retina preliminary

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RETINA - PRELIMINARY RAJVIN SAMUEL PONRAJ

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This presentation covers most of the basic sciences of Retina,...

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Page 1: Retina  preliminary

RETINA - PRELIMINARY

RAJVIN SAMUEL PONRAJ

Page 2: Retina  preliminary

Gross anatomy :

Inner most ,cup shaped , thin , transparent tunic of eyeball appears purplish red - underlying choroid

Divided into posterior pole and peripheral retina

Page 3: Retina  preliminary

Embryology: Neuroectoderm of optic cup Outer layer – pigment layer Inner layer – neural layer [mantle,matrix, marginal ] Location : sandwitched neuroepithelium b/w mesenchyme of vitreous and

choroid

Page 4: Retina  preliminary

Extent : anteriorly to ora serrata posteriorly to optic disc Distance from limbus : Equator=13.7 mm Ora serrata = 8 mm Circumference : Equatorial = 72 mm Ora serrata = 60 mm

Page 5: Retina  preliminary

Optic nerve head: measures 1.8 mm vertical 1.5 mm horizontally

- pinkish pale colour - terminaton of all layers except nerve fibre layer - cup and disc

Thickness of retina : optic disc =0.56 mm equator = 0.1 mm

Page 6: Retina  preliminary

MACULA : Round yellow area at posterior pole 5.5 mm size – 3 mm temporal 1 mm inferior to

disc

Page 7: Retina  preliminary

Fovea - 1.5 mm wide , thin bottom- 22” clivity thick basement margin - prone for macular holes -Henle’s layer-oblique

cones Foveola - 0.35 mm wide , thin pit , Densely

cones Bowing vitreally- fovea externa Umbo - Tiny depression - Foveal light reflex 0.15 mm - bouquet of cones - narrowed gateau nucleaire

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RETINA FOVEAvs

Page 10: Retina  preliminary

Blood -retinal barrier Outer =Rpe,choroid vessel bruch membrane Inner= Retinal vascular endothelium Water shed Zones 1.At outer plexiform,equator 2.Btw optic disc & macula

Capillary free zones 1.Foveal avascular zone 2. Periarterial zone

Page 11: Retina  preliminary

Retinal vasculature – supply prelaminar optic nerve, inner retinal quadrants - No anastamosis/ end arteries - Interdigitations among venules arterioles[common sheet] --adequate perfusion - Laminar meshwork of capillaries - Blood flow under Autoregulation-- pericytes,O2,CO2,Ph,…

Page 12: Retina  preliminary

Choroidal Vasculature : supply directly optic disc Circle of Zinn,major & minor arterial iridis peripapillary & sub macular Segmental supply-choriocapillary [functionally] Honey comb pattern[interconnect Blood flow regulation - sympathetic innervation

Page 13: Retina  preliminary

Reinal pigment epithelium : 4 to 6 million Single / cuboidal / hexagonal Tight junctions – Zonulae occludens Apical, mid,basal portions melanosomes

Functions : Vitamin A synthesis, phagocytosis

Page 14: Retina  preliminary

Number : Rods- 110 to 125 million

Cones- 6.3 to 6.8 million

Distribution :Rods 0 1,60,000 30,000 Cones 1.47,000 to 5000

Page 15: Retina  preliminary
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Outer segment- Discs

Cilium

Inner segment- Inner myeloid outer ellipsoid [ golgi,ER, mitochondria] Nucleus Synaptic process :Cone pedicle Rod spherule

Page 17: Retina  preliminary

Intracellular disk

Disk membrane

Connectingcilium

outer segmentouter segment

Disk membrane

Intracellularspace

Extracellularspace

Visualpigment

Visualpigment

Extracellularspace

Plasmamembrane

Intracellularspace

Connectingcilium

ROD CELL CONE CELLPHOTORECEPTORS IN THE EYE

Page 18: Retina  preliminary

VISUAL PIGMENTS :

Rhodopsin : Opsin [glycoprotein] & Retinal Two isomeric states of retinal –All trans form --11 cis form Unbleached rhodopsin – reddish purple [due to visual purple] Peak absorption at 500 nm wave length -coresponds to deep green,.. Photopsin : absorption peak at 565 nm blue sensitive - 440 nm green sensitive – 540 nm red sensitive - 570 nm

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In Dark :- Depolarisation - Na extruded from inner to outer [-40 mv] segment – open channel In light :- Hyperpolarization – Closure of Na channel [-70 mv]

Page 21: Retina  preliminary

Rhodopsin + transducin, GDP[complex] GDP exchanged with GTP Transducin released Activates Phosphodiesterase

Page 22: Retina  preliminary

Photo receptor cell renewal

1-2 Weeks - turnover , Daily 100 discs shedBalance maintained between-shedding and resynthesis of disc membranes.Discs engulfed by RPE into phagosome vesicles then digested by lysosomes and enter bruch membrane,..

Page 23: Retina  preliminary

Complex molecules of Gag’s surrounding pigment cells and photoreceptors.

Serve to attach Neural retina to RPE for physical support.

Other factors contributing for strength:-vitreous gel-Intraocular fluid pressure-RPE water transport mechanism.

Page 24: Retina  preliminary

Visual threshold : The minimal light intensity evoking a visual sensationIntensity of illumination From 0 to 9 log units

Sensitivity adjustment by: -pupil size -level of neural activity -steady state concentration of photosensitive pigments

Average level of sensitivity allows capture of image of max clarity & contrast detail

Page 25: Retina  preliminary

Rods – more light sensitive – Low illumination

Cones – less light sensitive – High illumination

Scotopic vision lack in spatial and Temporal acuity and colour perception.

