Editing Visual system (section)

===Eye===
{{Main|Eye|Anterior segment of eyeball}}
Light entering the eye is [[refracted]] as it passes through the [[cornea]]. It then passes through the [[pupil]] (controlled by the [[Iris (anatomy)|iris]]) and is further refracted by the [[lens (vision)|lens]]. The cornea and lens act together as a compound lens to project an inverted image onto the retina.

[[File:Cajal Retina.jpg|thumb|left|[[S. Ramón y Cajal]], ''Structure of the [[Mammal]]ian Retina, 1900'']]
====Retina====
{{Main|Retina}}
The retina consists of many [[photoreceptor cell]]s which contain particular [[protein]] [[molecule]]s called [[opsin]]s. In humans, two types of opsins are involved in conscious vision: [[Rod cell|rod opsins]] and [[Cone cell|cone opsins]]. (A third type, [[melanopsin]] in some [[Retinal ganglion cell|retinal ganglion cells]] (RGC), part of the [[body clock]] mechanism, is probably not involved in conscious vision, as these RGC do not project to the [[lateral geniculate nucleus]] but to the [[Pretectal area|pretectal olivary nucleus]].<ref>{{Cite journal | last = Güler | first = A.D.  |date= May 2008 | title =  Melanopsin cells are the principal conduits for rod/cone input to non-image forming vision | journal = Nature | volume = 453 | issue = 7191 | pages = 102–5 | pmid =  18432195 | doi = 10.1038/nature06829| bibcode =2008Natur.453..102G | format = Abstract | pmc = 2871301 |display-authors=etal}}</ref>) An opsin absorbs a [[photon]] (a particle of light) and transmits a signal to the [[cell (biology)|cell]] through a [[signal transduction pathway]], resulting in hyper-polarization of the photoreceptor.

Rods and cones differ in function. Rods are found primarily in the periphery of the retina and are used to see at low levels of light. Each human eye contains 120 million rods. Cones are found primarily in the center (or [[Fovea centralis|fovea]]) of the retina.<ref name="HyperPhysics">{{cite web |last1=Nave |first1=R |title=Light and Vision |url=http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html |access-date=2014-11-13 |publisher=[[HyperPhysics]]}}</ref> There are three types of cones that differ in the [[wavelengths]] of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones mediate day vision and can distinguish [[color]] and other features of the visual world at medium and high light levels. Cones are larger and much less numerous than rods (there are 6-7 million of them in each human eye).<ref name="HyperPhysics" />

In the retina, the photoreceptors [[synapse]] directly onto [[bipolar cell of the retina|bipolar cell]]s, which in turn synapse onto [[retinal ganglion cell|ganglion cell]]s of the outermost layer, which then conduct [[action potentials]] to the [[brain]]. A significant amount of [[visual processing]] arises from the patterns of communication between [[neuron]]s in the retina. About 130 million photo-receptors absorb light, yet roughly 1.2 million [[axons]] of ganglion cells transmit information from the retina to the brain. The processing in the retina includes the formation of center-surround [[receptive fields]] of bipolar and ganglion cells in the retina, as well as convergence and divergence from photoreceptor to bipolar cell. In addition, other neurons in the retina, particularly [[Horizontal cell|horizontal]] and [[amacrine cell]]s, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to [[motion (physics)|motion]] or sensitive to color and indifferent to motion.<ref name="Tov2008">{{harvnb|Tovée|2008}}</ref>

===== Mechanism of generating visual signals =====
The retina adapts to change in light through the use of the rods. In the dark, the [[chromophore]] [[retinal]] has a bent shape called cis-retinal (referring to a ''cis'' conformation in one of the double bonds). When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. This is called bleaching because the purified [[rhodopsin]] changes from violet to colorless in the light. At baseline in the dark, the rhodopsin absorbs no light and releases [[glutamate]], which inhibits the bipolar cell. This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases, thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected.<ref>Saladin, Kenneth D. ''Anatomy & Physiology: The Unity of Form and Function''. 5th ed. New York: [[McGraw Hill Education|McGraw-Hill]], 2010.</ref><ref>{{Cite web |url=http://webvision.med.utah.edu/GCPHYS1.HTM |title=Webvision: Ganglion cell Physiology |access-date=2018-12-08 |archive-url=https://web.archive.org/web/20110123202041/http://webvision.med.utah.edu/GCPHYS1.HTM |archive-date=2011-01-23 }}</ref>

The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain:<ref name=Tov2008 />
#M cells, with large center-surround receptive fields that are sensitive to [[Depth perception|depth]], indifferent to color, and rapidly adapt to a stimulus;
#P cells, with smaller center-surround receptive fields that are sensitive to color and [[shape]];
#K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
#[[Photosensitive ganglion cell|another population that is intrinsically photosensitive]]; and
#a final population that is used for eye movements.<ref name=Tov2008 />

A 2006 [[University of Pennsylvania]] study calculated the approximate [[Bandwidth (computing)|bandwidth]] of human retinas to be about 8,960 [[Kilobit|kilobits]] per second, whereas [[guinea pig]] retinas transfer at about 875 kilobits.<ref>{{cite web|url=https://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight|title=Calculating the speed of sight}}</ref>

In 2007 Zaidi and co-researchers on both sides of the Atlantic studying patients without rods and cones, discovered that the novel photoreceptive ganglion cell in humans also has a role in conscious and unconscious visual perception.<ref name="Zaidi, 2007">{{cite journal |display-authors=etal |vauthors=Zaidi FH, Hull JT, Peirson SN |date=December 2007 |title=Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina |journal=[[Curr. Biol.]] |volume=17 |issue=24 |pages=2122–8 |doi=10.1016/j.cub.2007.11.034 |pmc=2151130 |pmid=18082405|bibcode=2007CBio...17.2122Z }}</ref> The peak [[spectral sensitivity]] was 481&nbsp;nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.

====Photochemistry====
{{Main|Visual cycle}}
The functioning of a [[camera]] is often compared with the workings of the eye, mostly since both focus light from external objects in the [[field of view]] onto a light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the laws of optics, the eye functions as  a [[transducer]], as does a [[Charge-coupled device|CCD camera]].

In the visual system, '''retinal''', technically called ''[[retinene]]''<sub>1</sub> or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of the [[retina]].  Retinal is the fundamental structure involved in the transduction of [[light]] into visual signals, i.e. nerve impulses in the ocular system of the [[central nervous system]].  In the presence of light, the retinal molecule changes configuration and as a result, a [[nerve impulse]] is generated.<ref name=Tov2008 /><!-- look up page from Tovée2008 -->