4 layers of retina is supplied by choriocapillaris . The inner six layers get supply from central retinal artery which is a branch of ophthalmic artery. The outer plexiform layer gets partly by both the above. The fovea is avascular and is mainly supplied by choriocapillaris . The macula is perfused by temporal branches of the central retinal artery. In 30% of eyes, a cilioretinal artery, branching from the ciliary circulation, supplies the Macular Region. It helps to retain central vision in the event of occlusion of the Central Retinal artery. The retinal arteries are end arteries & have no anastomoses. The only place where the retinal system anastomoses is in the neighbourhood of lamina cribrosa . The veins of the retina unite to form Central retinal vein at the disc, which follows the corresponding artery. Empties into sup and inf ophthalmic veins which drain into Cavernous sinus.

52. ARRANGEMENT OF RETINAL CAPILLARIES 2 retinal capillary networks- superficial and deep. Superficial capillary network : at the level of RNFL Deep Capillary network : Between inner nuclear and outer plexiform layer. Parafoveal region : Well developed capillary network. FAZ of 500µm diameter Peripapillary region : 4 layered capillary network due to thick nerve fibre layer.

53. OPTIC NERVE INTRAOCULAR PART Surface Nerve Fibre Layer : Retinal arteriole Prelaminar region: Vessels of Ciliary region Laminar region: Short Posterior Ciliary arteries and arterial circle of Zinn -Haller Retrolaminar region: Ciliary circulation from recurrent Pial vessels and Retinal circulation from Central retinal artery.

54. INTRAORBITAL PART Periaxial System by 6 branches of Internal Carotid artery: Ophthalmic artery Long Posterior Ciliary arteries Short Posterior Ciliary arteries Lacrimal artery Central artery of Retina Circle of Zinn Axial system Intraneural branches of Central Retinal artery Central collateral arteries from Central Retinal artery Central artery of Optic nerve

55. INTRACANALICULAR PART Only by Periaxial system of vessels The Pial Plexus is formed mainly by branches from Ophthalmic artery. INTRACRANIAL PART Periaxial system of vessels The Pial Plexus is formed mainly by 4 sources: Internal Carotid artery Anterior cerebral artery Small recurrent br. from Ophthalmic artery Anterior communicating artery

56. VENOUS DRAINAGE Optic nerve head by Central Retinal Vein Orbital part by Peripheral Pial Plexus and Central Retinal Vein Intracranial part by Pial plexus draining into Anterior cerebral and Basal vein .

57. CENTRAL RETINAL VEIN OCCLUSION Venous occlusive disease of the retina Within the retrolaminar portion of the optic nerve, the central retinal artery and vein are aligned parallel to each other in a common tissue sheath where they are naturally compressed as they cross through the rigid sieve-like openings in the lamina cribrosa . These vessels may be subject to compression from mechanical stretching of the lamina, as with increase in intraocular pressure , which may cause a posterior bowing of the lamina and subsequent impingement on the central retinal vein.

58. CENTRAL RETINAL ARTERY OCCLUSION Central Retinal Artery ( Zinn’s Artery) is the first branch of Ophthalmic Artery Originates from the medial surface of the ophthalmic artery either in the optic canal, or after this vessel passes through the optic foramen and has entered the Dural sheath of optic nerve Complete or partial central artery occlusions most commonly occur at the level of the lamina cribrosa just before the artery enters the retina. Here, the central artery has the structure of a medium -sized artery and is subject to atherosclerosis.

59. Retrobulbar Neuritis and Sinus Infection Only a thin plate of bone separates the optic nerve and its meningeal sheaths from the sphenoidal and posterior ethmoidal air sinuses as they pass through the optic canal. A spread of infection at this site could cause optic neuritis .

60. OPTIC CHIASMA Superior aspect of Chiasma by Anterior cerebral and Anterior Communicating arteries Inferior aspect by br. from Internal Carotid artery, Anterior superior hypophyseal artery and Posterior communicating artery. VENOUS DRAINAGE Superior aspect by Superior Chiasmal vein draining into Anterior cerebral vein Inferior aspect by pre- infundibular vein draining into Basal vein.

61. OPTIC TRACT Pial plexus formed by Posterior communicating artery , Anterior choroidal artery and Middle cerebral artery. VENOUS DRAINAGE Superior aspect by Anterior cerebral vein Inferior aspect by Basal vein

62. Lateral Geniculate Body Posterior medial aspect by Posterior cerebral artery Anterolateral aspect by Anterior choroidal artery Hilum containing macular fibres by both Posterior cerebral artery and Anterior choroidal artery. Venous Drainage by Basal vein

63. Occlusion of Anterior choroidal artery produces an upper- and lower-sector field defect.

64. Optic Radiations Anteriorly by Anterior choroidal artery Middle part by Deep optic artery which is a br. of Middle cerebral artery Posteriorly by Calcarine branches of posterior cerebral artery and Perforating branches from Middle cerebral artery Venous drainage Mainly by Basal vein and in some part by Middle cerebral vein

65. Visual Cortex Calcarine artery, br. of Posterior cerebral artery Terminal br. of Middle cerebral artery Venous Drainage Medial aspect by Internal Occipital vein draining into Great cerebral vein of Galen and Straight sinus Superolateral aspect by Inferior cerebral vein which ends in the Cavernous sinus.

66. LESIONS OF VISUAL PATHWAY

67. Lesions of optic nerve Blindness on affected side Loss of direct Pupillary Light Reflex on ipsilateral side and indirect on contralateral side. Causes: optic atrophy, traumatic avulsion of optic nerve, acute optic neuritis. Sagittal lesion of chiasma Bitemporal hemianopia Bitemporal hemianopic paralysis of PR. Tumours of pituitary gland, glioma of third ventricle, craniopharyngioma Lateral chiasmal lesions Binasal hemianopia Binasal hemianopic paralysis of PR Atheroma of carotids or posterior communicating arteries

68. Lesions of optic tract Homonymous hemianopia Contralateral hemianopic PR(Wernicke’s reaction) Syphilitic meningitis, TB, Aneurysm of posterior cerebral artery. Lesions of LGB Homonymous hemianopia Sparing of PR Lesions of total optic radiations Complete Homonymous hemianopia Lesions of Parietal lobe Inferior quadrantic hemianopia(pie in the floor) Lesions of Temporal lobe Superior quadrantic hemianopia(pie in the sky)

69. Lesions of visual cortex Occlusion of Posterior cerebral artery supplying anterior part of occipital cortex Congruous Homonymous hemianopia (sparing the macula) Lesions of tip of occipital cortex due to head injury or gunshot Congruous Homonymous macular defect