PHYSIOLOGY OF VISUAL ANALYZER

 

The visual system includes the eyes, the accessory structures, and the optic nerves (II), tracts, and pathways. The eyes respond to light and initiate afferent action potentials, which are transmitted from the eyes to the brain by the optic nerves and tracts. The accessory structures, such as eyebrows, eyelids, eyelashes, and tear glands, help protect the eyes from direct sunlight and damaging particles.

Much of the information about the world around us is detected by the visual system. Our education is largely based on visual input and depends on our ability to read words and numbers.Visual input includes information about light and dark, color and hue.

Accessory Structures

Accessory structures protect, lubricate,move, and in other ways aid in the function of the eye. These structures include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles. The eyebrows protect the eyes by preventing perspiration, which can irritate the eyes, from running down the forehead and into them, and they help shade the eyes from direct sunlight. The eyelids, also called palpebrae, with their associated lashes, protect the eyes from foreign objects. The space between the two eyelids is called the palpebral fissure, and the angles where the eyelids join at the medial and lateral margins of the eye are called canthi. The medial canthus contains a small reddish-pink mound called the caruncle. The caruncle contains some modified sebaceous and sweat glands. The eyelids consist of five layers of tissue. From the outer to the inner surface, they are (1) a thin layer of integument on the external surface; (2) a thin layer of areolar connective tissue; (3) a layer of skeletal muscle consisting of the orbicularis oculi and levator palpebrae superioris muscles; (4) a crescent-shaped layer of dense connective tissue called the tarsal plate, which helps maintain the shape of the eyelid; and (5) the palpebral conjunctiva (described in the next section), which lines the inner surface of the eyelid and the anterior surface of the eyeball.

If an object suddenly approaches the eye, the eyelids protect the eye by rapidly closing and then opening (blink reflex). Blinking, which normally occurs about 25 times per minute, also helps keep the eye lubricated by spreading tears over the surface of the eye. Movements of the eyelids are a function of skeletal muscles. The orbicularis oculi muscle closes the lids, and the levator palpebrae superioris elevates the upper lid.

The eyelids also help regulate the amount of light entering the eye. Eyelashes are attached as a double or triple row of hairs to the free edges of the eyelids. Ciliary glands are modified sweat glands that open into the follicles of the eyelashes to keep them lubricated.When one of these glands becomes inflamed, it’s called a sty. Meibomian glands are sebaceous glands near the inner margins of the eyelids and produce sebum, which lubricates the lids and restrains tears from flowing over the margin of the eyelids. An infection or blockage of a meibomian gland is called a chalazion, or meibomian cyst. The conjunctiva is a thin, transparent mucous membrane. The palpebral conjunctiva covers the inner surface of the eyelids, and the bulbar conjunctiva covers the anterior surface of the eye. The points at which the palpebral and bulbar conjunctivae meet are the superior and inferior conjunctival fornices.

Lacrimal Apparatus

The lacrimal apparatus consists of a lacrimal gland situated in the superolateral corner of the orbit and a nasolacrimal duct beginning in the inferomedial corner of the orbit. The lacrimal gland is innervated by parasympathetic fibers from the facial nerve (VII). The gland produces tears, which leave the gland through several ducts and pass over the anterior surface of the eyeball.

Tears are produced constantly by the gland at the rate of about 1 mL/day to moisten the surface of the eye, lubricate the eyelids, and wash away foreign objects. Tears are mostly water, with some salts,mucus, and lysozyme, an enzyme that kills certain bacteria.Most of the fluid produced by the lacrimal glands evaporates from the surface of the eye, but excess tears are collected in the medial corner of the eye by the lacrimal canaliculi. The opening of each lacrimal canaliculus is called a punctum. The upper and lower eyelids each have a punctum near the medial canthus. Each punctum is located on a small lump called the lacrimal papilla. The lacrimal canaliculi open into a lacrimal sac, which in turn continues into the nasolacrimal duct. The nasolacrimal duct opens into the inferior meatus of the nasal cavity beneath the inferior nasal concha.

