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Retina

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   Human eye cross-sectional view. Courtesy NIH National Eye Institute.
   Many animals have eyes different from the human eye.
   Enlarge
   Human eye cross-sectional view. Courtesy NIH National Eye Institute.
   Many animals have eyes different from the human eye.

   The retina is a thin layer of neural cells that lines the back of the
   eyeball of vertebrates and some cephalopods. In vertebrate embryonic
   development, the retina and the optic nerve originate as outgrowths of
   the developing brain. Hence, the retina is part of the central nervous
   system (CNS). It is the only part of the CNS that can be imaged
   directly.

   The vertebrate retina contains photoreceptor cells ( rods and cones)
   that respond to light; the resulting neural signals then undergo
   complex processing by other neurons of the retina. The retinal output
   takes the form of action potentials in retinal ganglion cells whose
   axons form the optic nerve. Several important features of visual
   perception can be traced to the retinal encoding and processing of
   light.

   The unique structure of the blood vessels in the retina has been used
   for biometric identification.

Anatomy of vertebrate retina

   Section of retina.
   Enlarge
   Section of retina.

   The vertebrate retina has ten distinct layers. From innermost to
   outermost, they include:
    1. Inner limiting membrane - Műller cell footplates
    2. Nerve fibre layer
    3. Ganglion cell layer - Layer that contains nuclei of ganglion cells
       and gives rise to optic nerve fibers.
    4. Inner plexiform layer
    5. Inner nuclear layer
    6. Outer plexiform layer - In the macular region, this is known as the
       Fibre layer of Henle.
    7. Outer nuclear layer
    8. External limiting membrane - Layer that separates the inner segment
       portions of the photoreceptors from their cell nuclei.
    9. Photoreceptor layer - Rods / Cones
   10. Retinal pigment epithelium

Physical structure of human retina

   In adult humans the entire retina is 72% of a sphere about 22 mm in
   diameter. At the centre of the retina is the optic disc, sometimes
   known as "the blind spot" because it lacks photoreceptors. It appears
   as an oval white area of 3 mm². Temporal (in the direction of the
   temples) to this disc is the macula. At its centre is the fovea, a pit
   that is most sensitive to light and is responsible for our sharp
   central vision. Human and non-human primates possess one fovea as
   opposed to certain bird species such as hawks who actually are
   bifoviate and dogs and cats who possess no fovea but a central band
   known as the visual streak. Around the fovea extends the central retina
   for about 6 mm and then the peripheral retina. The edge of the retina
   is defined by the ora serrata. The length from one ora to the other (or
   macula), the most sensitive area along the horizontal meridian is about
   3.2 mm.
   Retina's simplified axial organisation. The retina is a stack of
   several neuronal layers. Light is concentrated from the eye and passes
   across these layers (from left to right) to hit the photoreceptors
   (right layer). This elicits chemical transformation mediating a
   propagation of signal to the bipolar and horizontal cells (middle
   yellow layer). The signal is then propagated to the amacrine and
   ganglion cells. These neurons ultimately may produce action potentials
   on their axons. This spatiotemporal pattern of spikes determines the
   raw input from the eyes to the brain. (Modified from a drawing by Ramón
   y Cajal.)
   Enlarge
   Retina's simplified axial organisation. The retina is a stack of
   several neuronal layers. Light is concentrated from the eye and passes
   across these layers (from left to right) to hit the photoreceptors
   (right layer). This elicits chemical transformation mediating a
   propagation of signal to the bipolar and horizontal cells (middle
   yellow layer). The signal is then propagated to the amacrine and
   ganglion cells. These neurons ultimately may produce action potentials
   on their axons. This spatiotemporal pattern of spikes determines the
   raw input from the eyes to the brain. (Modified from a drawing by Ramón
   y Cajal.)

   In section the retina is no more than 0.5 mm thick. It has three layers
   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 byproduct of evolution, the ganglion cells lie
   innermost in the retina while the photoreceptive cells lie outermost.
   Because of this arrangement, light must first pass through the
   thickness of the retina before reaching the rods and cones. However it
   does not pass through the epithelium or the choroid (both of which are
   opaque).

