ANATOMY OF RETINAPHYSIOLOGY OF VISION

VISUAL PATHWAYS

Dr. Sajjan SangaiDept. of Ophthalmology

MGM Medical College and Hospital

Embryology

• Structures that are derived from the optic vesicle

Sensory layer of retina (Pars optica retinae)

Epithelial linings of cilliary body/iris (Pars retinae cilliaris/ Pars retinae iridis)

Topography of Retina

• Diaphanous tissue: Purplish / red due to visual pigment (rhodopsin).

• Most highly developed tissue of the eye.• Extends from the optic disc to ora serrata.• Surface area: 266 mm2• Thickest near optic disc :0.56 mm• Thinner towards periphery: 0.18 mm - Equator 0.10 mm - Ora

Retinal Equator

• Retinal Equator – Line where 4 vortex veins exit , retina posterior to this is known as posterior retina.

Retina

Optic Disc

Macula lutea

Peripheral Retina

1. Optic Disc

• Colour :Pale pink• 1.5 mm diameter, well defined circular area• At the optic disc- All retinal layers terminate

except nerve fibres , which pass through the lamina cribrosa to run into optic nerve.

• Physiological Cup: Depression seen in it , central retinal vessels emerge through the centre of this cup.

2. Macula Lutea/ Area Centralis

• Yellow spot – due to presence of carotenoid pigment Xanthophyll.

• Dark area : 5.5 mm in diameter • Situated in posterior pole of the eyeball ,

temporal to optic disc.• Corresponds to 15 degrees of the visual field ,

accurate diurnal vision and colour discrimination are primary functions.

Fovea

• Approximate centre of area centralis• Posterior pole of globe• 3-4mm temporal to the centre of optic disc.• 0.8mm below horizontal meridian.• Diameter-1.85mm – 5 degrees of visual field • Avg. thickness- 0.25 mm

• Central concave indentation – Foveola is produced.

• Downward sloping border which meets the floor of foveal pit is known as CLIVUS.

• UMBO: Tiny depression in very centre of the foveola ( Visible foveolar reflex on Direct Ophthalmoscope).

Foveola

• Diameter: 0.35mm • Thickness:0.13mm • Highest visual acuity in retina corresponds to

only 1 degree of visual fields due toI. Sole presence of cone photoreceptors.II. Avascular nature.• Foveal Avascular Zone: Located inside fovea

but outside foveola (clinically can be rendered on FFA).

3. Peripheral Retina

• Increases the field of vision • Divided into 4 regionsI. Near periphery-1.5mm around area centralisII. Mid periphery- 3mm wide zone around near

peripheryIII. Far periphery- from optic disc- 9-10 mm on

temporal side and 16 mm on nasal side in the horizontal meridian(asymmetry due to optic disc on nasal side)

IV. Ora serrata

Ora serrata

• Serrated peripheral margins where the retina ends and cilliary body starts.

• 6mm nasally- 7mm temporally from limbus.• 6-8 mm from limbus• 25mm from optic nerve on nasal side

Ora serrata

Microscopic Architecture of Retina

• Contains 3 types of cells and their synapses arranged in following layers: (Sclerad to Vitread) are as follows:

Retinal Pigment Epithelium

Photoreceptor cell layer

External limiting membrane

Outer nuclear layer

Outer Plexiform layer

Inner nuclear layer

Inner Plexiform layer

Ganglion cell layer

Nerve fibre layer

Internal limiting membrane

Retinal Layers

Layer Degree of neurons

Pigment epithelium Neuroepithelial layer

Photoreceptor layer Neuron I (rod and cones)

Ext. limiting membrane (Percipient Elements)

Outer nuclear layer

Outer plexiform layer

Inner nuclear layer Neuron II Cerebral Layer

Inner plexiform layer Conductive and associative elements

( Bipolar cells, Horizontal cells, Amacrine cells , Centrifugal bipolars,

Ganglion cell layer Neuron III Muller fibres,

Nerve fibre layer conductive elements Astrocytes.)

Internal limiting membrane

I. Retinal Pigment Epithelium

• Fine mottling due to unequal distribution of pigmentation within individual cells, gives a granular appearance.

• Single layer- 5 million cells , firmly attached to its basement lamina, lamina vitrea of brusch’s membrane.

