Download 10.9 Sense of Sight The eye, the organ containing visual receptors

10.9 Sense of Sight
The eye, the organ containing visual receptors, provides vision, with the assistance of accessory
organs. These accessory organs include the eyelids and lacrimal apparatus, which protect the
eye, and a set of extrinsic muscles, which move the eye.
Visual Accessory Organs
The eye, lacrimal gland, and associated extrinsic muscles are housed in the orbital cavity, or
orbit, of the skull. Each orbit is lined with the periosteum of various bones, and also contains fat,
blood vessels, nerves, and connective tissues.
Each eyelid has four layers—skin, muscle, connective tissue, and conjunctiva. The skin of the
eyelid, which is the thinnest skin of the body, covers the lid’s outer surface and fuses with its
inner lining near the margin of the lid. The eyelids are moved by the orbicularis oculi muscle
(see fig. 8.17a, p. 207), which acts as a sphincter and closes the lids when it contracts, and by the
levator palpebrae superioris muscle, which raises the upper lid and thus helps open the eye (fig.
10.14). The conjunctiva is a mucous membrane that lines the inner surfaces of the eyelids and
folds back to cover the anterior surface of the eyeball, except for its central portion
(cornea).
FIGURE 10.14Sagittal section of the closed eyelids and anterior portion of the eye.
The lacrimal apparatus consists of the lacrimal gland, which secretes tears, and a series of
ducts that carry tears into the nasal cavity (fig. 10.15). The gland is located in the orbit and
secretes tears continuously. The tears exit through tiny tubules and flow downward and medially
across the eye.
FIGURE 10.15The lacrimal apparatus consists of a tear-secreting gland and a series of ducts.
Two small ducts (the superior and inferior canaliculi) collect tears, which flow into the lacrimal
sac, located in a deep groove of the lacrimal bone, and then into the nasolacrimal duct, which
empties into the nasal cavity. Secretion by the lacrimal gland moistens and lubricates the surface
of the eye and the lining of the lids. Tears also have an enzyme (lysozyme) that kills bacteria,
reducing the risk of eye infections.
The extrinsic muscles arise from the bones of the orbit and insert by broad tendons on the eye’s
tough outer surface. Six extrinsic muscles move the eye in different directions. Any given eye
movement may utilize more than one extrinsic muscle, but each muscle is associated with one
primary action. Figure 10.16 illustrates the locations of these extrinsic muscles, and table 10.2
lists their functions, as well as the functions of the eyelid muscles.
TABLE 10.2 Muscles Associated with the Eyelids and Eyes
Name
Innervation
Muscles of the Eyelids
Orbicularis oculi
Facial nerve (VII)
Levator palpebrae
Oculomotor nerve
superioris
(III)
Extrinsic Muscles of the Eyes
Oculomotor nerve
Superior rectus
(III)
Oculomotor nerve
Inferior rectus
(III)
Oculomotor nerve
Medial rectus
(III)
Lateral rectus
Abducens nerve (VI)
Superior oblique
Trochlear nerve (IV)
Inferior oblique
Oculomotor nerve
(III)
Function
Closes eye
Opens eye
Rotates eye upward and toward midline
Rotates eye downward and toward midline
Rotates eye toward midline
Rotates eye away from midline
Rotates eye downward and away from
midline
Rotates eye upward and away from midline
FIGURE 10.16
Extrinsic muscles of the right eye (lateral view).
APR Module 7: Nervous System: Dissection: Vision: Eye-lateral: Other: Muscles: Layer 1
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Are the extrinsic muscles of the eye under voluntary control?
Answer can be found in Appendix F on page 582.
One eye deviating from the line of vision may result in double vision (diplopia). If this condition
persists, the brain may suppress the image from the deviated eye. As a result, the turning eye
may become blind (suppression amblyopia). Treating eye deviation early in life with exercises,
eyeglasses, and surgery can prevent such monocular (one-eye) blindness.
