A third type of light-sensing cell, the photosensitive ganglion cell , is important for entrainment of circadian rhythms and reflexive responses such as the pupillary light reflex. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses that are sent to various visual centres of the brain through the fibres of the optic nerve.
Neural signals from the rods and cones undergo processing by other neurons, whose output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. In vertebrate embryonic development , the retina and the optic nerve originate as outgrowths of the developing brain, specifically the embryonic diencephalon ; thus, the retina is considered part of the central nervous system CNS and is actually brain tissue.
The vertebrate retina is inverted in the sense that the light sensing cells are in the back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones. In this region there are no photoreceptors, giving rise to the blind spot.
Because of this, cephalopods do not have a blind spot. Although the overlying neural tissue is partly transparent, and the accompanying glial cells have been shown to act as fibre-optic channels to transport photons directly to the photoreceptors,   light scattering does occur. This area, termed the fovea centralis , is avascular does not have blood vessels , and has minimal neural tissue in front of the photoreceptors, thereby minimizing light scattering.
The cephalopods have a non-inverted retina which is comparable in resolving power to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate retinal pigment epithelium RPE. Although their photoreceptors contain a protein, retinochrome, that recycles retinal and replicates one of the functions of the vertebrate RPE, one could argue that cephalopod photoreceptors are not maintained as well as in vertebrates and that, as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates.
The cephalopod retina does not originate as an outgrowth of the brain, as the vertebrate one does. It is arguable that this difference shows that vertebrate and cephalopod eyes are not homologous but have evolved separately. From an evolutionary perspective, a more complex structure such as the inverted retina can generally come about as a consequence of two alternate processes: a an advantageous "good" compromise between competing functional limitations, or b as a historical maladaptive relic of the convoluted path of organ evolution and transformation.
Vision is an important adaptation in higher vertebrates. A third view of the "inverted" vertebrate eye is that it combines two benefits: the maintenance of the photoreceptors mentioned above, and the reduction in light intensity necessary to avoid blinding the photoreceptors, which are based on the extremely sensitive eyes of the ancestors of modern hagfishes a fish that lives in very deep, dark water.
The vertebrate retina has ten distinct layers. These layers can be grouped into 4 main processing stages: photoreception; transmission to bipolar cells ; transmission to ganglion cells , which also contain photoreceptors, the photosensitive ganglion cells ; and transmission along the optic nerve.
At each synaptic stage there are also laterally connecting horizontal and amacrine cells. The optic nerve is a central tract of many axons of ganglion cells connecting primarily to the lateral geniculate body , a visual relay station in the diencephalon the rear of the forebrain. It also projects to the superior colliculus , the suprachiasmatic nucleus , and the nucleus of the optic tract. It passes through the other layers, creating the optic disc in primates.
Additional structures, not directly associated with vision, are found as outgrowths of the retina in some vertebrate groups. In birds , the pecten is a vascular structure of complex shape that projects from the retina into the vitreous humour ; it supplies oxygen and nutrients to the eye, and may also aid in vision. Reptiles have a similar, but much simpler, structure. The entire retina contains about 7 million cones and 75 to million rods. The optic disc, a part of the retina sometimes called "the blind spot" because it lacks photoreceptors, is located at the optic papilla , where the optic-nerve fibres leave the eye.
Temporal in the direction of the temples to this disc is the macula , at whose centre is the fovea , a pit that is responsible for our sharp central vision but is actually less sensitive to light because of its lack of rods. Human and non-human primates possess one fovea, as opposed to certain bird species, such as hawks, who are bifoviate, and dogs and cats, who possess no fovea but a central band known as the visual streak. The farthest edge of the retina is defined by the ora serrata.
The distance from one ora to the other or macula , the most sensitive area along the horizontal meridian is about 32 mm. In section, the retina is no more than 0. It has three layers of nerve cells and two of synapses , including the unique ribbon synapse. The optic nerve carries the ganglion cell axons to the brain, and the blood vessels that supply the retina. The ganglion cells lie innermost in the eye while the photoreceptive cells lie beyond.
