third photoreceptor

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Assignment 2

Third Photoreceptor

Beside two classic photoreceptor cells, rods and cones, third one aiding in

vision system is photosensitive ganglion cells. Its discovery process started in 1990s

(Wikipedia.org). In 2002, however, a third photoreceptor was discovered and referred

as intrinsically photosensitive retinal ganglion cell, or ipRGC (Umich.edu).

Background

This discovery has its origins in attempts to understand how endogenous 24-

hour body clocks (circadian clocks) are regulated by light. In the early 1990s, mice

homozygous for gene defects, e.g. retinal degeneration (rd), and lacking any visual

responses to light were examined to determine the impact of rod/cone loss on

photoentrainment. Remarkably, rd/rd mice lacking functional rods and most cones

showed normal circadian responses to light (Foster, 1991). These and a host of

subsequent experiments, including studies in humans with genetic defects of the eye,

David, et al., (1998) & Czeisler, et al., (1995) showed that the processing of light

information by the circadian and classic visual systems must be different, and raised

the possibility that the eye may contain an additional non-rod, non-cone

photoreceptor. This initial thought work aided in further research on it and attracted

funding bodies. The major assumption here was that only a small number of rods

and/or cones were necessary for normal photoentrainment of the clock. To test this

assumption, a mouse was engineered in which all rods and cones were ablated (rd/rd

cl). Such genetic lesions had little effect on circadian responses to light, although loss

of the eyes completely abolished this capacity (Freedman, et al., 1992; Lucas, et al.,


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1999). According to Foster (2009), in mammals light-induced pupil constriction is

regulated by the rods and cones, yet multiple studies have shown that pupil

constriction still occurs after profound damage to these photoreceptors. However, it is

vital to note that rods and cones have major contribution to pupil constriction. While

the novel receptors drive constriction under sustained bright light conditions. Lucas,

et al., (2001) tested the rd/rd cl mouse for this assumption. Pupil measurements were

undertaken in rd/rd cl mice and showed that these animals were fully able to constrict

their pupils under bright light conditions.

Characteristics

This third photoreceptor, say scientists at Brown resides deeper in the retina

than rods and cones and looks remarkably different, more like the underside of a

canopy of twisted tree branches. There are about 1.3 million ganglion cells in the

human visual system, 1 to 2% of them photosensitive (Wikipedia.org).

Pigment

The opsin found in the photosensitive ganglion cells of the retina that are

involved in various reflexive responses of the brain and body to the presence of

(day)light, such as the regulation ofcircadian rhythms, pupillary reflex and other non-

visual responses to light, is called melanopsin. In structure, it is an opsin,

a retinylidene protein variety. When light activates the melanopsin signaling system,

the melanopsin-containing ganglion cells discharge nerve impulses that are conducted

through their axons to specific brain targets (Wikipdia.org).

Function
http://en.wikipedia.org/wiki/Ganglion_cellhttp://en.wikipedia.org/wiki/Circadian_rhythmhttp://en.wikipedia.org/wiki/Pupillary_reflexhttp://en.wikipedia.org/wiki/Melanopsinhttp://en.wikipedia.org/wiki/Retinylidene_proteinhttp://en.wikipedia.org/wiki/Nerve_impulsehttp://en.wikipedia.org/wiki/Axonhttp://en.wikipedia.org/wiki/Axonhttp://en.wikipedia.org/wiki/Nerve_impulsehttp://en.wikipedia.org/wiki/Retinylidene_proteinhttp://en.wikipedia.org/wiki/Melanopsinhttp://en.wikipedia.org/wiki/Pupillary_reflexhttp://en.wikipedia.org/wiki/Circadian_rhythmhttp://en.wikipedia.org/wiki/Ganglion_cell


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Though the functions overlap of ganglion photoreceptors, rods & cones but

this novel photoreceptor have many unique functions associated. However, few are its

distinctive functions which are as follows;

It has contribution to our awareness of environmental light. According toKellogg scientist Kwoon Y. Wong, Ph.D. studies its main function is to gauge

ambient light intensity rather than analyze spatial details.

Wong believes ipRGCs absorb light and use the energy to generate nerveimpulses similar to the rods and cone. However unlike rods and cones, which

send their signals to regions of the brain that deal with conscious visual

perception, ipRGCs send their messages to other parts of the brain that

produce subconscious physiological responses to light. These responses affect

pupil constriction, enhance alertness, affect the release of hormones, and

synchronize daily rhythms like the sleepwake cycle to the environmental

lightdark cycle.

This receptor bestows individual a proper sense of time. It also turns lightenergy directly into brain signals. These signals govern the bodys 24-hour

clock. This third class of ocular photoreceptor is responsible for regulating

circadian clock (body clock). A blind individual sensitive to bright light

through pupil constriction should be encouraged to expose his or her eyes to

sufficient daytime light to maintain normal circadian entrainment and sleep

wake timing.

This subconscious visual system has been associated with many medicalconditions. For example, scientists have shown that if ipRGCs absorb too little

light during the day, depression and insomnia can occur. On the other hand, if
http://www.kellogg.umich.edu/bios/wong.htmlhttp://www.kellogg.umich.edu/bios/wong.html


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these cells receive too much light at night, diseases such as breast and prostate

cancer can develop. Subconscious vision has an enormous impact on our

well-being, says Dr. Wong. The discovery of ipRGCs has made it much

easier to investigate how different kinds of environmental light may influence

our body, and will expedite the development of new strategies to promote

health.

Most important, these newly discovered photoreceptors still function in manyblind patients whose rods and cones have completely degenerated

(Umich.edu).


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References

Czeisler, C.A., Shanahan, T.L., Klerman, E.B., et al. (1995). Suppression of

melatonin secretion in some blind patients by exposure to bright light.N Engl J

Med, 332, 611.

David, Z.K., Janssen, J.W., DeGrip, W.J., et al. (1998). Light detection in a blind

mammal.Nat Neurosci, 1, 6556.

Foster, R.G., Provencio, I., Hudson D, et al. (1991). Circadian photoreception in the

retinally degenerate mouse (rd/rd). J Comp Physiol, 169, 3950.

Foster, R.G. (2009). The Third Photoreceptor System of the Eye

Photosensitive Retinal Ganglion Cells.European Ophthalmic Review

2(1), 84-6

Freedman, M.S., Lucas, R.J., Soni, B., et al. (1992). Regulation of mammalian

circadian behavior by non-rod, non-cone, ocular photoreceptors. Science,

284, 5024.

Lucas, R.J., Freedman, M.S., Munoz, M., et al., (1999). Regulation of the

mammalian pineal by non-rod, non-cone, ocular photoreceptors. Science,

284, 5057.

Lucas, R.J., Douglas, R.H., Foster, R.G. (2001). Characterization of an ocular

photopigment capable of driving pupillary constriction in mice,Nat

Neurosci, 4, 6216.

http://brown.edu/Administration/News_Bureau/2001-02/01-080.html

http://www.kellogg.umich.edu/news/newsletter/fall2010/rodscones.html

http://en.wikipedia.org/wiki/Photoreceptor_cell
http://brown.edu/Administration/News_Bureau/2001-02/01-080.htmlhttp://www.kellogg.umich.edu/news/newsletter/fall2010/rodscones.htmlhttp://en.wikipedia.org/wiki/Photoreceptor_cellhttp://en.wikipedia.org/wiki/Photoreceptor_cellhttp://en.wikipedia.org/wiki/Photoreceptor_cellhttp://www.kellogg.umich.edu/news/newsletter/fall2010/rodscones.htmlhttp://brown.edu/Administration/News_Bureau/2001-02/01-080.html


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