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Retina

Retina

National Eye Institute. Many animals have eyes different from the human eye.]] The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which converts light into nervous signals. The retina contains photoreceptor cells (rods and cones) which receive the light; the resulting neural signals then undergo complex processing by other neurons of the retina, 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. In embryonal development, the retina and the optic nerve originate as outgrowths of the brain. The unique structure of the blood vessels in the retina have been used for biometric identification.

Retinal anatomy

The human retina has ten distinct layers: # Retinal pigment epithelium (RPE) # Photoreceptor layer # External limiting membrane - Layer that separates the inner segment portions of the photoreceptors from their cell nuclei. # Outer nuclear layer # Outer plexiform layer - In the macular region, this is known as the Fiber layer of Henle. # Inner nuclear layer # Inner plexiform layer # Ganglion cell layer - Layer that contains nuclei of ganglion cells and gives rise to optic nerve fibers. # Nerve fiber layer # Inner limiting membrane

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 nerve. This spot is known as the blind spot as 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 center 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 the hawk who actually are bifoviate and dogs and cats which possess no fovea but a central band known as the visual streak. 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 (or macula), the most sensitive area along the horizontal meridian is about 3.2 mm. meridian 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. Perhaps as a product 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 six million cones and a hundred and twenty-five 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.

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 vison 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 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 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 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

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.
- 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. histologic 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

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.

Bibliography

- S. Ramón 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.
- Schulz, H., Goetz, T., Kaschkoetoe, J., Weber B.H. (2004). [http://www.retinacentral.org The Retinome - defining a reference transcriptome of the adult mammalian retina/retinal pigment epithelium - RetinaCentral]. BMC Genomics. 2004 Jul 29;5(1):50. Institute of Human Genetics, Biocenter, University of Wuerzburg, D-97074 Wuerzburg, Germany. hschulz@biozentrum.uni-wuerzburg.de

External links

- Kolb, H., Fernandez, E., & Nelson, R. (2003). [http://webvision.med.utah.edu The neural organization of the vertebrate retina]. Salt Lake City, Utah: John Moran Eye Center, University of Utah. Retrieved July 19, 2004.
- [http://www.techreview.com/articles/04/09/demo0904.asp Demo: Artificial Retina], MIT Technology Review, September 2004. Reports on implant research.
- [http://bionic.gsbme.unsw.edu.au/ Australian Vision Prosthesis Group], Graduate School of Biomedical Engineering, University of New South Wales.
- [http://www.retinacentral.org RetinaCentral], Genetics and Diseases of the Human Retina.
- [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.figgrp.740 Retinal layers image.] NeuroScience 2nd Ed Category:Visual system Category:Eye ja:網膜