   The white blood cells in the capillaries in front of the photoreceptors
   can be perceived as tiny bright moving dots when looking into blue
   light. This is known as the blue field entoptic phenomenon (or
   Scheerer's phenomenon).

   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 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 seven million cones and a
   hundred million rods. At the centre of the macula 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 macula has a yellow pigmentation from screening pigments and is
   known as the macula lutea.

Difference between vertebrate and cephalopod retinas

   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.

Physiology

   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 respond to bright light and mediate high-resolution vision
   and colour vision. The rods respond to dim light and mediate
   lower-resolution, black-and-white, night vision. It is a lack of cones
   sensitive to red, blue, or green light that causes individuals to have
   deficiencies in colour vision or various kinds of colour blindness.
   Humans and old world monkeys have three different types of cones (
   trichromatic vision) while other mammals lack cones with red sensitive
   pigment and therefore have poorer (dichromatic) 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
   amacrine cells, which modify the synaptic signal before the ganglion
   cells. Rod and cone signals are intermixed and combine, although rods
   are mostly active in very poorly lit conditions and saturate in broad
   daylight, while cones function in brighter lighting because they are
   not sensitive enough to work at very low light levels.

   Despite the fact that all are nerve cells, only the retinal ganglion
   cells and few amacrine cells create action potentials. In the
   photoreceptors, exposure to light hyperpolarizes the membrane in a
   series of graded shifts. The outer cell segment contains a
   photopigment. Inside the cell the normal levels of cGMP keeps the Na
   channel open and in thus in the resting state the cell is depolarised.
   The photon causes the retinal bound to the receptor protien to
   isomerise to trans-retinal. This causes receptor to activate multiple
   G-proteins. This inturn causes the Ga-subunit of the protein to bind
   and degrade cGMP inside the cell which then cannot bind to the CNG Na
   channels. Thus the cell is hyperpolarised. 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 (see dark adaptation).

   In the retinal ganglion cells there are two types of response,
   depending on the receptive field of the cell. The receptive fields of
   retinal ganglion cells comprise a central approximately circular area,
   where light has one effect on the firing of the cell, and an annular
   surround, where light has the opposite effect on the firing of the
   cell. In ON cells, an increment in light intensity in the centre of the
   receptive field causes the firing rate to increase. In OFF cells, it
   makes it decrease. Beyond this simple difference ganglion cells are
   also differentiated by chromatic sensitivity and the type of spatial
   summation. Cells showing linear spatial summation are termed X cells
   (also called "parvocellular", "P", or "midget" ganglion cells), and
   those showing non-linear summation are Y cells (also called
   "magnocellular, "M", or "parasol" 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 10,000 points. The
   information capacity is estimated at 500,000 bits per second (for more
   information on bits, see information theory) without colour or around
   600,000 bits per second including colour.

Diseases and disorders

   There are many inherited and acquired diseases or disorders that may
   affect the retina. Some of them include:
     * 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.
     * Cone-rod dystrophy (CORD) describes a number of diseases where
       vision loss is caused by deterioration of the cones and/or rods in
       the retina.
     * 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.
     * Retinal diseases in dogs include retinal dysplasia, progressive
       retinal atrophy, and sudden acquired retinal degeneration.

Diagnosis and treatment

   A number of different instruments are available for the diagnosis of
   diseases and disorders affecting the retina. An ophthalmoscope is used
   to examine the retina. Recently, adaptive optics has 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, now becoming widely available, is optical
   coherence tomography (OCT). This non-invasive technique allows one to
   obtain a 3D volumetric or high resolution cross-sectional tomogram of
   the retinal fine structure with histologic-quality.
   OCT scan of a retina at 800nm with an axial resolution of 3µm
   Enlarge
   OCT scan of a retina at 800nm with an axial resolution of 3µm

   Treatment depends upon the nature of the disease or disorder.
   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.

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.

   A recent University of Pennsylvania study calculated the approximate
   bandwidth of human retinas is 8.75 megabits per second, whereas a
   guinea pig retinas transfer at 875 kilobits.
   Retrieved from " http://en.wikipedia.org/wiki/Retina"
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