RETINA

RETINAmonika pallan & afasana pathanANATOMY & PHYSIOLOGY

RetinaRetina is the innermost tunic of the eyeballThin,delicate,transparent membraneHighly developed tissue of the eyeAppears purplish red

Grosss anatomyThree distinct regions of retina:1.optic disc2.macula lutea3.peripheral retinaretina extends from optic disc to ora serrataSurface area:266sq/mm

Optic discPale pink in colour; well defined circular areaDiameter: 1.5mmAll the retina layers terminate here,except the nerve fibre which pass through the lamina cribrosa

Macula luteaThe macula lutea is comparatively dark area 5.5 mm in diameter , situated at the posterior pole of the eyeball , temporal to optic disc also called as yellow spot or area centralisPrimary function :- photoptic visionFovea centralis is central depression in macula;measurin 1.85 mmin diameter & 0.25 thicknessIt is most sensitive part of retina

FoveolaDiameter:- 0.35Central floor of foveaUmbo is the tiny depression in centre of foveola

maculaFovea centralisfoveolaumbo

Peripheral retinaThe peripheral retina can be divided into four regions Near periphery mid peripheryFar peripheryOra serrata

Microscopic structure of the retinaRetinal pigment epitheliumLayer of rods and conesExternal limiting membraneOuter nuclear layerOuter molecular (plexiform) layerInner nuclear layerInner molecular (plexiform) layerGanglion cell layerNerve fibre layerInternal limiting membrane

Retinal pigment epithelium (RPE)It is the outermost layer of retina Made up of single layer of hexagonal cellsThe cells contain pigmentsIt is firmly adherent to the underlying brush layer & loosely attached layer of rod and cone cellsSub-retinal space :- potential space betn RPE & retina

ContsRetinal detachment :- separation of RPE from retina Sub- retinal fluid :- fluid betn sensory retina & RPE. RPE cells are connected to each other by zonule occludents & zonule adherence

Functions of RPE

Renewal of photoreceptor & recycling of Vit AProvides mechanical support to process of photoreceptorsManufacturing of pigments absorbing lightRPE cells are phagocytic

Layer of Rods & Cone cellsRods & cones transform light energy into nerve impulsesRods contain photosensitive substance rhodopsin whereas cones contain photosensitive substance idopsinCone cells - photoptic vision rod cells - scotoptic visionCones :- 6.5 millionRods :- 120 million

ContsAt fovea:- max cones(199000cones/mm.sq) & rods are absentDensity:- 100000- 324000 cones/mm.sq 160000 rods/mm.sq

Structure of rod cell

Length :- 40-60 umOuter segment is cylindrical composed of numerous protein lamellar discDics :- 600- 1000/rodThickness of disc :- 22.5 -24.5 nmDisc contains 90% of visual purpleOuter segment is attached to inner segment via ciliumInner segment :- consist of ellipsoid & myoid region

Conts..Outer rod fibre arises from inner end of rod & passes through external limiting memebrane further swells into densely stained nucleusThe nucleus terminates further into inner rod fibreThe inner rod fibre ends as a bulb called rod spherule

Cone cellsLength :- 40-80umAt periphery :- 40um ( shortest)At fovea :- 80um (longest)Outer segment is conical :shorter than rodContains idopsin pigment packed in lamellar discDisc :- 1000-1200 disc/coneInner segment & cilium are similar to rodsInner segment is directly continous with nucleus A stout cone inner fibre runs from the nucleus & has lateral processes at the end called cone foot / cone pedicel

External limiting membraneFenestrated membrane extending from the ora serrata upto the edge of optic discProcesses of rods & cones pass through itThis layer is formed by the junction betn the cell membrane of photoreceptors & mullers cell

Outer nuclear layerMade up of the nuclei of rods & conesCone nuclei:- 6-7 um Rod nuclei :-5.5umThese nuclei lie in a single layer next to external limiting membraneRod nuclei form the bulk of this layer except at cone dominated foveal region

Variation in Thickness

Nasal to disc:- 45um ( 8-9 layers )Temporal to disc :- 22um (4 rows)Foveal region :- 50um (10 rows)Rest of retina except ora serrata 27 um (1 row of cones & 4 rows of rods)

Outer plexiform layerThis layer is made of synapses betn the rod spherules & cone pedicles with the dendrites of bipolar cells

Inner nuclear layerIt is very thin & is similar to outer nuclear layer It consists :-Bipolar cellsHorizontal cells Amacrine cellsMullers cellsCentral retinal vesselsOther glial cells This layer disappers at fovea

