A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells. The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. Rods primarily contribute to night-time vision (scotopic conditions) whereas cones primarily contribute to day-time vision (photopic conditions), but the chemical process in each that supports phototransduction is similar. A third class of mammalian photoreceptor cell was discovered during the 1990s the intrinsically photosensitive retinal ganglion cells. These cells are thought not to contribute to sight directly, but have a role in the entrainment of the circadian rhythm and pupillary reflex.

There are major functional differences between the rods and cones. Rods are extremely sensitive, and can be triggered by a single photon. At very low light levels, visual experience is based solely on the rod signal.

Cones require significantly brighter light (that is, a larger number of photons) to produce a signal. In humans, there are three different types of cone cell, distinguished by their pattern of response to light of different wavelengths. Color experience is calculated from these three distinct signals, perhaps via an opponent process. This explains why colors cannot be seen at low light levels, when only the rod and not the cone photoreceptor cells are active. The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths, so they may respectively be referred to as S-cones, M-cones, and L-cones.

In accordance with the 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 (that is, more "red"). Light of a shorter wavelength can also produce the same response from an L cone cell, but it must be much brighter to do so.

The human retina contains about 120 million rod cells, and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Certain owls, such as the nocturnal tawny owl, have a tremendous number of rods in their retinae. In the human visual system, in addition to the photosensitive rods & cones, there are about 2.4 million to 3 million ganglion cells, with 1 to 2% of them being photosensitive. The axons of ganglion cells form the two optic nerves.

Photoreceptor cells are typically arranged in an irregular but approximately hexagonal grid, known as the retinal mosaic.

Optometry: Open Access Journal deals with eyes and related structures which includes such as vision, processing information related to vision. Optometrists are health professionals provide care related to vision. They give a clear idea related to diagnosis, treatment and management of vision. Optometry journal deals with medications, vision tests and abnormalities of eye.

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Optometry: Open Access
ISSN: 2476-2075
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