Optic system of eyeball

Cornea allows light to enter the eyeball. Aqueous humor fills anterior and posterior chambers in front of lens. Crystalline lens is a transparent elastic and biconcave lens, which refracts light and focuses it on retina. Vitreous body is a transparent gel enclosed by vitreous membrane, which fills eyeball behind lens.

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The Cornea

The central part of the cornea receives oxygen from the outside air. Soft plastic contact lenses worn for long periods must therefore be permeable to air so that air can reach the cornea. The most common eye injuries are cuts or tears of the cornea caused by foreign objects like stones or sticks hitting the cornea. Extensive injury to the cornea may cause connective tissue deposition, thereby making the cornea opaque. The cornea was one of the first organs transplanted. Several characteristics make it relatively easy to transplant: It’s easily accessible and relatively easily removed; it’s avascular and therefore does not require as extensive circulation as do other tissues; and it’s less immunologically active and therefore less likely to be rejected than other tissues.

The middle tunic of the eyeball is called the vascular tunic because it contains most of the blood vessels of the eyeball. The arteries of the vascular tunic are derived from a number of arteries called short ciliary arteries, which pierce the sclera in a circle around the optic nerve. These arteries are branches of the ophthalmic artery, which is a branch of the internal carotid artery. The vascular tunic contains a large number of melanin-containing pigment cells and appears black in color. The portion of the vascular tunic associated with the sclera of the eye is the choroid. The term choroid means membrane and suggests that this layer is relatively thin (0.1–0.2 mm thick). Anteriorly, the vascular tunic consists of the ciliary body and iris. The ciliary body is continuous with the choroid, and the iris is attached at its lateral margins to the ciliary body. The ciliary body consists of an outer ciliary ring and an inner group of ciliary processes, which are attached to the lens by suspensory ligaments. The ciliary body contains smooth muscles called the ciliary muscles, which are arranged with the outer muscle fibers oriented radially and the central fibers oriented circularly. The ciliary muscles function as a sphincter, and contraction of these muscles can change the shape of the lens.

The ciliary processes are a complex of capillaries and cuboidal epithelium that produces aqueous humor. The iris is the “colored part” of the eye, and its color differs from person to person. Brown eyes have brown melanin pigment in the iris. Blue eyes are not caused by a blue pigment but result from the scattering of light by the tissue of the iris, overlying a deeper layer of black pigment. The blue color is produced in a fashion similar to the scattering of light as it passes through the atmosphere to form the blue skies from the black background of space. The iris is a contractile structure, consisting mainly of smooth muscle, surrounding an opening called the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae and a radial group called the dilator pupillae. The sphincter pupillae are innervated by parasympathetic fibers from the oculomotor nerve (III).When they contract, the iris decreases or constricts the size of the pupil. The dilator pupillae are innervated by sympathetic fibers. When they contract, the pupil is dilated. The ciliary muscles, sphincter pupillae, and dilator pupillae are sometimes referred to as the intrinsic eye muscles.

Lens

The lens is an unusual biologic structure. Transparent and biconvex, with the greatest convexity on its posterior side, the lens consists of a layer of cuboidal epithelial cells on its anterior surface and a posterior region of very long columnar epithelial cells called lens fibers. Cells from the anterior epithelium proliferate and give rise to the lens fibers at the equator of the lens. The lens fibers lose their nuclei and other cellular organelles and accumulate a special set of proteins called crystallines. This crystalline lens is covered by a highly elastic transparent capsule. The lens is suspended between the two eye compartments by the suspensory ligaments of the lens, which are connected from the ciliary body to the lens capsule.

The eye functions much like a camera. The iris allows light into the eye, and the lens, cornea, and humors focus the light onto the retina. The light striking the retina is converted into action potentials that are relayed to the brain.