Because of this counter-intuitive arrangement, light must first pass through and around the ganglion cells and through the thickness of the retina, including its capillary vessels, not shown before reaching the rods and cones. Light is absorbed by the retinal pigment 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 neuropil layer, the rods and cones connect to the vertically running bipolar cells , and the horizontally oriented horizontal cells connect to ganglion cells.
The central retina predominantly contains cones, while the peripheral retina predominantly contains rods. 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 narrow and long, and, arranged in a hexagonal mosaic , the most dense, in contradistinction to the much fatter cones located more peripherally in the retina.
The macula has a yellow pigmentation, from screening pigments, and is known as the macula lutea. The area directly surrounding the fovea has the highest density of rods converging on single bipolar cells. Since its cones have a much lesser convergence of signals, the fovea allows for the sharpest vision the eye can attain. Though the rod and cones are a mosaic of sorts, transmission from receptors, to bipolars, to ganglion cells is not direct.
Since there are about million receptors and only 1 million optic nerve fibres, there must be convergence and thus mixing of signals. Moreover, the horizontal action of the horizontal and amacrine cells can allow one area of the retina to control another e. This inhibition is key to lessening the sum of messages sent to the higher regions of the brain. In some lower vertebrates e.
Using optical coherence tomography OCT there are 18 layers that can be identified in the retina. The layers and anatomical correlation are as follows:   . Poorly distinguishable from RPE. Previously: "cone outer segment tips line" COST. The neural retina contains the retinal progenitor cells RPCs that give rise to the seven cell types of the retina.
Differentiation begins with the retinal ganglion cells and concludes with production of the Muller glia. In addition to guiding cell fate determination, cues exist in the retina to determine the dorsal-ventral D-V and nasal-temporal N-T axes. Additional gradients are formed within the retina. The retina is stratified into distinct layers, each containing specific cell types or cellular compartments  that have metabolisms with different nutritional requirements.
At first glance, one may think that the vertebrate retina is "wired wrongly" or "badly designed"; but in fact, the retina could not function if it were not inverted. The photoreceptor layer must be embedded in the retinal pigment epithelium RPE , which performs at least seven vital functions,  one of the most obvious being to supply oxygen and other necessary nutrients needed for the photoreceptors to function.
These nutrients include glucose, fatty acids, and retinal. The mammalian photoreceptor amplification process uses large quantities energy for vision in photopic conditions requiring less under scotopic conditions and, thus, requires the large supply nutrients supplied by the blood vessels in the choroid, which lies beyond the RPE.
When light strikes cis-retinal in the disks in the rods and cones , cis-retinal changes to all-trans-retinal which then triggers changes in the opsins. Now, the outer segments do not regenerate the retinal back into the cis- form once it has been changed by light.
Instead the retinal is pumped out to the surrounding RPE where it is regenerated and transported back into the outer segments of the photoreceptors. This recycling function of the RPE protects the photoreceptors against photo-oxidative damage   and allows the photoreceptor cells to have decades-long useful lives.
The bird retina is devoid of blood vessels, perhaps to give unobscured passage of light for forming images, thus giving better resolution. It is, therefore, a considered view that the bird retina depends for nutrition and oxygen supply on a specialized organ, called the "pecten" or pecten oculi , located on the blind spot or optic disk.
This organ is extremely rich in blood vessels and is thought to supply nutrition and oxygen to the bird retina by diffusion through the vitreous body. The pecten is highly rich in alkaline phosphatase activity and polarized cells in its bridge portion — both befitting its secretory role.
This is considered to enhance metabolic rate of the pecten, thereby exporting more nutritive molecules to meet the stringent energy requirements of the retina during long periods of exposure to light. The bifurcations and other physical characteristics of the inner retinal vascular network are known to vary among individuals,  and these individual variances have been used for biometric identification and for early detection of the onset of disease. The mapping of vascular bifurcations is one of the basic steps in biometric identification.
The retina translates an optical image into neural impulses starting with the patterned excitation of the colour-sensitive pigments of its rods and cones, the retina's photoreceptor cells. The excitation is processed by the neural 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 colour vision during daylight illumination also called photopic vision. The rod responses are saturated at daylight levels and don't contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination called scotopic vision. The illumination in most office settings falls between these two levels and is called mesopic vision.
