ANATOMY OF RETINA
ANATOMY OF RETINA
- Retina is the innermost tunic of eye.
- Major component , clinically viewed as the fundus.
- Retina extends from the optic disc to ora serrata & has a surface area of 266 mm2.
- Retina is thickest at posterior pole in the peripapillary region (0.56 mm).
- Thinnest at ora serrata (0.1mm).
Retina is divided into two distinct regions:
A) POSTERIOR POLE
- It is the part of retina posterior to the retinal equator.
- Examined most commonly by direct ophthalmoscopy.
- When additional magnification or binocular vision is required, by slit lamp indirect biomicroscopy.
Posterior pole includes two distinct areas:-
1. Optic disc (optic nerve head)
- Vertically oval area.
- Average dimension of 1.76 mm horizontally & 1.88mm vertically.
- The optic disc is placed 3-4 mm nasal to the fovea (fovea is temporal to optic disc).
- There is a central depression in the optic disc called physiological cup.
- Optic nerve head (optic disc) in a normal human eye carries 1-1.2 million neurons from the eye towards the brain.
- Optic disc is also the entry point for the major BV’s (CRA & CRV) that supply the retina.
- CRA & CRV emerge thriugh the physiological cup.
2. Macula lutea (Yellow spot)
- Small yellowish area 5.5 mm in diameter.
- Situated at the posterior pole of the eyeball , temporal to the optic disc.
- Fovea centralis is the central depressed part of the macula & has a diameter of 1.5 mm.
- It is situated about 2 disc diameter (3 mm) temporal to temporal margin of optic disc.
- Most sensitive part of retina.
- Lowest threshold for light & highest visual acquity because it contains only cones.
Following parts are related to fovea centralis:
- Foveola:Forms the central floor of the fovea (0.35 mm in diameter)
- Umbo: Is a tiny depression in the very centre of foveola.
- Foveal avascular zone (FAZ): Located inside the fovea but outside the foveola.
B) PERIPHERAL RETINA
- It is the area bounded posteriorly by the retinal equator & anteriorly by the ora serrata.
- Peripheral retina is visualised by indirect ophthalmoscopy, or
- with more magnified view by use of Goldmann three mirror contact lens.
Nutrition & blood supply of Retina
- Retina receives nutrition from two discrete circulatory system:-
- Retinal vascular system: Through central retinal artery.
- Choroidal vascular system: Anterior ciliary artery & posterior ciliary artery.
- The junction between Retina & Ciliary body is Ora serrata.
- The retina is an out growth of the Diencephalon.
- Retina is part of CNS.
- Broadest neuroretinal rim is seen in Inferior pole.
- Macula lutea is the most sensitive part of retina.
Layers of the retina, from the outside inwards:
- Retinal Pigment Epithelium
- Layer of rods and cones
- External (Outer) limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Layer of optic nerve fibres
- Internal (Inner) limiting membrane
- Photosensitive layer of retina is Layers of rods and cones.
- Visual axis of the eye meets the retina at a point which coincides with the fovea centralis.
- The optical axis of the eye meets the retina at a point which is nasal to the fovea centralis.
- Peripheral retina is visualised by indirect ophthalmoscopy.
- The most common site of obstruction in central retinal artery is at the lamina cribrosa.
- Normal A: V ratio of retinal blood vessels is 2 : 3.
- Major retinal vessels are present in the nerve fibre layer.