The vestibular organ and kinesthetic receptors help orient us, even in unusual situations.
the disorientation and dizziness of seasickness resemble the dizziness caused by poisoning. The body apparently vomits in response to dizziness regardless of the cause, just in case it is due to poisoning (Stern & Koch, 1996).
Kinesthetic Sense. Throughout the skin, muscles, joints, and tendons are kinesthetic receptors, that signal when they are moved. As the body walks, bends, and writes, these recep- tors provide information about the location and movement of each part of the body. Close your eyes, take off your shoes, and wiggle your toes. You can tell they are wiggling because of your kinesthetic sense. Unlike the vestibular organ, the kines- thetic receptors are individual receptors that are not clumped together into sense organs. The kinesthetic receptors provide detailed information on the orientation of the head and body, differences in pressure due to gravity and movement on dif- ferent parts of the body, the movement of each body part, and a host of other kinds of information (Gray, 2008; Sholl, 2008). As refl ected in the skilled movements of a musician, painter, or discus thrower, they are remarkably sensitive, allowing fi ne and complicated patterns of movement.
Skin Senses
We usually do not think of the skin as a sense organ, yet it’s capable of picking up many different kinds of sensory information. The skin can detect pressure, tempera- ture, and pain. Feeling a kiss on the cheek, cold in the winter, and all other sensa- tions involving the skin are made up of combinations of these three skin sensations.
Although the skin can detect only three kinds of sensory information, it has at least four types of receptors: the free nerve endings, the basket cells wound around the base of hairs, the tactile discs , and the specialized end bulbs . These are shown in fi gure 5.16 . It appears that all four types of receptors play a role in the sense of touch (pressure), with the free nerve endings being the primary receptors for temperature and pain (Hole, 1990).
Specialized end bulbs Tactile discs
Basket cell around hair
Free nerve endings
Hair
Figure 5.16
Diagram of the skin showing the major skin receptor cells.
free nerve endings Sensory receptor cells in the skin that detect pressure, temperature, and pain.
basket cells Sensory receptor cells at the base of hairs that detect pressure.
tactile discs (tak´til)Sensory receptor cells that detect pressure.
specialized end bulbs Sensory receptor cells that detect pressure.
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Pressure and Sensitivity. The skin is amazingly sensitive to pressure, but sensi- tivity differs considerably from one region of the skin to another depending on how many skin receptors are present. In the most sensitive regions—the fi ngertips, the lips, and the genitals—a pressure that pushes in the skin less than 0.001 mm can be felt, but sensitivity in other areas is considerably less (Schiffman, 1976). Perhaps the most striking example of the sensitivity of the skin is its ability to “read.” Many blind peo- ple can read books using the Braille alphabet, patterns of small raised dots that stand for the letters of the alphabet. An experienced Braille user can read up to 300 words per minute using the sensitive skin of the fi ngertips (see fi gure 5.17 ).
Temperature. When it is hot or cold outside, how do you sense this fact? It seems to most of us that the entire surface of the skin is able to detect temperature, but we actually sense skin temperature only through sensory receptors located in rather widely spaced “spots” on the skin. One set of spots detects warmth and one detects coldness.
The information sent to the brain by these spots creates the feeling of temperature across the entire skin surface.
When the skin is warmed (for example, by air, sunlight, or water), the recep- tors in the warm spots send messages about warmness to the brain; when the skin is cooled, the cold spots send messages about coldness. Recall from chapter 1 (p. 6) that the sensation of intense heat is created by stimulation of both the warm and cold spots. Although the cold receptors are generally responsive only to cold temperatures, extreme heat also makes them fi re. Therefore, high temperatures stimulate the recep- tors in both sets of spots to send messages simultaneously to the brain, which are interpreted as hotness.
Pain
Everyone experiences pain some of the time. Toes get stepped on, fi ngers are cut, and ankles are twisted from time to time. Even though it is unpleasant, pain is the useful signal that something bad has happened to a part of the body and it needs our attention.
What are the neural systems that underlie the experience of pain?
Free nerve endings throughout the body serve as nocioceptors —receptors for stimuli that are experienced as painful (Perl, 2007). Neural messages from the nocio- ceptors are transmitted to the brain along two distinct nerve pathways— rapid and slow neural pathways. This is why we often experience “fi rst and second pain” (Melzack &
Wall, 1983; Sternbach, 1978). The fi rst pain sensation is a clear, localized feeling that does not “hurt” much, but it tells us what part of the body has been hurt and what kind of injury occurred. The second pain is a more diffuse, long-lasting pain that hurts in the emotional sense. When you cut your fi nger with a knife, an initial sensation tells you that you have been cut and where the cut has occurred, followed a moment later by a more diffuse and painful sensation. The fi rst sensation makes you drop the knife and grab your fi nger; the second makes you jump up and down and scream!
