Photosensitive ganglion cell

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Photosensitive ganglion cells, or melanopsin-containing ganglion cells, are a recently discovered type of nerve cell in the retina of the eye which, unlike other retinal ganglion cells, are intrinsically photosensitive. This means that they are excited by light even when all influences from classical photoreceptors (rods and cones) are blocked, either by applying pharmacological agents or by dissociating the ganglion cell from the retina. Photosensitive ganglion cells contain the photopigment melanopsin. The giant retinal ganglion cells of the primate retina are examples of photosensitive ganglion cells.

Photosensitive ganglion cells play an important role in synchronizing circadian rhythms to the rising and setting of the sun. They send light information directly to the circadian pacemaker of the brain, the suprachiasmatic nucleus of the hypothalamus. The physiological properties of these ganglion cells match known properties of the synchronization or "light entrainment" mechanism regulating circadian rhythms. This is the mechanism that allows us to overcome jet lag.

Photosensitive ganglion cells also innervate various other brain targets, such as the center of pupillary control, the olivary pretectal nucleus of the midbrain. They contribute to the regulation of pupil size and other behavioral responses to ambient lighting conditions. Photosensitive ganglion cells are responsible for the persistence of circadian and pupillary light responses in mammals with degenerated rod and cone photoreceptors, such as humans suffering from retinitis pigmentosa.

The photopigment of these cells, melanopsin, absorbs light mainly in the blue portion of the visible spectrum (best wavelength = 480 nanometers). The phototransduction mechanism in these cells is not fully understood, but seems likely to resemble that in invertebrate rhabdomeric photoreceptors. Photosensitive ganglion cell respond to light by depolarizing and increasing the rate at which they fire nerve impulses. In addition to responding directly to light, these cells receive excitatory and inhibitory influences from rods and cones by way of synaptic connections in the retina.

[edit] References

Berson, D.M. (2003). Strange vision: ganglion cells as circadian photoreceptors. Trends in Neuroscience 26:314-320.
Dacey, D.M. et al. (2005). Melanopsin-expressing ganglion cells in primate retina signal colour and

irradiance and project to the LGN. Nature. 433(7027):749-54.