The electromagnetic spectrum is the entire range of wavelengths or frequencies of electromagnetic radiation from very short gamma waves at one end of the spectrum to the longest radio waves at the other end. Visible light is the portion of the electromagnetic spectrum that can be detected by the human eye. Light has characteristics of both particles (photons) and waves, with a wavelength between 400 and 700 nm. This range sometimes is called the range of visible light or, more correctly, the visible spectrum. Within the visible spectrum, each color has a different wavelength.

Light Refraction and Reflection

An important characteristic of light is that it can be refracted (bent). As light passes from air to a denser substance like glass or water, its speed is reduced. If the surface of that substance is at an angle other than 90 degrees to the direction the light rays are traveling, the rays are bent as a result of variation in the speed of light as it encounters the new medium. This bending of light is called refraction. If the surface of a lens is concave, with the lens thinnest in the center, the light rays diverge as a result of refraction. If the surface is convex, with the lens thickest in the center, the light rays tend to converge. As light rays converge, they finally reach a point at which they cross. This point is called the focal point, and causing light to converge is called focusing. No image is formed exactly at the focal point, but an inverted, focused image can form on a surface located some distance past the focal point. How far past the focal point the focused image forms depends on a number of factors. A biconvex lens causes light to focus closer to the lens than does a lens with a single convex surface. Furthermore, the more nearly spherical the lens, the closer to the lens the light is focused; the more flattened the biconcave lens, the more distant is the point where the light is focused.

If light rays strike an object that is not transparent, they bounce off the surface. This phenomenon is called reflection. If the surface is very smooth, such as the surface of a mirror, the light rays bounce off in a specific direction. If the surface is rough, the light rays are reflected in several directions and produce a more diffuse reflection. We can see most solid objects because of the light reflected from their surfaces.

Focusing of Images on the Retina

The focusing system of the eye projects a clear image on the retina. Light rays converge as they pass from the air through the convex cornea. Additional convergence occurs as light encounters the aqueous humor, lens, and vitreous humor. The greatest contrast in media density is between the air and the cornea; therefore, the greatest amount of convergence occurs at that point. The shape of the cornea and its distance from the retina are fixed, however, so that no adjustment in the location of the focal point can be made by the cornea. Fine adjustment in focal point location is accomplished by changing the shape of the lens. In general, focusing can be accomplished in two ways. One is to keep the shape of the lens constant and move it nearer or farther from the point at which the image will be focused, such as occurs in a camera, microscope, or telescope. The second way is to keep the distance constant and to change the shape of the lens, which is the technique used in the eye.

As light rays enter the eye and are focused, the image formed just past the focal point is inverted. Action potentials that represent the inverted image are passed to the visual cortex of the cerebrum, where they are interpreted by the brain as being right side up.

The focal point (FP) is where light rays cross. (a) Distant image. The lens is flattened, and the image is focused on the retina. (b) Accommodation for near vision. The lens is more rounded, and the image is focused on the retina.

When the ciliary muscles are relaxed, the suspensory ligaments of the ciliary body maintain elastic pressure on the lens, thereby keeping it relatively flat and allowing for distant vision. The condition in which the lens is flattened so that nearly parallel rays from a distant object are focused on the retina is referred to as emmetropia and is the normal resting condition of the lens. The point at which the lens does not have to thicken for focusing to occur is called the far point of vision and normally is 20 feet or more from the eye.

When an object is brought closer than 20 feet to the eye, three events occur to bring the image into focus on the retina: accommodation by the lens, constriction of the pupil, and convergence of the eyes.

Accommodation

When focusing on a nearby object, the ciliary muscles contract as a result of parasympathetic stimulation from the oculomotor nerve (III). This sphincterlike contraction pulls the choroid toward the lens to reduce the tension on the suspensory ligaments. This allows the lens to assume a more spherical form because of its own elastic nature. The more spherical lens then has a more convex surface, causing greater refraction of light. This process is called accommodation. As light strikes a solid object, the rays are reflected in every direction from the surface of the object. Only a small portion of the light rays reflected from a solid object, however, pass through the pupil and enter the eye of any given person. An object far away from the eye appears small compared to a nearby object because only nearly parallel light rays enter the eye from a distant object. Converging rays leaving an object closer to the eye can also enter the eye, and the object appears larger.

