touch / təch/ • v. [tr.] 1. come so close to (an object) as to be or come into contact with it: the dog had one paw outstretched, not quite touching the ground. ∎  bring one's hand or another part of one's body into contact with: he touched a strand of her hair | she lowered her head to touch his fingers with her lips. ∎  (touch something to) move a part of one's body to bring it into contact with: he gently touched his lips to her cheek. ∎  lightly press or strike (a button or key on a device or instrument) to operate or play it: he touched a button on the control pad. ∎  [intr.] (of two people or two or more things, typically ones of the same kind) come into contact with each other: for a moment their fingers touched. ∎  cause (two or more things, typically ones of the same kind) to come into contact: we touched wheels and nearly came off the road. ∎  Geom. be tangent to (a curve or surface) at a certain point. ∎ inf. reach (a specified level or amount): sales touched twenty grand last year. ∎  inf. be comparable to in quality or excellence: there's no one who can touch him at lightweight judo.2. handle in order to manipulate, alter, or otherwise affect, esp. in an adverse way: I didn't play her records or touch any of her stuff. ∎  cause harm to (someone): I've got friends who'll pull strings—nobody will dare touch me. ∎  take some of (a store, esp. of money) for use: in three years I haven't touched a cent of the money. ∎  consume a small amount of (food or drink): the beer by his right hand was hardly touched. ∎  used to indicate that something is avoided or rejected: he was good only for the jobs that nobody else would touch. ∎  (touch someone for) inf. ask someone for (money or some other commodity) as a loan or gift: he touched me for his fare.3. have an effect on; make a difference to: a tenth of state companies have been touched by privatization. ∎  be relevant to: some Canadian interests touched European powers. ∎  (usu. be touched) (of a quality or feature) be visible or apparent in the appearance or character of (something): the trees were beginning to be touched by the colors of autumn. ∎  reach and affect the appearance of: a wry smile touched his lips. ∎  (touch something in) chiefly Art lightly mark in features or other details with a brush or pencil. ∎  (often be touched) produce feelings of affection, gratitude, or sympathy in: she was touched by her friend's loyalty. ∎  [as adj.] (touched) inf. slightly insane.• n. 1. an act of bringing a part of one's body, typically one's hand, into contact with someone or something: her touch on his shoulder was hesitant | expressions of love through words and touch. ∎  [in sing.] an act of lightly pressing or striking something in order to move or operate it: you can manipulate images on the screen at the touch of a key. ∎  the faculty of perception through physical contact, esp. with the fingers: reading by touch. ∎  a musician's manner of playing keys or strings. ∎  the manner in which a musical instrument's keys or strings respond to being played: Viennese instruments with their too delicate touch. ∎  a light stroke with a pen, pencil, etc. ∎  [in sing.] inf., dated an act of asking for and getting money or some other commodity from someone as a loan or gift: I only tolerated him because he was good for a touch now and then. ∎  [in sing.] archaic a thing or an action that tries out the worth or character of something; a test: you must put your fate to the touch.2. a small amount; a trace: add a touch of vinegar he retired to bed with a touch of the flu. ∎  a detail or feature, typically one that gives something a distinctive character: the film's most inventive touch. ∎  [in sing.] a distinctive manner or method of dealing with something: later he showed a surer political touch. ∎  [in sing.] an ability to deal with something successfully: getting caught looks so incompetent, as though we're losing our touch.3. Bell-ringing a series of changes shorter than a peal.4. short for touch football.PHRASES: a touch to a slight degree; a little: the water was a touch too chilly for us.in touch1. in or into communication: she said that you kept in touch, that you wrote | ask someone to put you in touch with other suppliers.2. possessing up-to-date knowledge: we need to keep in touch with the latest developments. ∎  having an intuitive or empathetic awareness: you need to be in touch with your feelings.lose touch1. cease to correspond or be in communication: I lost touch with him when he joined the air force.2. cease to be aware or informed: we cannot lose touch with political reality.out of touch lacking knowledge or information concerning current events and developments: he seems surprisingly out of touch with recent economic thinking. ∎  lacking in awareness or sympathy: we have been betrayed by a government out of touch with our values.to the touch used to describe the qualities of something perceived by touching it or the sensations felt by someone who is touched: the silk was slightly rough to the touch the ankle was swollen and painful to the touch.touch base (with) see base¹ .touch bottom reach the bottom of a body of water with one's feet or a pole. ∎  be at the lowest or worst point: the housing market has touched bottom.touch a chordsee chord² .touch woodsee wood.would not touch something with a ten-foot pole inf. used to express a refusal to have anything to do with someone or something: relax, I wouldn't touch you with a ten-foot pole!PHRASAL VERBS: touch at (of a ship or someone in it) call briefly at (a port).touch down (of an aircraft or spacecraft) make contact with the ground in landing.touch something off cause something to ignite or explode by touching it with a match. ∎  cause something to happen, esp. suddenly: there was concern that the move could touch off a trade war. touch on (or upon) 1. deal briefly with (a subject) in written or spoken discussion: he touches upon several themes from the last chapter.2. come near to being: a self-confident manner touching on the arrogant.touch something up make small improvements to something: these paints are handy for touching up small areas on walls or ceilings.touch wood see knock on wood at wood.DERIVATIVES: touch·a·ble adj.touch·er n.