PRACTICE
23. Explain how the eyelid moves.
24. Describe the conjunctiva.
25. What is the function of the lacrimal apparatus?
Structure of the Eye
The eye is a hollow, spherical structure about 2.5 centimeters in diameter. Its wall has three
distinct layers—an outer (fibrous) layer, a middle (vascular) layer, and an inner (nervous) layer.
The spaces within the eye are filled with fluids that support its wall and internal parts that help
maintain its shape. Figure 10.17 shows the major parts of the eye.
FIGURE 10.17
Transverse section of the right eye (superior view).
APR Module 7: Nervous System: Dissection: Vision: Eye-lateral: Layer 2
APR Module 7: Nervous System: Histology: Vision - eye: LM: Low magnification
Outer Layer
The anterior sixth of the outer layer bulges forward as the transparent cornea (kor′ne-ah), which
is the window of the eye and helps focus entering light rays. The cornea is composed largely of
connective tissue with a thin surface layer of epithelium. It is transparent because it contains few
cells and no blood vessels, and the cells and collagenous fibers form unusually regular patterns.
The cornea is continuous with the sclera (skle′rah), the white portion of the eye. The sclera
makes up the posterior five-sixths of the outer layer and is opaque due to many large,
disorganized, collagen and elastic fibers. The sclera protects the eye and is an attachment for the
extrinsic muscles. In the back of the eye, the optic nerve and certain blood vessels pierce the
sclera. Clinical Application 10.2 describes headaches that include visual symptoms.
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Worldwide, the most common cause of blindness is loss of transparency of the cornea. Each
year, 40,000 corneal transplants are performed in the United States. Corneas are “immune
privileged,” not evoking an immune response, so anyone can donate to anyone. However,
corneal transplants are effective only if the transplanted tissue includes stem cells normally
found in a layer of cells, called the limbus, which separates the cornea from the conjunctiva.
(The cornea itself does not contain stem cells.) Researchers performed limbal cell transplants on
patients with corneal damage in one eye. They took the stem cells from the patients’ healthy
eyes, culturing the limbal cells into a bluish, translucent gel, which they then applied to the
affected eyes. Vision returned. Limbal stem cell transplants may prove to be more effective than
corneal transplants, which have been done since 1905.
Middle Layer
The middle layer of the wall of the eye includes the choroid coat, ciliary body, and iris (fig.
10.17). The choroid coat (ko′roid kōt), in the posterior five-sixths of the globe of the eye, is
loosely joined to the sclera and is honeycombed with blood vessels, which nourish surrounding
tissues. The choroid coat also has many pigment-producing melanocytes. The melanin that these
cells produce absorbs excess light, which helps keep the inside of the eye dark.
The ciliary body (sil′e-er′′e bod′e), which is the thickest part of the middle layer, extends
forward from the choroid coat and forms an internal ring around the front of the eye. Within the
ciliary body are many radiating folds called ciliary processes and groups of muscle fibers that
constitute the ciliary muscles.
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CLINICAL APPLICATION 10.2
Headache
Headaches are common. The cells of the nervous tissue in the brain lack pain receptors, but
nearly all the other tissues of the head, including the meninges and blood vessels, are richly
innervated, and can be the source of headache pain.
Many a migraine sufferer knows that an attack is imminent early in the morning, when an
ominous dull throbbing begins, often on one side of the head. Migraine is more than head pain—
the person feels unwell in a general sense, and an attack can be disabling, lasting from a few
hours to several days. In a migraine, certain cranial blood vessels constrict, producing a localized
cerebral blood deficiency. When vasodilation quickly follows, a severe headache results. Several
types of drugs, such as the triptans, effectively relieve migraines, but they are best taken at the
first sign of illness, and several drugs may need to be tried to find an effective one. For some
people, keeping a diary of conditions before an attack can reveal a trigger, such as eating
chocolate or exposure to low-pressure weather systems.
In some individuals, migraine begins with an “aura” of shimmery bright lights in the peripheral
vision. Accompanying the aura is often “photophobia,” which is head pain when exposed to
light. Photophobia arises from a group of brain neurons closely associated with the optic nerve
(see figure 10.26). Completely blind migraine sufferers, whose optic nerves do not function, do
not experience photophobia. But people who are “legally blind,” with partially degenerated
retinas but intact and functional optic nerves, do experience pain from light. Experiments on
animals localized neurons near the optic nerve that trigger photophobia when stimulated.