When rays from a distant object reach the lens, they don’t have to be refracted to any great extent to be focused on the retina, and the lens can remain fairly flat.When an object is closer to the eye, the more obliquely directed rays must be refracted to a greater extent to be focused on the retina. As an object is brought closer and closer to the eye, accommodation becomes more and more difficult because the lens cannot become any more convex. At some point, the eye no longer can focus the object, and it’s seen as a blur. The point at which this blurring occurs is called the near point of vision, which is usually about 2–3 inches from the eye for children, 4–6 inches for a young adult, 20 inches for a 45- year-old adult, and 60 inches for an 80-year-old adult. This increase in the near point of vision, called presbyopia, occurs because the lens becomes more rigid with increasing age, which is primarily why some older people say they could read with no problem if they only had longer arms.

Vision Charts

When a person’s vision is tested, a chart is placed 20 feet from the eye, and the person is asked to read a line of letters that is standardized for normal vision. If the person can read the line, the vision is considered to be 20/20, which means that the person can see at 20 feet what people with normal vision can see at 20 feet. If, on the other hand, the person can see words only at 20 feet that people with normal vision can see at 40 feet, the vision is considered 20/40.

Pupil constriction

Another factor involved in focusing is the depth of focus, which is the greatest distance through which an object can be moved and still remain in focus on the retina. The main factor affecting the depth of focus is the size of the pupil. If the pupillary diameter is small, the depth of focus is greater than if the pupillary diameter is large.With a smaller pupillary opening, an object may therefore be moved slightly nearer or farther from the eye without disturbing its focus. This is particularly important when viewing an object at close range because the interest in detail is much greater, and therefore the acceptable margin for error is smaller.When the pupil is constricted, the light entering the eye tends to pass more nearly through the center of the lens and is more accurately focused than light passing through the edges of the lens. Pupillary diameter also regulates the amount of light entering the eye.

The dimmer the light, the greater the pupil diameter must be. As the pupil constricts during close vision, therefore, more light is required on the object being observed.

Extrinsic Eye Muscles

Six extrinsic muscles of the eye cause the eyeball to move. Four of these muscles run more or less straight anteroposteriorly. They are the superior, inferior, medial, and lateral rectus muscles. Two muscles, the superior and inferior oblique muscles, are placed at an angle to the globe of the eye. The movements of the eye can be described graphically by a figure resembling the letter H. The clinical test for normal eye movement is therefore called the H test. A person’s inability to move his eye toward one part of the H may indicate dysfunction of an extrinsic eye muscle or the cranial nerve to the muscle. The superior oblique muscle is innervated by the trochlear nerve (IV). The nerve is so named because the superior oblique muscle goes around a little pulley, or trochlea, in the superomedial corner of the orbit. The lateral rectus muscle is innervated by the abducens nerve (VI), so named because the lateral rectus muscle abducts the eye. The other four extrinsic eye muscles are innervated by the oculomotor nerve (III).

Convergence

Because the light rays entering the eyes from a distant object are nearly parallel, both pupils can pick up the light rays when the eyes are directed more or less straight ahead. As an object moves closer, however, the eyes must be rotated medially so that the object is kept focused on corresponding areas of each retina. Otherwise the object appears blurry. This medial rotation of the eyes is accomplished by a reflex which stimulates the medial rectus muscle of each eye. This movement of the eyes is called convergence.

Convergence can easily be observed. Have someone stand facing you. Have the person reach out one hand and extend an index finger as far in front of his face as possible.While the person keeps his gaze fixed on the finger, have him slowly bring the finger in toward his nose until he finally touches it. Notice the movement of his pupils during this movement.What happens?

Retina

The retina is the innermost, nervous tunic of the eye. It consists of the outer pigmented retina, which is pigmented simple cuboidal epithelium, and the inner sensory retina, which responds to light. The sensory retina contains 120 million photoreceptor cells called rods and another 6 or 7 million cones, as well as numerous relay neurons. The retina covers the inner surface of the eye posterior to the ciliary body.

