Light sensitive receptors

There are two main types of light-sensitive receptor cells in the retina of the human eye, rod cells and cone cells. The 3 million or so rod cells are... 

The vertebrate retina contains two classes of light-sensitive receptor cells called rods and cones. The rod is an elongated cylindrical cell containing several hundred thylakoids which support the visual pigment. The pigment system in the rod is confined to internal membranes situated close to the outer membrane of the cell. In the other type of visual receptor, the cone, the pigment is situated in the external membrane itself. In the cone the external... 

Rod and cone cells are the light sensitive receptor cells in the retina of the human eye. About three million rod cells are responsible for our vision in dim light, whereas the hundred million cone cells are responsible for our vision in the bright light and for the perception of bright colours. In the rod cells, ll-cw-retinal is converted to rhodopsin. 

The prototype family A receptor, rhodopsin itself, clearly functions as a monomer despite its occurrence in very high density in the light-sensitive membranes however, it could be argued that rhodopsin should not be used as an example for receptors in general as it has very special requirements in respect to signaling due to its function as an ultra-rapid light sensor. 

Among the many sensory reactions Phycomyces displays, the study of the photoreceptor and adaptation deserves maximal attention, since Phycomyces shares these two attributes with a variety of other blue light sensitive organisms. Action-spectroscopy indicates a flavin as the photoreceptor of Phycomyces. /3-carotene was positively ruled out as a possible receptor, since mutants with no trace amounts of )3-carotene are phototropical normal. The photoreceptor has not yet been isolated. As in other systems the difficulty consists in distinguishing the flavin photoreceptor from the bulk flavoproteins in the cell. One therefore needs unambiguous criteria for the identification of the photoreceptor. The most promising approach for an isolation would be a photoreceptor mutant and we described the properties those mutants should have. Until now there is no firm evidence that the photomutants, madA or madB are defective in the photoreceptor. 

The cell illustrated opposite, a rod, has a structure divided by membrane discs into which the 7-helix receptor rhodopsin is integrated (see p. 224). In contrast to other receptors in the 7-helix class (see p. 384), rhodopsin is a light-sensitive chromoprotein. Its protein part, opsin, contains the aldehyde retinal (see p. 364)—an isoprenoid which is bound to the e-amino group of a lysine residue as an aldimine. 

Figure 2.2 The retina consists of three layers. Retinal ganglion cells are located at the top followed by a layer of bipolar cells and receptors at the bottom. Light has to pass through the top two layers to reach the light-sensitive sensors. Information then travels upward from the receptors to the bipolar cells and on to the retinal ganglion cells. Information is also exchanged laterally through amacrine and horizontal cells. (Retina illustration from LifeART Collection Images 1989-2001 by Lippincott Williams Wilkins, used by permission from SmartDraw.com.

Ying SW, Rusak B. 5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons. Brain Res 1997 755 246-254. 

Another unexpected role of some skeletal pieces has been described. Ophiuroids are a large group of Echinoderms that includes the brittlestars. They have five arms, superficially resembling true starfishes (Asteroidea), and can catch fast motile prey. Aizenberg and coworkers have found that calcite crystals in their skeletons act as optical receptors or lenses. However, it is not known whether the system of lenses and nerves is actually an eye . However, these structures are absent in closely related but not light sensitive species of brittlestars. 

Epinephrine. Epinephrine (Adrenalin) finds use in a number of situations because of its potent stimulatory effects on both a- and /3-adrcncrgic receptors. Like the other catecholamines, epinephrine is light sensitive and easily oxidized on exposure to air because of the catechol ring system. The development of a pink to brown color indicates oxidative breakdown. To minimize oxidation, solutions of the drug are. stabilized by the addition of reducing agents such as sodium bisulfite. As the free amine, it is used in aqueous solution for inhalation. Like other amines, it forms salts with acids, for example, those now used include the hydrochloride and the bitartratc. Epinephrine is destroyed readily in alkaline solutions and by metals (c.g.. Cu, Fe, Zn), weak... 

Cones Light sensitive photo-receptor cells in the retina, shaped like cones. Cones are good at recognising colour, but poor at distinguishing changes in brightness (see Rods). 

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