At mesopic light levels, both the rods and cones are actively contributing pattern information. What contribution the rod information makes to pattern vision under these circumstances is unclear. The response of cones to various wavelengths of light is called their spectral sensitivity. In normal human vision, the spectral sensitivity of a cone falls into one of three subtypes, often called blue, green, and red, but more accurately known as short, medium, and long wavelength-sensitive cone subtypes.
It is a lack of one or more of the cone subtypes that causes individuals to have deficiencies in colour vision or various kinds of colour blindness. These individuals are not blind to objects of a particular colour, but are unable to distinguish between colours that can be distinguished by people with normal vision. Humans have this trichromatic vision , while most other mammals lack cones with red sensitive pigment and therefore have poorer dichromatic colour vision.
However, some animals have four spectral subtypes, e. Some fish are sensitive to the polarization of light as well. In the photoreceptors, exposure to light hyperpolarizes the membrane in a series of graded shifts. The outer cell segment contains a photopigment.
The photon causes the retinal bound to the receptor protein to isomerise to trans-retinal. This causes the receptor to activate multiple G-proteins. 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.
When thus excited by light, the photoceptor sends a proportional response synaptically to bipolar cells which in turn signal the retinal ganglion cells. The photoreceptors are also cross-linked by horizontal cells and amacrine cells , which modify the synaptic signal before it reaches the ganglion cells, the neural signals being intermixed and combined.
Of the retina's nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials. 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.
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. In a linear model, this response profile is well described by a difference of Gaussians and is the basis for edge detection algorithms.
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 visual signals to the brain, the visual pathway , the retina is vertically divided in two, a temporal nearer to the temple half and a nasal nearer to the nose 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 million retinal receptors, there are only approximately 1. So, a large amount of pre-processing is performed within the retina. The fovea produces the most accurate information. Despite occupying about 0. The resolution limit of the fovea has been determined to be around 10, points. The information capacity is estimated at , bits per second for more information on bits, see information theory without colour or around , bits per second including colour. When the retina sends neural impulses representing an image to the brain, it spatially encodes compresses those impulses to fit the limited capacity of the optic nerve.
Compression is necessary because there are times more photoreceptor cells than ganglion cells. This is done by " decorrelation ", which is carried out by the "centre—surround structures", which are implemented by the bipolar and ganglion cells. There are two types of centre—surround structures in the retina — on-centres and off-centres. On-centres have a positively weighted centre and a negatively weighted surround.
Off-centres are just the opposite. Positive weighting is more commonly known as excitatory , and negative weighting as inhibitory. These centre—surround structures are not physical apparent, in the sense that one cannot see them by staining samples of tissue and examining the retina's anatomy. The centre—surround structures are logical i. It is believed that the connection strength between cells is caused by the number and types of ion channels embedded in the synapses between the bipolar and ganglion cells.
The centre—surround structures are mathematically equivalent to the edge detection algorithms used by computer programmers to extract or enhance the edges in a digital photograph. Thus, the retina performs operations on the image-representing impulses to enhance the edges of objects within its visual field. For example, in a picture of a dog, a cat and a car, it is the edges of these objects that contain the most information.
In order for higher functions in the brain or in a computer for that matter to extract and classify objects such as a dog and a cat, the retina is the first step to separating out the various objects within the scene. As an example, the following matrix is at the heart of a computer algorithm that implements edge detection. This matrix is the computer equivalent to the centre—surround structure. In this example, each box element within this matrix would be connected to one photoreceptor.
The photoreceptor in the centre is the current receptor being processed. The sum of all nine of these elements is finally calculated. This summation is repeated for every photoreceptor in the image by shifting left to the end of a row and then down to the next line.
The total sum of this matrix is zero, if all the inputs from the nine photoreceptors are of the same value. The zero result indicates the image was uniform non-changing within this small patch. Negative or positive sums mean the image was varying changing within this small patch of nine photoreceptors. The above matrix is only an approximation to what really happens inside the retina.
