Editing Photoreceptor cell (section)

== Histology ==
{{multiple image
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| image1    = Rod&Cone.jpg
| alt1      = Anatomy of a Rod Cell<ref>Human Physiology and Mechanisms of Disease by Arthur C. Guyton (1992) {{ISBN|0-7216-3299-8}} p. 373</ref>
| image2    = Cone cell en.png
| alt2      = Cone cell structure
| footer            = Anatomy of rods and cones varies slightly.
| footer_align      = center 
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[[Rod cell|Rod]] and [[Cone cell|cone]] photoreceptors are found on the outermost layer of the [[retina]]; they both have the same basic structure.  Closest to the visual field (and farthest from the brain) is the [[axon]] terminal, which releases a [[neurotransmitter]] called [[glutamate]] to [[bipolar cell]]s. Farther back is the [[Soma (biology)|cell body]], which contains the cell's [[organelle]]s. Farther back still is the inner segment, a specialized part of the [[cell (biology)|cell]] full of [[mitochondrion|mitochondria]]. The chief function of the inner segment is to provide [[Adenosine triphosphate|ATP]] (energy) for the [[sodium-potassium pump]]. Finally, closest to the [[brain]] (and farthest from the field of view) is the outer segment, the part of the photoreceptor that absorbs [[light]]. Outer segments are actually modified [[cilia]]<ref name=Richardson>{{cite journal|last1= Richardson|first1= T.M.|title = Cytoplasmic and ciliary connections between the inner and outer segments of mammalian visual receptors |journal= Vision Research|volume= 9|issue= 7|pages= 727–731| year= 1969|doi=10.1016/0042-6989(69)90010-8|pmid= 4979023}}</ref><ref name=Louvi>{{cite journal|last1= Louvi|first1= A.|last2 = Grove|first2= E. A.|title = Cilia in the CNS: The quiet organelle claims center stage|journal= Neuron|volume= 69|issue= 6|pages= 1046–1060| year= 2011|doi = 10.1016/j.neuron.2011.03.002|pmid=21435552|pmc=3070490}}</ref>  that contain disks filled with [[opsin]], the molecule that absorbs photons, as well as voltage-gated [[sodium channel]]s.

The membranous [[photoreceptor protein]] ''[[opsin]]'' contains a [[pigment]] molecule called ''[[retinal]]''. In rod cells, these together are called [[rhodopsin]]. In cone cells, there are different types of opsins that combine with retinal to form pigments called [[photopsin]]s. Three different classes of photopsins in the cones react to different ranges of light frequency, a selectivity that allows the visual system to [[Signal transduction|transduce]] [[color]]. The function of the photoreceptor cell is to convert the light information of the photon into a form of information communicable to the nervous system and readily usable to the organism: This conversion is called [[signal transduction]].

The opsin found in the intrinsically photosensitive ganglion cells of the retina is called [[melanopsin]]. These cells are involved in various reflexive responses of the brain and body to the presence of (day)light, such as the regulation of [[circadian rhythm]]s, [[pupillary reflex]] and other non-visual responses to light. Melanopsin functionally resembles invertebrate opsins.