477
CHAPTER TWELVE
Nervous System III
ply the receptor cells of the retina may squeeze shut, cutting
off the nutrient and oxygen supply. The result may eventu-
ally be permanent blindness.
Drugs, or traditional or laser surgery to promote the out-
ow of aqueous humor, can treat glaucoma if it is diagnosed
early. However, because glaucoma in its early stages typi-
cally produces no symptoms, discovery of the condition usu-
ally depends on measuring the intraocular pressure using an
instrument called a
tonometer.
A common eye disorder particularly in older people is
cataract.
The lens or its capsule slowly becomes cloudy,
opaque, and discolored, adding a yellowish tinge to a per-
son’s view of the world. Clear images cannot focus on the
retina, and in time, the person may become blind. Removing
the lens with a laser and replacing it with an artiF
cial implant
can treat cataract. Afterward, patients report that their sur-
roundings are no longer yellow.
Several conditions affect the retinas of an older person.
Age-related macular degeneration is an impairment of the
macula, the most sensitive part of the retina, which blurs
images. Retinal detachment becomes more common. People
with diabetes are at high risk of developing diabetic retinop-
athy, an interference with retinal function because of dam-
aged blood vessels or growth of new ones that block vision.
Despite these various problems, many older individu-
als continue to enjoy sharp, functional senses well into the
upper decades of life.
PRACTICE
47
Why do smell and taste diminish with age?
48
What are some causes of age-related hearing loss?
49
Describe visual problems likely to arise with age.
ticular sounds, such as
f, g, s, t, sh, th, z
and
ch.
Hearing loss
may also be due to a degeneration or failure of nerve path-
ways to the brain. This condition, called presbycusis, may
affect the ability to understand speech. It gradually worsens.
Tinnitus, a ringing or roaring in the ears, is also more com-
mon among older adults. Hearing aids can often restore some
hearing. A person to whom the ordinary sounds of life are
hopelessly garbled may show, understandably, symptoms of
paranoia, depression, or social withdrawal.
Vision may decline with age for several reasons. “Dry
eyes” are common. Too few tears, or poor quality tears, lead
to itching and burning eyes, and diminished vision. In some
cases, too many tears result from oversensitivity to environ-
mental effects, such as wind, intense light, or a change in
temperature.
With age, tiny dense clumps of gel or crystal-like deposits
form in the vitreous humor. When these clumps cast shadows
on the retina, the person sees small moving specks in the F eld
of vision. These specks, or
oaters,
are most apparent when
looking at a plain background, such as the sky or a blank wall.
Also with age, the vitreous humor may shrink and pull away
from the retina. This may mechanically stimulate receptor
cells of the retina, and the person may see fl ashes of light.
The inability to read small print up close, called presby-
opia, results from a loss of elasticity in the lens, preventing it
from changing shape easily. After age seventy, the iris can-
not dilate as well as it once did, halving the amount of light
that can enter the eye. Brighter lights can counter this effect.
Glaucoma
develops in the eyes as a person ages when
the rate of aqueous humor formation exceeds the rate of its
removal. ±luid accumulates in the anterior chamber of the
eye, and the fl uid pressure rises. As this pressure is transmit-
ted to all parts of the eye, in time, the blood vessels that sup-
(4) Mechanoreceptors, sensitive to mechanical
forces.
(5) Photoreceptors, sensitive to light.
2. Sensory impulses
a. When receptors are stimulated, membrane
potentials change.
b. Receptor potentials are transferred to nerve F
bers,
triggering action potentials.
3. Sensation and perception
a. Sensation is a feeling resulting from sensory
stimulation.
b. Perception is when a particular part of the sensory
cortex interprets the sensory stimulation.
c. The cerebral cortex projects a sensation back to the
region of stimulation.
4. Sensory adaptations are adjustments of sensory
receptors to continuous stimulation. Impulses are
triggered at slower rates.
CHAPTER SUMMARY
12.1
INTRODUCTION (PAGE 438)
Sensory receptors are sensitive to internal and external
environmental changes and initiate impulses to the brain
and spinal cord.
12.2
RECEPTORS, SENSATION, AND
PERCEPTION (PAGE 438)
1. Receptor types
a. Each type of receptor is sensitive to a distinct type
of stimulus.
b. The major types of receptors include:
(1) Chemoreceptors, sensitive to changes in
chemical concentration.
(2) Pain receptors (nociceptors), sensitive to tissue
damage.
(3) Thermoreceptors, sensitive to temperature
changes.
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