Inner plexiform layerConsists of synapse betn bipolar cell & dendrites of ganglion cellAbsent at fovea

Ganglion cell layerConsist of cell body & nuclei of ganglion cell ( 2nd order neurons)Absent at foveaVarious types of ganglion cell:-W,X,Y ganglion cellP(P1 &P2) & M GANGLION CELLOff centre & on centre cellMonosynaptic & polysynaptic cell

Nerve fibre layerConsists of unmyelinated axons of ganglion cells which converge at optic nerve headThickness:- 0.5-2 umThickness decreases from optic disc to lateral sideFibres show definite pattern of arrangement

Internal limiting membraneConsists of following elements:-1) collagen fibrils2) proteoglycans of vitreous3) basement membrane 4) plasma membrane of muller cells 5) other glial cells

Blood supply of retinaOuter 4 layers :- choriocapillaries Inner 6 layer:- central retinal arteryFovea :- choriocapillariesMacular region:- superior & inferior temporal branches of central retinal artery

PYSIOLOGY OF RETINA

PHYSIOLOGY OF VISIONIt is a complex phenomenon Main mechanism concerned with vision :-1)Transduction2) Transmission of visual sensation3) Visual perceptions

transductionRods & cones serve as a sensory nerve ending for visual senasation Stimulus inadequateadequate

Inadequate stimulus4 types of stimulus:-Pressure phosphaneMovement phosphaneElectrical phosphaneRadiation phosphane

Adequate stimulusVisible portion of EM spectrum forms the adequate stimulusVisible portion of light lies betn uv rays & infrared raysVisible portion :-400 nm -750nmThe visible consists of VIBGYOR ( 7 colours)

Transmission of visual sensetion

Photochemical changes take placeLight absorbed by rods and cones pigmentElectrical impluses are arise Information is forwarded thuru ganglion cells

Optic nerveOptic chiasmaOptic tractLGBOptic radiationVisual cortex

Visual perceptionsFunctional elements of vision1)light sense:- appreciation of light2)Form sense :- most acute at fovea3)Constrast sense :- differentiate bet luminance which are not separated by definite borders4) Colour sense :- discriminate betn colours

Photochemistry of visionLight falling on the retina is absorbed by rods & cones , further photochemical changs takes placeIt includes:-Vit AVisual pigmentsLight induced changes

Vitamin A(food) Dietary source of vit A

(Intestine) Digestion,absorption& transport

(Liver) 1.Storage of vit A 2.Production of retinol in liver cells as binding protein retinol

(Blood) Transport of retinol bound to retinol- binding protein

(eye) 1.formation of 2.maintainance of healthy rhodopsin corneal & conjunctival cells

Vitamin A sources:- carrort,fish oil,milk products, different fruits &vegetables37

Visual pigmentsThese pigments absorbs lightThe peak of each pigment s absorption is called its absorption maximum

Peak absorptionWavelength (nm)% of aborption

Rhodopsin pigmentAlso known as visual purplePresent in the outer segment of rods photopsin + retinol RhodopsinRhodopsin protein is insoluble in water ; sensitive to strong acids & alkalisPeak sensitivity : 493- 505nmAbsorbs yellow wavelength of light;transmitts violet to red colour ,hence appears visual purple

Cone pigments3 kinds of cone pigmentsThese are different from rhodopsin pigmentsResponsible for colour visionCone pigments are different from rhodopsin in opsin portion;11 cis retinol is same as rhodopsinPeak absorbance:-Blue sensitive cones:- 435 nmGreen sensitive cones:- 535 nmRed sensitive cones:-580 nm

Light induced changesRhodopsin bleaching Rhodpsin regenerationVisual cyclePhotochemistry of photoptic vision

rhodopsin bleaching& regeneration RhodopsinBarthorhodopsinLumirhodopsinMetarhodopsin |Metarhodopsin ||All trans-retinalAll trans -retinolisomerase11 cis -retinol11 cis-retinalopsin

NADHNADNADHNAD

Visual cycleRate of photodecomposition = rate of photoregeneration

rhodopsin

All trans retinal11 cis -retinalopsinLight energyopsin

Photochemistry of photopic vision Like rhodopsin ,cone pigments also consists of protein opsin i.e photopsin & retinine Photopsin differs slightly from rhodopsin 3 classes of photopsin :-Erythrolabe red sensitiveChlorolabe green sensitiveCyanolabe- blue sensitive

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