FIGURE 10.26
optic radiations.
A visual pathway includes the optic nerve, optic chiasma, optic tract, and
APR Module 7: Nervous System: Animation: Vision
Many headaches are associated with stressful life situations that cause fatigue, emotional tension,
anxiety, or frustration. These conditions can trigger various physiological changes, such as
prolonged contraction of the skeletal muscles in the forehead, sides of the head, or back of the
neck, which stimulate pain receptors and produce a tension headache. More severe vascular
headaches accompany constriction or dilation of the cranial blood vessels. For example, the
throbbing headache of a “hangover” from drinking too much alcohol may be due to blood
pulsating through dilated cranial vessels. Other causes of headaches include sensitivity to food
additives, high blood pressure, increased intracranial pressure due to a tumor or to blood
escaping from a ruptured vessel, decreased cerebrospinal fluid pressure following a lumbar
puncture, and sensitivity to or withdrawal from certain drugs.
Many strong but delicate fibers, called suspensory ligaments, extend inward from the ciliary
processes and hold the transparent lens in position (fig. 10.18). The distal ends of these fibers
attach along the margin of a thin capsule that surrounds the lens. The body of the lens lies
directly behind the iris and pupil and is composed of highly specialized epithelial cells called
lens fibers. The cytoplasm of these cells is the transparent substance of the lens.
FIGURE 10.18Lens and ciliary body viewed from behind.
More than 90% of the proteins in a lens cell are lens crystallins, which aggregate into fibers.
These proteins, along with the absence of organelles that scatter light (mitochondria,
endoplasmic reticula, and nuclei), account for the transparency of the lens.
An eye disorder common in older people is cataract. The lens or its capsule slowly becomes
cloudy and opaque. Cataracts are treated on an outpatient basis with surgery to remove the
clouded lens and replace it with an artificial lens. Without treatment, cataracts eventually cause
blindness.
The ciliary muscles and suspensory ligaments, along with the structure of the lens itself, enable
the lens to adjust shape to facilitate focusing, a phenomenon called accommodation (ah-kom′′oda′shun). The lens is enclosed by a clear capsule composed largely of elastic fibers. This elastic
nature keeps the lens under constant tension, and enables it to assume a globular shape. The
suspensory ligaments attached to the margin of the capsule are also under tension. When they
pull outward, flattening the capsule and the lens inside, the lens focuses on distant objects (fig.
10.19a). However, if the tension on the suspensory ligaments relaxes, the elastic lens capsule
rebounds, and the lens surface becomes more convex—focused for viewing closer objects (fig.
10.19b).
FIGURE 10.19Accommodation. (a) The lens thins as the ciliary muscle fibers relax. (b) The lens
thickens as the ciliary muscle fibers contract.
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The ciliary muscles control the actions of the suspensory ligaments in accommodation. For
example, one set of these muscle fibers extends back from fixed points in the sclera to the
choroid coat. When the fibers contract, the choroid coat is pulled forward, and the ciliary body
shortens. This relaxes the suspensory ligaments, and the lens thickens in response (see fig.
10.19b). When the ciliary muscles relax, tension on the suspensory ligaments increases, and the
lens becomes thinner and less convex again (see fig. 10.19a).
PRACTICE
26. Describe the outer and middle layers of the eye.
27. What factors contribute to the transparency of the cornea?
28. How does the shape of the lens change during accommodation?
29. Why would reading for a long time cause eye fatigue, while looking at a distant scene is
restful?
The iris (i′ris) is a thin diaphragm composed mostly of connective tissue and smooth muscle
fibers. From the outside, the iris is the colored part of the eye. The iris extends forward from the
periphery of the ciliary body and lies between the cornea and lens (see fig. 10.17). The iris
divides the space (anterior cavity) separating these parts into an anterior chamber (between the
cornea and the iris) and a posterior chamber containing the lens (between the iris and the
vitreous body).
