2nd lecture on physiology of eye by dr. roomi

7/28/2019 2nd Lecture on Physiology of Eye by Dr. Roomi

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PHYSIOLOGY OF EYE

BY

DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)


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10 Layers of retina


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Retinal DetachmentInjury to the eyeball Fluid or blood may be collected between the neural

retina and the pigment epithelium.

Cause: Contracture of fine collagenous fibrils in the

vitreous humor These fibrils pull the retina toward the interior of the globe.

The detached retina can resist degeneration for days because of:

1. Diffusion across the detachment gap

2. Independent blood supply by the Retinal artery

(Early surgical placement may save the permanent loss of vision)


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Retinal Detachment


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The Fovea centralis

The fovea lies slightly below and to one side of the

optic disc. It is found in the centre of a shallowdepression or pit (the macula lutea).

Thefovea is a minute area in the center of the retinaoccupying a total area a little more than 1 square

millimeter

It is especially capable of acute and detailed vision.


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The Fovea centralis

The central fovea, only 0.3 millimeter in diameter, iscomposed almost entirely of cones

These cones have a special structure that gives a cleardetail of the image.

The foveal cones have especially long and slender bodies,in the foveal region, ( Peripheral retina has cones with fatbodies)

Fovea also has blood vessels, ganglion cells and innernuclear layer of cells all arranged in manner that lightpasses unimpeded to reach the cones.

Only cones are present at the fovea which haveindividual connections with the bipolar and ganglioncells, hence the fovea gives us our most sensitive andacute vision


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Peripheral Retina

There are hardly any cones in the

peripheral retina, but many rods.

The rods here are also shorter and wider

than in the central retina.

Receptive fields at the periphery are very

large with many rods converging onto one

ganglion cell.


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The Rods and Cones

Photo receptors present in the outer nuclear layer or ReceptorLayer of Retina

The human receptor layer consists of

approximately 120 million rods and 6

million cones arranged side by side.

The distribution of these

photoreceptors varies across the surface

of the retina. There are no rods at all inthe fovea, and very few cones are found

at the periphery, where rods

predominate.


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Rods and Cones

Rods and cones (thenames reflect theirrespective shapes) containlight sensitive pigments.

Each photoreceptor

consists of an outersegment which containshundreds of thin plates ofmembrane (lamellae ordiscs).

The outer segment isconnected by a cilium to an

inner segment whichcontains a nucleus. Rods are about 500 times

more sensitive to light thancones, but cones give uscolour vision.


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Structure of Rod/Cone

The light-sensitive

photochemical is found inthe outer segment.

In rods, this is Rhodopsin

In cones, it is one of threecolor photochemicals,(color pigments)

that function almost

exactly the same asrhodopsin

except for differences inspectral sensitivity.


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Discs/Lamellae of Rods and

Cones

Large numbers ofdiscs are

present in the outer segments of

the rods and cones.

Each of the discs is an infolded

shelf of cell membrane.

There are as many as 1000 discs

in each rod or cone.


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Pigment layer of the retina

Melanin pigment layer is absent in albino.

Light reflected in all directions inside the eye ball by

unpigmented surfaces of the retina & sclera.

Light excites many receptors Visual acuity of albinos badly affected 20/100 to 20/200.


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Photochemistry of vision


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The principal steps in phototransduction


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The Dark Current

In the dark an inward current(thedarkcurrent) carried bythe Na+ ions flows into theouter segment of the rod.

Figure 50-6;Guyton & Hall


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The Rod Receptor Potential

Normally about -40 mV

Normally the outer segment of the rod is verypermeable to Na+ ions.

In the dark an inward current (the dark current)carried by the Na+ ions flows into the outersegment of the rod.

The current flows out of the cell, through the

efflux of Na+

, ions in the inner segment of the rod.


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Rod Receptor Potential (Contd)

When rhodopsin decomposes it causes ahyperpolarization of the rod by decreasing Na+permeability of the outer segment.

The Na+

pump in the inner segment keepspumping Na+ out of the cell causing themembrane potential to become more negative(hyperpolarization).

The greater the amount of light the greater theelectronegativity.


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NIGHT BLINDNESS / NYCTALOPIA

It occurs by severe vitamin A deficiency.

Retinal & rhodopsin formed in the absence of Vitamin A areseverely depressed and insufficient.

Amount of light at night is too little to permit adequate visionin vitamin A deficient person.

Dietary deficiency of Vitamin A occurs in months.

Recovery in Night Blindness takes place in Less than one hourby I/V Vitamin A


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Dark and Light Adaptation

In light conditions most of the rhodopsin has beenreduced to retinal so the level of photosensitivechemicals is low. In dark conditions retinal isconverted back to rhodopsin. Therefore, the

sensitivity of the retinal automatically adjusts tothe light level.

Opening and closing of the pupil also contributes toadaptation because it can adjust the amountentering the eye.

Neuronal inhibition or excitation is also involved inthe adaptaion process


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Importance of Dark and Light Adaptation

The detection of images on the retina is a function ofdiscriminating between dark and light spots.

It is important that the sensitivity of the retina beadjusted to detect the dark and light spots on theimage.

Enter the sun from a movie theater, even the darkspots appear bright leaving little contrast.

Enter darkness from light, the light spots are not light

enough to register.


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2nd lecture on physiology of eye by dr. roomi

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