Vision in Humans
The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive
..... Click the link for more information. and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other arthropods. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. This information stimulates the hypothalamus, which passes the information on to the pineal gland. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock (see biorhythm biorhythm or biological rhythm, cyclic pattern of changes in physiology or in activity of living organisms, often synchronized with daily, monthly, or yearly environmental changes.
..... Click the link for more information. ).
The Human Eye
Anatomy and Function
The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The sclera is the outermost layer of eye tissue; part of it is visible as the "white" of the eye. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. A delicate membrane, the conjunctiva, covers the visible portion of the sclera.
Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue.
The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.
Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears tears, watery secretion of the lacrimal gland, which is located at the outer corner of the eye socket immediately above the eyeball. Tearing, or lacrimation, is a continuous and largely involuntary process stimulated by the autonomic nervous system.
..... Click the link for more information. , keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions.
Eye Disorders
In addition to errors of refraction (astigmatism astigmatism (əstĭg`mətĭz'əm)
..... Click the link for more information. , farsightedness farsightedness or hyperopia, condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either
..... Click the link for more information. , and nearsightedness nearsightedness or myopia, defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the
..... Click the link for more information. ), the human eye is subject to various types of injury, infection, and changes due to systemic disease. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person. Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. Eyes that are used in various ways for surgical repairs are supplied by eye banks. People can arrange to have their eyes donated to such organizations after their death.
Eyes in Other Animals
The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.
Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.
The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.
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Eye (invertebrate)
An aggregation of photoreceptor cells together with any associated optical structures. Eyes occur almost universally among animals, and are possessed by some species of virtually every major animal phylum. However, the complexity of eyes varies greatly, and this sense organ undoubtedly evolved independently a number of times within the animal kingdom.
The simplest invertebrate organs that might be considered to be eyes are clusters of photoreceptor cells located on the surface of the body. Pigment cells are usually interspersed among the photoreceptors, giving the eye a red or black color. Accessory structures, such as the lens and cornea, are usually absent. Simple eyes of this type, called pigment spot ocelli, are found in such invertebrates as jellyfish, flatworms, and sea stars.
The most basic image-forming type of invertebrate eye probably arose from such patches of photoreceptor cells by an in-sinking of the sensory epithelium to form a cup, which may have become closed in conjunction with the evolution of a cornea and lens. Such an evolutionary history is clearly suggested by the embryology and comparative anatomy of many invertebrates.
In bilateral cephalic invertebrates, the eyes are typically paired and located at the anterior end of the body. Although one pair is usual, as in mollusks and many arthropods, multiple pairs are not uncommon. Some polychaete annelids have 4 eyes, and scorpions may have as many as 12. The greatest number of eyes is found in marine flatworms, where there may be over 100 ocelli scattered over the dorsal anterior surface and along the sides of the body. The occurrence of eyes on parts of the body other than the head is usually correlated with radial symmetry or unusual modes of existence.
The primitive function of animal eyes was merely to provide information regarding the intensity, direction, and duration of environmental light. The perception of objects is dependent upon several factors, namely, the number of photoreceptors in the retina, the quality of the optics, and central processing of visual information.
Image formation has evolved as an additional capacity of the eyes of some invertebrates. The number of photoreceptor cells composing the retinal surface is of primary importance, since each photoreceptor cell or group of cells acts as the detector for one point of light. An image is formed by the retina through the association of points of light of varying intensity, much as an image is produced by an array of pixels on a computer monitor. The ability of an eye to form an image and the coarseness or fineness of the image are, therefore, dependent upon the number of points of light that are distinguished which, in turn, is dependent upon the number of photoreceptor cells composing the retina. A large number of photoreceptor cells must be present to produce even a coarse image. The great majority of invertebrate eyes cannot form a detailed image because they do not possess a sufficient number of photoreceptor cells. The number of photoreceptor cells might be sufficient to detect movement of an object, but is inadequate to provide much information about the object's form. See Photoreception
The focusing mechanisms of invertebrate eyes vary considerably. The focus of arthropod eyes tends to be fixed, that is, the distance between the optical apparatus and the retina cannot be changed. Thus objects are in focus only at a certain distance from the eye, determined by the distance between the lens and the retina.
The oceanic family of swimming polychaete worms, Alciopidae, have eyes of that are focused hydrostatically. A bulb to one side of the eye injects fluid into the space between the retina and the lens, forcing the lens outward. Another mechanism is employed in octopods whereby lens movement is brought about by a ciliary muscle attached to the lens (as in aquatic vertebrates, like fish).