The pupil appears black when you look into a person’s eye because of the pigment in the choroid and the pigmented portion of the retina. The eye is a closed chamber, which allows light to enter only through the pupil. Light is absorbed by the pigmented inner lining of the eye; thus looking into it is like looking into a dark room. If a bright light is directed into the pupil, however, the reflected light is red because of the blood vessels on the surface of the retina. This is why the pupils of a person looking directly at a flash camera often appear red in a photograph. People with albinism lack the pigment melanin, and the pupil always appears red because no melanin is present to absorb light and prevent it from being reflected from the back of the eye. The diffusely lighted blood vessels in the interior of the eye contribute to the red color of the pupil.

When the posterior region of the retina is examined with an ophthalmoscope, several important features can be observed.Near the center of the posterior retina is a small yellow spot approximately 4 mm in diameter, the macula lutea. In the center of the macula lutea is a small pit, the fovea centralis. The fovea and macula make up the region of the retina where light is focused. The fovea is the portion of the retina with the greatest visual acuity, the ability to see fine images because the photoreceptor cells are more tightly packed in that portion of the retina than anywhere else. Just medial to the macula lutea is a white spot, the optic disc, through which blood vessels enter the eye and spread over the surface of the retina. This is also the spot where nerve processes from the sensory retina meet, pass through the outer two tunics, and exit the eye as the optic nerve. The optic disc contains no photoreceptor cells and does not respond to light; therefore it’s called the blind spot of the eye.

Structure and Function of the Retina

The retina of each eye, which gives us the potential to see the whole world, is about the size and thickness of a postage stamp. The retina consists of a pigmented retina and a sensory retina. The sensory retina contains three layers of neurons: photoreceptor, bipolar, and ganglionic. The cell bodies of these neurons form nuclear layers separated by plexiform layers, where the neurons of adjacent layers synapse with each other. The outer plexiform (plexuslike) layer is between the photoreceptor and bipolar cell layers. The inner plexiform layer is between the bipolar and ganglionic cell layers.

The pigmented retina, or pigmented epithelium, consists of a single layer of cells. This layer of cells is filled with melanin pigment and, together with the pigment in the choroid, provides a black matrix, which enhances visual acuity by isolating individual photoreceptors and reducing light scattering. Pigmentation is not strictly necessary for vision, however. People with albinism (lack of pigment) can see, although their visual acuity is reduced because of some light scattering.

The layer of the sensory retina nearest the pigmented retina is the layer of rods and cones. The rods and cones are photoreceptor cells, which are sensitive to stimulation from “visible” light. The light-sensitive portion of each photoreceptor cell is adjacent to the pigmented layer.

Rods

Rods are bipolar photoreceptor cells involved in noncolor vision and are responsible for vision under conditions of reduced light. The modified, dendritic, light-sensitive part of rod cells is cylindrical, with no taper from base to apex. This rod-shaped photoreceptive part of the rod cell contains about 700 double-layered membranous discs. The discs contain rhodopsin, which consists of the protein opsin covalently bound to a pigment called retinal (derived from vitamin A). In the resting (dark) state, the shape of opsin keeps 11-cis-retinal tightly bound to the internal surface of opsin. As light is absorbed by rod cells, opsin changes shape from 11-cis-retinal to all-trans-retinal. These changes activate the attached G protein, called transducin, which closes Na+ channels, resulting in hyperpolarization of the cell.

This hyperpolarization in the photoreceptor cells is somewhat remarkable, because most neurons respond to stimuli by depolarizing. When photoreceptor cells are not exposed to light and are in a resting, nonactivated state, some of the Na+ channels in their membranes are open, and Na+ flow into the cell. This influx of Na+ causes the photoreceptor cells to release the neurotransmitter glutamate from their presynaptic terminals.

