ANATOMY AND PHYSIOLOGY OF MACULA

Dr.Pragnya Rao

CONTENTS • ANATOMY OF MACULA Embryology

macula lutea

retinal pigment epithelium

neuroepithelium

Blood supply

Applied anatomy

PHYSIOLOGY OF MACULA

photochemistry of vision: rods and cones

Visual cycle

Photopic vision

Colour vision

light adaptation

ANATOMY OF MACULA Macula lutea is a 5.5mm circular area deeper red than rest of the

fundus at the posterior pole of retina, lying inside the temporal vascular arcades, 2 disc diameters temporal to the optic disc.

• Other names: area centralis/ yellow spot/central retina.

• Subserves central 15 – 20 degrees of visual field.

• Photopic and color vision are primary functions of this area.

• Dark appearance is due to

difference in pigmentation

absence of retinal vessels in fovea

EMBRYOLOGY:

• Macula develops from the optic cup

Inner wall: Neurosnsory retina

Outer wall: RPE

• Development of retina starts by 6 weeks, all adult layers become recognisable by 5 ½ months.

• Macular development is delayed till 8 months and specialization

continues till several months after birth.

Macula lutea can be divisible into

• Foveola• Fovea• Perifoveal region• Para foveal region

Fovea centralis Specially differentiated central 1.5mm of macula in width and 0.25

• It is the most sensitive part of macula and corresponds to 5 degree of visual field.

• Layers: RPE.photoreceptors,external limiting,outer nuclear and plexiform, ganglion cells, nerve fibre layer, internal limiting layer

Outer plexiform layer in macula is oblique (hence CME appears stellate in macula & Honeycomb appears outside macula)

Xanthophyll carotenoid pigments: Located in fovea, most probably in the outer plexiform layer.

- Leutin

- Zeaxanthin

responsible for the characteristic dark appearance of macula

in normal angiograms.

Foveola

• 0.35mm wide Depression or pit in the Central floor of fovea• Most sensitive part of retina

• Situation:about 2 disc diameters (3mm) away from the temporal edge of optic disc & 1mm below the horizontal meridian.

• Thinnest part of retina, devoid of ganglion cells. Has only cones and muller cells. Vision is most acute at this area.

Umbo: • it’s a tiny depression in the centre of foveola.• corresponds to the ophthalmoscopically visible

foveal reflex.

Foveal reflex: intensely bright spot of light visible ophthalmoscopically visible due to reflexion of light from the wall of foveal depression.

Loss of FR maybe an early sign of damage.

Foveal avascular zone (FAZ) located inside the fovea but outside the foveola.

Extending about 0.4 – 0.6 mm in diameter, can be

known by FFA.

it varies with age and disease states.

Fluorescent fundus angiography (FFA)

• PARAFOVEAL AREA: portion of macula 0.5 mm in width surrounding the fovea.

• PERIFOVEAL AREA: 1.5 mm wide ring zone surrounding the parafoveal area.

RPE

• Outermost layer of retina with single layer of hexagonal cells containing pigment.

• Shows fine mottling due to unequal pigmentation and responsible for granular appearance of fundus.

• Its adherent firmly to underlying Bruch,s membrane,• loosely attached to neuroepithelium. Space between the two is

called SUBRETNAL SPACE filled with Subretinal fluid.

• in foveal region, RPE cells are taller, thinner and • contain larger xanthophyll granules

dark appearance in fundus

FUNCTIONS:

• RPE cells are connected to each other by tight junctions:

- zona occludens and zona adherens outer blood retinal barrier• maintains integrity of Sub retinal space,by avtive pumping of ions

and water out of it.• Phagocytic action• Storage, metabolism and transport of vitaminA • Preservation of optimal retinal mileu(inward transport of metabolites

and outer transport of wastes)• Absorption of stray light• Mechanical support to photoreceptors.

Neuroepithelium• Layer of rods and cones.

Photoreceptors( rods and cones): are the end organs of vision which transform light energy into visual /nerve impulse.

