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.
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.