TOUCH

Two bodies are said to be touching if there is no spatial gap between some point on the surface of one and some point on the surface of the other. If one of the touching bodies is that of a sentient being, it may be aware of certain properties of the other body: for instance, that it is hot or cold, rough or smooth, wet or dry, hard or soft, sweet or sour. The sentient being is said to be aware of an object's sweetness or sourness by taste. (Aristotle attributes our distinguishing taste from touch to the fact that only a part of our flesh is sensitive to flavor.) The remaining properties the sentient being is said, in common speech, to be aware of by touch. Accordingly, touch appears in the traditional list of senses, with sight, hearing, and so on.

Aristotle

Aristotle remarks that in the case of touch the contraries hot-cold, dry-moist, and hard-soft do not seem to have a single subject in the way in which the single subject of the properties acute-grave and loud-soft is sound, which is perceived by hearing. This may lead one to say that there are really a number of different senses that are mistakenly referred to as one sense, touch, perhaps because the body of a sentient being must touch an object in order for it to be aware by any of them of that object's properties. Or one may say that there is a single subject of the different contraries, namely, a material thing, and that there is only one sense, touch, whereby we are aware of the different properties of which the material thing is a subject. If one takes the latter course, it may appear that touch is the only sense whose proper object is the material world.

Locke, Berkeley, and Condillac

To John Locke, it seemed that "the idea most intimately connected with and essential to body, so as nowhere else to be found or imagined, but only in matter" was the idea of solidity. This idea is received by touch and "arises from the resistance which we find in body to the entrance of any other body into the place it possesses."

As Locke held it to be by touch that we receive the idea of solidity, the idea essential to body, so George Berkeley, in his Essay towards a New Theory of Vision, held it to be touch alone that directly acquaints us with the external world. He abandoned this view in The Principles of Human Knowledge, maintaining that the objects of touch are as much sensations as are the objects of sight.

Locke regarded solidity as a "simple idea": "If anyone asks me what this solidity is, I send him to his senses to inform him." Later philosophers have tried to explain what is involved in the sensation of solidity. Étienne Bonnot de Condillac distinguished it from the sensations of sound, color, and smell, since a person knows his own body by it. If a person presses his hand against his chest, his hand and chest "will be distinguished from one another by the sensation of solidities which they mutually give each other." Thus, involved in the notion of a sensation of solidity is the notion of the recognition as such of a feeling given to a part of the body. If organic sensations were not localized in the body, a person could never know his own or any other body by touching it, for "it is only with extension that we can construct extension, just as it is only with objects that we can construct objects."

H. H. Price

H. H. Price carried the analysis a step further. He divided touch "into three distinct types of sensation: contact sensation proper, muscular sensation, and the sensation of temperature." The perception of solidity involves both contact sensation proper and muscular sensation. The latter is "essentially a modification of the voluminous life-feeling [that] might also be described as our sense of embodiment." Muscular strain is felt at a place in the body and as having vectorial character, that is, originating from or tending toward a certain direction. A person experiences the solidity of something when the resistance he feels on pressing it "is actually felt as coming from within the closed boundary which contact-sensation reveals.… Thus the tactual conception of Matter is strictly speaking tactuo-muscular or contactuomuscular."