• There are about 120 million rods and 6.5 million cones

RODS: help in scotopic (low illumination) vision and contain rhodopsin pigment (visual purple).

• Rods are absent in the fovea, (0.35mm area- rod free zone) which corresponds to 1.250 of the visual field.

• Abundant in an area 5-6 mm from the fovea (160,000/mm2)• Maximum below the optic disc with number reducing towards

periphery.• Nasal retina has 20-25% more than temporal retina.• Superior retina has 2% more than inferior retina.

ROD cell:

Outer segment:

• 40-50 mm long, cylindrical, highly refractile, transversely striated• Has numerous lipid protein lamellar dics stacked one on another,

surrounded by cell membrane, varying in number 600-1000 disc/rod• Disc thickness: 22.5-24.5 nm• Discs contain 90% of rhodopsin, remaining is scaterred in

plasmalemma.

.

Inner segment:• outer segment is attached to it by cilium.

Has 2 regions:

Ellipsoid(outer portion) has abundant mitochondria

Myoid(inner): contains glycogen and usual organelles• an outer rod fibre arises from inner end of rod.• Passes through External limiting membrane• Swells into Rod granule (lies in outer nuclear layer)• Terminates into an inner rod fibre (outer plexiform layer), with Rod

sperule at the end that is in contact with cone foot.

CONES:• Contain a photosensitiv substance- photopsin.• Primarily responsible for discriminatory central vision (photopic vision) and color

vision.• Highest density in Fovea- avg of 199,000/mm2 .• Their number is highly variable 100,000- 324000/mm2 . • Density falls rapidly outside fovea: 6000/mm2 3mm away from fovea 4000/mm2

10mm away• Density is nasal>temporal retina (40-45%), inferior> superior

Cone cell:

40 – 80mm long, largest at fovea

Outer segment:• conical, shorter than rods, contains Idopsin• Lamellar discs are narrower than rods.• 1000-1200discs/cone

Inner segment:

• Cilium is similar to rods• Ellipsoid is plump and has large number of mitochondria.• Innr segment of cone is directly continuous with its nucleus and lies

in outer nuclear layer.• Cone foot/pedicle at the end of inner fibre lies in outer plexiform

layer.

Interphotoreceptor matrix (IPM)• Space between photoreceptor outer membrane and RPE.• Consists of proteins, glycoproteins, GAGs and proteoglycans like

chondroitin sulphate.

FUNCTIONS:• Retinal attachment• Molecular trafficking• Facilitation of phagocytosis• Photoreceptor outersegment alignment.

Interphotoreceptor binding protein(IBRP)• Accounts for 70% of soluble proteins in IPM• Produced by photoreceptors( mainly cones)• Binds to all trans retinol, 14 cis retinal, retinoic acid,cholesterol.• FUNCTIONS:

- transport of retinoids bw photoreceptors and RPE

- minimise fluctuations in retinoid availability

- protect plasma membrane from toxic effects of high retinoid concentration.

• Cellular retinoid binding protein:

subgroup of fatty acid binding proteins

- orchestrate reisomerisation in RPE

- role in early development of retina

- protect retina from toxic effects of fatty acids

Blood supply

• Supplied by small twigs of Superior and inferior temporal branches of central retinal artery.

• Cilioretinal artery ( branch from ciliary system) occassionally is seen originating in a hook shapewithin temporal margin of disc.

when present – retention of central vision in CRAO.

Outer 4 layers: choriocapillaries

Inner 6 layers: Central retinal artery.

Applied anatomy• Foveola: most sensitive part of retina• Absent FR• FAZ: accurately seen on FFA, • Blue laser is undesirable in treating macular disease• Ciioretinal artery: Spared central vision in CRAO• CME: stellate appearance due to obliquely placed

PHYSIOLOGY OF MACULA

• Macula is the most sensitive part of retina.• Primary visual direction: occulocentric direction of an object fixated

along primary line of sight, and is imaged on the centre of fovea.

