Touch, Sense of
TOUCH, SENSE OF
Sensations of touch arise by the activation of sensory receptors located in the skin that are responsive to mechanical stimuli. Proprioception, the perception of the position and movement of the limbs of the body, arises as a result of neural activity in sensory receptors located in and around the muscles underneath the skin. Advancing age is associated with diminished functioning of sensory systems, and the cutaneous and proprioceptive senses are no exception. For example, the ability to detect mechanical disturbances on the skin, such as those produced by a vibrating probe, and the ability to discriminate changes in the spatial patterns of stimulation of the skin, such as those produced by introducing a spatial gap between two objects pressed against the skin, tend to decline with age. An elderly person who has difficulty with the perception of the position and movement of limbs, or who has problems in moving quickly and accurately, may be experiencing an age-related decline in proprioception. The effects of aging on tactile sensitivity and proprioception can best be understood in light of what is known about the anatomy and physiology of receptors and how they conduct neural information about tactile and proprioceptive stimuli to the brain.
The sensory receptors for touch and proprioception are complex in structure, but the basic organization is that of a neuron that has an ending, endings responsible for mechano-electric transduction. Once the mechanical stimulus is transduced into an electrical impulse, the neuron transmits this information very quickly to the spinal cord and then to the brain. Information arising from the mechanoreceptors of the body and face goes to specific regions within the brain that interpret the signals in terms of tactile perceptions. The cortical regions devoted to this function have many independent representations of the body surface.
Many types of mechanical stimuli are used to understand how the tactile and proprioceptive systems work. For example, mechanical stimuli produced by pins or probes applied perpendicularly or tangentially to the skin have been used to determine the basic properties of the transduction of mechanical stimuli to electrochemical neural responses, as well as the subsequent transmission of these neural responses to the central nervous system. These stimuli indent the skin and can be either of a vibratory nature or of the ramp-and-hold variety. Vibratory stimuli are delivered by a probe that moves the skin at a particular amplitude and frequency of oscillation. Ramp-and-hold stimuli consist of an initial dynamic (ramp) indentation of the skin by the probe, followed by static (hold) indentation of the probe until it is withdrawn. The rate and depth of the initial ramp indentation and the duration of the hold state can be varied widely, as can the rate of withdrawal. Other types of stimuli that have been used to understand taction include periodic and aperiodic gratings moved across the skin surface, airpuffs, embossed letters, and everyday items such as sandpaper, cloth, and steel wool.
The classification of mechanoreceptors both in the periphery and in the central nervous system is based on the receptor’s responses to ramp-and-hold-like stimuli. Mechanoreceptors have been found to be either fast adapting (FA) or slowly adapting (SA). Here, adaptation refers to the rate of decline in neural activity with time in response to ramp-and-hold-like stimuli. There are two subclasses of FA and SA mechanoreceptors: FA I and FA II, and SA I and SA II. It has been fairly well established that the FA Is are the Meissner corpuscles and the hair receptors, the FA IIs are the Pacinian corpuscles, the SA Is are the Merkel cell-neurite complexes and the touch pads and the SA IIs are the Ruffini endings. They are defined historically. One could use the phrasiology-corpuscles first discovered by Meisser, corpuscles described by Pacini and the cell-neurite complexes as shown by Merkel. Ruffini was the first to show the existence of another tactile ending.
In psychophysical tasks involving the detection of vibration on the skin, it is possible, by carefully choosing the frequency of vibration, the size of the stimulus, and the site of stimulation, to examine the effects of aging on each of four information processing channels designated as the P, NP I, NP II, and NP III channels. Each of these channels has, as its input stage, one of the four receptor types described above. Experiments on the effects of aging have revealed that the sensitivity of each channel declines with age, especially for the P channel, a finding that can be explained by understanding the functional and structural characteristics of the channels.