Local Sign Theory

The analyses of both Condillac and Price specify organic sensations as being localized. As Condillac expressed it, to know its body the child must "perceive its sensations, not as modifications of its soul, but as modifications of the organs which are their occasional causes." Condillac cannot explain "how the self which is only in the soul appears to be found in the body … it is enough that we observe this fact." The alternatives are either that a person is born with the capacity to locate organic sensations or that he acquires this capacity. Most philosophers hold the capacity to be acquired, although they differ widely in the accounts they give of how it is acquired; whether by the person's learning to interpret some feature of the sensation as a sign of its location (the so-called local sign) or in some other way.

Movement and Touch

Perhaps the most important recent contribution to the problem of how touch mediates awareness of its objects was made by David Katz in "Der Aufbau der Tastwelt." Summarizing Katz's conclusions, Maurice Merleau-Ponty expresses the crux of the matter as being that "the movement of one's body is to touch what lighting is to vision.… When one of my hands touches the other, the hand that moves functions as subject and the other as object. There are tactile phenomena, alleged tactile qualities, like roughness and smoothness, which disappear completely if the exploratory movement is eliminated. Movement and time are not only an objective condition of knowing touch, but a phenomenal component of tactile data. They bring about the patterning of tactile phenomena, just as light shows up the configuration of a visible surface."

Body-Object Relation

With the view that the objects of touch are physical objects may be contrasted the view that we are not aware of the object we touch but of a relation holding between our body and that object. It is a fact that how warm an object feels to an observer depends causally on the warmth of the part of the observer's body with which he is touching it. We notice the temperature of a hand that is colder or warmer than our own. Aristotle explains this in terms of his theory of sensation as the assimilation in form of the organ to the object. D. M. Armstrong mentions it, together with the fact that a person can say immediately with what portion of his body he is in contact with an object perceived by touch, in support of his theory that all immediate tactual perception involves perception of a relation holding between the observer's body and the object he is touching. As evidence for his theory, Armstrong holds that "hardness and softness as immediately perceived by touch, are obviously relative to the hardness or softness of our flesh." It is unclear from this evidence whether Armstrong is justified in claiming more than that how things feel to us depends on the condition of the part of the body with which we feel them.

See also Aristotle; Armstrong, David M.; Berkeley, George; Colors; Condillac, Étienne Bonnot de; Locke, John; Merleau-Ponty, Maurice; Sensa; Sound.

Bibliography

Aristotle. De Anima. Book II, Ch. 11.

Armstrong, D. M. Bodily Sensations. London and New York: Routledge and Paul, 1962. Chs. 3–5.

Berkeley, George. An Essay towards a New Theory of Vision. Dublin, 1709. Secs. 45–49.

Condillac, E. B. de. Treatise on the Sensations. Paris and London, 1754. Part II, Chs. 1–5.

Grice, H. P. "Some Remarks about the Senses." In Analytic Philosophy. 1st series. Oxford: Blackwell, 1962.

Katz, David. "Der Aufbau der Tastwelt." Zeitschrift für Psychologie 11 (1925).

Locke, John. An Essay concerning Human Understanding. London, 1690. Book II, Ch. 4.

Merleau-Ponty, Maurice. Phenomenology of Perception, 313–317. New York: Humanities Press, 1962.

O'Shaughnessy, B. "The Sense of Touch." Australasian Journal of Philosophy 67 (1989): 37–58.

Perkins, M. Sensing the World. Indianapolis: Hackett, 1983.

Price, H. H. "Touch and Organic Sensation." PAS 44 (1943–1944).

Urmson, J. O. "The Objects of the Five Senses." Proceedings of the British Academy 54 (1968): 117–131.

Vesey, G. N. A. The Embodied Mind. London: Allen and Unwin, 1965. Ch. 4. Contains an exposition and criticism of the local sign theory and references to other work on the subject.

Warnock, G. J. Berkeley. Harmondsworth, U.K., 1953. Ch. 3.

G. N. A. Vesey (1967)

Bibliography updated by Benjamin Fiedor (2005)

Touch

How we feel the outside world

Touch and health

Resources

Touch is one of the five senses (the others being smell, taste, vision, and hearing) through which animals and people interpret the world around them. While the other senses are localized primarily in a single area (such as vision in the eyes or taste in the tongue), the sensation of touch (or contact with the outside world) can be experienced anywhere on the body, from the top of the head to the tip of the toe. Touch is based on nerve receptors in the skin which send electrical messages through the central nervous system to the cerebral cortex in the brain, which interprets these electrical codes. For the most part, the touch receptors specialize in experiencing either hot, cold, pain, or pressure. Arguably, touch is the most important of all the senses; without it animals would not be able to recognize pain (such as scalding water), which would greatly decrease their chances for survival. Research has also shown that touch has tremendous psychological ramifications in areas like child development, persuasion, healing, and reducing anxiety and tension.

