473
CHAPTER TWELVE
Nervous System III
Albinism
is an inherited condition in which an enzyme necessary to
produce pigment is missing, causing very pale, highly sun-sensitive
skin. More severe forms of albinism also aF
ect the eyes, making vision
blurry and intolerant to light. A person may squint even in very faint
light. This separate extrasensitivity is because light re±
ects inside the
lenses, over-stimulating visual receptors. The eyes of many people with
albinism also dart about uncontrollably, a condition called
nystagmus.
Cones provide sharp images, whereas rods produce
more general outlines of objects. This is because nerve F
bers
from many rods may converge, and their impulses may be
transmitted to the brain on the same nerve F ber (see chapter
10, p. 375). Thus, if light stimulates a rod, the brain cannot
tell which one of many receptors has been stimulated. Such
a convergence of impulses occurs to a much lesser degree
among cones, so when a cone is stimulated, the brain is able
to pinpoint the stimulation more accurately
(f g. 12.38)
.
The area of sharpest vision, the fovea centralis in the
macula lutea, lacks rods but has densely packed cones with
few or no converging F bers. Also, the overlying layers of the
retina, as well as the retinal blood vessels, are displaced to
the sides in the fovea, which more fully exposes the recep-
tors to incoming light. Consequently, to view something in
detail, a person moves the eyes so that the important part of
an image falls upon the fovea centralis.
The concentration of cones decreases in areas farther
away from the macula lutea, whereas the concentration of
rods increases in these areas. Also, the degree of convergence
among the rods and cones increases toward the periphery
of the retina. As a result, the visual sensations from images
focused on the sides of the retina are blurred compared with
those focused on the central portion of the retina.
The bony orbit usually protects the eye, but a forceful blow can dis-
place structures in and around the eye. The suspensory ligaments
may tear and the lens become dislocated into the posterior cavity, or
the retina may pull away from the underlying vascular choroid coat.
Once the retina is detached, photoreceptor cells may die because of
lack of oxygen and nutrients. Unless such a
detached retina
is repaired
surgically, this injury may cause visual loss or blindness.
Visual Pigments
Rods and cones contain light-sensitive pigments that decom-
pose when they absorb light energy. The light-sensitive pig-
ment in rods is
rhodopsin
(ro-dop
sin), or visual purple,
and it is embedded in membranous discs stacked in these
receptor cells
(f g. 12.39)
. A single rod cell may have 2,000
interconnected discs, derived from the cell membrane. In
the presence of light, rhodopsin molecules break down into
molecules of a colorless protein called
opsin
and a yellowish
organic molecule called
retinal
(retinene) synthesized from
vitamin A.
cortex of the cerebrum interprets such an image, it corrects
this, and objects are seen in their real positions.
Light waves coming from objects more than 20 feet away
are traveling in nearly parallel lines, and the cornea and the
lens in its more fl attened or “at-rest” condition focuses the light
waves on the retina. Light waves arriving from objects less
than 20 feet away, however, reach the eye along more diver-
gent lines—the closer the object, the more divergent the lines.
Divergent light waves focus behind the retina unless
something increases the refracting power of the eye.
Accommodation accomplishes this increase, thickening
the lens. As the lens thickens, light waves converge more
strongly so that diverging light waves coming from close
objects focus on the retina.
PRACTICE
40
What is refraction?
41
What parts of the eye provide refracting surfaces?
42
Why is it necessary to accommodate for viewing close objects?
Visual Receptors
The photoreceptors of the eye are modiF ed neurons of two
distinct types. One group of receptor cells, called
rods,
have
long, thin projections at their terminal ends. The cells of the
other group, called
cones,
have short, blunt projections. The
retina has about 100 million rods and 3 million cones.
Rods and cones occupy a deep layer of the retina, closely
associated with a layer of retinal pigment epithelium (see
F
gs. 12.32 and 12.33). The projections from the receptors
extend into the pigmented layer and contain light-sensitive
visual pigments.
The retinal pigment epithelium absorbs light waves that
the receptor cells do not, and with the pigment of the choroid
coat, keeps light from refl
ecting off the surfaces inside the
eye. The retinal pigment epithelium also stores vitamin A,
which the receptor cells use to synthesize visual pigments.
Researchers can grow retinal pigment epithelium cells in laboratory
cultures, and the cells retain their pigment. This means that someday
scientists may be able to grow tissue that can be implanted into a
person’s eye to treat some forms of blindness.
The visual receptors are stimulated only when light
reaches them. When a light image is focused on an area of
the retina, some receptors are stimulated and send impulses
to the brain. However, the impulse leaving each activated
receptor provides only a small portion of the information
required for the brain to interpret a total scene.
Rods and cones function differently. Rods are hundreds
of times more sensitive to light than are cones, and as a
result, rods provide vision in dim light. In addition, rods pro-
duce colorless vision, whereas cones can detect colors.
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