Glutamate binds to receptors on the postsynaptic membranes the bipolar cells of the retina, causing them to hyperpolarize. Thus, glutamate causes an inhibitory postsynaptic potential (IPSP) in the bipolar cells.

When photoreceptor cells are exposed to light, the Na+ channels close, fewer Na+ enter the cell, and the amount of glutamate released from the presynaptic terminals decreases.As a result, the hyperpolarization in the bipolar cells decreases, and the cells depolarize sufficiently to release neurotransmitters, which stimulate ganglionic cells to generate action potentials. The number of Na+ channels that close and the degree to which they close is proportional to the amount of light exposure.

At the final stage of this light-initiated reaction, retinal is completely released from the opsin. This free retinal may then be converted back to vitamin A, from which it was originally derived. The total vitamin A/retinal pool is in equilibrium so that under normal conditions the amount of free retinal is relatively constant.

To create more rhodopsin, the altered retinal must be converted back to its original shape, a reaction that requires energy. Once the retinal resumes its original shape, its recombination with opsin is spontaneous, and the newly formed rhodopsin can again respond to light.

Light and dark adaptation is the adjustment of the eyes to changes in light. Adaptation to light or dark conditions, which occurs when a person comes out of a darkened building into the sunlight or vice versa, is accomplished by changes in the amount of available rhodopsin. In bright light excess rhodopsin is broken down so that not as much is available to initiate action potentials, and the eyes become “adapted” to bright light. Conversely, in a dark room more rhodopsin is produced, making the retina more light-sensitive. If breakdown of rhodopsin occurs rapidly and production is slow, do eyes adapt more rapidly to light or dark conditions? Light and dark adaptation also involves pupil reflexes. The pupil enlarges in dim light to allow more light into the eye and contracts in bright light to allow less light into the eye. In addition, rod function decreases and cone function increases in light conditions, and vice versa during dark conditions. This occurs because rod cells are more sensitive to light than cone cells and because rhodopsin is depleted more rapidly in rods than in cones.

Cones

Color vision and visual acuity are functions of cone cells. Color is a function of the wavelength of light, and each color results from a certain wavelength within the visible spectrum. Even though rods are very sensitive to light, they cannot detect color, and sensory input that ultimately reaches the brain from these cells is interpreted by the brain as shades of gray. Cones require relatively bright light to function. As a result, as the light decreases, so does the color of objects that can be seen until, under conditions of very low illumination, the objects appear gray. This occurs because as the light decreases, fewer cone cells respond to the dim light.

Cones are bipolar photoreceptor cells with a conical lightsensitive part that tapers slightly from base to apex. The outer segments of the cone cells, like those of the rods, consist of double-layered discs. The discs are slightly more ous and more closely stacked in the cones than in the rods. Cone cells contain a visual pigment, iodopsin, which consists of retinal combined with a photopigment opsin protein. Three major types of color-sensitive opsin exist: blue, red, and green; each closely resembles the opsin proteins of rod cells but with somewhat different amino acid sequences. These color photopigments function in much the same manner as rhodopsin, but whereas rhodopsin responds to the entire spectrum of visible light, each iodopsin is sensitive to a much narrower spectrum.

Most people have one red pigment gene and one or more green pigment genes located in a tandem array on each X chromosome. An enhancer gene on the X chromosome apparently determines that only one color opsin gene is expressed in each cone cell. Only the first or second gene in the tandem array is expressed in each cone cell, so that some cone cells express only the red pigment gene and others express only one of the green pigment genes.

Although considerable overlap occurs in the wavelength of light to which these pigments are sensitive, each pigment absorbs light of a certain range of wavelengths. As light of a given wavelength, representing a certain color, strikes the retina, all cone cells containing photopigments capable of responding to that wavelength generate action potentials. Because of the overlap among the three types of cones, especially between the green and red pigments, different proportions of cone cells respond to each wavelength, thus allowing color perception over a wide range. Color is interpreted in the visual cortex as combinations of sensory input originating from cone cells. For example, when orange light strikes the retina, 99% of the red-sensitive cones respond, 42% of the green-sensitive cones respond, and no blue cones respond.When yellow light strikes the retina, the response is shifted so that a greater number of green-sensitive cones respond. The variety of combinations created allows humans to distinguish several million gradations of light and shades of color.