• Cobble stone appearance ( 4.2 million – 6.1 million)

• Area centralis- 12-18 µm width, 10-14 µm height.• Ora- 60 µm in width

Functions:• Photoreceptor

renewal and recycling of vitamin A.

• Maintain integrity of subretinal space.

• Transport of nutrients and metabolites.

• Mechanical support to the processes of photoreceptor.

II. Photoreceptor layer of Rods and Cones

• 77.9 -107.3 million ( avg.92 Million) Rods• 4.08 – 5.29 million ( avg. 4.6 million ) Cones• Arranged as mosaics with variation.• Cone Density and Distribution: Maximally at Fovea ( avg. 1,99,000 cones/mm2) with increasing eccentricity from fovea , density of cones decrease rapidly.

• Rod density and distribution:Avg. Horizontal diameter of rod free area at fovea is 0.35 mm 1.25 degrees of visual fields Present in large number (1,60,000 / mm2) in a ring shaped zone 5-6 mm from fovea.

Visual Pigments

Rods•Scotopic vision •Greatest sensitivity for blue – green light ( 493 nm)•Visual Purple/ Rhodopsin.

Cones

•Trichromatic pigments•Short-Blue- 440 nm•Medium- Green- 540nm•Long- Orange- 577 nm

• Structure of Photoreceptor (in relation to various Layers):

Part of photoreceptor Layer

Cell body and Nucleus Outer Nuclear Layer

Cell process Outer Plexiform layer

Inner and outer segments Layer of rods and cones

Rod Cells

• 40-60 µm • Cylindrical , highly

retractile , transversely striated and contains visual purple.

• Has rod spherules.

Cone Cells

• 40-80 µm, Largest at fovea and shortest at periphery.

• Contains iodopsin.• Has cone pedicles/

cone foot.

III. External Limiting Membrane

• From optic disc to ora serrata and becomes continuous with the basal lamina between the pigmented and non pigmented portions of cilliary epithelium.

• Functions: 1. Selective barrier for nutrients.2. Stabilization of position of the transducing

portion of photoreceptor.

IV. Outer Nuclear Layer

• Contains: soma and nuclei of photoreceptor cells • Width varies.• Nasal to disc: 45 µm – 8-9 rows of nuclei.• Temporal to disc- 22 µm – 4 rows of nuclei.• Fovea- 50 µm – 10 rows• Different nuclei can be differentiated because

nuclei stains with Mallory stain:

Rod nuclei forms the bulk of this multi-layered outer nuclear layer except in foveal region

Rods Nuclei Orange

Cones Nuclei Red

V. Outer Plexiform Layer

• Synapses between rod spherules and cone pedicles with dendrites of bipolar cells and processes of horizontal cells

• Marks the junction of end organs of vision and 2nd order neurons in retina.

• Thickest at macula 51 µm consists of predominantly of oblique fibres that have deviated from fovea also known as HENLE’S LAYER.

Outer 2/3rd Layer:Inner fibres ( Axons) of photoreceptors.

Inner 1/3rd Layer: Dendrites of bipolar and horizontal cells as well as Müller cells processes.

VI. Inner Nuclear Layer

• Very thin layer.• Disappears at fovea• Consists of : a) Bipolar cells b) Horizontal cellsc) Amacrine cellsd) Soma of Müller cellse) Capillaries of central retinal arteries

Outermost Layer- Horizontal Cell Nuclei

Outer Intermediate Layer-Bipolar Cell nuclei

Inner Intermediate Layer-Muller nuclei

Innermost Layer-Amacrine and inter plexiform cell nuclei

• Neuronal interconnections between photoreceptors and bipolar cells .

• Synapses with the processes of Amacrine cells and cell bodies of the diffuse ganglion cells.

• Contribute to vertical communication within the retinal layer.

• Carry out paracrine functions.

• As principal glial cells, act as a supportive framework and a nutritive function.

Horizontal cells Bipolar cells

Amacrine cellsMüller cells

• Horizontal Cell:Flat cells having numerous horizontal

associative and neuronal interconnections between photoreceptor and bipolar cells in outer plexiform layer.

Unlike bipolar cells information is relayed radially through the retina , horizontal form a network of fibres that integrate the activity of photoreceptor cell horizontally.

• Highest at fovea, number decreases towards periphery but processes branch extensively from centre to ora.

