The somesthetic senses consist of the cutaneous (skin) senses (pressure, pain, warmth, cold), the kinesthetic sense, and the vestibular senses. Receptors for the kinesthetic senses, which provide information on the position of the limbs, are located in the muscles and joints. Receptors for the vestibular senses, which provide information on the position and movement of the head, are located in the inner ear. The receptors for the vestibular senses consist of hair cells in the semicircular canals and the vestibular sacs. The hair-cell receptors in the semicircular canals are sensitive to movement of the head in three dimensions—the so-called rotational sense; the hair-cell receptors in the vestibular sacs, from which crystals of calcium carbonate, or otoliths, are suspended, provide information on the position of the head and linear movement.
Even after many decades of research, the receptors for the cutaneous and kinesthetic senses have still not been clearly differentiated. Examination of cross-sections of the skin reveals various structures that have been proposed as receptors for the respective sensations: Meissner corpuscles—touch, Pacinian corpuscles — deep pressure, Ruffini cylinders—warmth, Krause end bulbs — coldand free nerve endings—touchand pain. In addition, structures designated as annulospiral endings and flower—spray endings, which are found in muscles and joints, have been proposed as the receptors for kinesthetic sensitivity. Unfortunately, the validity of these matchings has not been completely confirmed.
Concerning changes in the various somesthetic senses with aging, touch sensitivity has been found to remain relatively unchanged until the early fifties, after which the absolute threshold for touch increases. The increase in touch thresholds with age is greater in the smooth (nonhairy) portions of the skin than in the hair—covered parts (Axelrod & Cohen, 1961; Kenshalo, 1977). Furthermore, Corso (1977) demonstrated that touch sensitivity on the ankles, knees, and other lower areas in the lower extremities is more impaired with aging than touch sensitivity to the wrists, shoulders, and other areas in the upper extremities of the body.
Sensitivity to vibratory stimuli of the sort used in a massage or neurologi— cal exam also declines with aging, particularly when the vibrations are of high frequency (Verrillo, 1980). The effects of aging per se should be distin— guished from those of disorders such as thiamin deficiency and diseases such as anemia and diabetes, which are also associated with reduced vibratory sensitivity.
The results of research on age—related changes in temperature sensitivity are mixed, but, in general, there appear to be slight increases in the absolute thresholds for both warmth and cold (Schieber, 1992; Whitbourne, 1985). Furthermore, the ability of the body to regulate extremes of temperature declines with aging. After age 65, there is a significant decrease is the temperature of the skin and body core (Verrillo & Verrillo, 1985).
Impairments in sensitivity of both the kinesthetic and vestibular senses have been observed in later life. In summarizing findings on active and passive movement of the muscles and joints, Ochs, Newberry, Lenhardt, and Harkins (1985) concluded that with increasing age, there is greater deterioration in the perception of movement for the great toe, several joints, the knees, and the hips, whereas judgments of tension with active movement of these structures are relatively unaffected. Decrements in the sense of balance occurring with aging are associated with feelings of dizziness and sensations of spinning (vertigo). Such decrements, which are accompanied by increased body sway, can result in the loss of balance and severe falls. However, vision can help compensate for decrements in vestibular sensations and enable the person to keep from falling.
Of all the somesthetic senses, the greatest amount of research on age-related changes has been conducted on the sense of pain. The results of earlier investigations suggested that older adults are less sensitive than younger adults to pain (Kenshalo, 1977; Whitbourne, 1985). This conclusion continues to enjoy fairly wide support. Recent studies, however, have pointed to a confounding variable, namely, the tendency on the part of older adults to underreport lower levels and overrate higher levels of pain (Harkins, Price, & Martinelli, 1986). Older adults tend to be more cautious in their reports than younger adults, reporting that they feel pain only when they are certain that it is present. Changes in pain sensitivity with age also vary with the area of the body stimulated, the type of stimulation, and various personal and social factors. Attention, attitudes, beliefs, emotions, ethnic background, motivation, personality traits, prior experience, socioeconomic status, and suggestion are also related to the perception of pain.
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