Retina 48334 225416832 2008-07-13T16:00:28Z CopperKettle 1434427 /* Anatomy of vertebrate retina */ wfy Muller cell as Muller glia {{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|NIH]] [[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 vertebrate '''retina''' is a light sensitive part inside the inner layer of the [[eye]]. Two of its three types of [[photoreceptor cell]]s, [[rod cell|rods]] and [[cone cell|cones]], receive light and transform it into image-forming signals which are transmitted through the [[optic nerve]] to the [[brain]]. In this respect, the retina is comparable to the film in a camera. The third and more recently discovered category of [[Photosensitive ganglion cell|photosensitive cells]] is probably not involved in image-forming vision. These are a small proportion, about 2% in humans, of the retina's ganglion cells, themselves photosensitive through the photopigment [[melanopsin]], which transmit information about light through the [[retinohypothalamic tract|RHT]] (retinohypothalamic tract) directly to the [[suprachiasmatic nucleus|SCN]] (suprachiasmatic nucleus) and other brain structures. Signals from these ganglion cells are used to adjust the size of the pupil, [[Entrainment (chronobiology)|entrain]] the body's [[circadian rhythm]]s and acutely suppress the [[Pineal gland|pineal]] hormone [[melatonin]], processes which in fact function in many blind people who do not have functioning rods and cones. While rods and cones respond maximally to wavelengths around 555 nanometers (green), the light sensitive ganglion cells respond maximally to about 480nm (blue-violet). There are several different photopigments involved. Neural signals from the rods and cones undergo complex processing by other [[neuron]]s of the retina. The output takes the form of [[action potential]]s in retinal ganglion cells whose [[axon]]s form the [[optic nerve]]. Several important features of [[visual perception]] can be traced to the retinal encoding and processing of light. 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 unique structure of the [[blood vessel]]s in the retina has been used for [[Retinal scan|biometric identification]]. ==Anatomy of vertebrate retina== [[Image:Gray881.png|thumb|300px|Section of retina.]] The vertebrate retina has ten distinct layers.<ref>[http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/RETINA.HTM The Retinal Tunic<!-- Bot generated title -->]</ref> From innermost to outermost, they include: # ''[[Inner limiting membrane]]'' - [[Muller glia|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|macular region]], 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|Rods]] / [[Cone cell|Cones]] # ''[[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 [[primate]]s 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)|meridian]] is about 3.2 mm. [[Image:Fig_retine.png|thumb|left|400px|Retina's simplified axial organization. The retina is a stack of several neuronal layers. Light is concentrated from the eye and passes across these layers (from left to right) to hit the photoreceptors (right layer). This elicits chemical transformation mediating a propagation of signal to the bipolar and horizontal cells (middle yellow layer). The signal is then propagated to the amacrine and ganglion cells. These neurons ultimately may produce action potentials on their axons. This spatiotemporal pattern of spikes determines the raw input from the eyes to the brain. (Modified from a drawing by [[Santiago Ramón y Cajal|Ramón y Cajal]].)]] In section the retina is no more than 0.5 mm thick. It has three layers of [[nerve]] cells and two of [[synapse]]s. The optic nerve carries the [[ganglion cell]] [[axon]]s 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 cell]]s 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 [[neuropil]]s 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. ===Vertebrate and cephalopod retina differences=== 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 rods and cones. 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)|homologous]] but have evolved separately. ==Physiology== An image is produced by the "patterned excitation" of the cones and rods in the retina. 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|colour 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 cell]]s which in turn signal the retinal ganglion cells. The receptors are also 'cross-linked' by [[horizontal cell]]s and [[amacrine cell]]s, 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|saturate]] 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 [[cyclic guanosine monophosphate]] (cGMP) keep 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|isomerise]] to [[retinal|trans-retinal]]. This causes receptor to activate multiple [[G-protein]]s. This in turn causes the Ga-subunit of the protein to bind and degrade cGMP inside the cell which then cannot bind to the Na+ [[cyclic nucleotide-gated ion channel]]s (CNGs). 