The compound eye of crustaceans, insects, centipedes, and horseshoe crabs has a sufficiently different construction from that of other invertebrates to warrant separate discussion. The structural unit of the compound eye is called an ommatidium (see illustration). The outer end of the ommatidium is composed of a cornea, which appears on the surface of the eye as a facet. Beneath the cornea is an elongated, tapered crystalline cone; in many compound eyes the cornea and cone together function as a lens. The receptor element at the inner end of the ommatidium is composed of one or more central translucent cylinders (rhabdome), around which are located several photoreceptor cells (typically 7 or 8).
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The rhabdome is the initial photoreceptive element, and it in turn stimulates the adjacent photoreceptor cells to depolarize. The photoreceptor element of each ommatidium functions as a unit and can respond only to one point of light. Thus image formation is dependent upon the number of photoreceptor units present. The number of ommatidia composing a compound eye varies greatly.
Pigment granules surround the ommatidium proximally and distally, forming a light screen that separates one ommatidium from another. The pigment granules migrate, depending upon the amount of light. In bright light the ommatidium is adapted by funneling light directly down to the rhabdome, by extending the pigment screen, so that light received by one ommatidium is prevented from stimulating the rhabdome of another. Under these conditions the image produced is said to be appositional, or mosaic. The term mosaic has been misinterpreted to mean that a given ommatidium forms a separate image, even if only a part of the image. In general, however, the compound eyes function like any other eye—each photoreceptor unit represents one point in visual space. It is not obvious whether or not compound eyes have any special advantages over other eye designs, despite their universal occurrence in crustaceans and insects. However, in many arthropods the total corneal surface is greatly convex, resulting in a wide visual field.
Many invertebrate eyes are capable of seeing and analyzing patterns of polarized light in nature. This capacity reaches its apex in compound eyes, as well as in the simple eyes of cephalopods. Cuttlefish are known to communicate with each other with displays produced on their body surfaces that are visible only to animals that have polarization vision. Most invertebrates with polarization vision, however, use this ability to navigate with the assistance of patterns of polarization in the sky that occur naturally due to scattering of sunlight by the atmosphere. Bees and ants can find their way back to their nests or hives using only these celestial polarization cues. See Eye (vertebrate)
Eye (vertebrate)
A sense organ that acts as a photoreceptor capable of image formation. The eye of vertebrates is constructed along a basic anatomical pattern which, in the diversification of animals, has undergone a variety of structural and functional modifications associated with different ecologies and modes of living. Often compared with a camera, the vertebrate eye is conveniently described in terms of its wall, cavities, and lens (see illustration).
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Wall
The wall of the eye consists of three distinct layers or tunics which, from outward to inward, are termed the fibrous, vascular, and sensory tunics.
Fibrous tunic
This continuous, outermost fibrous tunic comprises a transparent anterior portion, the cornea, and a tough posterior portion, the sclera. In the human, the cornea represents about one-sixth of the fibrous tunic, the sclera five-sixths.
The vertebrate cornea exhibits very few modifications in structure regardless of environmental influences. Its major constituent is connective tissue (both cells and fibers), regularly arranged and bordered on both anterior and posterior surfaces by an epithelium. The anterior epithelium is stratified, ectodermal in origin, and continuous with the (conjunctival) epithelium lining the eyelids. The transparency of the cornea is attributed to the geometric organization of its connective tissue elements, its constant state of deturgescence, and its chemical composition. It is the first ocular component traversed by the incoming light.
The sclera, a touch connective tissue tunic, provides support for the eye and serves for the attachment (insertions) of the muscles that move it.
The limbus is located at the angle of the anterior chamber. This small, circular transitional zone between the cornea and the sclera houses the major route for the discharge of aqueous humor from the anterior chamber.
Vascular tunic
The vascular tunic or uvea makes up the middle layer of the wall of the eye. It does not form a continuous layer around the eye but is deficient anteriorly, where the opening is termed the pupil. Beginning at the pupil, three continuous components of the uvea can easily be recognized: the iris, ciliary body, and choroid.
The iris is a spongy, circular diaphragm of loose, pigmented connective tissue separating the anterior and posterior chambers and housing a hole, the pupil, in its center. When heavily pigmented, the human iris appears brown; when lightly pigmented, blue.
The ciliary body is continuous with the root of the iris. The posterior epithelium of the iris continues along the internal surface of the ciliary body as a double layer of cells (ciliary part of the retina) which assumes many folds for the attachment of the suspensory ligament of the lens. This ligament holds the lens in position and shape, and marks the posterior boundary of the posterior chamber. The inner layer of the ciliary epithelium contains no pigment. It produces aqueous humor which flows into the posterior chamber and thence into the anterior chamber (via the pupil). The continual production and removal of this fluid maintain the intraocular pressure of the eye (which is increased in glaucoma).