Eye

: This article refers to the sight organ. See Eye (disambiguation) for other usages An eye is an organ that detects light. Different kinds of light-sensitive organs are found in a variety of creatures. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision. Many complex organisms including some mammals, birds, reptiles and fish have two eyes which may be placed on the same plane to be interpreted as a single three-dimensional "image" (binocular vision), as in humans; or on different planes producing two separate "images" (monocular vision), such as in rabbits and chameleons.
Varieties of eyes
chameleon chameleon]] In most vertebrates and some mollusks the eye works by allowing light to enter it and project onto a light-sensitive panel of cells known as the retina at the rear of the eye, where the light is detected and converted into electrical signals, which are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the intensity of the light that enters the eye. The eyes of cephalopods, fish, amphibians, and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens (similar to how a camera focuses). Compound eyes are found among the arthropods and are composed of many simple facets which give a pixelated image (not multiple images as is often believed). Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360 degree field of vision. Compound eyes are very sensitive to motion. Some arthropods (many Strepsiptera) have compound eye composed of a few facets each with a retina capable of creating an image, which does provide muliple image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain providing a very wide angle high resolution image. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye. Some of the simplest eyes, called ocelli, can be found in animals like snails, who can not actually "see" in the common sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark (day and night), but no more. This enables snails to keep out of direct sunlight. Jumping spiders have simple eyes that are so large, supported by an array of other smaller eyes, that they can get enough visual inputs to hunt and pounce on their prey. Some insect larvae like caterpillars have a different type of single eye (stemmata) which gives a rough image.
Evolution of eyes
How a complex structure like the projecting eye could have evolved is often said to be a difficult question for the theory of evolution. Darwin famously treated the subject of eye evolution in his Origin of Species: :To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. Despite the precision and complexity of the eye, computer models of eye evolution, developed by Dan-Erik Nilsson and Susanne Pelger, demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection. Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans and some, like diurnal birds of prey, can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, suggesting that the development of eyes through evolution might not be so improbable as it might seem. However, the development of the eye is considered to be monophyletic; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago (Mya).
Anatomy
monophyletic monophyletic The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. The cornea and lens help to converge light rays to focus onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain. Light enters the eye from an external medium such as air or water, passes through the cornea, and into the first of two humours, the aqueous humour. Most of the light refraction occurs at the cornea which has a fixed curvature. The first humour is a clear mass which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the convergence of light at the lens) and provides the corneal endothelium with nutrients. The iris, between the lens and the first humour, is a coloured ring of muscle fibres. Light must first pass though the centre of the iris, the pupil. The size of the pupil is actively adjusted by the circular and radial muscles to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina, too little light would be blinding. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina. To clearly see an object far away, the circularly arranged ciliary muscles will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more convex, form. Humans gradually lose this flexibility with age, resulting in the inability to focus on nearby objects, which is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism. On the other side of the lens is the second humour, the vitreous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. Wrapped around these tissues are three layers of tissue surrounding the vitreous humour. The outermost is the sclera which gives the eye most of its white colour. It consists of fibrin connective tissue and both protects the inner components of the eye and maintains its shape. On the inner side of the sclera is the choroid, which contains blood vessels that supply the retinal cells with necessary oxygen and removes the waste products of respiration. Within the eye, only the sclera and ciliary muscles contain blood vessels. The choroid gives the inner eye a dark colour, which prevents disruptive reflections within the eye. The inner most layer of the eye is the retina, containing of the photosensitive rod and cone cells, and neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does have two points at which it is different; the fovea and blind spot. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for colour vision in humans, and enables high acuity, such as is necessary in reading. The blind spot is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus "blind". In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.
Other articles regarding eye anatomy
Aqueous humour, Anterior chamber, Blind spot, Canal of Schlemm, Ciliary body, Ciliary muscle, Cornea, Conjunctiva, Choroid, Fovea, Iris, Lens, Macula, Optic disc, Optic nerve, Ora serrata, Posterior chamber, Pupil, Retina, Sclera, Suspensory ligament, Tapetum lucidum, Trabecular meshwork, Vitreous humour, Zonular fibers.
Cytology
The retina contains two forms of photosensitive cells - rods and cones. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, they do not distinguish between colours, and have low visual acuity (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colours (wavelengths) of light, which allows an organism to see colour. The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires looking at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" where rods also exist, and where the light is sufficient to stimulate cells, allowing the individual to observe distant stars. Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to electromagnetic radiation of a particular wavelength and intensity (ie. a colour visible light) the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The opsin in both opens ion channels on the cell membrane which leads to the generation of an action potential (an impulse which will eventually get to the visual cortex in the brain). This is the reason why cones and rods enable organisms to see in dark and light conditions - each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell and information is relayed to the visual cortex. Whereas, a single cone cell is connected to a single bipolar cell. Thus, action potentials from rods share neurons, where those from cones are given their own. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential. Because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapse of the bipolar cell to attain the threshold level to generate an action potential. Furthermore, colour is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents - resulting in different colours being seen. If, for example, the red and green cones are stimulated to the same extent, and no blue cones are stimulated, yellow is seen. For this reason red, green and blue are called primary colours and the products of mixing two secondary colours. The secondary colours can be further complimented with primary colours to see tertiary colours.
Acuity
Visual acuity can be measured with several different metrics. Cycles per degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from angular resolution. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.73 millimeters at a distance of one meter. A horse can resolve about 14 CPD (0.62 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m). A diopter is the unit of measure of focus.
Dynamic range
At any given instant, the retina can resolve a contrast ratio of around 100:1 (about 6 1/2 stops). As soon as your eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Hence, over time, a contrast ratio of about 1,000,000:1 (about 20 stops) can be resolved.
Adnexa and related parts

