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.
Video
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
When an object is brought
closer than
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,
Vision Charts
When a person’s vision is
tested, a chart is placed
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
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.