At the level of the peripheral nervous system, the inputs to the P channel are FA II nerve fibers of Pacinian corpuscles. This channel is extremely sensitive at the optimal frequency of vibration of 250 Hz, with psychophysical thresholds in young adults being as low as 0.1 micrometers in the amplitude of vibration required to be detected. The exquisite sensitivity of the P channel is attributed partially to the capacity of this channel for spatial summation, which is the improvement in sensitivity that results as the size of the stimulus is increased, activating an increasing number of sensory receptors. The other three information-processing channels, NP I, NP II, and NP III, with their inputs from FAI, SA II, and SA I peripheral nerve fibers, respectively, are less sensitive than the P channel, mainly due to their inability to exhibit spatial summation. The fact that the deleterious effects of aging are substantially greater in the P channel than in any of the three NP channels is due, in part, to this unique capacity for spatial summation. Specifically, as people age, mechanoreceptors die, resulting in a progressive reduction in receptor density that becomes profound by about sixty-five or seventy years of age. Because one mechanism of spatial summation in the P channel is the integration of neural activity over a large number of receptors, the reduction in the density of Pacinian corpuscles has a particularly severe effect on sensitivity. Reduced neural input to the central nervous system from receptors—resulting from a reduction in the number of Pacinian corpuscles—results in elevated detection thresholds in older individuals. The smaller loss of sensitivity with aging found in the NP channels is thought to be due to the fact that the sensitivities of these channels, which are not dependent on spatial summation, are less affected by the reduction of receptor density.
Other factors associated with aging known to affect tactile sensitivity include changes in the physical properties of skin (such as reduced skin compliance) and changes in the peripheral and central nervous systems, resulting in some cases from a reduced blood supply to neurons, which can be due to a variety of vascular problems, including atherosclerosis. At a practical level, a decreased touch sensitivity in elderly individuals can cause a wide range of problems, including the inability to recognize objects by touch and an impaired ability to detect an object that has come into contact with the skin.
Proprioception is mediated by proprioceptors that are located in muscles and joints. The proprioceptive endings are: (1) the muscle spindles located in the muscles themselves, (2) Golgi tendon organs, which attach the muscles to bone, and (3) joint capsules that contain a group of endings similar in structure to the tactile receptors. The decline in proprioception in older individuals is often manifested in dramatic effects on motor performance, including very long reaction times and inaccurate and highly variable motor responses, such as those involved in walking, picking up objects, and driving a car. Of course, a decline in motor performance may result from factors other than, or in addition to, the loss of sensory feedback provided to the brain by proprioceptors. For example, motor performance may decline as a result of impairment of the brain areas associated with movement, cognition, and balance.
George A. Gescheider Stanley J. Bolanowski
See also Balance, Sense of; Motor Performance; Skin.
Bolanowski, S. J.; Gescheider, G. A.; Verrillo, R. T.; and Checkosky, C. M. ‘‘Four Channels Mediate the Mechanical Aspects of Touch.’’ Journal of the Acoustical Society of America 84 (1988): 1680–1694.
Cauna, N. ‘‘The Effects of Aging on the Receptor Organs of the Human Dermis.’’ In Advances in Biology of the Skin, Vol. 6 Aging. Edited by W. Montagna. Elmsford, N.Y.: Pergamon Press, 1965. Pages 63–96.
Gescheider, G. A.; Bolanowski, S. J.; Hall, K. L.; Hoffman, K.; and Verrillo, R. T. ‘‘The Effects of Aging on Information Processing Channels in the Sense of Touch: Absolute Sensitivity.’’ Somatosensory and Motor Research 11 (1994): 345–357.
Stevens, J. C., and Patterson, M. Q. ‘‘Dimensions of Spatial Acuity in the Touch Sense: Changes over the Life Span.’’ Somatosensory and Motor Research 12 (1995): 29–47.
Verrillo, R. T., and Violet, V. ‘‘Sensory and Perceptual Performance.’’ In Aging and Human Performance. Edited by N. Charness. Chichester, U.K.: Wiley, 1985. Pages 1–46.
Shaving cream is a substance applied to the skin to facilitate removal of hair. Shaving cream softens and moistens the skin and the hair, thus making shaving more comfortable and contributing to smoother skin. The advantages of using shaving cream, rather than soap, oil, or just water, are many. Shaving with a modern bar of soap approximates shaving with cream but doesn't provide all of the benefits: soap is only one element of many in a modern shaving preparation.
According to Burma Shave chronicler Frank Rowsome, Jr., modern shaving cream began with Burma Shave, which achieved high sales volume almost immediately after it was introduced. Prior to that time, lather was produced from a bar, and was basically another form of soap.