The orbit
In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets. This placement of the eyes helps to protect them from injury.
Eyebrows
In humans, the eyebrows redirect flowing substances (usually rainwater) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid, and its beneficial effects, and can damage the cornea, due to osmotic differences between tear fluid and freshwater.
Eyelids
In many animals, including humans, eyelids wipe the eye and prevent the eyes from dehydration. They spread tear fluid on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above water and below it. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex.
Eyelashes
In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.
Eye movement
Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body. The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second (Westheimer and McKee, 1954). Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand. Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vison would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.
Extraocular muscles
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single joint in the center of the eye.
Rapid eye movement
Rapid eye movement typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.
Saccades
Saccades are rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900°/s in adult humans.
Microsaccades
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.
Vestibulo-ocular reflex
Many animals can look at something while turning their heads. The eyes are automatically rotated to remain fixed on the object, directed by input from the organs of balance near the ears.
Smooth pursuit movement
The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.
Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).
Vergence movement
feedback When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from eachother' (divergence). Exaggerated convergence is called cross eyed viewing (focussing on the nose for example) . When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge. Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.
Accommodation
To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.
Diseases, disorders, and age-related changes
There are many diseases and disorders that may affect the eyes. As the eye ages certain changes occur that can be attributed to solely the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older person requires extra lighting. With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis,and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities - visible as floaters gradually increase in number.
See also

- WikiSaurus:eye — the WikiSaurus list of synonyms and slang words for eyes in many languages
- Adaptation
- Binocular vision
- Corrective lens
- Crystallin
- Evil eye
- Eye color
- Eye contact
- Eye tracking
- Eyeglass prescription
- Macropsia
- Micropsia
- Nictitating membrane
- Ocular tremor
- Ophthalmology
- Optician
- Optometry
- Persistence of vision
- Phosphenes
- Snellen chart
- Staring contest
- Tears
- Visual perception
External links

- [http://www.djo.harvard.edu/ DJO | Digital Journal of Ophthalmology]
- [http://www.afb.org/eyeconditions.asp Glossary of Eye Conditions]
- [http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html Evolution of the Eye]
- [http://www.eyetopics.com eye Topics]
- [http://webvision.med.utah.edu/anatomy.html Diagram of the eye]
- [http://webvision.med.utah.edu/ Webvision. The organisation of the retina and visual system.]
References

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- [http://soma.npa.uiuc.edu/courses/bio303/Ch11b.html Internet lecture on eye types in animal kindom] # [http://www.agingeye.net/ AgingEye Times] Category:Visual system Category:Head and neck Category:Ophthalmology ms:Mata ja:目 zh-min-nan:Ba̍k-chiu