The choroid is the most posterior portion of the uvea. It is directly continuous with the subepithelial portion of the ciliary body and consists primarily of blood vessels embedded within deeply pigmented connective tissue.
Sensory tunic
The retina is the sensory tunic of the eye. It has the form of a cup closely applied to the inner portion of the choroid, and, internally, it is slightly adherent to the semisolid vitreous body. The vertebrate retina contains the light-sensitive receptors (visual cells) and a complex of well-organized impulse-carrying nerve cells (neurons), all arranged into discrete layers.
The pigment epithelium forms an important barrier between the light-sensitive receptors (visual cells) and their blood supply, the choroid. As in the choroid, the pigmentation serves to absorb light and prevent its reflection.
The rods and cones of vertebrates generally occur as single units, but combinations of each type are frequently encountered in several vertebrate classes. Cones appear to be adapted for photopic, or daylight, vision, based on correlations with the visual habits of the animals involved. Rods, which predominate in nocturnal vertebrates, are adapted for scotopic, or night, vision. Except for their external process, the structure of these cells does not reflect these differences. See Photoreception; Vision
An important adaptation for improving visual detail in vertebrates is the formation of circumscribed thickenings of the retina resulting from localized increases in the number of visual cells and the other retinal neurons associated synaptically with them. Such thickenings, termed areas of acute vision, appear in some members of all vertebrate classes and reach their greatest development in birds, in which one to three distinct areas may be found in the same retina. Only a single area occurs in humans; it is colored yellow and is called the macula. The macula is situated in the center of the fundus and contains only cones.
Cavities
Three cavities or chambers are present within the vertebrate eye: anterior, posterior, and vitreous. The anterior and posterior chambers are continuous with one another at the pupil and are filled with the aqueous humor. The eye is normally maintained in a distended state by the (intraocular) pressure created by this fluid. The vitreous cavity, on the other hand, is filled with a semisolid material, the vitreous body, which is fixed in amount and relatively permanent. Its consistency is not uniform in all vertebrates, however.
Lens
The lens is a transparent body, supported by thin suspensory fibers and by the vitreous body behind and by the iris in front. It is completely cellular, the anterior cells forming a thin epithelium, and the posterior cells, much elongated, forming the so-called lens fibers. The entire lens is surrounded by an elastic capsule which serves for the attachment of the ciliary zonule. In all vertebrates the lens functions in accommodation, either by moving backward and forward or by changing its shape. An opacity of the lens is termed a cataract.
Electrophysiology of rods and cones
Visual information perceived by the vertebrate eye is fed to the brain in the form of coded electrical impulses that are initiated by the light-sensitive, visual-pigment-containing outer segments of the rods and cones. Light striking the outer segments is absorbed by these pigments, resulting—in the case of rhodopsin, for example—in the isomerization of the 11-cis-retinal chromophore to all trans-retinal. The outcome of this photolytic process is a change in electrical activity at the plasma membrane enclosing the outer segments, and a sudden and drastic decrease in its permeability (particularly to Na+). The net result is a hyperpolarization response, or increased negativity of membrane potential. Hyperpolarization generates a membrane current that spreads to the inner segment and finally to the synaptic terminal, where it regulates the release of neurotransmitter and thus controls the flow of information from the visual cells to other retinal cells (bipolars, horizontals, other photoreceptors).
Cyclic GMP is directly responsible for regulating the permeability of the plasma membrane by opening ionic channels (in the light). Its concentration is controlled by a peripheral membrane enzyme, phosphodiesterase, which in turn is activated by transducin, an intracellular messenger protein generated by a photolytic intermediate of rhodopsin. Since one molecule of photoactivated rhodopsin can react with many molecules of transducin, an amplification of the visual cells' response is produced, the final amplitude being enhanced by breakdown of cyclic GMP by phosphodiesterase and subsequent closure of outer segment ionic channels and hyperpolarization.
Since photoreceptors are depolarized in the dark, their axon terminals continually release a transmitter that hyperpolarizes (inhibits) the bipolar cell, and since this cell is hyperpolarized in the dark, it is prevented from releasing its excitatory transmitter at the ganglion cell synapse so that the synapse is not excited. In the light, hyperpolarization of the visual cells causes a decrease in the amount of inhibitory transmitter released at the bipolar synapse, leading to a depolarization of the latter, which in turn increases the amount of excitatory transmitter released at the bipolar-ganglion synapses and affecting the ganglion cells.
A change in the light energy taking place across the retina also initiates a transient complex of electrical waveforms, the electroretinogram, which is recorded as a difference in potential between the cornea and the back of the eye.
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