The Eye 2

The eye

Clinical Method for Stating Visual Acuity.
The chart for testing eyes usually consists of letters of different sizes placed 20 feet away from the person being tested. If the person can see well the letters of a size that he should be able to see at 20 feet, the person is said to have 20/20 vision—that is, normal vision.
If the person can see only letters that he should be able to see at 200 feet, the person is said to have 20/200 vision. In other words, the clinical method for expressing visual acuity is to use a mathematical fraction that expresses the ratio of two distances, which is also the ratio of one’s visual acuity to that of a person with normal visual acuity.
Cataracts
“Cataracts” are common eye abnormality that occurs mainly in older people. A cataract is a cloudy or opaque area in the lens, because the proteins in the lens fibers become denatured
The condition can be corrected by surgical removal of the lens.
Fluid System of the Eye
Intraocular Fluid
The eye is filled with intraocular fluid, which maintains sufficient pressure in the eyeball to keep it distended.
This fluid can be divided into two portions: aqueous humor, which lies infront of the lens, and vitreous humor, which is between the posterior surface of the lens and the retina.
Aqueous humor is continually being formed and reabsorbed ,iIts formed by the Ciliary Body at an average rate of 2 to 3 microliters / minute .The balance between formation and reabsorption regulates the total volume and intraocular pressure. This fluid move from the posterior chamber to the anterior chamber of through the pupil. From here, the fluid flows to the meshwork of trabeculae, finally entering the canal of Schlemm, which empties into extraocular veins.
Intraocular Pressure
The average normal intraocular pressure is about 15 mm Hg, with a range from 12 to 20 mm Hg., this pressure is measured clinically by using a “tonometer”
The level of this pressure is determined mainly by the resistance to outflow of aqueous humor from the anterior chamber into the canal of Schlemm. When large amounts of debris are present in the aqueous humor, the debris is accumulated in the trabecular spaces will prevent adequate reabsorption of fluid causing “glaucoma,”
Glaucoma can sometimes be treated by placing drops in the eye that contain a drug reduces the secretion or increases the absorption of aqueous humor. When drug therapy fails operative techniques to open the spaces of the trabeculae
The retina
The retina is the light-sensitive portion of the eye that contains (1) the cones, which are responsible for color vision, and (2) the rods, which are mainly responsible for black and white vision and vision in the dark.
The functional components of the retina which are arranged in layers from the outside to the inside as follows:
(1) pigmented layer
(2) layer of rods and cones projecting to the pigment,
(3) outer nuclear layer containing the cell bodies of the rods and cones, (4) outer plexiform layer,
(5) inner nuclear layer,
(6) inner plexiform layer,
(7) ganglionic layer,
(8) layer of optic nerve fibers
(9) inner limiting membrane.
Foveal Region of the Retina :
Macula yellow central portion of the retina about 6 mm in diameter for clearest and distinct vision .The central part of macula called fovea is a minute area occupying a total area a about 1 square millimeter; it is especially capable of acute and detailed vision, its composed almost entirely of cones . Also in the foveal region, the blood vessels, ganglion cells, inner nuclear layer of cells and plexiform layers are all displaced to one side rather than resting directly on top of the cones. This allows light to pass directly to the cones.


Rods and Cones.
The essential components of a photoreceptor (either a rod or a cone):
1-The outer segment.
2-Inner segment.
3-Nucleus.
4-Synaptic body.
The light-sensitive photochemical is found in the outer segment , in the case of the rods, called rhodopsin; and in the cones, it is called color pigments(usually three color pigments ).Both rhodopsin and the color pigments are conjugated proteins.
The inner segment of the rod or cone contains the usual cytoplasm with cytoplasmic organelles. Particularly important are the mitochondria, which provide energy for function of the photoreceptors.
The synaptic body is the portion of the rod or cone that connects with subsequent neuronal cells, (the horizontal and bipolar cells).
Pigment Layer of the Retina.
The black pigment melanin in the pigment layer prevents light reflection throughout the globe of the eyeball; this is extremely important for clear vision. Its absence in albinos, people who are hereditarily lacking in melanin pigment in all parts of their bodies. When an albino enters a bright room, light that impinges on the retina is reflected in all directions inside the eyeball by the unpigmented surfaces of the retina so that a single is reflected everywhere and excites many receptors.
Blood Supply of the Retina:
The nutrient blood supply for the internal layers of the retina is derived from the central retinal artery.The outer layers of the retina, especially the outer segments of the rods and cones, depend mainly on diffusion from the choroid blood vessels for their nutrition.
Retinal Detachment: The neural retina occasionally detaches from the pigment epithelium. In some instances the cause of such detachment is injury to the eyeball that allows blood to collect between the neural retina and the pigment epithelium. the retina will be destroyed and will be unable to function even after surgical repair if not correct early .

