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Retina

The retina is the part of the eye which translates the visual image into a neural signal. It terminates the optic nerve to the photoreceptive cells, retinal receptors (known as rods and cones), which translate the light coming into the eye into biochemical signals and finally to nerve impulses sent back up to the brain. The retina not only detects light, it also plays a significant part in visual perception.

Using a ophthalmoscope[?] an ophthalmologist can see through to the back of your eye to determine the health of the retina.

The unique structure of the blood cells that are formed in the retina makes it a good way of doing biometric identification.

Image of a human eye, showing the retina -- note that many animals have eyes with different anatomy

Physical structure

In adult humans the entire retina is a disc, around 40 mm in diameter. At the centre of the retina is the optic nerve. it appears as an oval white area of only 3 mm^2. To the left of the disc is the fovea[?] or macula, around the fovea extends the central retina for about 6mm and then the peripheral retina. The edge of the retina is defined by the ora serrata[?].

In section the retina is no more than 0.5 mm thick. It has five layers, three of nerve cells and two of synapses. The optic nerve carries the ganglion cell[?] axons to the brain and the blood vessels that open into the retina. As a product of evolution the the ganglion cells lie innermost in the retina while the photoreceptive cells lie outermost against the epithelium and choroid. Because of this light must first pass through the thickness of the retina before reaching the rods and cones.

Between the ganglion cell layer and the rods and cones there are two layers of neuropils where synaptic contacts are made. The neuropil layers are the outer plexiform layer and the the inner plexiform layer. In the outer the rod and cones connect to the vertically running bipolar cells and the horizontally oriented horizontal cells connect to ganglion cells.

The central retina is cone-dominated and the peripheral retina is rod-dominated, in total there are about a million cones and a hundred million rods. At the centre of the fovea is the foveal pit where the cones are smallest and in a hexagonal mosaic, the most efficient and highest density. Below the pit the other retina layers are displaced, before building up along the foveal slope until the rim of the fovea or parafovea which is the thickest portion of the retina. The fovea has a yellow pigmentation from screening pigments and is known to ophthalmologists as the macula lutea.

Operation

An image is produced by the "patterned excitation" of the retinal receptors, the cones and rods. The excitation is processed by the neuronal system and various parts of the brain working in parallel to form a representation of the external environment in the brain.

The cones are associated with bright light and colour vision, the rods with black-and-white, night vision. It is a lack of cones sensitive to red, blue or green spectrum light that causes individuals to have deficiencies in colour vision. When light falls on a receptor it sends a proportional response synaptically to bipolar cells[?] which in turn signal the retinal ganglion cells. The receptors are also 'cross-linked' by horizontal cells[?] and ammacrine cells[?], which modify the synaptic signal before the ganglion cells. Rod and cone signals are intermixed and combine.

Despite all being nerve cells only the retinal ganglion cells create action potentials. In the receptor cells the exposure to light hyperpolarizes the membrane in a series of graded shifts. The outer cell segment contains a photopigment and the process leads to a change in levels of cyclic GMP, altering the sodium conductance of the membrane. The amount of neurotransmitter released is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away in bright light and only replaced as a chemical process, so in a transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity.

In the retinal ganglion cells there are two types of response, depending on the receptive field of the cell. One response is by an increase in light intensity over background and the other by a decrease. These cells are sometimes termed 'on' and 'off' centre cells. Beyond this simple difference ganglion cells are also differentiated by chromatic sensitivity and the type of spatial summation. With spatial summation cells showing linear summation are termed X cells and those showing non-linear summation are Y cells.

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[?].

While there are a hundred million retinal receptors there are only a million or so fibres in the optic nerve so a large amount of pre-processing is performed within the retina so only the most significant information is passed to the brain. The fovea produces the most accurate information, despite occupying less than 2 of the visual field the rest of the retina produces only ten times the information generated at 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.

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