Physical structure of human retina
Inside big human being the entire retina is 72% of a sphere astir Twenty-two millimeter from side to side. At a centre of the retina is the optic nerve. This spot is referred to as a blind spot as it lacks photoreceptors. It appears as an elliptic whiten vicinity of Terzetto millimetre^Two. Temporal (in the counsel of the temples) to this disc is the macula. At its center is the fovea, a pit that is virtually all sensitive to weak & is responsible my acutely central vision. Individual & non-person primates possess 1 fovea centralis centralis when opposed to certain bird mintage like a hawk world health organization actually come bifoviate & dogs & cats which possess there are no fovea however a central band called the ocular streak. Around a fovea centralis extends a central retina for astir 6mm so a peripheral retina. A edge of the retina is defined per ora serrata. A length from either a single ora to a more (or even macula), a virtually all sensitive locality along the horizontal meridian is about Troika.Ii millimeter.

Within division a retina is no extra than Cipher.Fin millimeter midst. It has deuce-ace shells of nerve cells and ii of synapses. A optic nerve carries a ganglion cell axons to a brain and a blood vessels that open into the retina. Maybe as a product of evolution, a ganglion cells lie innermost in the retina when the photoreceptive cells lie outmost. Because of this arrangement, weak must foremost pass through a thickness of the retina prior to reaching the rods & cones. Still it doesn't pass through the epithelium or the choroid (both of which are then unintelligible).

A white blood cells in the capillaries in front of the photoreceptors can be perceived whenever midget brightly moving dots when seeking into blue weak. This is referred to as a blue field entoptic phenomenon (or Scheerer's phenomenon).

Between a ganglion cell layer & a rods & cones there are ii shells of neuropils where synaptic contacts come processed. A neuropil shells come a outer plexiform layer and a inner plexiform layer. In a outer a rod & cones attach to the vertically heading bipolar cells & the horizontally orientated horizontal cells attach to ganglion cells.

A central retina is cone-dominated & a peripheral retina is rod-dominated. Around number there are astir six million cones & the hundred & twenty-5 million rods. At a centre of a macula is the foveal pit around which the cones come little & in a hexangular mosaic, the virtually all effective & greatest density. Beneath a pit a more retina shells come displaced, prior even to building higher along a foveal slope until a rim of the fovea centralis or parafovea which is the thickest portion of the retina. a macula has a yellow pigmentation from either screening pigments & is referred to as the macula lutea.

Physiology
An image is by a "patterned excitation" of a retinal receptors, the cones & rods. the excitation is made per neural models & various area of the brawithin working in parallel to form a representation of the external environment in the brain.

A cones respond to brilliantly weak & in-between high-guide vison & colour vision. A rods respond to dim weak & in-between lower-guide, black-&-white, nighttime vision. These are the deficiency of cones sensitive to red, blue, or even even green weak that is the causal agent of souls to use at times deficiencies within colour vision or various kinda colour blindness. Homo & old globe monkeys use ternion different types of cones (trichrome vision) when more mammalian lack cones by having red sensitive pigment & so use poorer (bichrome) colour vision. Once weak lessens in the receptor it sends the proportional response synaptically to bipolar cells which in turn signal a retinal ganglion cells. A receptors come besides 'cross-linked' by horizontal cells and amacrine cells, which modify a synaptic signal prior to the ganglion cells. Rod & cone signals come intermixed & combine, although rods come mostly active withwithin super ill lit conditions & saturate in wide daylightly, spell cones are non sensitive plenty to function at super great light levels.

Despite tons existence neuron, sole a retinal ganglion cells & pack amacrine cells produce action potentials. within the photoreceptors, exposure to weak hyperpolarizes the membrane in a series of stratified shifts. the outer cell section contains a photopigment and a run leads to the vary around levels of cyclic GMP, altering the na conductance of the membrane. A total of neurotransmitter freed is reduced within brilliantly weak & increases when weak levels fall. a actual photopigment is bleached away inside brightly weak & merely replaced as a chemical action, and so inside a transition from either brilliantly weak to darkness the eye potty choose as much as xxx proceedings to email to the full sensitivity (look at dark adaptation).