Manufacturing soap itself is an ancient craft—the word comes from the Old English word sape. By the seventh century, Italian soapmakers were organized in a guild, and, in the next century, the Holy Roman Emperor Charlemagne recognized soapmakers as craftsman. In the fourteenth and fifteenth centuries soap was made at Savona, Italy. The modern French, Spanish and German words for soap (savon, jabon, and seife, respectively) are cognates of the name of that town.
The early American settlers manufactured soap at home, using a method which called for mixing and heating animal fat with lye in a pot set over a fire, usually outdoors. This "open kettle" method of soap making was popular for years. Later adapted for large scale production, its use continued through the first half of the twentieth century.
By the eighteenth century, soap makers realized that they could enhance their product by improving the quality of the fat and the purity of the lye they used. Castile soap, made in Spain and still available today, soon achieved eminence as a face soap because of its smoothness and quality. Castile soap originally used olive oil rather than animal fat, and the modern version uses other fats and oils in addition to olive oil.
Although Americans continued to make their own soap at home for many years, they also began to manufacture soap commercially during the late seventeenth and early eighteenth centuries. Because they utilized similar materials and methods, soap makers were frequently in partnership with candle and tallow makers. The first soap maker to render (purify by melting) fats at his own operation was William Colgate, who had learned his trade in the early 1800s in New York City. The company that today bears his name is a Major producer of soap and other cosmetic preparations. In the nineteenth century storekeepers purchased soap from manufacturers in large blocks, from which their customers in turn cut smaller chunks. Jesse Oakley of Newburgh, New York, became the first manufacturer to sell wrapped soap in a cake form that was a good size for home use.
Soap was used for shaving through the early 1800s. In 1840, a concentrated soap that foamed was sold in tablets by Vroom and Fowler, whose Walnut Oil Military Shaving Soap was probably the first soap made especially for shaving. A century later, as the United States entered World War II, animal fats of relatively uncontrolled type and quality were still being used to make soap. To help supply American troops with soap, women were urged to save cans of cooking fat, and then bring them to local butchers who collected and delivered the fat to soap manufacturers. Because contaminants were inevitable in ingredients collected so haphazardly, the soap makers had to heat, strain, and reheat the fats—a process both inefficient and expensive. However, by the end of the war, mounting questions about purity and consistency led to the creation of the modern, regulated soap and cosmetic industry.
In addition to raising concerns about the quality of soap, World War II contributed to the invention of the spray can. Aerosol containers were first invented during the war as a device for dealing with insects carrying malaria and other diseases. Initially assigned to the Secretary of Agriculture, the patent for this "bug bomb" was released to American industry after the war. When the first aerosol shaving cream appeared in 1950, it captured almost one fifth of the market for shaving preparations within a short time. Today, aerosol preparations dominate the shaving cream market.
The goal of any shaving preparation is to wet and soften the hair to be shaved, cushion the effect of the razor, and provide a residual film to soothe the skin. This film should be of the proper pH value: neither excessively alkaline nor overly acidic, it should correspond to the skin's pH level.
Many manufacturers would have us believe that the recipes for shaving cream are carefully guarded secrets. However, the secrecy revolves mostly around the quantities in which standard ingredients are used, and the choice of substitutes for the few ingredients that are variable. By law, ingredients are listed right on the container, except for perfumes. Actual recipes are easily found in industrial chemistry textbooks available at many libraries. A standard recipe contains approximately 8.2 percent stearic acid, 3.7 percent triethanolamine,. 5 percent lanolin, 2 percent glycerin, 6 percent polyoxyethylene sorbitan monostearate, and 79.6 percent water.
Two major ingredients in this formula are common in many of today's preparations. Stearic acid is one of the main ingredients in soap making, and triethanolamine is a surfactant, or surface-acting agent, which does the job of soap, albeit much better. While one end of a surfactant molecule attracts dirt and grease, the other end attracts water. Lanolin and polyoxyethylene sorbitan monostearate are both emulsifiers which hold water to the skin, while glycerin, a solvent and an emollient, renders skin softer and more supple.
