Eye physiology from guyton and halls physiology Part 2

BY Muhammad Ramzan Ul Rehman Nishtar Ken 1

BY Muhammad Ramzan Ul Rehman Nishtar Ken

2

PHYSIOLOGY OF EYE


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10 Layers of retina


Retinal DetachmentInjury to the eyeballFluid 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)


Retinal Detachment


The Fovea centralisThe fovea lies slightly below and to one side of the optic disc. It is found in the centre of a shallow depression or pit (the macula lutea).

The fovea is a minute area in the center of the retina occupying a total area a little more than 1 square millimeter

It is especially capable of acute and detailed vision.


The Fovea centralis The central fovea, only 0.3 millimeter in diameter, is composed almost

entirely of cones These cones have a special structure that gives a clear detail of the image. The foveal cones have especially long and slender bodies, in the foveal

region, ( Peripheral retina has cones with fat bodies) Fovea also has blood vessels, ganglion cells and inner nuclear layer of cells

all arranged in manner that light passes unimpeded to reach the cones. Only cones are present at the fovea which have individual connections with

the bipolar and ganglion cells, hence the fovea gives us our most sensitive and acute vision


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.


The Rods and ConesPhoto receptors present in the outer nuclear layer or

Receptor Layer 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 in the fovea, and very few cones are found at the periphery, where rods predominate.


Rods and Cones Rods and cones (the names

reflect their respective shapes) contain light sensitive pigments.

Each photoreceptor consists of an outer segment which contains hundreds of thin plates of membrane (lamellae or discs).

The outer segment is connected by a cilium to an inner segment which contains a nucleus.

Rods are about 500 times more sensitive to light than cones, but cones give us colour vision.


Structure of Rod/ConeThe light-sensitive photochemical is found in the outer segment.

In rods, this is Rhodopsin

In cones, it is one of three “color” photochemicals, (color pigments)

that function almost exactly the same as rhodopsin

except for differences in spectral sensitivity.


Discs/Lamellae of Rods and Cones Large numbers of discs 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.


Rhodopsin and Colour Pigments

Conjugated proteins.

They are present in the membranes

of the discs in the form of transmembrane proteins.

These Proteins ( Rhodopsin and Colour Pigments) constitute about 40 per cent of the entire mass of the outer segment.


Pigment layer of the retinaBlack pigment melanin.Prevents light reflection through out the globe of the eye ball.Clear vision. It stores large amount of vit A that is an important precursor of photosensitive chemicals of rods and cones.


Optic Disc This is the point at which axons leave the eyeball and join

the optic nerve. Also, arteries enter and veins leave the retina at the optic disc.

There are no photoreceptors here, hence it is known as the 'blind spot'. It is a pinky-yellow oval, approximately 2mm in diameter


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


The principal steps in phototransduction


The Dark Current

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

Figure 50-6;Guyton & Hall


The Rod Receptor PotentialNormally about -40 mVNormally the outer segment of the rod is very permeable to Na+ ions.

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

The current flows out of the cell, through the efflux of Na+, ions in the inner segment of the rod.


Rod Receptor Potential (Cont’d)When rhodopsin decomposes it causes a hyperpolarization of the rod by decreasing Na+ permeability of the outer segment.

The Na+ pump in the inner segment keeps pumping Na+ out of the cell causing the membrane potential to become more negative (hyperpolarization).

The greater the amount of light the greater the electronegativity.


NIGHT BLINDNESS / NYCTALOPIA

It occurs by severe vitamin A deficiency.

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

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

Dietary deficiency of Vitamin A occurs in months.

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


Dark and Light Adaptation

In light conditions most of the rhodopsin has been reduced to retinal so the level of photosensitive chemicals is low. In dark conditions retinal is converted back to rhodopsin. Therefore, the sensitivity of the retinal automatically adjusts to the light level.

Opening and closing of the pupil also contributes to adaptation because it can adjust the amount entering the eye.

Neuronal inhibition or excitation is also involved in the adaptaion process


Importance of Dark and Light Adaptation

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

It is important that the sensitivity of the retina be adjusted to detect the dark and light spots on the image.

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

Enter darkness from light, the light spots are not light enough to register.


29 Thank You

By Muhammad Ramzan Ul Rehman

Eye physiology from guyton and halls physiology Part 2

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Transcript of Eye physiology from guyton and halls physiology Part 2