• Consists of mainly cones, hence has a major role to play in

- Photopic vision

- color vision

- shape sense

Photochemistry of vision• Visual pigments present in the photoreceptors have the property to

absorb light (visible portion of electromagnetic spectrum)• If a substace absorbs

– all wavelengths of light equally: grey/black

- all wavelengths except green : green appearance• Absorption maximum: peak of absorption curve of each pigment

• Pigments:

- Rhodopsin

- Photopsin

Rhodopsin (visual purple)

Photosensitive visual pigment membrane bound glycolipid present in discs of rod outer segments.

• It consists of : - protein opsin (scotopsin)

- carotenoid retinal (aldehyde of VitaminA)• Molecular weight: 40,000• Insoluble in water,sensitive to heat and chemical agents that

denature proteins.• It primarily absorbs yellow wavelength, transmitting violet and red to

appear purple – visual purple

Cone pigments• Visual pigment present in the outer segments of cones• They respond to specific wavelengths of light , hence help in color

vision.• They differ from rod pigment with respect to opsin component.• Peak absorbance wavelength

- blue sensitive cones: 435 nm

- green sensitive cones: 535nm

- red sensitive cones : 580nm

PHOTOPIC VISION

Neurophysiology1. changes in photoreceptors

2. Processing and transmission of visual impulse in retina

3. Processing and transmission of visual implse in visual pathway

4. Analysis of impulse in visual cortex

5. 3 part system hypothesis of visual perception.

-movement, location, spatial organisation

-color

- shapes

Photochemical changes:

• photopic vision is a main function of cones, functioning during brightly lightened conditions.

• The photochemical changes occuring in cones is very similar to that in rods.

• Differential bleaching quality of cones helps in photopic vision.

near total bleaching of rods before significant cone

bleaching

Photochemical changes:

Phototransduction

Conduction along visual pathway

• Axons of ganglion cell layer continue as optic nerve fibres carrying total input from retina

• Optic nerve fibres decussate partially at OPTIC CHIASMA

bicortical representation of nasal fibre and foveal visual field• Uncrossed &crossed fibres beyond optic chiasma OPTIC TRACT

• Optic tracts end in LGB (lateral geniculate body)

1) relay station: relay visual information from optic tract to LGB via geniculo-calcarine tract.

2) To gate the transmission of signals: recieves gating control signals from - corticofugall fibres from V1

- reticular area of mesencephalon• OPTIC RADIATIONS:

-central fibres: macular fibres

- dorsal fibres: upper retina

- ventral fibres: lower retinal quadrants

Visual cortex VISUAL CORTEX: • Primary visual cortex:

Brodmann,s cortical area 17/ visual area I/ V-1/ striate cortex

• Secondary visual cortex:

visual association areas that lie anterior, superior and inferior to V-1.

secondary signals are transmitted in these areas for further analysis of visual meaning.

- Brodmann,s visual area 18. area II/ V-2

- Brodmann,s visual area 19/ visual area III/ V-3

• Striate cortex:

-simple cells

- complex cells

- hypercomplex cells

• Simple cells:

-found in layer IV of area 17, form 1st relay station with visual cortex.

- detection of lines and borders and their orientation.

• Complex cells:

- found in cortical layers above and below layer IV of area 17, and area 18, 19

- detection of lines, bars, edges- especially when they are moving.

• Hypercomplex cells:

- found in cortical layers II and III of area 17,18,19

- properties of complex cells

- recognition of lines of specific lengths, angles

shapes

• Orientation columns:

vertical grouping of cells with identical orientation specificity.

-constant depth column and

-constant direction column(views points perpendicular to centre of contralateral eye help in 3 dimensional perception/ depth

• Occular dominance:

although most cortical neurons are binocularly activated, strong monocular dominance exists.

ocular dominance column: binocular complex and hypercomplex cells from layers II, III, V ,VI that receive strong signals from one of the 2 eyes, along with simple cells in layer VI with input from same eye.

helps in binocular steroscopic vision.