Common substitutes for the third, fourth, and fifth ingredients listed above include laureth 23 and lauryl sulfate (both sudsing and foaming agents), waxes, cocamides (which cleanse and aid foaming), and lanolin derivatives (emulsifiers). Most ingredients are powdered or flaked, although lanolin, lanolin derivatives, and cocamides are liquids.
The differences between one brand of shaving cream and another amount to adjustments in the proportions of ingredients and in the processing method (longer or shorter heating times, storage of the finished product, and so on), and choice of ingredients such as emulsifiers or perfumes. Also important is the choice of aerosol propellant. Some mixtures contain more than one propellant; most common are butane, isobutane, and propane. Though the wide range of choices for ingredients is well known, the exact combinations of ingredients represent the highest level of "magic" in modern chemistry.
The modern manufacture of shaving cream is a carefully controlled process. Although carried out on a large scale, its manufacture resembles a laboratory procedure involving only small quantities of ingredients. There are two main phases to the manufacturing process.
- In the first phase, the fatty or oily portions of the formula—stearic acid, lanolin, and polyoxyethylene sorbitan monostearate—are heated in a jacketed kettle to a temperature of approximately 179 to 188 degrees Fahrenheit (80 to 85 degrees Celsius). The jacketed kettle, which can hold as little as 300 gallons or as much as 10,000 gallons, resembles a double boiler: one container, placed inside another, is heated when steam is circulated through the outer container. Inside the interior kettle are blades that revolve to mix the oils as they are heated.
- After the first group of ingredients has turned smooth over a period of roughly 40 minutes, the steam is released from the outer container of the kettle, and the mixture is allowed to cool.
- The second phase of manufacture begins when the mixture has cooled to about 152 degrees Fahrenheit (65 degrees Celsius). Most of the remaining ingredients—water, glycerin, and triethanolamine—are added now, and mixing continues for approximately 40 minutes.
- When the mixture reaches a temperature of 125 to 134 degrees Fahrenheit (50 to 55 degrees Celsius), perfumes or other scents can be added. Because perfumes consist primarily of highly volatile oils, they would evaporate if added when the blend was still warm. The formulas for perfumes, which can contain more than 200 different ingredients, come closer to being trade secrets than information about shaving cream itself (though textbook and handbook formulas for perfume are not hard to come by). In recognition of this, manufacturers do not have to disclose information about fragrances.
- The mixture, still being stirred, is allowed to cool further, until it reaches a temperature of 89 degrees Fahrenheit (30 degrees Celsius). Now a thickening white mass of highly viscous liquid, it is forced through a silk or stainless steel screen to eliminate any lumps that may have formed in the mixing process, and to catch the rare impurity or foreign object such as a small wood splinter.
- If this particular mixture is designated for tube packaging, it is now placed in a tube and fitted with a cap. After the bottom of the tube has been crimped, the product is ready for shipment and stocking on a store shelf.
- When the desired product is an aerosol spray, the shaving cream is poured into an open can. Next a valve and a cover are fitted onto the can and forced downward to form a seal. Propellant is then forced into the can through the valve. Most shaving preparations contain between four and five percent propellant; a larger amount would dry the shaving cream as it came out of the can, rendering it unusable. A small amount of material is intentionally released (purged) to relieve excess pressure, and the can is tested in water to make sure that the valve is holding tightly. The can is now ready to be shipped.
Today's soaps, shaving creams, and lotions are all manufactured under strict quality control, and regulated by various federal agencies including the Food and Drug Administration (FDA). Some states have their own regulatory agencies, though state agencies are more likely to focus on environmental concerns than product safety. Batches of shaving cream are examined and analyzed both at the manufacturing site and in the laboratory. Individual containers of shaving preparations are coded so that a manufacturer knows exactly which batch any given can or tube came from, and can identify its distribution history.
A manufacturer of shaving cream needs to be certain that each batch meets quality standards. Among the things tested for are pH value (the acidity or alkalinity of the product), the height of the foam when sprayed, and its absorption rate (spray the foam on a piece of paper—how long does it take till the bottom of the paper shows moisture?).
Water quality must also be checked carefully. Most manufacturers make sure the water they use is pure by exposing the water to ultraviolet light or using distilled water. Having a microbiologist on site to test the water and the final product is common in the industry.