Vertebrate

Conodonta
Hyperoartia
:Petromyzontidae (lampreys)
Pteraspidomorphi (early jawless fish)
Thelodonti
Anaspida
Cephalaspidomorphi (early jawless fish)
:Galeaspida
:Pituriaspida
:Osteostraci
Gnathostomata (jawed vertebrates)
:Placodermi
:Chondrichthyes (cartilaginous fish)
:Acanthodii
:Osteichthyes (bony fish)
::Actinopterygii (ray-finned fish)
::Sarcopterygii (lobe-finned fish)
:::Actinistia (coelacanths)
:::Dipnoi (lungfish)
:::Tetrapoda ::::Amphibia
::::Amniota
:::::Sauropsida/(Reptiles)
::::::Aves (Birds)
:::::Synapsida
::::::Mammalia Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. Vertebrates started to evolve about 530 million years ago during the Cambrian explosion, which is part of the Cambrian period (first known vertebrate is Myllokunmingia). The bones of the spinal column (or vertebral column) are called vertebrae. Vertebrata is the largest subphylum of chordates, and contains most animals with which people are generally familiar (except insects). Fish (including lampreys, but traditionally not hagfish, though this is now disputed), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Additional characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone. The internal skeleton which defines vertebrates consists of cartilage or bone, or in some cases both. The skeleton provides support to the organism during the period of growth. For this reason vertebrates can achieve larger sizes than invertebrates, and on average vertebrates are in fact larger. The skeleton of most vertebrates, that is excluding the most primitive ones, consists of a skull, the vertebral column and two pairs of limbs. In some forms of vertebrates, one or both of these pairs of limbs may be absent, such as in snakes or whales. These limbs have been lost in the course of evolution. The skull is thought to have facilitated the development of intelligence as it protects vital organs such as the brain, the eyes and the ears. The protection of these organs is also thought to have positively influenced the development of high responsiveness to the environment often found in vertebrates. Both the vertebral column and the limbs support the body of the vertebrate overall. This support facilitates movement. Movement is normally achieved with muscles that are attached directly to the bones or cartilages. The contour of the body of a vertebrate is formed by the muscles. A skin covers the inner parts of a vertebrate's body. The skin sometimes acts as a structure for protective features, such as horny scales or fur. Feathers are also attached to the skin. The trunk of a vertebrate is hollow and houses the internal organs. The heart and the respiratory organs are protected in the trunk. The heart is located behind the gills, or where there are lungs, in between the lungs. The central nervous system of a vertebrate consists of the brain and the spinal cord. Both of these are characterized by being hollow. In lower vertebrates the brain mostly controls the functioning of the sense organs. In higher vertebrates the size of the brain relative to the size of the body is larger. This larger brain enables more intensive exchange of information between the different parts of the brain. The nerves from the spinal cord, which lies behind the brain, extend to the skin, the inner organs and the muscles. Some nerves are directly connected to the brain, linking the brain with the ears and lungs. Vertebrates have been traced back to the ostracoderms of the Silurian Period (444 million to 409 million years ago) and the conodonts, a group of eel-like vertebrates characterized by multiple pairs of bony toothplates. All vertebrates have: the ability to form bones; paired, specialised sensory organs and a brain.

External links

- [http://tolweb.org/tree?group=Amniota&contgroup=Terrestrial_vertebrates Tree of Life]
- [http://reference.allrefer.com/encyclopedia/categories/vertz.html Vertebrate Zoology] Category:Chordates ko:척추동물 ms:Vertebrata ja:脊椎動物 simple:Vertebrate th:สัตว์มีกระดูกสันหลัง

Light

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. The three basic dimensions of light (i.e., all electromagnetic radiation) are:
- Intensity (or brilliance or amplitude), which is related to the human perception of brightness of the light,
- Frequency (or wavelength), perceived by humans as the color of the light, and
- Polarization (or angle of vibration), which is not perceptible by humans under ordinary circumstances. Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Visible electromagnetic radiation

Visible light is the portion of the electromagnetic spectrum between the frequencies of 380 THz (3.8×10¹⁴ hertz) and 750 THz (7.5×10¹⁴ hertz). The speed (

c

), frequency (

f

\nu

), and wavelength (

\lambda

) of a wave obey the relation: :

c = f~\lambda \,\!

Because the speed of light in a vacuum is fixed, visible light can also be characterised by its wavelength of between 400 nanometres (abbreviated 'nm') and 800 nm (in a vacuum). Light entering the eye is absorbed by light-sensitive pigments within the rod cells and cone cells in the retina, triggering a cascade of events that creates electrical nerve impulses that travel through the optic nerve to the brain, producing vision.

Speed of light

Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term. The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (a Danish physicist), in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000 kilometres per second (approximately 141,050 miles per second). The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second. Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.

Refraction

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). When light passes through a transparent substance, such as air, water or glass, its speed is reduced, and it undergoes refraction. The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as: :

n = \frac \;\!

Thus, n=1 in a vacuum and n>1 in matter. When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. If the incident beam is not orthogonal to the edge between the media, the direction of the beam will change. Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.

Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

Color and wavelengths

The different wavelengths are detected by the human eye and then interpreted by the brain as colors, ranging from red at the longest wavelengths of about 700 nm. (lowest frequencies) to violet at the shortest wavelengths of about 400 nm. (highest frequencies). The intervening frequencies are seen as orange, yellow, green, cyan, blue, and, conventionally, indigo.
indigo
The wavelengths of the electromagnetic spectrum immediately outside the range that the human eye is able to perceive are called ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength (low frequency) end. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see infrared light. UV radiation is not normally directly perceived by humans except in a very delayed fashion, as overexposure of the skin to UV light can cause sunburn, or skin cancer, and underexposure can cause vitamin D deficiency. However, because UV is a higher frequency radiation than visible light, it very easily can cause materials to fluoresce visible light. Cameras that can detect IR and convert it to light are called, depending on their application, night-vision cameras or infrared cameras. These are different from image intensifier cameras, which only amplify available visible light. When intense radiation (of any frequency) is absorbed in the skin, it causes heating which can be felt. Since hot objects are strong sources of infrared radiation, IR radiation is commonly associated with this sensation. Any intense radiation that can be absorbed in the skin will have the same effect, however.

Measurement of light

The following quantities and units are used to measure the quantity or "brightness" of light. Light can also be characterised by:
- amplitude,
- color, wavelength, or frequency, and
- polarization (or angle of vibration).

Light sources

polarization There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". The blue color is most commonly seen in a gas flame or a welder's torch. Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be be stimulated, as in a laser or a microwave maser. Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation. Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions. Certain other mechanisms can produce light:
- scintillation
- scintillator
- electroluminescence
- sonoluminescence
- triboluminescence
- radioactive decay
- particle-antiparticle annihilation

Theories about light

Early Greek ideas

In 55 BC Lucretius, continuing the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles. Ptolemy also wrote about the refraction of light.

10th century optical theory

The scientist Abu Ali al-Hasan ibn al-Haytham (965-c.1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light. Alhazen's work did not become known in Europe until the late 16th century.

The 'plenum'

René Descartes (1596-1650) held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light travelled faster in a denser medium, by analogy with the behaviour of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.

Particle theory

Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether. Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye. Another supporter of the wave theory was Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory. Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt by the Michelson-Morley experiment. Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845, Faraday discovered that the angle of polarisation of a beam of light as it passed through a polarising material could be altered by a magnetic field, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether. Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. The technology of radio transmission was, and still is, based on this theory. The constant speed of light predicted by Maxwell's equations contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. A solution to this contradiction would later be found by Albert Einstein.

Particle theory revisited

The wave theory was accepted until the late 19th century, when Einstein described the photoelectric effect, by which light striking a surface caused electrons to change their momentum, which indicated a particle-like nature of light. This clearly contradicted the wave theory, and for years physicists tried in vain to resolve this contradiction.

Quantum theory

In 1900, Max Planck described quantum theory, in which light is considered to be as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by :

E_f = hf = \frac \,\!

where h is Planck's constant,

\lambda

is the wavelength and c is the speed of light. As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.

Wave-particle duality

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

A light wave

1929 that oscillate perpendicular to each other and to the direction of motion (a transverse wave).]] The electric and magnetic fields are perpendicular to the direction of travel and to each other. This picture depicts a very special case, linearly polarized light. See Polarization for a description of the general case and an explanation of linear polarization. While these relations of the electric and magnetic fields are always true, the subtle difference in the general case is that the direction and amplitude of the magnetic (or electric) field can vary, in one place, with time, or, in one instant, can vary along the direction of propagation.

Photoreceptor

A photoreceptor is a light-sensitive protein involved in the function of photoreceptor cells. Some examples are rhodopsin and photopsins in the human retina, phytochrome in plants, and bacteriorhodopsin in some bacteria. A photoreceptor is also the cell found in ommatidia and retinae that contain these photosensitive pigments (see photoreceptor cell). Category:Sensory receptors

Cone cell

__NOTOC__ Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea. They gradually become more sparse towards the outside of the retina. Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods (Kandel et al., 2000). Because humans (normally) have three kinds of cones, with different photopsins, which have different response curves (that is, they respond to variation in color in different ways), we have trichromatic vision.

