Retina

cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye.]] The retina is the part of the eye which transduces light into neural signals. Neural signals begin in the photoreceptors (rods and cones), undergo complex processing by other neurons, and are transformed into action potentials in retinal ganglion cells whose axons form the optic nerve. The retina not only detects light, it also plays a significant part in visual perception. Using a ophthalmoscope an ophthalmologist can see the retina of your eye to determine its health. Recently, adaptive optics have been used to image individual rods and cones in the living human retina. The unique structure of the blood cells that are formed in the retina makes it a good way of doing biometric identification.

Physical structure

\nIn adult humans the entire retina is 72% of a sphere about 22 mm in diameter. At the centre of the retina is the optic nerve. It appears as an oval white area of only 3 mm². Temporal to 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. The length from one ora to the other along the horizontal meridian is about 3.2 mm. [[Image:Fig_retine.png|thumb|left|400px|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 Ramon y Cajal.)]] 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 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 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 six million cones and a hundred and twenty-five 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

\nAn 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 repond to bright light and mediate high-resolution vison and colour vision. The rods repond 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. 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. Despite all being nerve cells only the retinal ganglion 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 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 (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 of the firing of the cell, and an annular surround, where light has the opposite effect of the firing of the cell. One response, from on cells, is to increase the rate of firing to increases in light intensity in the centre of the receptive field. The other response, from off cells, is to decrease the rate of firing to increases in light intensity in the centre of the receptive field. 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. 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.

See also

\n* Ignipuncture

External links

\n* Kolb, H., Fernandez, E., & Nelson, R. (2003). The neural organization of the vertebrate retina. Salt Lake City, Utah: John Moran Eye Center, University of Utah. Retrieved July 19, 2004, from [[1]. Everything you wanted to know about eye and retina.

Bibliography

• S. R. Y. Cajal, Histologie du Système Nerveux de l'Homme et des Vertébrés, Maloine, Paris, 1911.
• M. Meister and M. J. B. II, The neural code of the retina, Neuron, vol. 22 p. 435-50, 1999.
• R. W. Rodieck, Quantitative analysis of cat retinal ganglion cell response to visual stimuli, Vision Research, vol. 5 p. 583-601, 1965.
• J. J. Atick and A. N. Redlich, What does the retina know about natural scenes?, Neural Computation, p. 196-210, 1992.

\n\nCategory:Visual systemCategory:Computer vision