Where To Learn More
DeNavarre, M. G. The Chemistry and Manufacture of Cosmetics. Van Nostrand, 1962.
Lubowe, Irwin I. Cosmetics and the Skin. Reinhold Publishing Corp., 1964.
Men's Shaving Products Market. Frost & Sullivan, 1990.
Winter, Ruth. A Consumer's Dictionary of Cosmetic Ingredients, Crown, 1989.
Brooks, Geoffrey J. and Fred Burmeister. "Preshave and Aftershave Products." Cosmetics and Toiletries. April, 1990, pp. 67-69.
"Creams and Lotions Formulary." Cosmetics and Toiletries. November, 1986, pp. 139-70.
"Deodorants, Antiperspirants and Shaving Products Formulary." Cosmetics and Toiletries. April, 1990, pp. 75-87.
—Lawrence H. Berlow
One of the primary reasons why travelers who live in northern climates head off to fair-weather vacation spots is to smooth on suntan lotion, pass hours soaking in sunshine, and emerge with their skin browned by the sun's ultraviolet (UV) rays. Not everyone who desires tanned skin has the time or inclination to stay in the sun for the time needed to obtain brown skin. As a result, artificial means have been devised to tan skin even during the coldest and bleakest weather.
Sunlamps are the primary non-natural method of acquiring a tan. A sunlamp is a source of light that generates UV rays, resulting in an artificially produced but natural-looking tan. Some sun-lamps feature adjustable lamp heads that can be pointed at any angle, so that the user can focus the light on a specific body part. Smaller lamps are specifically designed as facial tanners.
Sunlamps became fashionable during the 1960s, when beach culture was popularized in the California-oriented songs of such rock groups as the Beach Boys and Jan and Dean and on screen in such teen-oriented movies as Beach Party (1963), Muscle Beach Party (1964), Bikini Beach (1964), and Beach Blanket Bingo (1965). Teens and young adults wished to look as tan and attractive as Frankie Avalon (1940–) and Annette Funicello (1942–), the popular stars of the Beach Party films. If they did not live in warm climates and have daily access to the sun, they could purchase sunlamps and tan themselves indoors. During the 1960s artificial tanning creams also became available. Such products as Rapid Tan, QT (Quick Tan), Tan-O-Rama, and Man-Tan featured dihydroxyacetone, a colorless substance that turned the skin dark brown. The downside to such products was that they irritated the skin and stained clothing, and the tans they produced often were uneven or blotchy.
In the 1970s and 1980s more and more Americans became concerned with feeling fit and looking good and, as a result, indoor tanning salons opened up across the country. Tanning salons were businesses that featured tanning beds, located in separate booths or rooms to insure privacy. Customers relaxed on the clamshell-shaped beds, while their bodies were exposed to the artificial sunlight generated by the tubular bulbs that surrounded them. Booths were equipped with timers to prevent overexposure to the light. At this time tanned skin became so associated with physical fitness and vigor that tanning beds and sun lamps even were featured in health clubs, which primarily existed to allow their members to lift weights, run on treadmills, ride stationary bicycles, or play tennis.
Tanned skin had developed a reputation as a signal of health, but by the mid-1970s that idea had started to be challenged. Scientists discovered that although exposure to sunlight or the artificial light produced by sunlamps may allow the body to manufacture Vitamin D, which plays a primary role in building bones and teeth, only a tiny quantity of light is required for all the Vitamin D the body needs. Scientists also determined that even a moderate amount of the UV radiation that causes the skin to darken also harms the body's immune system. Exposure to UV rays has been linked to the early aging of skin, causing it to look rough and leathery and, more seriously, can cause malignant melanoma, a deadly skin cancer. The negative effects of tanning often are not immediately apparent. Young people in their teens or twenties may not suffer the ill-effects of tanned skin until middle or old age.
Despite these health concerns, tanning salons remain popular. In the early twenty-first century over 28,000 tanning salons were open for business across the United States. Additionally, relaxing and playing in the sun continue to be favorite pastimes, and beach resorts remain popular vacation destinations.
