438
UNIT THREE
12.1
INTRODUCTION
Our senses not only make our lives meaningful, connecting
us to the sights, sounds, smells, tastes, and textures of the
outside world, but also help our bodies maintain homeosta-
sis by providing information about what is happening on the
inside. Sensory receptors are the portals that link our ner-
vous systems to all of these events. The
general senses
are
those with receptors widely distributed throughout the body,
including the skin, various organs, and joints. The
special
senses
have more specialized receptors and are conF
ned to
structures in the head, such as the eyes and ears.
All senses work in basically the same way. Sensory
receptors are specialized cells or multicellular structures
that collect information from the environment and stimulate
neurons to send impulses along sensory F bers to the brain.
There the cerebral cortex forms a perception, a person’s par-
ticular view of the stimulus.
Table 12.1
outlines the path-
ways from sensation to perception that describe an apple.
These are special senses.
THE WORLD WITHOUT COLOR
Recall from chapter 11 (p. 412) that the terms “axon” and
“nerve F ber” are used synonymously. Also recall that unipo-
lar neurons, which include most sensory neurons, have an
unusual structure in which the portion of the neuron associ-
ated with the dendrites, called a peripheral process, is consid-
ered to function like an axon (see F g. 10.7). Because of this,
and for simplicity, the neuron processes that bring sensory
information into the CNS will be called sensory F bers or affer-
ent F bers, no matter what type of neuron is involved.
12.2
RECEPTORS, SENSATION,
AND PERCEPTION
Sensory receptors are diverse but share certain features. Each
type of receptor is particularly sensitive to a distinct type of
environmental change and is much less sensitive to other
forms of stimulation. The raw form in which these receptors
send information to the brain is called
sensation.
The way
our brains interpret this information is called
perception.
J
ohn Dalton, a famous English chemist, saw things diF
erently than
most people. In a 1794 lecture, he described his visual world.
Sealing wax that appeared red to other people was as green as
a leaf to Dalton and his brother. Pink wild±
owers were blue, and
Dalton perceived the cranesbill plant as “sky blue” in daylight, but
“very near yellow, but with a tincture of red” in candlelight. He concluded,
“ . . . that part of the image which others call red, appears to me little more
than a shade, or defect of light.” The Dalton brothers, like 7% of males and
0.4% of females today, had the inherited trait of colorblindness.
Dalton was very curious about the cause of his colorblindness, so he
made arrangements with his personal physician, Joseph Ransome, to dissect
his eyes after he died. Ransome snipped oF
the back of one eye, removing
the retina, where the cone cells that provide color vision are nestled among
the more abundant rod cells that impart black-and-white vision. Ransome
could see red and green normally when he peered through the back of his
friend’s eyeball, so he concluded that it was not an abnormal ²
lter in front of
the eye that altered color vision.
³ortunately, Ransome stored the eyes in dry air, where they remained
relatively undamaged. In 1994, Dalton’s eyes underwent DNA analysis at
London’s Institute of Ophthalmology. The research showed that Dalton’s
remaining retina lacked one of three types of pigments, called photopig-
ments, that enable cone cells to capture certain incoming wavelengths
of light.
Although people have studied colorblindness for centuries, we are still
learning more about it. Recently, researchers investigated why colorblind
men lacking cones that capture green light are aF
ected to diF
erent degrees.
They discovered that colorblind men who can discern a few shades of green
have red cone cells that can detect some wavelengths of light that fall within
the green region of the spectrum. Color vision may be more complex than
we had thought.
People who are colorblind must function in a multicolored world.
To help
them overcome the disadvantage of not seeing important color diF
erences,
researchers have developed computer algorithms that convert colored video
pictures into shades those with colorblindness can see. This circle of dots is a
test to determine whether someone is colorblind. AF
ected individuals cannot
see a diF
erent color in certain of the dots in such a drawing. As a result, their
brains cannot perceive the embedded pattern that forms the number 16 that
others can see. The above has been reproduced from
Ishihara’s Tests for Colour
Blindness
published by Kanehara & Co. Ltd. Tokyo, Japan, but tests for colour
blindness cannot be conducted with this material. ³or accurate testing, the
original plates should be used.
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