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Chapter: Essentials of Anatomy and Physiology: The Senses

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Characteristics of Sensations

Certain characteristics of sensations will help you understand how the sensory areas work with informa-tion from the receptors.

CHARACTERISTICS OF SENSATIONS

 

Certain characteristics of sensations will help you understand how the sensory areas work with informa-tion from the receptors.

 

1. Projection—the sensation seems to come from the area where the receptors were stimulated. If you touch this book, the sensation of touch seems to be in your hand but is actually being felt by your cere-bral cortex. That it is indeed the brain that feels sensations is demonstrated by patients who feel phantom pain after amputation of a limb. After loss of a hand, for example, the person may still feel that the hand is really there. Why does this hap-pen? The receptors in the hand are no longer pres-ent, but the severed nerve endings continue to generate impulses. These impulses arrive in the parietal lobe area for the hand, and the brain does what it has always done and creates the projection, the feeling that the hand is still there. For most amputees, phantom pain diminishes as the severed nerves heal, but the person often experiences a phantom “presence” of the missing part. This may be helpful when learning to use an artificial limb.

 

2. Intensity—some sensations are felt more distinctly and to a greater degree than are others. A weak stimulus such as dim light will affect a small num-ber of receptors, but a stronger stimulus, such as bright sunlight, will stimulate many more recep-tors. When more receptors are stimulated, more impulses will arrive in the sensory area of the brain. The brain “counts” the impulses and projects a more intense sensation.

 

3. Contrast—the effect of a previous or simultaneous sensation on a current sensation, which may then be exaggerated or diminished. Again, this is a func-tion of the brain, which constantly compares sensa-tions. If, on a very hot day, you jump into a swimming pool, the water may feel quite cold at first. The brain compares the new sensation to the previous one, and since there is a significant differ-ence between the two, the water will seem colder than it actually is.

 

4. Adaptation—becoming unaware of a continuing stimulus. Receptors detect changes, but if the stim-ulus continues it may not be much of a change, and the receptors will generate fewer impulses. The water in the swimming pool that seemed cold at first seems to “warm up” after a few minutes. The water has not changed temperature, and the recep-tors for cold have no changes to detect, therefore they generate fewer impulses. The sensation of cold lessens, and we interpret or feel that as increasing warmth. For another example, look at your left wrist (or perhaps the right one). Many of us wear a watch and are probably unaware of its presence on the arm most of the time. The cuta-neous receptors for touch or pressure adapt very quickly to a continuing stimulus, and if there is no change, there is nothing for the receptors to detect.


5. After-image—the sensation remains in the con-sciousness even after the stimulus has stopped. A familiar example is the bright after-image seen after watching a flashbulb go off. The very bright light strongly stimulates receptors in the retina, which generate many impulses that are perceived as an intense sensation that lasts longer than the actual stimulus.

 

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