Purkinje shift: Change in spectral sensitivity with intensity of illumination.Wavelength of light that Is maximally sensitive Is longer for photopic vision than scotopic vision.

Page 26: Retina  preliminary

When one enters a dark room from bright light ,sensitivity of rods gradually increase and adaptation reached after 20-30 mins,..

Page 27: Retina  preliminary

- Ability to discriminate btw shapes of objects - Both retinal function and psychological - Cones play a major role /high foveal activity

Visual acuity : measure of spatial resolution /ability to discriminate btw points.

Page 28: Retina  preliminary

To achieve resolution of 2 targets/edges ,one unstimulated cone btw 2 cones needed…

Two cones separated by 2 um space subtends 25 sec arc

Minimal resolvable –The minimum separation btw 2

points to discriminate between. The resolving power of the eye to produce a minimum image size of an object 0.004 mm and the object subtends 1 minute angle at the nodal point

-Minimal angle of resolution

Page 29: Retina  preliminary

Resolution of eye due to Cones--

- finite size [1-2 um] - tight arrangement at fovea The orientation as light pipes / fibre optic directed

to second nodal point on lens.

The configuration of foveal pit with small concavity - walls which prevents glare striking the centre

Yellow macular filters blue light [short wavelenth] -prevents chromatic aberration

Page 30: Retina  preliminary

Ability of eye to perceive slight changes in luminance between regions not separated by definite borders.

Types : Letter contrast sensitivity contrast sensitivity gratings

Also distances between repeats of patterns taken into account

Page 31: Retina  preliminary

Letter contrast Contrast sensitivity

sensitivity grating

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Spherules & pedicles in outer plexiform layer A typical synaptic triad – 2 axon terminal [horizontal cells] 1 dendritic terminal [bipolar cells] The dendrites of flat midget , flat top , and giant bistratified

bipolar cells - basal junctions with cone pedicles. Rod bipolar cell dendrites end as central elements at

ribbon synaptic complex of rod spherules.

Basal processes from cone pedicles contact the cone pedicles, and rod spherules forming gap junctions.

Page 34: Retina  preliminary

Receptive field of visual neuron Locus of all points within Visual field [Spatial/

temporal/colour in which light stimuli effective in eliciting response from the

cell…

CONNECTIONS1. The photoreceptors themselves—the rods and cones—

outer plexiform layer, where they synapse with bipolar cells [inner nuclear layer]

Page 35: Retina  preliminary

2. The horizontal cells, in the [outer plexiform layer] from the rods and cones to bipolar cells

3. The bipolar cells vertically from the rods, cones, and horizontal cells to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells

Page 36: Retina  preliminary

4. The amacrine cells, which transmit signals in two directions, either directly from bipolar cells to ganglion cells or horizontally within the [inner plexiform layer] from axons of the bipolar cells to dendrites of the ganglion cells or to other amacrine cells

 5. The ganglion cells, which transmit

output signals from the retina through the optic nerve into the brain

Page 37: Retina  preliminary

- Provide opposing excitatory and inhibitory signals in the visual pathway:

- That is, some bipolar cells depolarize when the rods and cones are excited, and others hyperpolarize when they are inhibited.[Level of polarisation]

- Response by slow electric potentials by Centre – surround behaviour [contrast-sensitivity]

Centre - Hyperpolarisation

Surround - Depolarisation

Page 38: Retina  preliminary

They connect laterally between the synaptic bodies of the rods and cones, as well as connecting with the dendrites of the bipolar cells. The outputs of the horizontal cells are always inhibitory. This is called Lateral inhibition.

Page 39: Retina  preliminary

Response based on intensity of light & wavelength of different colour stimuli

Page 40: Retina  preliminary

Amacrine cell responds strongly at the onset of a continuing visual signal, but the response dies rapidly.

Respond strongly at the offset of visual signals.

Respond when a light is turned either on or off, signalling simply a change in illumination.

Responds to movement of a spot across the retina in a specific direction; therefore, these amacrine cells are said to be directional sensitive.

Page 41: Retina  preliminary

Determines rate of firing rather than level of polarisation All or none spikes of electrical activity. X cells[parvo/midget]- small dendritic fields for fine details image[sustained response] Y CELLS-[Magno]- broader dendritic fields for spatial summation[transient response] The above 2 are relatively brisk response,… W CELLS - large receptive fields Respond sluggishly-slow axon conduction

Page 42: Retina  preliminary

Sensation of colour is subjective Colour perceived depends on the following: -wavelengths from object ,other objects and wavelengths before looking at the object. White light split into its component

wavelengths –Property of Dispersion by prism

Page 43: Retina  preliminary

Retina contains 3 classes of cones with different but overlapping spectral sensitivity.

Page 44: Retina  preliminary

[cone classes] [spectral sensitivity peaks]

Blue receptors – 440 to 450 nm Green receptors - 535 to 555 nm Red receptors - 570 to 590 nm

Page 45: Retina  preliminary

There are three opponent channels that exist.

1.White vs black 2.Red vs green 3. Yellow vs blue

Response to one colour in the opponent channel

Is antagonistic to the other, that is excitatory by one and inhibitory by the other.

These mechanisms to receive and process information at a complex level.

Page 46: Retina  preliminary

Double opponent cell is a cell which has a centre and a surround to its receptive field and both are colour coded.

They are concerned with simultaneous colour contrast

Opponent colour cells are found among ganglion cells of retina and lateral geniculate

nucleus. Double opponent cells are found in the

striate cortex area 17.

Page 47: Retina  preliminary

THANK YOU