Types

The three kinds of cones typically respond most to yellowish-green (long wavelength or L), green (medium or M), and bluish-violet (short or S) light (peak wavelengths of 564 nm, 534 nm, and 420 nm respectively). The difference in the signals received from the three kinds allows the brain to perceive a wide gamut of different colors through the opponent process of color vision. The color yellow, for example, is perceived when the yellowish-green receptor is stimulated slightly more than the green receptor, and the color red is perceived when the yellowish-green receptor is stimulated significantly more than the green receptor. Similarly, blues are perceived when the bluish-violet receptor is stimulated more than the other two. The S (bluish-violet) cones are sensitive to light at wavelengths shorter than 400 nm, but the lens and cornea of the human eye are increasingly absorbative to these wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm (the onset of ultraviolet light). The eye is more sensitive to green light than other colors: when looking at a green light and a red light of the same intensity, the green light will appear to be brighter.

Structure

Cone cells are larger and less numerous than rods. Structurally, cone cells have a cone-like shape at one end where the pigment that filters incoming light, giving them their different response curves. They are typically 50 µm long, and their diameter varies from 1.0 to 4.0 µm, being smallest and most tightly packed at the center of the eye (the fovea). The blue-sensitive cells are a little larger than the others, and an order of magnitude less common. Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. The inner and outer segments are connected by a cilium (Kandel et al., 2000). The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials (Kandel et al., 2000). Like rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. Photopigments exist as transmembrane proteins within these disks, which provide more surface area for light to affect the pigments. In cones, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rods nor cones divide, but their membranous disks wear out and are sloughed off at the end of the outer segment, to be consumed and recycled by phagocytic cells.

Table

Comparison of rod and cone cells, from Kandel et al., 2000.

Reference

- Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science, 4th ed., pp.507-513. McGraw-Hill, New York.

External links

- [http://webvision.umh.es/webvision/photo1.html Webvision's Photoreceptors] Category:Photoreceptor cells Category:Eye

Neuron

Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.

Classes

There are three classes of neurons: afferent neurons, efferent neurons, and interneurons.
- Afferent neurons convey information from tissues and organs into the central nervous system.
- Efferent neurons transmit signals from the central nervous system to the effector cells.
- Interneurons connect neurons within the central nervous system. Structural classification
- Pseudounipolar- single dendrite longer than single axon
- Bipolar - single axon and single dendrite equal length
- Multipolar - more than two dendrites
- Golgi I- pyramidal cell
- Golgi II- granule (stellate) cell

Anatomy and histology

Golgi II Many highly specialized types of neurons exist, and these differ widely in appearance. Characteristically, neurons are highly asymmetric in shape. Neurons consist of:
- The dendrite, a short, branching arbour of cellular extensions. Each neuron has very many dendrites with profuse dendritic branches. These structures form the main information receiving network for the neuron.
- The soma, or cell-body, the relatively large central part of the cell between the dendrites and the axon.
- The axon, a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. This is the structure which carries nerve signals away from the neuron. Neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. Axons and dendrites in the central nervous system are typically only about a micrometre thick, while some of those in the peripheral nervous system are much thicker. The soma is usually about 25 micrometres in diameter and not much larger than the cell nucleus it contains. The axon of a human motoneuron can be over a metre long, reaching from the base of the spine to the toes, while giraffes have single axons running along the whole length of their necks, several feet in length. Much of what we currently know about axonal transport comes from studying the squid neuron, an ideal neuron for research due to it's relatively immense size (0.5 - 1 millimetres thick, several centimetres long).

Connectivity

Neurons communicate with one another and to other cells through synapses, where the axon tip of one cell impinges upon a dendrite or soma of another, or less commonly to an axon. Neurons of the cortex in mammals, such as the Purkinje cells, can have over 1000 dendrites each, enabling connections with tens of thousands of other cells.