The differences are:. Once the image is spatially encoded by the centre—surround structures, the signal is sent out along the optic nerve via the axons of the ganglion cells through the optic chiasm to the LGN lateral geniculate nucleus. The exact function of the LGN is unknown at this time. The output of the LGN is then sent to the back of the brain.
Specifically, the output of the LGN "radiates" out to the V1 primary visual cortex. There are many inherited and acquired diseases or disorders that may affect the retina. Some of them include:. In addition, the retina has been described as a "window" into the brain and body, given that abnormalities detected through an examination of the retina can discover both neurological and systemic diseases.
A number of different instruments are available for the diagnosis of diseases and disorders affecting the retina. Ophthalmoscopy and fundus photography have long been used to examine the retina. Recently, adaptive optics has been used to image individual rods and cones in the living human retina, and a company based in Scotland has engineered technology that allows physicians to observe the complete retina without any discomfort to patients.
The electroretinogram is used to non-invasively measure 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 3D volumetric or high resolution cross-sectional tomogram of the fine structures of the retina, with histologic quality.
Retinal vessel analysis is a non-invasive method to examine the small arteries and veins in the retina which allows to draw conclusions about the morphology and the function of small vessels elsewhere in the human body. It has been established as a predictor of cardiovascular disease  and seems to have, according to a study published in , potential in the early detection of Alzheimer's disease.
Gene therapy holds promise as a potential avenue to cure a wide range of retinal diseases. This involves using a non-infectious virus to shuttle a gene into a part of the retina. Recombinant adeno-associated virus rAAV vectors possess a number of features that render them ideally suited for retinal gene therapy, including a lack of pathogenicity, minimal immunogenicity, and the ability to transduce postmitotic cells in a stable and efficient manner.
Each cell type can be specifically targeted by choosing the appropriate combination of AAV serotype , promoter, and intraocular injection site. Several clinical trials have already reported positive results using rAAV to treat Leber's congenital amaurosis , showing that the therapy was both safe and effective.
The methods used varied among the three trials, but included both functional methods such as visual acuity    and functional mobility    as well as objective measures that are less susceptible to bias, such as the pupil's ability to respond to light   and improvements on functional MRI.
The unique architecture of the retina and its relatively immune-privileged environment help this process. The highly compartmentalized anatomy of the eye facilitates accurate delivery of therapeutic vector suspensions to specific tissues under direct visualization using microsurgical techniques.
In addition, the eye and the visual system can be routinely and easily monitored for visual function and retinal structural changes after injections with noninvasive advanced technology, such as visual acuities, contrast sensitivity , fundus auto-fluorescence FAF , dark-adapted visual thresholds, vascular diameters, pupillometry, electroretinography ERG , multifocal ERG and optical coherence tomography OCT.
This strategy is effective against a number of retinal diseases that have been studied, including neovascular diseases that are features of age-related macular degeneration , diabetic retinopathy and retinopathy of prematurity. Since the regulation of vascularization in the mature retina involves a balance between endogenous positive growth factors , such as vascular endothelial growth factor VEGF and inhibitors of angiogenesis , such as pigment epithelium-derived factor PEDF , rAAV-mediated expression of PEDF, angiostatin, and the soluble VEGF receptor sFlt-1, which are all antiangiogenic proteins, have been shown to reduce aberrant vessel formation in animal models.
Neurotrophic factors have the ability to modulate neuronal growth during development to maintain existing cells and to allow recovery of injured neuronal populations in the eye. AAV encoding neurotrophic factors such as fibroblast growth factor FGF family members and GDNF either protected photoreceptors from apoptosis or slowed down cell death.
Organ transplantation Transplantation of retinas has been attempted, but without much success. At MIT , The University of Southern California, RWTH Aachen University, 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.
Around BCE , Herophilos identified the retina from dissections of cadaver eyes. He called it the arachnoid layer, from its resemblance to a spider web, and retiform , from its resemblance to a casting net. The top portion of the band divided by the top left and right slits is for connecting to the CDMA network.