• Bipolar cells:2 Degree neuron in visual circuitry.35,676,000 bipolar cellsOriented radially- Perikarya- Inner nuclear layer- Processes- Outer/ inner plexiform layer 9 µm at fovea, 5 µm at periphery.

Types of Bipolar CellsRod Bipolar cells

Invaginating midget

Invaginating diffuse

Flat midget

Flat Diffuse

On centre blue cone bipolar cell

Off centre blue cone bipolar cell

Giant bi-stratified bipolar cells

Giant Diffuse Invaginating

Bipolar CellsType Connections Peculiarity

1. Rod Bipolar Cells20%, Large soma profuse dendrites

Arborize only with rod spherules

Axons of these bipolar cells have synapses with soma up to 4 ganglion cells

2. Midget Bipolar cellsSmall

Make connections only in triads of cone pedicle

Invaginating- Deeply invaginate cone pedicleFlat- Makes superficial contact with cone pedicleAxons synapses with SINGLE ganglion cell.

3. Diffuse- Makes contact with cone pedicles only

Not with their triads Axons synapse with number of ganglion cells of all types.

4. Blue cone bipolar cells Innervate more than one cone pedicle

5. Giant Bipolar cells Distinguished by extent of their dendritic spread

VII. Inner Plexiform Layer

• Synapses between:

Axons of Bipolar cells ( 2nd order

neurons)

Ganglion cells( 3rd

order neurons)

• Amacrine cells also mediate interactions within the layer and the interplexiform cells receive input from the Amacrine cells

• Also contains processes of Muller cells , abundant microvasculature , occasional displaced nucleus of a ganglion / Amacrine cells.

• Layer is absent at foveola• Thicker: 18-36 µm• More synapses per unit area >2 million /mm2.

VIII. Ganglion Cell Layer

• Mainly composed of cell bodies 3rd order cells.• Others: Processes of Müller cells, other

neuroglia, branches of retinal vessels are also present.

• Layer structure:

Layers Situation

Single layer Peripheral retina

2 layers Temporal side of optic disc

6-8 layers Edge of foveola

At foveola and optic nerve head, ganglion cell layer is absent.

1.2 million ganglion cells present in retina

Each produce a single axon

Converge and exit from the eye as OPTIC NERVE

• Can be classified according to size, degree of arborisation, spread of their dendrites, pattern of synaptic connections with Amacrine and bipolar cells.

• 2 major types:

M cells (PARASOL)

•Project to magnocellular layer of lateral geniculate body and exhibit non opponent responses

P cells•P1 – Midget cells-contribute 90% of total ganglion cell layer @ foveola•P2- Small Bi- Stratified

IX. Nerve Fibre LayerUnmyelinated axons of ganglion cells

Converge at optic nerve head

Pass through lamina cribrosa

Become ensheathed by myelin posterior to lamina cribrosa

• Contents:1. Axons of ganglion cells- Centripetal nerve fibre.2. Centrifugal nerve fibre( Thicker than

centripetal).3. Processes of Müller cells which interweave

with axons of ganglion cells .4. Neuroglia cells present in nerve fibre.5. Retinal vessels: Do not project on surface of

retina , rich bed of superficial capillary network.

• 0.6- 2 µm non myelinated fibres. • Contains microtubules, mitochondria, smooth

endoplasmic reticulum.

• Arrangement of nerve fibre in Retina:• 1. Nasal Half: Directly to optic disc as superior

and inferior radiating fibres (Srf & Irf).

• 2. Macular region: Pass straight in temporal part of disc as papillomacular bundle.(PMB)

• 3. Temporal retina: Arch above and below the macular/ papillomacular bundle as superior / inferior arcuate fibres. (Saf & Iaf).

• Arrangement in optic nerve:Fibres from peripheral part

Deep in retin

a

Occupy most

peripheral

part of optic discFibres

closer to optic

disc

Superficially in retina and

occupy more central

(deep) portion of

disc.

X. Internal Limiting Membrane

• Pas positive true basement membrane.• Forms interface between retina and vitreous.• Consists of 4 elements:1. Collagen fibrils2. Proteoglycans3. Basement membrane4. Plasma membrane 5. Plasma membrane of Müller cells and other glial

cells.