Color blobs:

special column like areas interspersed among primary visual

columns, respond specifically to color signals.

also found in secondary visual areas.

EXTRASTRIATE CORTEX:

- color processing area: area V4 in rhesus monkey

- Motion processing area- MT area (in middle portion of temporal lobe)

- stereoscopic depth perception area: V2, V#

PHSYCOPHYSIOLOGICAL aspect of vision:

visual cortex connects with tactile sensory,motor,auditory,olfactory and speech centres to build an overall picture.

Color vision• Humans can distinguish 2 monochromatic lights as close as 2mm

apart, giving effective color palette of upto 300 distinct spectral colors.

• It’s a PERCEPTUAL phenomenon and is Subjective.

it depends on

– spectral component of light from the object

- spectral component of light from visual surroundings

- state of light adaptation of eye.

TRICHROMATIC THEORY: (Young Helmholtz theory) • Trichromacy results from physiology of retina and not a property of

light.• Color vision in humans is TRICHROMATIC due o 3 types of cones.

-erythrothrolabe/ LWS - 565nm

- chlorolabe/ MWS -535nm

- cyanolabe/SWS - 440nm

OPPONENT COLOR THEORY:

Some colors appear to be mutually exclusive, maybe due to subsequent neural processing beyond photoreceptors

• PHOTOCHEMISTRY:

- the 3 different types of cones are bleached by lights of different wavelength.

- cone receptor potential has sharp onset and offset unlike rods. (sharp onset, slow offset)

• TRANSMISSION OF SIGNALS IN RETINA:

transmitted by electronic conduction to other cells in retina via photoreceptor synapses.

IN HORIZONTAL CELLS:

-luminosity response: hyperpolarisation response with broad spectral functions.

-chromatic response: hyperpolarisation to part

of spectrum & depolarisation for the remainder.

• IN GANGLION CELLS:

- all 3 types of cones stimulate same ganglion cell resultant signal is white.

- Opponent color cell: some ganglion cells are excited by one color type of cone and inhibited by othe color types.

- Double opponent color cell:

response is on for 1 color in the centre and off for it in the surrounding.and response maybe off for 2nd color in the centre and on in the surrounding.

* this suggest that process of color analysis begins in retina.

DISTRIBUTION:

- trichomatic color vision exists 20-300 from the point of fixation.

-peripheral to it red & green become indistinguishable.

- in far periphery color sense is lost.

-very centre of fovea is blue blind.

Phenomenon associated with color sense• Simultaneous color contrast:

perception of particular coloured spot against coloured background.

eg: grey spot appears green in red background, red in green background(complementary colors)

• Successive color contrast:

-phenomenon of colored after images.(opponent cell system)

-one sees a green spot for long and then looks at a grey card, he sees a red dot.

• Color constancy:

eye continues to percieve the color of the object unchanged even after spectral composition of light falling is altered.

observed by Edvrin land while develpoing

polaroid camera.

LIGHT ADAPTATION• Ability of the visual system to function normally over an exceedingly

wide range of illumination – visual adaptation• Light adaptation: process in which retina adapts itself to bright light

• MECHANISM:

Photochemical adaptation:

-large proportion of photochemical is reduced to retinals and opsins.

- retinal in both rods and cones is further converted to vitaminA

- rod sensitivity decreases, threshold increases

- cones continue to adapt and respond to bright light

Neural adaptation:

- horizontal cells give negative feedback

- pupillary constriction

SUMMARY

• Macula is the most sensitive part of retina,subserving central 15-200 of vision

• Develops from optic cup.• Primarily contributes to

-photopic vision

- color vision - trichomatic

-Shape sense,

- light adaptation.

• Vision: photochemical changes in photoreceptors conducted to cells in the retina and then to the optic tract and finally to striate cortex.

References

• Anatomy and physiology of eye- A.K Khurana• Adler,s physiology of eye

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