Photochemistry of Vision: rhodopsin is a combination of the protein scotopsin and retinal, the retinal is a type called 11-cis retinal.
When light energy is absorbed by rhodopsin, the rhodopsin begins to decompose within a fraction of a second, into bathorhodopsin. Bathorhodopsin is extremely unstable and decays in to lumirhodopsin. This then decays in to metarhodopsin I, then to metarhodopsin II, and finally into scotopsin and all-trans retinal. The all-trans retinal into 11-cis retinal. This process requires the enzyme retinal isomerase. Once the 11-cis retinal is formed, it automatically recombines with the scotopsin to re-form rhodopsin.
Role of Vitamin A for Formation of Rhodopsin: a second chemical route by which all-trans retinal can be converted into 11-cis retinal. This is by conversion of the all-trans retinal first into all-trans retinol, which is one form of vitamin A. Then the all-trans retinol is converted into 11-cis retinol under the influence of the enzyme isomerase Finally, the 11-cis retinol is converted into 11-cis retinal, which combines with scotopsin to form new rhodopsin. Vitamin A is present both in the cytoplasm of the rods and in the pigment layer of the retina.Therefore, vitamin A is normally available to form new retinal when needed.
Rhodopsin Light energy Bathorhodpsin
(prelumirhodopsin)


Lumirhodopsin

Metarhodopsin I
Scotopsin
Metarhodopsin II
Enzyme Isomearse
Enzyme Isomearse

11-cis-retinal All trans- retinal

11-cis-retinol Isomearse All trans- retinol (Vit A)

Photochemistry of Color Vision by the Cones
The only difference is that the protein portions, or the opsins—called photopsins in the cones
Excitation of the Rod When Rhodopsin Is Activated by Light
Excitation of the rod causes increased negativity of the cell membrane , which is a state of hyperpolarization meaning that there is more negativity than normal inside the cell membrane. This is exactly opposite to the decreased negativity (the process of “depolarization”) that occurs in almost all other sensory receptors.
The inner segment continually pumps sodium from inside the rod to the outside, thereby creating a negative potential on the inside the cell. However, the outer segment of the rod, is very leaky to sodium ions. Therefore, positively charged sodium ions continually leak back to the inside of the rod and thereby neutralize much of the negativity( in the dark state).
Then, when the rhodopsin in the outer segment of the rod is exposed to light, the rhodopsin begins to decompose, and this decreases the outer segment membrane conductance of sodium to the interior of the rod, even though sodium ions continue to be pumped outward through the membrane of the inner segment. Thus, more sodium ions now leave the rod than leak back in. Because they are positive ions, their loss from creates increased negativity inside the membrane and state of hyperpolarization.
Light and Dark Adaptation
If a person has been in bright light for hours, large portions of the photochemicals will have been reduced to retinal and opsins. Furthermore, much of the retinal of both the rods and the cones will have been converted into vitamin A. Because of these two effects, the concentrations of the photosensitive chemicals remaining in the rods and cones are reduced,and the sensitivity of the eye to light is correspondingly reduced. This is called light adaptation.
Conversely, if a person remains in darkness for a long time, the retinal and opsins are converted back into the light-sensitive pigments. Furthermore, vitamin A is converted back into retinal to give still more light-sensitive pigments. Now because these two effects, the visual receptors be come so sensitive that even the minutest amount of light causes excitation. This is called dark adaptation.
Other Mechanisms of Light and Dark Adaptation.
The first of these is a change in pupillary size, because of changes in the amount of light allowed through the pupillary opening.
The other mechanism is neural adaptation, when the intensities of the signals transmitted is very intestine, then the intensities of these signals all decrease rapidly.
Night Blindness.
Night blindness occurs in any person with severe vitamin A deficiency. The simple reason for this is that without vitamin A, the amounts of retinal and rhodopsin that can be formed are severely depressed. This condition is called night blindness because the amount of light available at night is too little to permit adequate vision.