How we feel the outside world

Our sense of touch is based primarily in the outer layer of skin called the epidermis. Nerve endings that lie in or just below the epidermis cells respond to various outside stimuli, which are categorized into four basic stimuli: pressure, pain, hot, and cold. Animals experience one or a combination of these sensations through a complex neural network that sends electrical impulses through the spinal cord to the cerebral cortex in the brain. The cerebral cortex, in turn, contains brain cells (neurons) arranged in columns that specialize in interpreting specific types of stimuli on certain parts of the body.

The sensation of touch begins with various receptors in the skin. Although these receptors appear to specialize in reacting to certain sensations, there is some debate concerning this specificity since most touch stimuli are a combination of some or all of the four major categories.

Scientists have identified several types of touch receptors. Free nerve ending receptors, located throughout the body at the bases of hair, are associated primarily with light pressure (such as wind) and pain. Meissner corpuscles are nerve endings contained in tiny capsules and are found primarily in the fingertips and areas especially sensitive to touch (in the form of low-frequency vibrations), like the soles of the feet and the tongue. The Pacinian corpuscles look like the cross section of an onion and are found in deep tissues in the joints, the genitals, and the mammary glands. They are extremely sensitive to pressure and are also stimulated by rapid movement of the tissues and vibrating sensations. Ruffini endings, which are also located in the deeper layers of the skin, respond to continuous stimulation, like steady pressure or tension within the skin. Merkel disks, are found near the base of the epidermis and also respond to continuous stimulation or pressure. The skin also contains specific thermoreceptors for sensing hot and cold and nociceptors that identify high intensity stimulation in the form of pain.

Most, if not all of these receptors, are designed to adapt or become accustomed to the specific stimulation they interpret. In other words, the receptor does not continue to register a constant “feeling” with the same intensity as when it first begins and may even shut off the tactile experience. Imagine, for example, putting on a wool sweater over bare skin. The initial prickly sensation eventually abates, allowing the wearer to become accustomed to the feeling. Other examples include wearing jewelry such as rings, necklaces, and watches.

These receptors are also found in greater numbers on different parts of the body. For example, peoples’ backs are the least sensitive to touch, while their lips, tongue, and fingertips are most sensitive to tactile activity. Most receptors for cold are found on the surface of the face while thermoreceptors for warmth usually lie deeper in the skin and are fewer in number. A light breeze on the arm or head is felt because there tend to be more sense receptors at the base of the hairs than anywhere else.

Touch and health

Touch has a tremendous impact on most animals’ physical and psychological well being. Numerous studies of humans and other animals have shown that touch greatly impacts how we develop physically and respond to the world mentally. For example, premature babies that receive regular massages will gain weight more rapidly and develop faster mentally than those who do not receive the same attention. When baby rats are separated from their mothers for only 45 minutes, they undergo physiological or biochemical changes, specifically a reduction in a growth hormone. Touching of premature babies can also stimulate growth hormones (such as the hormone needed to absorb food) that occur naturally in healthy babies.

A baby does not have to be premature or sickly to benefit from touch. Even healthy babies show benefits from touch in terms of emotional stability. Difficult children often have a history of abuse and neglect. The reason is that touch serves as a type of reassurance to infants that they are loved and safe, which translates into emotional well being. In general, babies who are held and touched more tend to develop better alertness and cognitive abilities over the long run.

Touch continues to have a great psychological impact throughout peoples’ lives. Even adults who are hospitalized or sick at home seem to have less anxiety and tension headaches when they are regularly touched or caressed by caretakers or loved ones. Numerous studies have shown that touch also has a healing power. Researchers have found that touch reduces rapid heart beats and irregular heart beats

KEY TERMS

Biochemical —The biological or physiological chemicals of living organisms.

Central nervous system —The brain and spinal cord components of the nervous system, which controls activities of internal organs, movements, perceptions, thoughts, and emotions.

Cerebral cortex —The external gray matter surrounding the brain and made up of layers of nerve cells and fibers, thought to process sensory information and impulses.

