Developmental psychology.pdf

Sensory Processes 137
PROPRIOCEPTEVE SENSES
The story of the frog can indicate our progress. We began with the problem of awareness
of stimulation, emphasizing the questions of detection and discrimination, as illustrated
in the frog's plight. Then we examined several sensory capacities: the visual
system, appropriate in the frog for catching bugs and avoiding overhead predators;
auditory ability, extremely limited in the frog; olfactory and gustatory capabilities, also
undeveloped in comparison with human beings; and temperature sensitivity, certainly
sufficient to detect the danger in boiling water.
All of these sensory abilities are oriented to stimuli outside the body, making
the frog reasonably adept at avoiding birds, snakes, some fish, and even certain mammals
that prey upon it. But a major danger to the frog comes from another source, the
human being, who is interested in a fancy meal or a standard laboratory subject (Romer,
1971).
The frog also has receptors for stimuli originating from within it. The term
proprioception refers to these senses, for the root proprio means "belonging to the body."
We now consider three of these: kinesthesis, concerning the sensations of body movements
and positions; balance, involving the equilibrium of the body; and passive motion,
in which the body experiences motion without any activity on its own part.
This research carries us well beyond Aristotle's five senses. The total today
stands at eight to twelve, depending upon whether certain systems, such as cutaneous
sensitivity, are considered one or several senses.
Kinesthesis
In leaping from the pot, the frog probably will land on its front and hind legs together,
a feat that requires some practice and perhaps a bit of luck as well. But how does the
animal ease its fall? How does it know just how hard to push against the terrain to
absorb the force of its landing?
Similarly, if the frog is stimulated on its breast, it will wipe that spot with
either of its front legs. If its back is touched, the frog will use a front or hind leg,
depending upon where the point is located. And if the outside of one front leg is irritated,
perhaps by a bit of acid on the skin, the frog will wipe that area with the hind
foot on the same side.
The animal knows which area to wipe through cutaneous receptors, but how
does it reach the correct spot? The answer to successful wiping and also to a safe landing
lies in kinesthetic sensitivity, which guides the limbs in extension or flexion, forward
or backward, to one side or the other, through feedback from within the body itself.
Structures and Functions This capacity for sensing the voluntary movements of
one's body and the positions of its parts was first brought to scientific attention early
in the nineteenth century by Sir Charles Bell. He called it the muscle sense; later it
was known as the sixth sense; and today we refer to it as kinesthesis or kinesthetic
sensitivity, from the Greek root kinesis meaning "movement."
Information concerning these body movements and positions is made possible
by receptor mechanisms in the muscles, tendons, and joints. They are subject to pressure
and release of pressure as parts of the body are moved or held in a firm position,
and the resulting nerve impulses travel to the brain. Impulses are then sent back to
the muscles, tendons, and joints, stimulating further activity. Through this feedback
mechanism, motor activities act as stimuli for their own rearousal, and when carefully
or habitually practiced, they proceed more or less automatically.