Sensory Physiology
227
bulb neurons, allowing the brain to determine which recep-
tors have been stimulated. The codes used to transmit olfac-
tory information probably use both spatial (which neurons
are fi ring?) and temporal (what is the frequency of the action
potential responses?) components.
Information passes from the olfactory bulbs to the olfac-
tory cortex and other parts of the limbic system. This region is
intimately associated with emotional, food-getting, and sexual
behaviors and may play a role in processing olfactory informa-
tion relevant to these activities. Some of the areas of the olfac-
tory cortex then send projections to other regions of the frontal
cortex. Different odors elicit different patterns of electrical
activity in several cortical areas, allowing humans to discrimi-
nate between some 10,000 different odorants even though
they have only 1000 or so different olfactory receptor types.
Olfactory discrimination varies with attentiveness, hun-
ger (sensitivity is greater in hungry subjects), gender (women
in general have keener olfactory sensitivities than men), smok-
ing (decreased sensitivity has been repeatedly associated with
smoking), age (the ability to identify odors decreases with
age, and a large percentage of elderly persons cannot detect
odors at all), and state of the olfactory mucosa (as we have
mentioned, the sense of smell decreases when the mucosa is
congested, as in a head cold).
Hearing and Balance: Losing Both at Once
A patient comes to the doctor with complaints of dizziness
and ringing in one of his ears. He tells the doctor that these
symptoms come and go and have been present for about a
year. Recently, these attacks have increased in severity and
have been accompanied with loss of balance and vomiting.
The symptoms usually last for four to fi ve hours, during
which time he lies on the bed with his affected ear uppermost.
Based on these symptoms and after many tests to rule out
disorders of the cerebellum, the doctor diagnoses
Ménière’s
disease
.
This is a disease of the auditory and vestibular
systems that results from increased pressure in the inner ear,
including the cochlea and the semicircular canals. The disease
is believed to be caused by defi cits in absorption and renewal
of the endolymph and the distension of the structures in these
areas. As a result, both hearing and vestibular disruptions
occur, and complete or partial hearing loss may be permanent
in the affected ear. Ménière’s disease may sometimes resolve
itself without treatment after months or years, but in many
cases it is permanent.
Color Blindness
At high light intensities, as in daylight vision, most people—92
percent of the male population and over 99 percent of the
female population—have normal color vision. However,
there are several types of defects in color vision that result
from mutations in the cone pigments. The most common
form of
color blindness,
red-green color blindness, is present
predominantly in men, affecting 1 out of 12. Color blindness
in women is extremely rare (1 out of 200). Men with red-green
color blindness either lack the red or the green cone pigments
entirely or have them in an abnormal form. Because of this, the
discrimination between shades of these colors is poor.
Color blindness results from a recessive mutation in one
or more genes encoding the cone pigments. Genes encoding
the red and green cone pigments are located very close to
each other on the X chromosome, while the gene encoding
the blue chromophore is located on chromosome 7. Because
of this close association of the red and green genes on the
X chromosome, there is a greater likelihood that crossover
will occur during meiosis, thus eliminating or changing
the spectral characteristics of the red and green pigments
ADDITIONAL CLINICAL EXAMPLES
produced. This, in part, accounts for the fact that red-green
defects are not always complete, and that some color-blind
individuals under some conditions can distinguish shades
of red or green. In males, the presence of only a single X
chromosome means that a single recessive allele from the
mother will result in color blindness, even though the mother
herself may have normal color vision due to having one
normal X chromosome. It also means that 50 percent of the
male offspring of that mother will be expected to be color
blind. Individuals who have red-green color blindness will not
be able to see the number in
Figure 7–46
.
Figure 7–46
Image used for testing red-green color vision. With normal color
vision the number 57 is visible, while no number is apparent to those
with a red-green defect.
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