Epidermis —The outer layer of the skin consisting of dead cells. It is the primary protective barrier against sunlight, chemicals, and other possible harmful agents. The epidermal cells are constantly being shed and replenished.

Free nerve endings —Touch receptors in the skin that detect light pressure.

Glucocorticoids —A steroid or hormone like compound that affects metabolism and can have an anti-inflammatory effect.

Meissner corpuscles —Touch receptors in the skin that are sensitive to touch. Named after German histologist Georg Meissner.

Neurons —Nervous system unit that includes the nerve cell, dendrites, and axons.

Pacinian corpuscles —Touch receptors in the skin that sense pressure and rapid or vibrating movement of the tissues, named after Italian anatomist Filippo Pacini.

(arrhythmias). Another study showed that baby rats who are touched often during infancy develop more receptors to control the production of biochemicals called glucocorticoids, which are known as stress chemicals because of their ability to cause muscle shrinkage, high blood pressure, elevated cholesterol, and more.

Touch’s psychological impact goes beyond physical and mental health. Researchers have shown that touch is a powerful persuasive force. For example, studies have shown that touch can have a big impact in marketing and sales. Salespeople often use touch to establish a camaraderie and friendship that can result in better sales. In general, people are more likely to respond positively to a request if it is accompanied by a slight touch on the arm or hand. In a study of waiters and waitresses, for example, those that lightly touched a patron often received better tips.

See also Perception.

Resources

BOOKS

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

PERIODICALS

Ponte, Lowell. “The Sense That Shapes Our Future.” Readers Digest. (January 1992): 21-26.

Weider, Betty. “Therapeutic Touch.” Shape. (May 1992): 32.

David Petechuk

Touch

Touch is one of the five senses (the others being smell , taste , vision , and hearing ) through which animals and people interpret the world around them. While the other senses are localized primarily in a single area (such as vision in the eyes or taste in the tongue), the sensation of touch (or contact with the outside world) can be experienced anywhere on the body, from the top of the head to the tip of the toe. Touch is based on nerve receptors in the skin which send electrical messages through the central nervous system to the cerebral cortex in the brain , which interprets these electrical codes. For the most part, the touch receptors specialize in experiencing either hot, cold, pain , or pressure . Arguably, touch is the most important of all the senses; without it animals would not be able to recognize pain (such as scalding water ), which would greatly decrease their chances for survival. Research has also shown that touch has tremendous psychological ramifications in areas like child development, persuasion, healing, and reducing anxiety and tension.

How we feel the outside world

Our sense of touch is based primarily in the outer layer of skin called the epidermis. Nerve endings that lie in or just below the epidermis cells respond to various outside stimuli, which are categorized into four basic stimuli: pressure, pain, hot, and cold. Animals experience one or a combination of these sensations through a complex neural network that sends electrical impulses through the spinal cord to the cerebral cortex in the brain. The cerebral cortex, in turn, contains brain cells (neurons) arranged in columns that specialize in interpreting specific types of stimuli on certain parts of the body.

The sensation of touch begins with various receptors in the skin. Although these receptors appear to specialize in reacting to certain sensations, there is some debate concerning this specificity since most touch stimuli are a combination of some or all of the four major categories.

Scientists have identified several types of touch receptors. Free nerve ending receptors, located throughout the body at the bases of hair, are associated primarily with light pressure (such as wind ) and pain. Meissner corpuscles are nerve endings contained in tiny capsules and are found primarily in the fingertips and areas especially sensitive to touch (in the form of low-frequency vibrations), like the soles of the feet and the tongue. The Pacinian corpuscles look like the cross section of an onion and are found in deep tissues in the joints, the genitals, and the mammary glands . They are extremely sensitive to pressure and are also stimulated by rapid movement of the tissues and vibrating sensations. Ruffini endings, which are also located in the deeper layers of the skin, respond to continuous stimulation, like steady pressure or tension within the skin. Merkel disks, are found near the base of the epidermis and also respond to continuous stimulation or pressure. The skin also contains specific thermoreceptors for sensing hot and cold and nociceptors that identify high intensity stimulation in the form of pain.

Most, if not all of these receptors, are designed to adapt or become accustomed to the specific stimulation they interpret. In other words, the receptor does not continue to register a constant "feeling" with the same intensity as when it first begins and may even shut off the tactile experience. Imagine, for example, putting on a wool sweater over bare skin. The initial prickly sensation eventually abates, allowing the wearer to become accustomed to the feeling. Other examples include wearing jewelry such as rings, necklaces, and watches.

