Neural impulses containing sensory information are conveyed from specialized receptors by way of efferent (sensory) nerves to the central nervous system (spinal cord and brain). Then, neural impulses are conveyed from the central nervous system by way of efferent (motor) nerves to the muscles and glands. Combined with the secretions of certain glands, the two types of peripheral nerves—afferent and efferent—make up the information highway of the body. The efficiency with which sensory information is received, conducted, processed, and acted upon determines how effectively the individual is able to sustain and enhance his or her existence. Conducting information from the external world to the spinal cord and brain is, of course, not only a matter of the intactness of neural transmission pathways but also the specialized receptors connected to those pathways.
At least since Aristotle's time, over 2000 years ago, it has been commonly asserted that there are five senses—vision, hearing, taste, smell, and touch. Any educated person of today realizes, however, that there are many more senses than the traditional five. There are at least four skin senses alone— pressure, pain, warmth, and cold—in addition to senses of balance, position, gravity, rotary motion, and linear motion. Similar to the internal organs and systems discussed in the last section, the sensory receptors and connecting peripheral nerves are functioning at peak efficiency in early adulthood. The corresponding sensory thresholds are lowest and the ability to distinguish between sensations of different magnitude, quality, and complexity is greatest during the twenties. Consequently, a typical 20-year-old is more alert and perceptive of changes in the environment than a typical 60- or 70-year old. Beginning in the thirties and forties and increasing in severity during the sixties and seventies, however, sensations become duller and are responded to more slowly. One effect of declining sensory acuity is the perception of older adults as inattentive, distractible, and dull rather than merely less perceptive.
Though the decline in sensory abilities is to some extent inevitable with aging, the range of individual differences in this regard is extensive. Some older adults take pride in the fact that they can see or hear "as well as when I was young." In addition, older adults can learn to compensate for normal sensory losses by using good judgment and relying on their past experiences. They can wear appropriate sensory aids, avoid situations that demand acute sensory receptivity, and "fill in the gaps" created by undetected sensory information.
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