Distribution of Rods and Cones in the Retina

Cones are involved in visual acuity, in addition to their role in color vision. The fovea centralis is used when visual acuity is required, such as for focusing on the words of this page. The fovea centralis has about 35,000 cones and no rods. The 120 million rods are 20 times more plentiful than cones over most of the remaining retina, however. They are more highly concentrated away from the fovea and are more important in low-light conditions.

Inner Layers of the Retina

The middle and inner nuclear layers of the retina consist of two major types of neurons: bipolar and ganglion cells. The rod and cone photoreceptor cells synapse with bipolar cells, which in turn synapse with ganglion cells. Axons from the ganglion cells pass over the inner surface of the retina, except in the area of the fovea centralis, converge at the optic disc, and exit the eye as the optic nerve (II). The fovea centralis is devoid of ganglion cell processes, resulting in a small depression in this area; thus the name fovea, meaning small pit. As a result of the absence of ganglion cell processes in addition to the concentration of cone cells mentioned previously, visual acuity is further enhanced in the fovea centralis because light rays don’t have to pass through as many tissue layers before reaching the photoreceptor cells.

Rod and cone cells differ in the way they interact with bipolar and ganglion cells. One bipolar cell receives input from numerous rods, and one ganglion cell receives input from several bipolar cells so that spatial summation of the signal occurs and the signal is enhanced, thereby allowing awareness of stimulus from very dim light sources but decreasing visual acuity in these cells. Cones, on the other hand, exhibit little or no convergence on bipolar cells so that one cone cell may synapse with only one bipolar cell. This system reduces light sensitivity but enhances visual acuity.

Within the inner layers of the retina, association neurons are present also, which modify the signals from the photoreceptor cells before the signal ever leaves the retina. Horizontal cells form the outer plexiform layer and synapse with photoreceptor cells and bipolar cells. Amacrine cells form the inner plexiform layer and synapse with bipolar and ganglion cells. Interplexiform cells form the bipolar layer and synapse with amacrine, bipolar, and horizontal cells to form a feedback loop. Association neurons are either excitatory or inhibitory on the cells with which they synapse. These association cells enhance borders and contours, thereby increasing the intensity at boundaries, such as the edge of a dark object against a light background.

Neuronal Pathways for Vision

The optic nerve (II) leaves the eye and exits the orbit through the optic foramen to enter the cranial cavity. Just inside the vault and just anterior to the pituitary, the optic nerves are connected to each other at the optic chiasm. Ganglion cell axons from the nasal retina (the medial portion of the retina) cross through the optic chiasm and project to the opposite side of the brain. Ganglion cell axons from the temporal retina (the lateral portion of the retina) pass through the optic nerves and project to the brain on the same side of the body without crossing.

Beyond the optic chiasm, the route of the ganglionic axons is called the optic tract. Most of the optic tract axons terminate in the lateral geniculate nucleus of the thalamus. Some axons do not terminate in the thalamus but separate from the optic tract to terminate in the superior colliculi, the center for visual reflexes. Neurons of the lateral geniculate ganglion form the fibers of the optic radiations, which project to the visual cortex in the occipital lobe. Neurons of the visual cortex integrate the messages coming from the retina into a single message, translate that message into a mental image, and then transfer the image to other parts of the brain, where it is evaluated and either ignored or acted on.

The visual fields of the eyes partially overlap. The region of overlap is the area of binocular vision, seen with two eyes at the same time, and it is responsible for depth perception, the ability to distinguish between near and far objects and to judge their distance. Because humans see the same object with both eyes, the image of the object reaches the retina of one eye at a slightly different angle from that of the other.With experience, the brain can interpret these differences in angle so that distance can be judged quite accurately.