Adaptations to carrying action potentials

The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although thicker axons convey the impulses more rapidly, generally speaking. Many neurons have insulating sheaths of myelin around their axons. The sheaths are formed by glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables the action potentials to travel faster than in unmyelinated axons of the same diameter whilst simultaneously spending less energy to "recharge" the action potential after. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Multiple sclerosis is a neurological disorder which results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons, and recent experimental results have suggested that glial cells play a vital role in information processing among neurons. It has been estimated that Glial cells outnumber neurons by as many as 50:1.

Histology and internal structure

Nerve cell bodies stained with basophilic dyes will show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomes. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large numbers of protein synthesis. The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).

Neurons of the brain

The nematode worm (Caenorhabditis elegans) has 302 neurons. Scientists have mapped all of the nematode's neurons. As a result, such worms are ideal candidates for neurobiological experiments and tests. The human brain has about 100 billion (

10^

) neurons and 100 trillion (

10^

) connections (synapses) between them.

External links

- [http://primate-brain.org High-Resolution Cytoarchitectural Primate Brain Atlases]
- [http://purl.net/net/neurowiki NeuroWiki], a wiki website for Neuroscience related topics.
- [http://ccdb.ucsd.edu/CCDB/index.shtml Cell Centered Database] UC San Diego images of neurons. Category:Neuroscience Category:Neurons ja:ニューロン simple:Neuron

Axon

An axon, or "nerve fibre," is a long slender projection of a nerve cell, or "neuron", which conducts electrical impulses away from the neuron's cell body or soma. Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves. Individual axons are microscopic in diameter--typically about one micrometre across-- but may extend to macroscopic lengths. The longest axons in the human body, for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot. These single-cell fibers may extend a metre or even longer. In vertebrates generally, the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly spaced intervals, enabling an especially rapid mode of electrical impulse propagation called saltation. The demyelination of axons is what causes the multitude of neurological symptoms found in the disease Multiple Sclerosis. The axons of some neurons branch to form axon collaterals, along which the bifurcated impulse travels simultaneously to signal more than one other cell.

Growth & Development

Growing axons move through their environment via the growth cone, which is at the tip of the axon. The growth cone has a broad sheet like extension called lamellipodia which contain protrusions called filopodia. The filopodia are the mechanism by which the the entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Environments with high levels of cell adhesion molecules or CAM's create an ideal environment for axonal growth. This seems to provide a "sticky" surface for axons to grow along. Examples of CAM's specific to neural systems include N-CAM, neuroglial CAM or NgCAM, TAG-1, MAG, and DCC, all of which are part of the immunoglobulin superfamily. Another set of molecules called extracellular matrix adhesion molecules also privde a sticky substrate for axons to grow along. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Some of these are surface bound to cells and thus act as short range attractants or repellants. Others are difusible ligands and thus can have long range effects. Cells called "guidepost cells" assist in the guidance of neuronal axon growth. These cells are typically other, sometimes immature, neurons.

History

Some of the first intracellular recordings in a nervous system were made in the late 1930's by K. Cole and H. Curtis. Alan Hodgkin and Andrew Huxley also employed the squid axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential. Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. (Squid have very large axons, which allowed individual cells to be manipulated even before the invention of the microelectrode).

External links

- http://www.sfn.org/wrensite/projects/patch_clamp/index.htm Category:Neuroscience Category:Neurons

Visual perception

Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. Vision has a specific sensory system, the visual system. There is disagreement as to whether or not this constitutes one, two or even three distinct senses. Some people make a distinction between "black and white" vision and the perception of colour, and others point out that vision using rod cells uses different physical detectors on the retina from cone cells. Some argue that the perception of depth also constitutes a sense, but others argue that this is really cognition (that is, post-sensory) function derived from having stereoscopic vision (two eyes) and is not a sensory perception as such. Many people are also able to perceive the polarization of light.

The visual system

thumbnailThe eye is the light-sensitive organ that is the first component of the visual system. The eye's retina performs the first stages of visual perception processing, with the remaining stages of visual perception occurring in the optic nerve, the lateral geniculate nucleus, and the visual cortex of the brain.