The epithelium on the inner surface of the ciliary body secretes a watery fluid called aqueous
humor (a′kwe-us hu′mor) into the posterior chamber. The fluid circulates from this chamber
through the pupil (pu′pil), a circular opening in the center of the iris, and into the anterior
chamber. Aqueous humor fills the space between the cornea and lens, helps nourish these parts,
and aids in maintaining the shape of the front of the eye. Aqueous humor leaves the anterior
chamber through veins and a special drainage canal, the scleral venous sinus (canal of Schlemm),
which is in the wall of the anterior chamber at the junction of the cornea and the sclera.
An eye disorder called glaucoma develops when aqueous humor forms faster than it is removed.
As fluid accumulates in the anterior chamber of the eye, fluid pressure rises and is transmitted to
all parts of the eye. In time, the building pressure squeezes shut blood vessels that supply the
receptor cells of the retina. Cells that are robbed of nutrients and oxygen in this way may die, and
permanent blindness can result.
When diagnosed early, glaucoma can usually be treated successfully with drugs, laser surgery, or
traditional surgery, all of which promote the outflow of aqueous humor. Since glaucoma in its
early stages typically produces no symptoms, discovery of the condition usually depends on
measuring intraocular pressure, using an instrument called a tonometer.
The smooth muscle fibers of the iris are organized into two groups, a circular set and a radial
set. These muscles control the size of the pupil, through which light passes as it enters the eye.
The circular set of muscle fibers acts as a sphincter. When the muscle fibers contract, the pupil
gets smaller, and less light enters. Bright light stimulates the circular muscles to contract, which
decreases the intensity of light entering the eye. Conversely, when the radial muscle fibers
contract, the pupil’s diameter increases, and more light enters (fig. 10.20). Dim light stimulates
the radial muscles to contract, which dilates the pupil, allowing more light into the eye.
FIGURE 10.20Dim light stimulates the radial muscles of the iris to contract, and the pupil
dilates. Bright light stimulates the circular muscles of the iris to contract, and the pupil constricts.
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Inner Layer
The inner layer of the wall of the eye consists of the retina (ret′ı˘-nah), which contains the visual
receptor cells (photoreceptors). The retina is a nearly transparent sheet of tissue continuous with
the optic nerve in the back of the eye and extending forward as the inner lining of the eyeball.
The retina ends just behind the margin of the ciliary body.
The retina is thin and delicate, but its structure is quite complex. It has a number of distinct
layers, as figures 10.21 and 10.22 illustrate.
FIGURE 10.21The retina consists of several cell layers. Light waves penetrate a layer of
connecting neurons to impinge on the rods and cones, which are the photoreceptors. The retinal
pigment epithelium absorbs stray light rays.
FIGURE 10.22Retinal structure. Note the layers of cells and nerve fibers in this light micrograph
of the retina (75×).
APR Module 7: Nervous System: Histology: Vision - retina: LM: High Magnification
In the central region of the retina is a yellowish spot called the macula lutea. A depression in its
center, called the fovea centralis (fo′ve-ah sen-tral′is), is in the region of the retina that produces
the sharpest vision (see figs. 10.17 and 10.23).
FIGURE 10.23The retina. (a) Major features of the retina. (b) Nerve fibers leave the retina of the
eye in the area of the optic disc (arrow) to form the optic nerve in this magnified view of the
retina (53×).
Just medial to the fovea centralis is an area called the optic disc (op′tik disk) (fig. 10.23). Here,
nerve fibers from the retina leave the eye and join the optic nerve. A central artery and vein also
pass through the optic disc. These vessels are continuous with the capillary networks of the
retina, and along with vessels in the underlying choroid coat, they supply blood to the cells of the
inner layer. Because the optic disc region lacks photoreceptors, it is commonly known as the
blind spot of the eye.
The space bounded by the lens, ciliary body, and retina is the largest compartment of the eye and
is called the posterior cavity (see fig. 10.17). It is filled with a transparent, jellylike fluid called
vitreous humor (vit′re-us hu′mor), which along with collagen fibers forms the vitreous body.
The vitreous body supports the internal parts of the eye and helps maintain its shape.