There are two reasons that we experience these two somewhat separate pain sen- sations in sequence. First, the two sensations travel on different neural pathways that have different speeds of transmission. The rapid pathway neurons are thicker and sheathed in myelin (see p. 51), which speeds transmission. The slow pathway neurons, in contrast, are smaller and slower, unmyelinated neurons. The second reason that we experience fi rst and second pain is that the two neural pathways travel to different parts of the brain. The rapid pathway travels through the thalamus to the somatosensory area. If you recall from chapter 3 (p. 62), this is the part of the parietal lobe of the cere- bral cortex that receives and interprets sensory information from the skin and body.
When the information transmitted to this area on the rapid pathway is interpreted, we know what has happened and where it has happened, but the somatosensory area
A B C D E
F G H I J
K L M N O
P Q R S T
Z
U V W X Y
Figure 5.17
Raised dots used in the Braille alphabet are “read” with the fi ngertips.
Robots are increasingly important in industry. Using what is known about the sense of touch, the inventors of the “Salisbury Hand” have given it kinesthetic sensors that simulate tension on the tendons of the hand and stimulation of the fi ngertips.
(Photo of Salisbury Hand at MI AI lab courtesy of David Lampe, MIT)
nocioceptors (no¯ ´´-see-oh-sep´-turs) Receptors for stimuli that are experienced as painful.
does not process the emotional aspects of the experience of pain. Information that travels on the slower pathway is routed to the limbic system (p. 63). Here, in the brain system that mediates emotion, the emotional “ouch” part of the experience of pain is processed.
Pain Gates. Pain involves much more than the sim- ple transmission of neural messages from nocioceptors to the brain. There is not a direct relationship between the stimulus and the amount of pain experienced. Under certain circumstances, pain messages can even be sup- pressed. For example, a football player whose attention is focused on a big game may not notice a painful cut until after the game is over. The pain receptors transmit the pain message during the game, but the message is not fully processed by the brain until the player is no longer concentrating on the game (Keefe & France, 1999).
Pain signals are regulated in three parts of the nervous system: the brain stem, the spinal cord, and in the peripheral pain receptors (Melzack, 1999; Perl, 2007):
1. Regulation of pain in the brain stem. A matrix of neurons in the brain stem regulates the transmission of impulses from the nocioceptors to the cerebral cortex (see fi gure 5.18 ) (Christie & Mallet, 2009; Melzack & Wall, 1983). All messages from the body’s nocioceptors travel to the brain through the brain stem. The slow- pain neural fi bers pass through “pain gates” in the brain stem that can be either
“opened” or “closed.” That is, the pain gates can make us more or less sensitive to stimulation of the nocioceptors. The pain gates can be infl uenced to allow more slow-pain neural transmission along the slow-pain pathways to the limbic system.
When pain messages are received from nocioceptors—say, from a bruised toe—they sensitize the pain gates and make them transmit slow-pain impulses more readily.
This is because the neuropeptide involved in slow-pain transmission—called sub- stance P (for pain)—sometimes diffuses across to nearby neurons in the brain stem that ordinarily do not carry pain messages, causing them to transmit pain impulses, too (Hopkins, 1997; Kobayashi & others, 2010).
Fortunately, the pain gates also can be “closed” to make them less likely to transmit slow-pain impulses to the limbic system. For example, placing a sore foot in warm water helps close the gate on pain from the toe. Similarly, sweet tasting liquids and sweet aromas associated with them can reduce the sensation of pain (Prescott &
Wilkie, 2007). Even looking at the part of the body that hurts in a mirror reduces the sensation of pain by closing the pain gates (Longo & others, 2009).
The pain gates appear to be operated by specialized neurons that block trans- mission in the neurons that carry “second pain” messages to the brain. As shown in fi gure 5.18 , the gate neurons inhibit the pain neurons using substances called endor- phins. When signaled by other sensory neurons or by neural fi bers from the cortex to close the pain gate, the gate neuron inhibits the pain neuron and stops the pain message from reaching the brain.
Some pain-killing drugs, such as the opiate morphine, operate by duplicating the effects of the endorphins in inhibiting pain neurons (Perl, 2007). Indeed, the term endorphin means endogenous (produced inside the body) morphine.
The endorphins may explain why acupuncture is successful for some indi- viduals. Pain often can be reduced by this procedure. In acupuncture, needles are inserted in the skin at special points and then twirled or heated. There is little doubt that some persons receiving acupuncture experience less pain, but the question is why. One possibility is that the needles indirectly stimulate the production of endor- phins, which block the pain in the pain gates (Goldman & others, 2010). To test this
Guy Gertsch, who fi nished the 1982 Boston Marathon in a respectable 2 hours and 47 minutes, discovered at the fi nish line that he had run the last 19 miles with a broken thigh bone.
