556
UNIT 3
Regulation and Integration of the Body
15
Myopia
(mi-o
9
pe-ah; “short vision”) occurs when distant ob-
jects focus in front of the retina, rather than on it (
Figure 15.14
,
leF). Myopic people see close objects without problems because
they can focus them on the retina, but distant objects are blurred.
Te common name for myopia is
nearsightedness
. (Notice that
this terminology names the aspect of vision that is
unimpaired
.)
Myopia typically results from an eyeball that is too long.
Correction has traditionally involved using concave lenses
that diverge the light before it enters the eye. Laser procedures
to flatten the cornea slightly offer other treatment options.
Hyperopia
(hy
9
per-o
0
pe-ah; “far vision”), or
farsightedness
,
occurs when the parallel light rays from distant objects focus
behind
the retina (±igure 15.14, right). Hyperopic individuals
can see distant objects perfectly well because their ciliary mus-
cles contract almost continuously to increase the light-bending
power of the lens, which moves the focal point forward onto
the retina. However, diverging light rays from
nearby
objects
focus so far behind the retina that the lens cannot bring the fo-
cal point onto the retina even at its full refractory power. As a
result, close objects appear blurry, and convex corrective lenses
are needed to converge the light more strongly for close vision.
Hyperopia usually results from an eyeball that is too short.
Unequal curvatures in different parts of the cornea or lens
also lead to blurry images. Tis refractory problem is
astig-
matism
(
astigma
5
not a point). Special cylindrically ground
lenses or laser procedures are used to correct this problem.
Check Your Understanding
4.
Arrange the following in the order that light passes through
them to reach the photoreceptors (rods and cones) in the
retina: lens, bipolar cells, vitreous humor, cornea, aqueous
humor, ganglion cells. (Hint: See Figure 15.6 if you need a
reminder of where ganglion cells and bipolar cells are.)
5.
You have been reading this book for a while now and your
eyes are beginning to tire. Which intrinsic eye muscles are
relaxing as you stare thoughtfully into the distance?
6.
Why does your near point of vision move farther away as
you age?
For answers, see Appendix H.
Photoreceptors and Phototransduction
Describe the events that convert light into a neural signal.
Compare and contrast the roles of rods and cones in vision.
Compare and contrast light and dark adaptation.
Once light is focused on the retina, the photoreceptors come
into play. ±irst, we will describe the functional anatomy of the
rod and cone photoreceptor cells, then the chemistry and re-
sponse of their visual pigments to light, and finally, photorecep-
tor activation, phototransduction, and information processing
in the retina.
Phototransduction
is the process by which light
energy is converted into a graded receptor potential.
Accommodation of the lenses.
Accommodation
is the pro-
cess that increases the refractory power of the lens. Te ciliary
muscles contract, pulling the ciliary body anteriorly toward
the pupil and inward, releasing tension in the ciliary zonule.
No longer stretched, the elastic lens recoils and bulges, pro-
viding the shorter focal length needed to focus the image of
a close object on the retina (±igure 15.13b). Parasympathetic
fibers of the oculomotor nerves control the contraction of
the ciliary muscles.
Te closest point on which we can focus clearly is called
the
near point of vision
, and it represents the maximum
bulge the lens can achieve. In young adults with emmetropic
vision, the near point is 10 cm (4 inches) from the eye. How-
ever, it is closer in children and gradually recedes with age, ex-
plaining why children can hold their books very close to their
faces while many elderly people must hold the newspaper at
arm’s length. Te gradual loss of accommodation with age
reflects the lens’s decreasing elasticity. In many people over
age 50, the lens is nonaccommodating, a condition known as
presbyopia
(pres
0
be-o
9
pe-ah), literally “old person’s vision.”
Constriction of the pupils.
Te sphincter pupillae muscles of
the iris enhance the effect of accommodation by reducing the
size of the pupil toward 2 mm (see ±igure 15.5). Tis
accom-
modation pupillary reflex
, mediated by parasympathetic
fibers of the oculomotor nerves, prevents the most diver-
gent light rays from entering the eye. Such rays would pass
through the extreme edge of the lens and would not focus
properly, causing blurred vision.
Convergence of the eyeballs.
Te visual goal is always to
keep the object being viewed focused on the retinal foveae.
When we look at distant objects, both eyes are directed either
straight ahead or to one side to the same degree, but when
we fixate on a close object, our eyes converge.
Convergence
,
controlled by somatic motor fibers of the oculomotor nerves,
is medial rotation of the eyeballs by the medial rectus mus-
cles so that each is directed toward the object being viewed.
Te closer that object, the greater the degree of convergence
required. ±or example, when you focus on the tip of your
nose, you “go cross-eyed.”
Reading or other close work requires almost continuous ac-
commodation, pupillary constriction, and convergence. Tis is
why prolonged periods of reading tire the eye muscles and can
result in eyestrain. When you read for an extended time, it helps
to look up and stare into the distance occasionally to relax your
intrinsic eye muscles.
Homeostatic Imbalance
15.8
Teoretically, visual problems related to refraction could re-
sult from a hyperrefractive (overconverging) or hyporefractive
(underconverging) lens or from structural abnormalities of the
eyeball. In practice, 99% of refractive problems are related to
eyeball shape—either too long or too short.
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