Sel fotoreseptor: Perbedaan antara revisi

Cones require significantly brighter light (i.e., a larger numbers of photons) in order to produce a signal. In humans, there are three different types of cone cell, distinguished by their pattern of response to different wavelengths of light. Color experience is calculated from these three distinct signals, perhaps via an [[opponent process]].<ref name=Hurvich>{{cite book|last1= Hurvich|first1= Leo|title= Color Vision|url= https://archive.org/details/colorvision00hurv| publisher= Sinauer|address = Sunderland, MA|year= 1981}}</ref> The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths. Note that, due to the [[W. A. H. Rushton#Principle of Univariance|principle of univariance]], the firing of the cell depends upon only the number of photons absorbed. The different responses of the three types of cone cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths. So, for example, an L cone cell contains a photoreceptor protein that more readily absorbs long wavelengths of light (i.e., more "red"). Light of a shorter wavelength can also produce the same response, but it must be much brighter to do so.

When light hits a photoreceptive pigment within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the plasma membrane), attached to a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes it to activate a regulatory protein called [[transducin]], which leads to the activation of [[cGMP phosphodiesterase]], which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters.<ref name="Kandel">{{cite book |last= Kandel |first= E. R. |coauthors= Schwartz, J.H.; Jessell, T.M. |title= Principles of Neural Science |url= https://archive.org/details/principlesneural00kand_658 |edition= 4th |year= 2000 |publisher= McGraw-Hill |location= New York |isbn= 0-8385-7701-6 |pages= 507–513[https://archive.org/details/principlesneural00kand_658/page/n436 507]–513 }}</ref> The entire process by which light initiates a sensory response is called [[visual phototransduction]].

* {{en}} {{cite book|author=Campbell, Neil A., and Reece, Jane B.|title=Biology|url=https://archive.org/details/biologyc00camp|publisher=Benjamin Cummings|location=San Francisco|year=2002|pages=1064–1067[https://archive.org/details/biologyc00camp/page/1064 1064]–1067|isbn=0-8053-6624-5}}