Developmental psychology.pdf

136 Modes of Awareness
-J-
Hair
Free nerve
ending
Figure 5.12
Skin Receptors. Dendntes around
hair follicles and other unknown
mechanisms apparently mediate light
pressure. Pacinian corpuscles have
been presumed to be receptors for
heavy pressure, and free nerve
endings mediate pain sensitivity. The
middle photo shows a hair follicle and
the other shows a Pacinian
corpuscle
One condition for pain is high stimulus intensity, as we saw in the terminal
threshold, and another is the stimulation of free nerve endings, found just below the
skin surface almost everywhere on the body. Some of these nerves appear to transmit
pain without being associated with any other sensitivity; hence they are likely candidates
for pain receptors (Sweet, 1959).
The structural bases of temperature sensitivity are not known either, even
though human beings and many animals, including the frog, possess a sensitivity to
warmth and cold just a few degrees above or below normal body temperature. At this
time free nerve endings and the microscopic blood vessels that run to every part of the
skin are currently mentioned as possible mediators, but progress in understanding temperature
receptors has been slow (Hahn, 1974).
With this lack of success in identifying receptors for cutaneous sensitivity, there
have been few significant advances of a theoretical nature. Explorations of the human
skin instead have told us about the distribution of these receptors rather than their
mode of operation. Sensitivity to light pressure, for example, is greatest on the most
mobile body parts. The tips of the fingers and lips serve well in explorations based on
touch.
Pain is experienced at many points but not in the cerebral cortex, certain visceral
organs, or the mucous lining of the cheek, where there are no free nerve endings.
Receptors for cold are much more numerous than those for warmth throughout most
of the human body, but the cold spots themselves are not evenly distributed. There are
many more per square centimeter on the tip of the nose than on the forearm.
Research and Demonstrations The experience of pain can be markedly changed
by one's thoughts and other sensations. Dentists use recorded music as an anesthetic,
and acupuncture involves the insertion of needles in the human body to relieve pain.
This predominantly Eastern practice has been used as an anesthetic in surgery and in
the treatment of arthritis and headaches, but its physiological basis also remains
essentially unknown.
The puzzling nature of our cutaneous receptors was dramatically illustrated
some years ago amid startling reports of people being able to "see" colors by using
their fingers. When this claim was tested under carefully controlled conditions, it was
found that the reports were much exaggerated. Some people could make these discriminations
but only at a minimal level. Furthermore, the capacity being used was
not vision but temperature sensitivity. Certain people, some of the time, apparently
can tell one colored cloth from another by detecting small differences in reflected warmth
from the different-colored surfaces (Youtz, 1965; Makous, 1966).