In the retinal ganglion cells there are deuce types of response, based on the receptive field of the cell. a open fields of retinal ganglion cells comprise a central close to round front yard, in which weak has a single burden on the firing of the cell, & an ring-shaped environment, in which weak has the paired result on the firing of the cell. Withwithin In cells, an increment in candlepower in a centre of the open field is the causal agent of the firing rate to increase. Within OFF cells, it makes it decrease. Beyond this elementary difference ganglion cells come besides differentiated by chromatic sensitivity & a nature and severity of spacial summation. Cells showing linear spacial summation come termed 10 cells (too known as "parvocellular", "P", or even "midget" ganglion cells), & victims showing non-linear summation come Y cells (too known as "magnocellular, "M", or "sunshade" retinal ganglion cells), although the correspondence between X and Y cells (in the cat retina) and P and M cells (in the primate retina) is not as simple as it once seemed.

In the transfer of signal to the brain, the visual pathway, the retina is vertically divided in two, a temporal half and a nasal half. The axons from the nasal half cross the brain at the optic chiasma to join with axons from the temporal half of the other eye before passing into the lateral geniculate body.

Although there are more than 130 million retinal receptors, there are only approximately 1.2 million fibres (axons) in the optic nerve so a large amount of pre-processing is performed within the retina. The fovea produces the most accurate information. Despite occupying about 0.01% of the visual field (less than 2° of visual angle), about 10% of axons in the optic nerve are devoted to the fovea. The resolution limit of the fovea has been determined at around 10⁴ points. The information capacity is estimated at 5 x 10⁵ bits per second (for more information on bits, see information theory) without colour or around 6 x 10⁵ bits per second including colour.

Diseases and disorders
Retinitis pigmentosa is a genetic disease that affects the retina and causes the loss of peripheral vision. Macular degeneration describes a group of diseases characterized by loss of central vision because of death or impairment of the cells in the macula. In retinal separation, the retina detaches from the back of the eyeball. Ignipuncture is an outdated treatment method. Both hypertension and diabetes mellitus can cause damage to the tiny blood vessels that supply the retina, leading to hypertensive retinopathy and diabetic retinopathy. Retinoblastoma is a cancer of the retina.

Diagnosis and treatment
An ophthalmoscope is used to examine the retina. Recently, adaptive optics have been used to image individual rods and cones in the living human retina. The electroretinogram is used to measure non-invasively the retina's electrical activity, which is affected by certain diseases. A relatively new technology, which is recently becoming widespreadly available is optical coherence tomography (OCT). This non-invasive technique allows to obtain a 3D volumetric or high resolution cross-sectional tomogram of the retinal fine structure with histologic-quality.

Transplantation of retinas has been attempted, but without much success.

At MIT and the University of New South Wales, an "artificial retina" is under development: an implant which will bypass the photoreceptors of the retina and stimulate the attached nerve cells directly, with signals from a digital camera.

Difference between vertebrate and cephalopod retinas
As described above, the vertebrate retina is inverted in the sense that the light sensing cells sit at the back side of the retina, so that light has to pass through a layer of neurons before it reaches the photoreceptors. By contrast, the cephalopod retina is everted: the photoreceptors are located at the front side of the retina, with processing neurons behind them. Because of this, cephalopods do not have a blind spot.

The cephalopod retina does not originate as an outgrowth of the brain, as the vertebrate one does. This shows that vertebrate and cephalopod eyes are not homologous but have evolved separately.

Research
George Wald, Haldan Keffer Hartline and Ragnar Granit won the 1967 Nobel Prize in Physiology or Medicine for their scientific research on the retina.