As a person ages, tiny, dense clumps of gel or deposits of crystal-like substances form in the
vitreous humor. When these clumps cast shadows on the retina, the person sees small, moving
specks in the field of vision, called floaters.
PRACTICE
30. Explain the source of aqueous humor, and trace its path through the eye.
31. How does the pupil respond to changes in light intensity?
32. Describe the structure of the retina.
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Light Refraction
When a person sees an object, either the object is giving off light, or it is reflecting light waves
from another source. These light waves enter the eye, and an image of the object is focused on
the retina. Focusing bends the light waves, a phenomenon called refraction (re-frak′shun).
Optics
Refraction occurs when light waves pass at an oblique angle from a medium of one optical
density into a medium of a different optical density. This happens at the curved surface between
the air and the cornea and at the curved surface of the lens itself. A lens with a convex surface (as
in the eye) causes light waves to converge (fig. 10.24).
FIGURE 10.24A lens with a convex surface causes light waves to converge. The lens of the eye
functions the same way.
The convex surface of the cornea refracts light waves from outside objects. The convex surface
of the lens and, to a lesser extent, the surfaces of the fluids in the chambers of the eye then refract
the light again.
If eye shape is normal, light waves focus sharply on the retina, much as a motion picture image is
focused on a screen for viewing. Unlike the motion picture image, however, the image that forms
on the retina is upside down and reversed from left to right. The visual cortex interprets the
image in its proper position.
PRACTICE
33. What is refraction?
34. What parts of the eye provide refracting surfaces?
Photoreceptors
Photoreceptors are modified neurons of two distinct kinds, as figure 10.21 illustrates. One group,
called rods (rodz), have long, thin projections at their ends, and provide black and white vision.
The other group, cones (kōnz), have short, blunt projections, and provide color vision. There is
only one type of rod, but three types of cones.
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Rods and cones are in a deep part of the retina, where they project into an adjacent layer of
retinal pigment epithelium (RPE) that absorbs light waves the photoreceptors do not absorb.
With the pigment of the choroid coat, the RPE keeps light from reflecting off surfaces inside the
eye (see fig. 10.22).
Photoreceptors are stimulated only when light reaches them. A light image focused on an area of
the retina stimulates some photoreceptors, and impulses travel from them to the brain. However,
the impulses leaving each activated photoreceptor deliver only a fragment of the information
required for the brain to interpret a complete scene.
Rods and cones contribute to different aspects of vision. Rods are hundreds of times more
sensitive to light than cones and therefore can provide vision in dim light, without color. Cones
detect color.
A human eye has 125 million rods and 7 million cones. A cat has three types of cone cells, but
sees mostly pastels. A dog has two types of cone cells, and its visual world is much like that of a
person with colorblindness. Researchers corrected colorblindness in monkeys by introducing the
genes for human cone pigments into their eyes.
Rods and cones also differ in the sharpness of the perceived images, or visual acuity. Cones
provide sharp images, and rods provide more general outlines of objects. Rods give less precise
images because axon branches from many rods converge, so that their impulses are conducted to
the brain on the same nerve fiber (fig. 10.25a). Thus, if a point of light stimulates a rod, the brain
cannot tell which one of many receptors has been stimulated. Convergence of impulses is less
common among cones. When a cone is stimulated, the brain can pinpoint the stimulation more
accurately (fig. 10.25b).
FIGURE 10.25Rods and cones are photoreceptors. (a) A single sensory nerve fiber transmits
impulses from several rods to the brain. (b) Separate sensory nerve fibers transmit impulses from
cones to the brain. (c) Scanning electron micrograph of rods and cones (1,350×).
The fovea centralis, the area of sharpest vision, lacks rods but contains densely packed cones
with few or no converging fibers (see fig. 10.17). Also in the fovea centralis, the overlying layers
of the retina and the retinal blood vessels are displaced to the sides, more fully exposing
photoreceptors to incoming light. Consequently, to view something in detail, a person moves the
eyes so that the important part of an image falls on the fovea centralis.