Retinal Layers Foveola Fovea Parafoveolar region

Perifoveolar region

RPE + +( Dark Area) + +

Photoreceptor layer

C++++ C+++ C++R+ C++R+

ELM + + + +

ONL + + + +

OPL - + Thickest +

INL - + Thickest +

IPL - + + +

Ganglion cell layer

- Multi-layered Thickest Multi-layered

Nerve fibre layer

- + + +

Internal Limiting membrane

+ (Thin) + + +

Blood Supply

• Outer four layers – Choriocapillaries• Inner six layers- Central retina Artery• Retina is supplied by Central Retinal ArteryEnters optic nerve on lower surface 15-20 mm behind the globe.Retinal arteries are end arteries and have no anastomosis at ora serrata.

Arteries are distinguished from veins by being brighter red and narrower.

Veins have purplish tint and are duller and of wide calibre.

Choroidal vessels: Broader, ribbon like. Without any central streak Anastomose freely Easily visible in myopes and albinos.

Physiology of Vision

Initiation of vision (Photo transduction)

Processing and transmission of visual sensation

Visual Perception

• Phototransduction:

Retina

Light falling upon the retina causes photochemical changes

Photochemical changes

1.Rhodopsin Bleaching2. Rhodopsin regeneration3. Visual cycle

Electrical changes

Generation of receptor potential

Rhodopsin Bleaching: Rhodopsin- the visual pigment present in rods

for scotopic vision. Maximum absorption spectrum- 500 nm

Rhodopsin

Protein- Opsin

Carotenoid- Retinine( Vit. A aldehyde / 11 Cis Retinal)

11 Cis Retinal All Trans Retinal

OpsinThis process is known as photodecomposition and rhodopsin is said to be bleached.

Rhodopsin

Bathorhodopsin

Lumirhodopsin

Metarhodopsin

All trans retinal

11 Cis retinalLight induced changes in Rhodopsin

Rhodopsin regenration:

All trans retinal +Vitamin A ( retinal)

from blood

11 Cis Retinal + Opsin in rod outer

segmentRhodopsin

Visual Cycle: Equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle.

All trans retinal

11 Cis retinal

Rhodopsin

Excitation of nerve

OpsinOpsin

Magnocellular, Parvocellular and Koniocellular pathways

P cells/ Parvocellular

Smaller, thinner axons of smaller calibre

Colour sensitive with High Spatial resolution

Visual Perception

Stimulation

Sensation1.Light sense2. Form Sense

3. Sense of contrast 4. Colour sense

• Light Sense:Light falling upon retina is gradually reduced in intensity , there comes a point when it is no longer perceived. This is known as Light minimum.• Measured when eye is dark adapted for 20-30

minutes.• Light minimum for fovea is considerably

higher than for the Para central and peripheral parts.

• Form sense:Cones play major role and most acute at fovea where it is most closely set and highly differentiated.

Visual acuity is measured in a variety of ways1. Recognition- Snellen’s Chart, Landolt C chart .2. Resolution- Acuity grating3. Localisation- Vernier grating.

• Sense of Contrast:• Ability to perceive slight change in

luminance between regions which are not separated by definite borders.

• Measurement of contrast sensitivity:

1. Pelli- Robson’s Contrast sensitivity chart.

2. Cambridge low-contrast gratings.3. Arden gratings.4. Functional acuity contrast test.

• Colour Sense: Ability of the eye to discriminate between colours excited by light of different wavelengths.

Cones

Short

Stimulated by blue light (440nm)

Medium

Stimulated by green light (540 nm)

Long

Stimulated by red light (577

nm)

White colour can be formed from combination of these colours in suitable proportions hence normal colour vision is trichromatic.

Theories of colour vision:a) Young- Helmholtz theoryb) Opponent colour theory of Herring

Visual Pathway

Neural epithelium

of rods and cones(

end organs)

Bipolar cells in inner nuclear

layer with its axons in

inner reticular

layer

Ganglion cells in retina

Optic nerve,

Chiasm to lateral

geniculate body

Optic radiations to Visual

cortex

• Fibres from peripheral regions in retina forms 2 distinct groups corresponding to nasal and temporal half of retina.

Fibres from temporal half

Optic Chiasma

Optic tract of same side

Fibres from nasal half

Optic Chiasma

Optic tract of opposite side