Retina

Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Retina shopping experience:

1. Compare - without doubt the biggest advantage that the Retina offers shoppers today is the ability to compare thousands of Retina at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Retina? Wrong! If the Retina is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Retina then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Retina? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Retina and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Retina wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Retina then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Retina site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Retina, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Retina, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

For the moth genus, see Retina (moth).

{{Infobox Anatomy | Name = {{PAGENAME--> | Latin = | GraySubject = 225 | GrayPage = 1014 | Image = Human_eye_cross-sectional_view_grayscale.png | Caption = Right human [eye cross-sectional view. Courtesy [National Institutes of Health [National Eye Institute. Many animals have eyes different from the human eye. | Image2 = | Caption2 = | Precursor = | System = | Artery = [central retinal artery | Vein = | Nerve = | Lymph = | MeshName = Retina | MeshNumber = A09.371.729 | DorlandsPre = r_10 | DorlandsSuf = 12705919 | -->The retina is a thin layer of neural cells that lines the back of the eyeball of vertebrates and some cephalopods. It is comparable to the film in a camera. In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain. Hence, the retina is part of the central nervous system (CNS). It is the only part of the CNS that can be imaged directly.

The vertebrate retina contains photoreceptor cells (rod cell and cone cell) that respond to light; the resulting neural signals then undergo complex processing by other neurons of the retina. The retinal output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light.

The unique structure of the blood vessels in the retina has been used for Retinal scan.

Anatomy of vertebrate retina The vertebrate retina has ten distinct layers.http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/RETINA.HTM From innermost to outermost, they include:

Inner limiting membrane - Müller cell footplates

Nerve fiber layer

Ganglion cell layer - Layer that contains nuclei of ganglion cells and gives rise to optic nerve fibers.

Inner plexiform layer

Inner nuclear layer

Outer plexiform layer - In the macula, this is known as the Fiber layer of Henle.

Outer nuclear layer

External limiting membrane - Layer that separates the inner segment portions of the photoreceptors from their cell nuclei.

Photoreceptor layer - Rod cell / Cone cell

Retinal pigment epithelium

Physical structure of human retina In adult humans the entire retina is 72% of a sphere about 22 mm in diameter. An area of the retina is the optic disc, sometimes known as "the blind spot" because it lacks photoreceptors. It appears as an oval white area of 3 mm². Temporal (in the direction of the temples) to this disc is the macula. At its center is the fovea, a pit that is most sensitive to light and is responsible for our sharp central vision. Human and non-human primates possess one fovea as opposed to certain bird species such as hawks who actually are bifoviate and dogs and cats who possess no fovea but a central band known as the visual streak. Around the fovea extends the central retina for about 6 mm and then the peripheral retina. The edge of the retina is defined by the ora serrata. The length from one ora to the other (or macula), the most sensitive area along the horizontal meridian (eye) is about 3.2 mm.

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In section the retina is no more than 0.5 mm thick. It has three layers of nerve cells and two of synapses. The optic nerve carries the ganglion cell axons to the brain and the blood vessels that open into the retina. As a byproduct of evolution, the ganglion cells lie innermost in the retina while the photoreceptive cells lie outermost. Because of this arrangement, light must first pass through the thickness of the retina before reaching the rods and cones. However it does not pass through the epithelium or the choroid (both of which are opaque).

The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or Scheerer's phenomenon).

Between the ganglion cell layer and the rods and cones there are two layers of neuropils where synaptic contacts are made. The neuropil layers are the outer plexiform layer and the inner plexiform layer. In the outer the rod and cones connect to the vertically running bipolar cells and the horizontally oriented horizontal cells connect to ganglion cells.

The central retina is cone-dominated and the peripheral retina is rod-dominated. In total there are about seven million cones and a hundred million rods. At the centre of the macula is the foveal pit where the cones are smallest and in a hexagonal mosaic, the most efficient and highest density. Below the pit the other retina layers are displaced, before building up along the foveal slope until the rim of the fovea or parafovea which is the thickest portion of the retina. The macula has a yellow pigmentation from screening pigments and is known as the macula lutea.

Difference between vertebrate and cephalopod retinas The vertebrate retina is inverted in the sense that the light sensing cells sit at the back side of the retina, so that light has to pass through a layer of neurons before it reaches the photoreceptors. By contrast, the cephalopod retina is everted: the photoreceptors are located at the front side of the retina, with processing neurons behind them. Because of this, cephalopods do not have a blind spot.

The cephalopod retina does not originate as an outgrowth of the brain, as the vertebrate one does. This shows that vertebrate and cephalopod eyes are not homology (biology) but have evolved separately.

Physiology An image is produced by the "patterned excitation" of the retinal receptors, the cones and rods. The excitation is processed by the neuronal system and various parts of the brain working in parallel to form a representation of the external environment in the brain.

The cones respond to bright light and mediate high-resolution vision and colour vision. The rods respond to dim light and mediate lower-resolution, black-and-white, night vision. It is a lack of cones sensitive to red, blue, or green light that causes individuals to have deficiencies in colour vision or various kinds of color blindness. Humans and old world monkeys have three different types of cones (trichromatic vision) while other mammals lack cones with red sensitive pigment and therefore have poorer (dichromatic) colour vision.

When light falls on a receptor it sends a proportional response synaptically to bipolar cells which in turn signal the retinal ganglion cells. The receptors are also 'cross-linked' by horizontal cells and amacrine cells, which modify the synaptic signal before the ganglion cells. Rod and cone signals are intermixed and combine, although rods are mostly active in very poorly lit conditions and color saturation in broad daylight, while cones function in brighter lighting because they are not sensitive enough to work at very low light levels.

