The Skin Senses
The Greek philosopher Aristotle believed that all of the senses from the skin were encompassed in the broad category of touch. Today we know that the so-called skinsenses include several distinct subsystems, each giving rise to a distinct sensation—including pressure, temperature, and pain (Figure 4.11). Not surprisingly, some parts of the body have greater skin sensitivity than others—it’s especially high in the hands and fingers, the lips and tongue, and the genital areas.
Among the various senses, the skin senses may be the best example of specificity cod-ing (or “labeled lines”), whereby distinct types of receptors are associated with different sensations. Some of the receptors respond to sustained pressure or very low-frequency vibration. A second type of receptors respond to faster vibrations. Yet another type, called the Ruffini endings, respond to sustained downward pressure or stretching of the skin; among other functions, these latter receptors probably play a key role in helping us mon-itor and control our finger positions.
Still other receptors are responsible for our sensitivity to temperature—and even here, we encounter specialization. One type of receptor fires whenever the temperature increases in the area immediately surrounding the receptor; a different (and more numerous) type of receptor does the opposite—firing in response to a drop in skin temperature. It turns out that, in most cases, neither of these receptor types is especially active. This is because many mechanisms inside the body work to maintain a constant body temperature, and so neither receptor type is triggered. But if you move close to the radiator or step into cold water, these receptors immediately respond, informing you about these events.
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