retina preliminary
DESCRIPTION
This presentation covers most of the basic sciences of Retina,...TRANSCRIPT
RETINA - PRELIMINARY
RAJVIN SAMUEL PONRAJ
Gross anatomy :
Inner most ,cup shaped , thin , transparent tunic of eyeball appears purplish red - underlying choroid
Divided into posterior pole and peripheral retina
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
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
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
MACULA : Round yellow area at posterior pole 5.5 mm size – 3 mm temporal 1 mm inferior to
disc
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
RETINA FOVEAvs
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
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,…
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
Reinal pigment epithelium : 4 to 6 million Single / cuboidal / hexagonal Tight junctions – Zonulae occludens Apical, mid,basal portions melanosomes
Functions : Vitamin A synthesis, phagocytosis
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
Outer segment- Discs
Cilium
Inner segment- Inner myeloid outer ellipsoid [ golgi,ER, mitochondria] Nucleus Synaptic process :Cone pedicle Rod spherule
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
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
In Dark :- Depolarisation - Na extruded from inner to outer [-40 mv] segment – open channel In light :- Hyperpolarization – Closure of Na channel [-70 mv]
Rhodopsin + transducin, GDP[complex] GDP exchanged with GTP Transducin released Activates Phosphodiesterase
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,..
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.
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
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.
When one enters a dark room from bright light ,sensitivity of rods gradually increase and adaptation reached after 20-30 mins,..
- 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.
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
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
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
Letter contrast Contrast sensitivity
sensitivity grating
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.
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]
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
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
- 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
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.
Response based on intensity of light & wavelength of different colour stimuli
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.
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
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
Retina contains 3 classes of cones with different but overlapping spectral sensitivity.
[cone classes] [spectral sensitivity peaks]
Blue receptors – 440 to 450 nm Green receptors - 535 to 555 nm Red receptors - 570 to 590 nm
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.
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.
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