FOR MORE INFORMATION
Sweet, Cheryl A. "Healthy Tan"—A Fast-Fading Myth. Rockville, MD: Department of Health and Human Services, Public Health Service, Food and Drug Administration, 1990.
Waud, Sydney P. Sunbathing: The Healthy Way to a Perfect Tan. New York: Mayflower Books, 1979.
For many years, suntanned skin symbolized health and sex appeal; a tan first gained status in the 1920s and reached a peak in the 1970s. Movie star Douglas Fairbanks Sr. (1883–1939) was among the first to popularize the suntan when he began appearing in films with gleaming bronzed muscles. His tanned skin became a symbol of his physical fitness. Before this time, creamy white skin had been a mark of high fashion and social status because those with enough money could stay inside, away from the heat of the day, while field laborers were darkened by the sun. In the early twentieth century, however, more and more laborers moved to the cities and began working long hours indoors. Suntanned skin became a distinction available only to the wealthy with enough leisure time and money to travel to a sunny locale during the dreary winter months. Like Fairbanks, fashion designer Coco Chanel (1882–1971) established a trend for tanned skin in the 1920s. Returning from vacation with a deep, dark tan, Chanel began using tanned models and mannequins for her designs. By 1946, women could wear tiny bikinis to tan as much skin as possible.
Tanning had become a national obsession by the 1970s. Magazines featured articles about achieving the perfect tan. Numerous methods for obtaining a tan were used. People would coat their skin with baby oil, lay on aluminum reflectors, or bake in indoor tanning booths. By the late 1980s, indoor tanning became one of the fastest growing industries in North America, according to U.S. News and World Report.
But by the late 1980s, warnings about the risks of sun bathing came to public attention. Incidences of melanoma (a form of skin cancer) had increased more than tenfold from 1930 to the late 1980s. Soon, protective sun lotions were marketed to protect sunbathers from burning. Sunless tanning lotions offered the "look" without the damage. Despite warnings, the numbers of people with the worst form of skin cancer, melanoma, continued to rise at a rate of 7 percent each year since 1981. By the end of the twentieth century, well-known people like U.S. senator John McCain (1936–) of Arizona made public their struggles with skin cancer and warned against overexposure. Appealing public figures, such as Gwyneth Paltrow (1972–) and Nicole Kidman (1967–), were helping to make alabaster skin a sign of sex appeal and status again.
For More Information
Alder, Jerry, et al. "The Dark Side of the Sun." Newsweek (June 9, 1986): pp. 60–64.
"Brown as a . . . " Newsweek (August 1, 1966): pp. 58–59.
"Choose Your Cover: Facts and Statistics about Skin Cancer." NationalCenter for Chronic Disease Prevention and Health Promotion: Cancer Prevention and Control.http://www.cdc.gov/ChooseYourCover/skin.htm (accessed April 1, 2002).
Levine, Art. "A New Bronze Age for the Tanning Industry: Indoor Tanning Salons Take on Their Critics." U.S. News and World Report (September 8, 1997): p. 48.
Sweet, Cheryl A. "'Healthy Tan'—A Fast-Fading Myth." Rockville, MD: U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration, 1990.
tanning, process by which skins and hides are converted into leather. Vegetable tanning, a method requiring more than a month even with modern machinery and tanning liquors, employs tannin; its use is shown in Egyptian tomb paintings dating from 3000 BC Mineral tanning includes tawing, or alum tanning, another ancient method, and chrome tanning, the process most common today, based on the use of chrome salts and requiring only a few hours. Known as early as 1856, chrome tanning was first patented in the United States by Augustus Schultz in 1884. In oil tanning, or chamoising, the pelts are treated with fats and hung to dry; the leather is commonly napped on both sides and is very absorbent. The most recently developed tanning process employs artificial agents (syntans). Most heavy leathers, such as sole leather, are vegetable tanned; many light leathers are chrome tanned. The Native Americans of North America used the chamois method, employing the fat, livers, and brains of animals. Their tanned white buckskin was highly esteemed, especially for clothing, both by Native Americans and by colonial pioneers. In the tanyards of European settlers tanners used oak and hemlock bark; gallnuts; the wood, nuts, and leaves of the chestnut tree; and the leaves of sumac.