Despite the fact that all are nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials. In the photoreceptors, exposure to light hyperpolarizes the membrane in a series of graded shifts. The outer cell segment contains a photopigment. Inside the cell the normal levels of cGMP keeps the Na+ channel open and thus in the resting state the cell is depolarised. The photon causes the retinal bound to the receptor protein to Isomerism to retinal. This causes receptor to activate multiple G-proteins. This in turn causes the Ga-subunit of the protein to bind and degrade cGMP inside the cell which then cannot bind to the CNG Na+ channels. Thus the cell is hyperpolarised. The amount of neurotransmitter released is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away in bright light and only replaced as a chemical process, so in a transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity (see dark adaptation).

In the retinal ganglion cells there are two types of response, depending on the receptive field of the cell. The receptive fields of retinal ganglion cells comprise a central approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect on the firing of the cell. In ON cells, an increment in light intensity in the centre of the receptive field causes the firing rate to increase. In OFF cells, it makes it decrease. Beyond this simple difference ganglion cells are also differentiated by chromatic sensitivity and the type of spatial summation. Cells showing linear spatial summation are termed X cells (also called "parvocellular", "P", or "midget" ganglion cells), and those showing non-linear summation are Y cells (also called "magnocellular, "M", or "parasol" retinal ganglion cells), although the correspondence between X and Y cells (in the cat retina) and P and M cells (in the primate retina) is not as simple as it once seemed.

In the transfer of signal to the brain, the visual pathway, the retina is vertically divided in two, a temporal half and a nasal half. The axons from the nasal half cross the brain at the optic chiasma to join with axons from the temporal half of the other eye before passing into the lateral geniculate body.

Although there are more than 130 million retinal receptors, there are only approximately 1.2 million fibres (axons) in the optic nerve so a large amount of pre-processing is performed within the retina. The fovea produces the most accurate information. Despite occupying about 0.01% of the visual field (less than 2° of visual angle), about 10% of axons in the optic nerve are devoted to the fovea. The resolution limit of the fovea has been determined at around 10,000 points. The information capacity is estimated at 500,000 bits per second (for more information on bits, see information theory) without colour or around 600,000 bits per second including colour.

Spatial Encoding The retina, unlike a camera, does not simply relay a picture to the brain, it first spatially encodes the image to fit the limited capacity of the optic nerve (there are 100 times less ganglion cells than photoreceptors). The retina employs spatial encoding (which involves sampling every region in the image, recording its value/colour), but it also aims to decorrelate incoming spatial images. This is carried out by the center surround inhibition of the bipolar and ganglion cells, which is based on the assumption that neighboring areas on an image are more likely to be the same colour/intensity. Once spatially encoded, the signal is sent to the LGN where it will be temporally encoded.

Diseases and disorders There are many inherited and acquired diseases or disorders that may affect the retina. Some of them include:

Retinitis pigmentosa is a group of genetic diseases that affect the retina and causes the loss of night vision and peripheral vision.
Macular degeneration describes a group of diseases characterized by loss of central vision because of death or impairment of the cells in the macula.
Cone-rod dystrophy (CORD) describes a number of diseases where vision loss is caused by deterioration of the Cone cell and/or Rod cell in the retina.
In retinal separation, the retina detaches from the back of the eyeball. Ignipuncture is an outdated treatment method.
Both hypertension and diabetes mellitus can cause damage to the tiny blood vessels that supply the retina, leading to hypertensive retinopathy and diabetic retinopathy.
Retinoblastoma is a cancer of the retina.
Retinal diseases in dogs include retinal dysplasia, progressive retinal atrophy, and sudden acquired retinal degeneration.

Diagnosis and treatment A number of different instruments are available for the diagnosis of diseases and disorders affecting the retina. An ophthalmoscope is used to examine the retina. Recently, adaptive optics has been used to image individual rods and cones in the living human retina.The Electroretinography is used to measure non-invasive (medical) the retina's electrical activity, which is affected by certain diseases. A relatively new technology, now becoming widely available, is optical coherence tomography (OCT). This non-invasive technique allows one to obtain a dimension volumetric or high resolution cross-sectional tomogram of the retinal fine structure with histologic-quality.

Treatment depends upon the nature of the disease or disorder. Transplantation of retinas has been attempted, but without much success. At Massachusetts Institute of Technology, The University of Southern California, and the University of New South Wales, an "artificial retina" is under development: an implant which will bypass the photoreceptors of the retina and stimulate the attached nerve cells directly, with signals from a digital camera.

Research George Wald, Haldan Keffer Hartline and Ragnar Granit won the 1967 Nobel Prize in Physiology or Medicine for their scientific research on the retina.

A recent University of Pennsylvania study calculated the approximate bandwidth of human retinas is 8.75 megabits per second, whereas a guinea pig retinas transfer at 875 kilobits. http://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight.html

Robert MacLaren and colleagues at University College London and Moorfields Eye Hospital in London showed in 2006 that photoreceptor cells could be transplanted successfully in the mouse retina if donor cells were at a critical developmental stage. http://www.nature.com/nature/journal/v444/n7116/abs/nature05161.html

References

S. Ramón y Cajal, Histologie du Système Nerveux de l'Homme et des Vertébrés, Maloine, Paris, 1911.
J. J. Atick and A. N. Redlich, What does the retina know about natural scenes?, Neural Computation, p. 196-210, 1992.