These receptors are also found in greater numbers on different parts of the body. For example, peoples' backs are the least sensitive to touch, while their lips, tongue, and fingertips are most sensitive to tactile activity. Most receptors for cold are found on the surface of the face while thermoreceptors for warmth usually lie deeper in the skin and are fewer in number. A light breeze on the arm or head is felt because there tend to be more sense receptors at the base of the hairs than anywhere else.

Touch and health

Touch has a tremendous impact on most animals' physical and psychological well being. Numerous studies of humans and other animals have shown that touch greatly impacts how we develop physically and respond to the world mentally. For example, premature babies that receive regular massages will gain weight more rapidly and develop faster mentally than those who do not receive the same attention. When baby rats are separated from their mothers for only 45 minutes, they undergo physiological or biochemical changes, specifically a reduction in a growth hormone. Touching of premature babies can also stimulate growth hormones (such as the hormone needed to absorb food) that occur naturally in healthy babies.

A baby does not have to be premature or sickly to benefit from touch. Even healthy babies show benefits from touch in terms of emotional stability. Difficult children often have a history of abuse and neglect. The reason is that touch serves as a type of reassurance to infants that they are loved and safe, which translates into emotional well being. In general, babies who are held and touched more tend to develop better alertness and cognitive abilities over the long run.

Touch continues to have a great psychological impact throughout peoples' lives. Even adults who are hospitalized or sick at home seem to have less anxiety and tension headaches when they are regularly touched or caressed by caretakers or loved ones. Numerous studies have shown that touch also has a healing power. Researchers have found that touch reduces rapid heart beats and irregular heart beats (arrhythmias). Another study showed that baby rats who are touched often during infancy develop more receptors to control the production of biochemicals called glucocorticoids, which are known as stress chemicals because of their ability to cause muscle shrinkage, high blood pressure, elevated cholesterol , and more.

Touch's psychological impact goes beyond physical and mental health. Researchers have shown that touch is a powerful persuasive force. For example, studies have shown that touch can have a big impact in marketing and sales. Salespeople often use touch to establish a camaraderie and friendship that can result in better sales. In general, people are more likely to respond positively to a request if it is accompanied by a slight touch on the arm or hand. In a study of waiters and waitresses, for example, those that lightly touched a patron often received better tips.

See also Perception.

Resources

books

Ackerman, Diane. A Natural History of the Senses. New York: Vintage Books, 1991.

Bennet, Thomas L. The Sensory World. An Introduction to Sensation and Perception. Monterey, CA: Brooks/Cole Publishing Company, 1978.

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

periodicals

Ponte, Lowell. "The Sense That Shapes Our Future." ReadersDigest (January 1992): 21-26.

Weider, Betty. "Therapeutic Touch." Shape (May 1992): 32.

David Petechuk

KEY TERMS

Biochemical

—The biological or physiological chemicals of living organisms.

Central nervous system

—The brain and spinal cord components of the nervous system, which controls activities of internal organs, movements, perceptions, thoughts, and emotions.

Cerebral cortex

—The external gray matter surrounding the brain and made up of layers of nerve cells and fibers, thought to process sensory information and impulses.

Epidermis

—The outer layer of the skin consisting of dead cells. It is the primary protective barrier against sunlight, chemicals, and other possible harmful agents. The epidermal cells are constantly being shed and replenished.

Free nerve endings

—Touch receptors in the skin that detect light pressure.

Glucocorticoids

—A steroid or hormone like compound that affects metabolism and can have an anti-inflammatory effect.

Meissner corpuscles

—Touch receptors in the skin that are sensitive to touch. Named after German histologist Georg Meissner.

Neurons

—Nervous system unit that includes the nerve cell, dendrites, and axons.

Pacinian corpuscles

—Touch receptors in the skin that sense pressure and rapid or vibrating movement of the tissues, named after Italian anatomist Filippo Pacini.

Touch

Touch is the sense that enables an organism to get information about things that are in direct contact with its body. As one of the five senses, touch allows a person to feel heat, cold, pain, and pressure. Touch is a very important sense, since it tells an organism a great deal about its immediate environment.