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 [[Adaptation (eye)]]). 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. 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 130 million retinal receptors, there are only approximately 1.2 million fibres (axons) in the optic nerve; 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== [[Image:Receptive field.jpg|frame|right|On-centers and off-centers of the retina]] The retina, unlike a camera, does not simply send a picture to the brain. The retina spatially encodes (compresses) the image to fit the limited capacity of the optic nerve. Compression is necessary because there are 100 times more [[Photoreceptor cell]]s than ganglion cells as mentioned above. The retina does so by [[Decorrelation |"decorrelating"]] the incoming images in a manner to be described below. These operations are carried out by the center surround structures as implemented by the bipolar and ganglion cells. There are two types of center surround structures in the retina -- on-centers and off-centers. On-centers have a positively weighted center and a negatively weighted surround. Off-centers are just the opposite. Positive weighting is more commonly known as [[Chemical_synapse#Receptor_binding|excitatory]] and negative weighting is more commonly known as [[Chemical_synapse#Receptor_binding|inhibitory]]. These center surround structures are not physical in the sense that you cannot see them by staining samples of tissue and examining the retina's anatomy. The center surround structures are logical (i.e., mathematically abstract) in the sense that they depend on the connection strengths between ganglion and bipolar cells. It is believed that the connection strengths between cells is caused by the number and types of [[ion channel]]s embedded in the [[synapse]]s between the ganglion and bipolar cells. [[Stephen Kuffler]] in the 1950s was the first person to begin to understand these center surround structures in the retina of cats. See [[Receptive field]] for figures and more information on center surround structures. See chapter 3 of David [[Hubel]]'s on-line book (listed below) for an excellent introduction. The center 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 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 (mathematics)|matrix]] is at the heart of the computer [[algorithm]] that implements edge detection. This matrix is the computer equivalent to the center surround structure. In this example, each box (element) within this matrix would be connected to one photoreceptor. The photoreceptor in the center is the current receptor being processed. The center photoreceptor is multiplied by the +1 weight factor. The surrounding photoreceptors are the "nearest neighbors" to the center and are multiplied by the -1/8 value. 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 the same value. The zero result indicates the image was uniform (non-changing) within this small patch. Negative or positive sums mean something was varying (changing) within this small patch of nine photoreceptors. <TABLE BORDER=1> <TR><TD BGCOLOR=lightyellow>-1/8<TD BGCOLOR=lightyellow>-1/8<TD BGCOLOR=lightyellow>-1/8 <TR><TD BGCOLOR=lightyellow>-1/8<TD BGCOLOR=lightblue> +1 <TD BGCOLOR=lightyellow>-1/8 <TR><TD BGCOLOR=lightyellow>-1/8<TD BGCOLOR=lightyellow>-1/8<TD BGCOLOR=lightyellow>-1/8 </TABLE> The above matrix is only an approximation to what really happens inside the retina. First, the above example is called "balanced". The term balanced means that the sum of the negative weights is equal to the sum of the positive weights so that they cancel out perfectly. Retinal ganglion cells are almost never perfectly balanced. Second, the table is square while the center surround structures in the retina are circular. Third, neurons operate on spike trains traveling down nerve cell [[axons]]. Computers operate on a single number that is essentially constant from each input [[pixel]]. (The computer pixel is basically the equivalent of a biological photoreceptor.) Fourth, the retina performs all these calculations in parallel while the computer operates on each pixel one at a time. There are no repeated summations and shifting as there would be in a computer. Finally, the [[horizontal cell|horizontal]] and [[amacrine cell]]s play a significant role in this process but that is not represented here. Here is an example of an input image and how edge detection would modify it. [[Image:Edge-detection-2.jpg]] Once the image is spatially encoded by the center surround structures, the signal is sent out the optical 