Sources of information

To perform its task, visual perception takes into account not only patterns of illumination on the retina, but also our other senses and our past experiences. Consider the task of bird sighting (an instance of object recognition): to be able to identify a bird against a background of tree and brushes, one needs prior exposure to general properties of the bird category. From past experiences, we expect birds to have a certain shape, color, etc. Hearing a sound that is characteristic of birds, a song for example, will help us locate one: information from the other senses is used in visual perception. In this case, locational information from the auditory domain is used.

Individual and group differences in visual perception

Most of the general processes of visual perception have been shown to be universal, as opposed to being dependant on culture, although there are specific instances where cultural variability appears to come into play. It has also been shown that certain individual differences such as impairment of sight and spatial skills can also affect our visual perception. There are also other factors that influence how we perceive things such as personality, cognitive styles, gender, occupation, age, values, attitudes, motivation, religious beliefs, economic status, education and habits.

Theoretical perspectives in the study of visual perception

Unconscious inference

Hermann von Helmholtz is often credited with the founding of the scientific study of visual perception. Helmholtz held vision to be a form of unconscious inference: vision is a matter of deriving a probable interpretation for incomplete data. The general goal of vision is to identify, as accurately as possible, the features of our environment: roughly, what objects are present where. Other features are irrelevant to this task : illumination patterns, viewing position, etc. Those are confounding variables. Call S = (F,C) the scene, with F the features we’re interested in and C the confounding variables. S determines I, the pattern of illumination on the retina, which is all the information our visual system has on the current scene. The task is to find S given I. This problem is under-constrained: many different S correspond to the same I, and many I could correspond to the same S. One of the reasons is that much information is lost when a 3-dimensional world is collapsed into a 2-dimensional array. To see why, consider the figure of a circle such as this one: O. It could correspond to an infinity of ellipses viewed at a certain slant. But we always interpret it as a circle viewed on the frontal plane – the explanation we infer from the data for this particular stimulus. Inference requires prior assumptions about the world: two well-known assumptions that we make in processing visual information are that light comes from above and that objects are viewed from above not below. The study of visual illusions (cases when the inference process goes wrong) has yielded a lot of insight into what sort of assumptions the visual system makes.

Gestalt

Psychologists of the Gestalt school have raised a large part of the research questions that still preoccupy vision scientists today. The so-called Gestalt Laws of Organisation have broadened the study of how people perceive objects to be organized patterns or wholes, instead of collections of many separate parts. Gestalt is a German word that translates to "configuration or pattern". According to this theory, there are four main factors that determine how we group things according to visual perception.
- Proximity – Depending on how close object are to one other, we tend to group the ones closest to each other as a group.
- Similarity – If objects are similar in shape or size to one another we tend to group them together.
- Closure – How we complete a pattern because of how the items are grouped together even though the pattern is not complete.
- Simplicity – How we group items according to symmetry, regularity, and smoothness.

Ecological psychology

Psychologist James J. Gibson developed a theoretical perspective on vision that is radically different from that of Helmholtz. Gibson considers that enough visual perception is available in normal environments to allow for veridical perception (accurate perception of the world). Gibson replaces inference with information pickup. Although most researchers today feel closer to Helmholtz's unconscious inference theory, Gibson has done much in identifying what sort of information is available to the visual system.

Types of visual perception

- Black and white vision
- Color vision
- Gestalt perception
- Motion perception

Disorders/Dysfuntions

- Achromatopsia
- Color blindness
- Scotopic Sensitivity Syndrome

References

- Rudolph Arnheim (1954). Art and Visual Perception: A Psychology of the Creative Eye. Berkeley: University of California Press.
- Lothar Kleine-Horst (2001). Empiristic Theory of Visual Gestalt Perception. Hierarchy and Interactions of Visual Functions. Koeln: Enane. ISBN 3-928955-42X

External links

- [http://enane.de/cont.htm Empiristic theory of visual gestalt perception]
- [http://www.aber.ac.uk/media/Modules/MC10220/visper03.html Visual Perception 3 - Cultural and Environmental Factors]
- [http://www.sapdesignguild.org/resources/optical_illusions/gestalt_laws.html Gestalt Laws]
- [http://www.aber.ac.uk/media/Modules/MC10220/visper04.html Visual Per