The right side is not an antenna but serves to cosmetically mirror the left side and also to create a similar look to the GSM version of the phone. The internal components are situated between two panels of aluminosilicate glass, described by Apple as being "chemically strengthened to be 20 times stiffer and 30 times harder than plastic," theoretically allowing it to be more scratch-resistant and durable than the prior models.
In fall , pentalobe screws started to replace the Philips screws used in post-repair units in the US and in production units in Japan. The iPhone 4 supports up to iOS 7 , released in September Due to the relatively aged hardware of the iPhone 4, certain features available on newer iPhone models are not available on the device; as of iOS 7, they include Siri , 3D maps and turn-by-turn navigation, AirDrop , AirPlay mirroring, live camera filters, panorama mode and certain visual effects introduced by iOS 7, such as the blurring of translucent interface elements, the parallax effect on the home screen, and live wallpapers.
An iPhone 4 Bumper is a ring of rubber and plastic that surrounds the edge of the iPhone 4, to protect the phone. The inner part is rubber with an external band made of plastic. The Bumper wraps around the edges of the device protecting the edges, and to a lesser extent, the screen. The Bumper does not cover the front or rear of the phone, though it does slightly raise the iPhone off whatever surface it is sitting on.
The six holes in the Bumper enable access to the mute switch, dock connector, headphone jack, speaker, and microphones. Apple updated the bumper with a wider volume switch hole with the release of the iPhone 4 for CDMA networks. The updated bumper is also compatible with iPhone 4S , the next-generation iPhone that has the same exterior design and measurements as the CDMA model of the iPhone 4, but with a SIM card slot as well.
Pre-release reception was largely positive. Fox News commented that "seeing it in action is far more informative than staring at Gizmodo's photos of a busted test unit". The article noted how the iPhone 4, unlike current cameras, can record HD content and then edit it from the same device, labeling it a "true mobile editing suite". Reviews of the iPhone 4 were largely positive.
Joshua Topolsky of Engadget described the device's industrial design as being "more detailed and sophisticated" than its predecessor. Some users reported a yellow discoloration of the screen that disappeared after several days, which was attributed to the glass lamination glue that was used. It did not have time to dry fully before the unit reached consumers due to the speed of manufacturing. Shortly after the iPhone 4 was launched, some consumers reported that signal strength of the phone was reduced when touching the lower left edge of the phone, bridging one of the two locations which separates the two antennas, resulting in dropped calls in some areas with lower signal reception.
The legal challenge was started by a law firm, who set up a website to recruit disenchanted iPhone 4 buyers for a lawsuit against Apple. Apple explained how the formula it used to calculate the number of bars to display was "wrong". Apple promised to correct the issue and release a software update within a few weeks that would address the "mistake", which had been present since the original iPhone.
Consumer Reports initially stated that the iPhone 4's signal issues are not "unique, and may not be serious" and it continued to mention that signal loss is a problem that is faced by the entire smartphone industry. It was such a problem that Apple made a formal apology onstage.
The next day, the magazine altered their stance after encountering instances of dropped calls. It sarcastically pointed out that using a piece of tape to cover the lower left antenna gap was one way to fix the problem, but recommended that consumers who "want an iPhone that works well without a masking-tape fix" purchase the iPhone 3GS instead. On July 16, , at a press conference which included a private tour of Apple's antenna design lab for journalists,  Steve Jobs announced that Apple would provide all iPhone 4 owners with a free case to help solve the antenna issue and a refund to those users who had already purchased a Bumper.
The free case offer would be valid until September 30, , when Apple would re-evaluate the situation. Jobs also announced that Apple could not produce enough Bumpers for all owners of the phone, but would source a supply and offer a range of cases. Consumer Reports noted that the solution was not permanent, though a good first step. From Wikipedia, the free encyclopedia. This is the latest accepted revision , reviewed on 27 April For the first 4G iPhone, see iPhone 5.
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The retina (from Latin: rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the. The third-generation MacBook Pro was released in , marketed as the "MacBook Pro with Retina display" to differentiate it from the previous model: the On October 30, , Apple released the third generation MacBook Air, with Amber Lake processors, a inch Retina display with a resolution of ×