Touch is one of the least-thought about senses, perhaps because it does not involve a complex and highly specialized organ or specific organs that are identified with that sense. The eyes, ears, nose, and mouth are linked immediately and automatically with vision, hearing, smell, and taste. Yet the sense of touch is identified with the largest organ in the human body—the skin.

HOW THE SENSE OF TOUCH WORKS

Touch is detected by sensory nerves in the skin called "mechanoreceptors." These receptors are designed to respond to specific stimuli, because touch actually involves a variety of very different sensations. On the human body, there are well more than 500,000 sensory cells unevenly distributed over the skin's surface, and not all of them are alike. There are at least a half dozen different types of known touch receptor cells in humans. Some respond to light pressure, others to vibrations, and others to pain or temperature changes.

Each of these different types of receptors has a different structure. For example, capsule-like receptors called "Merkel's discs" are located in parts of the body that are extremely sensitive, such as the fingertips. As one would expect, there are far fewer of these type of cells where there is less need for them, such as on a person's leg or shoulders. Certain other parts of the body have uncovered nerve endings called "free endings." The skin around the hair roots has the largest amount of these free nerve endings.

In order for a person to feel sensation, one of these points on the skin must be stimulated in some way. When this happens, the receptor cells respond by generating a nerve impulse. The impulse is transmitted along pathways via the spinal cord to the brain. The nerves that carry impulses or information from the skin to the spinal cord are called spinal nerves. Each pair of these nerves transmits signals from a particular skin area. The first stop in the brain is the thalamus. The thalamus directs the signals to the appropriate part of the brain's cerebral cortex (the outer layer of gray matter covering the cerebrum) responsible for processing touch information. The brain then decides whether to pay attention to the signal and if so, how to respond to it. These steps all must happen instantaneously, since any sort of delay could mean serious injury or worse.

Little is known about the exact way that the brain interprets what is basically a coded impulse, which it then converts into the actual sensation that we experience or feel. The normal brain correctly lets people feel pain when they burn their finger, and pleasure when a sore muscle is massaged. It also allows people to gently squeeze a fruit and tell if it is ripe, sense an ant crawling up their leg, check a child's temperature with their hand, or "read" the raised dots of a Braille book. Touch also acts as an early warning system, informing people of serious or dangerous conditions before they encounter them fully.

THE SENSE OF TOUCH IN ANIMALS

The sense of touch is no less important for all other organisms. The variety found in the sense organs of animals is based on evolution according to the particular needs of the species. In other words, nature keeps what works best, even if it is a little different. Examples from nature make this point very well. The overly large ears of most rabbits best serve an animal whose first and only defense is a quick getaway. Rabbits' acute hearing sometimes gives then a slight edge over predators. As for the sense of touch, certain animals rely more on this sense than others. An earthworm that spends its life crawling about and has a minimal amount of specialized organs has special hairs that generate a nerve impulse when they contact something. Some insects have similar hairs on their legs and antennas that "touch" the air and sense changes in currents. This is why it is so difficult to swat a housefly, which knows what is coming by the moving air it senses.

Fish also have a sense of touch founded in their lateral line. These are nerves running lengthwise on their bodies that allow them to detect the presence of nearby organisms or objects. The tongues and beaks of many birds have a well-developed sense of touch. The whiskers of a cat or a rat enable them to "feel" things with their long bristles almost in the way humans do with their fingertips.

For all organisms, including humans, touch allows us to coordinate bodily movements. Because of touch, people unconsciously sense where their legs, arms, head, eyes, muscles, and internal organs are. Touch disorders can range from the minor but troublesome itching caused by allergies, diseases, infections, or bites. Numbness or pins-and-needles can signal a more serious nerve injury or can result from a blood clot, tumor, heart attack, or stroke. Chronic pain, which has no explanation, is another type of touch disorder.

THE BENEFITS OF TOUCH

Science has only recently realized that touching and being touched has important psychological and physical advantages, especially to mammals. It is now known that the stimulation of the skin an infant receives by being held while feeding is as important as the food. Studies have shown that in cases where mammals, like a cow or horse, lick their newborn, licking the skin actually stimulates the nervous system and gets it working. Newborn animals that are not licked usually die, because their intestinal or urinary systems never begin working. Human babies that were never held often display signs of "failure to thrive." Finally, humans have a psychological need to touch and be touched. For example, studies have shown that seniors who live alone but have an animal to care for and "pet," are in better health and spirits than those who do not.

[See alsoOrgan; Sense Organ ]