Photopigments
Both rods and cones contain light-sensitive pigments that decompose when they absorb light
energy. The light-sensitive biochemical in rods is called rhodopsin (ro-dop′sin), or visual
purple. In the presence of light, rhodopsin molecules are broken down into a colorless protein
called opsin and a yellowish substance called retinal (retinene) that is synthesized from vitamin
A.
Decomposition of rhodopsin molecules activates an enzyme that initiates a series of reactions
altering the permeability of the rod cell membrane. As a result, a complex pattern of nervous
stimulation originates in the retina. The impulses are conducted away from the retina along the
optic nerve into the brain, where they are interpreted as vision.
In bright light, nearly all of the rhodopsin in the rods of the retina decomposes, greatly reducing
rod sensitivity. In dim light, however, regeneration of rhodopsin from opsin and retinal is faster
than rhodopsin breakdown. ATP provides the energy required for this regeneration (see chapter
4, p. 91).
Poor vision in dim light, called nightblindness, results from vitamin A deficiency. Lack of the
vitamin reduces the supply of retinal, rhodopsin production falls, and rod sensitivity is low.
Supplementing the diet with vitamin A is used to treat nightblindness.
In one form of the inherited condition, Leber congenital amaurosis, absence of an enzyme in the
RPE prevents production of retinal from vitamin A, causing nightblindness that progresses to full
blindness by early adulthood. Gene therapy that introduces functional copies of the mutant gene
into the RPE has enabled more than 200 people receiving the experimental treatment to see.
The light-sensitive pigments in cones, as in the rods, are composed of retinal and protein. In
cones, however, three types of opsin proteins, different from the protein portion of rhodopsin
found in rods, combine with retinal to form the three cone pigments. The three types of cones
each contain one of these three photopigments.
The wavelength of light determines the color that the brain perceives from it. For example, the
shortest wavelengths of visible light are perceived as violet, and the longest are perceived as red.
One type of cone pigment (erythrolabe) is most sensitive to red light waves, another (chlorolabe)
to green light waves, and the third (cyanolabe) to blue light waves. The color a person perceives
depends on which set of cones or combination of sets the light in a given image stimulates. If all
three sets of cones are stimulated, the person senses the light as white, and if none are stimulated,
the person senses black. Different forms of colorblindness result from lack of different types of
cone pigments.
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Visual Pathways
Visual pathways conduct impulses from the retina to the visual cortex, where they are perceived
as vision. The pathways begin as the axons of the retinal neurons leave the eyes to form the optic
nerves (fig. 10.26). Just anterior to the pituitary gland, these nerves give rise to the X-shaped
optic chiasma (op′tik ki-az′mah), and within the chiasma, some of the fibers cross over. More
specifically, the fibers from the nasal (medial) half of each retina cross over, but those from the
temporal (lateral) sides do not. Thus, fibers from the nasal half of the left eye and the temporal
half of the right eye form the right optic tract, and fibers from the nasal half of the right eye and
the temporal half of the left eye form the left optic tract.
Just before the nerve fibers reach the thalamus, a few of them enter nuclei that function in
various visual reflexes. Most of the fibers, however, enter the thalamus and synapse in its
posterior portion (lateral geniculate body). From this region, the visual impulses enter nerve
pathways called optic radiations, which lead to the visual cortex of the occipital lobes.
PRACTICE
35. Distinguish between the rods and cones of the retina.
36. Explain the roles of visual pigments.
37. Trace an impulse from the retina to the visual cortex.
10.9 Sense of Sight
1. Match the visual accessory organ with its function
2. Name the three layers of the eye wall and describe the functions of each layer.
3. Explain why looking at a close object causes fatigue, in terms of how accommodation is
accomplished.
4. Explain the mechanisms of pupil constriction and pupil dilation.
5. All of the following are compartments within the eye. In which one is vitreous humor found
a. anterior chamber
b. posterior chamber
c. anterior cavity
d. posterior cavity
6. Distinguish between the fovea centralis and the optic disc.
7. Explain how light is focused on the retina.
8. Distinguish between rods and cones.
9. Explain why cone vision is generally more acute than rod vision.
10. Explain why rod vision may be more important under dim light conditions.