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]]. Simplified Signal Flow: Photoreceptors ==> Bipolor ==> Ganglion ==> Chiasm ==> LGN ==> V1 cortex [[Image:ERP_-_optic_cabling.jpg|500px]] ==Diseases and disorders== {{main|List of eye 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|cones]] and/or [[Rod cell|rods]] 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 [[dog]]s 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|electroretinogram]] is used to measure [[non-invasive (medical)|non-invasively]] 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|3D]] volumetric or high resolution cross-sectional [[tomogram]] of the retinal fine structure with [[histologic]]-quality. [[Image:Retina-OCT800.png|thumb|OCT scan of a retina at 800nm with an axial resolution of 3µm]] Treatment depends upon the nature of the disease or disorder. [[Organ transplant|Transplantation]] of retinas has been attempted, but without much success. At [[Massachusetts Institute of Technology|MIT]], 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 (computing)|bandwidth]] of human retinas is 8.75 megabits per second, whereas a [[guinea pig]] retinas transfer at 875 kilobits. <ref>[http://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight.html Calculating the speed of sight - being-human - 28 July 2006 - New Scientist<!-- Bot generated title -->]</ref> [[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. <ref>[http://www.nature.com/nature/journal/v444/n7116/abs/nature05161.html Retinal repair by transplantation of photoreceptor precursors : Abstract : Nature<!-- Bot generated title -->]</ref> == References == <div class="references-small"> {{reflist}} </div> * [[S. Ramón y Cajal]], ''Histologie du Système Nerveux de l'Homme et des Vertébrés'', Maloine, Paris, 1911. * {{cite journal |author=Meister M, Berry MJ |title=The neural code of the retina |journal=Neuron |volume=22 |issue=3 |pages=435–50 |year=1999 |pmid=10197525| doi = 10.1016/S0896-6273(00)80700-X <!--Retrieved from CrossRef by DOI bot-->}} * {{cite journal |author=Rodieck RW |title=Quantitative analysis of cat retinal ganglion cell response to visual stimuli |journal=Vision Res. |volume=5 |issue=11 |pages=583–601 |year=1965 |pmid=5862581| doi = 10.1016/0042-6989(65)90033-7 <!--Retrieved from CrossRef by DOI bot-->}} * {{cite book |author=Wandell, Brian A. |title=Foundations of vision |publisher=Sinauer Associates |location=Sunderland, Mass |year=1995 |pages= |isbn=0-87893-853-2 |oclc= |doi=}} * J. J. Atick and A. N. Redlich, ''What does the retina know about natural scenes?'', Neural Computation, p. 196-210, 1992. * {{cite journal |author=Schulz HL, Goetz T, Kaschkoetoe J, Weber BH |title=The Retinome - defining a reference transcriptome of the adult mammalian retina/retinal pigment epithelium |journal=BMC Genomics |volume=5 |issue=1 |pages=50 |year=2004 |pmid=15283859 |doi=10.1186/1471-2164-5-50}} ==See also== *[[Charles Schepens]] - "the father of modern retinal surgery" ==External links== *[http://neuro.med.harvard.edu/site/dh/index.html Eye, Brain, and Vision - online book - by David Hubel] * Kolb, H., Fernandez, E., & Nelson, R. (2003). [http://webvision.med.utah.edu The neural organization of the vertebrate retina]. Salt Lake City, Utah: John Moran Eye Center, [[University of Utah]]. Retrieved July 19, 2004. * [http://www.techreview.com/articles/04/09/demo0904.asp Demo: Artificial Retina], MIT Technology Review, September 2004. Reports on implant research at [[Technology Review]] * [http://www.technologyreview.com/Biotech/17768 Successful photoreceptor transplantation], MIT Technology Review, November 2006. How stem cells might restore sight [[Technology Review]] * [http://bionic.gsbme.unsw.edu.au/ Australian Vision Prosthesis Group], Graduate School of Biomedical Engineering, [[University of New South Wales]] * [http://www.retinacentral.org RetinaCentral], Genetics and Diseases of the Human Retina at [[University of Würzburg]] * [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.figgrp.740 Retinal layers image.] NeuroScience 2nd Ed at [[United States National Library of Medicine]] * [http://webvision.med.utah.edu/ The Organization of the Retina and Visual System] at [[University of Utah]] * {{BUHistology|07901loa}} {{Eye}} {{Retina}} [[Category:Visual system]] [[Category:Eye]] [[ar:شبكية]] [[ca:Retina]] [[cs:Sítnice]] [[de:Netzhaut]] [[et:Võrkkest]] [[es:Retina]] [[eo:Retino]] [[eu:Erretina]] [[fr:Rétine]] [[ko:망막]] [[hr:Mrežnica (oko)]] [[io:Retino]] [[id:Retina]] [[it:Retina]] [[he:רשתית]] [[lt:Tinklainė]] [[nl:Netvlies]] [[ja:網膜]] [[no:Netthinne]] [[pl:Siatkówka (anatomia)]] [[pt:Retina]] [[ro:Retină]] [[ru:Сетчатка]] [[simple:Retina]] [[sk:Sietnica]] [[sl:Mrežnica]] [[fi:Verkkokalvo]] [[sv:Näthinna]] [[tr:Retina]] [[uk:Сітківка]] [[zh:视网膜]]