A woman is being treated for pain by an acupuncturist. One theory for the effectiveness of acupuncture is that the needles stimulate the release of pain-blocking endorphins.
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hypothesis, the drug naloxone, which blocks the action of en dorphins, was admin- istered to persons undergoing acupuncture. While the en dorphin-blocking drug was active, the person receiving acupuncture experienced the normal level of pain (Price, 1988). This suggests that acupuncture may close the pain gates by stimulating the endorphins.
2. Regulation of pain in the spinal cord. Linda Watkins and Steven Maier (2003) of the University of Colorado have provided evidence that glial cells regulate the transmission of pain messages in the spinal cord. As described in the last chapter, some glial cells surround the synaptic gap and infl uence the likelihood that a neural signal will cross the gap. One function of glial cells is to regulate the transmission of pain signals in this way. For example, glial cells increase the transmission of pain signals when viral or bacterial infections are in the body (causing you to ache when you have the fl u). In addition, several neurotransmitters cause glial cells to increase the transmission of pain signals, raising the possibility that stress may infl uence pain sensations in this way (Watkins & Maier, 2003).
3. Peripheral regulation of pain. Under some circumstances, nocioceptors outside the spinal column can be made more sensitive to the stimuli that we sense as painful. If you have been cut on the hand, you may notice that even a light touch near
Somatosensory area of cortex
Pathway of fast-pain fibers Limbic system
Pathway of slow-pain fibers Area of pain gates
Direction of pain message
Endorphin Endorphin receptor Neurotransmitter molecules in axon of slow- pain neuron Stimulation of endorphin receptors inhibits firing of axon of slow-pain neuron.
Close-up view of inhibitory pain gates Neuron in
slow-pain fiber
Inhibitory pain gate neuron
Axon of inhibitory pain gate neuron
Figure 5.18 The operation of inhibitory pain gates. Secretion of endorphins by inhibitory gate neurons inhibits the fi ring of the axon of the neuron that transmits the pain message. Pain gate neurons regulate only the transmission of slow-pain fi bers and are located primarily in the brain stem and spinal cord. Unfortunately, other pain gate neurons can make transmission of slow-pain messages to the limbic system more likely under some circumstances.
the cut feels painful. The infl ammation (swelling) around the cut leads to the sensiti- zation of nocioceptors near the cut, which results in the light touch being experienced as painful. This occurs in two ways: First, infl ammation makes nocioceptors them- selves so sensitive that they fi re even when stimulated by a light touch. Second, the release of substance P can turn nearby nerve endings that normally play a role in the sense of touch into nocioceptors (Hopkins, 1997). Usually, this sensitization of nerve endings reverses itself as the wound or bruise heals, but for persons with arthritis and other chronic infl ammatory diseases, the phenomenon of peripheral sensitization can be debilitating. Researchers hope that our growing understanding of the three levels of pain regulation will lead to new ways to control pain.
Phantom Limbs. A sad and curious phenomenon often occurs when persons have lost limbs. Amazingly, many amputees experience their missing arm or leg as if it were still there. They feel a missing arm, for example, as if it were hanging by their side when they sit still, and swinging in coordination with their other arm and legs when they walk. This phantom limb is experienced not as a memory of the lost limb but as a clear and realistic sensation that the missing limb is actually there (Melzack, 1992).
Over 60% of amputees experience pain in the phantom limb (Bosmans & others, 2010).
Phantom-limb pain is especially common in women and when the limb was an arm (Bosmans & others, 2010). A friend of mine recently wrote to say: “My mother, who lost her left leg to polio in her early 20s, is now 73, and sometimes when I ask how her arthritis is, she often responds, ‘the foot I don’t have aches as much as my good one.’”
Fortunately, phantom-limb pain often improves over time (Bosman & others, 2010).
How is it possible to feel sensations from a limb that does not exist and, therefore, cannot be transmitting sensations to the brain? A team of researchers from Germany and the United States appears to have provided the answer: When sensory and pain neurons from one part of the body have been cut, the area of the somatosensory cortex that served that part of the body becomes sensitive to input from parts of the body that activate nearby portions of the somatosensory cortex (Flor & others, 1995). For exam- ple, in a woman whose left arm was amputated, the portion of somatosensory cortex that formerly served her left arm may begin to receive input from her face. (Look back to fi gure 3.15, p. 67, to see that the area of the somatosensory cortex that serves the face is next to the area that receives input from the arm.) In addition, cutting sensory neurons from one part of the body tends to reduce the effi ciency of the pain gates. This suggests that phantom pain that is perceived as being in a missing arm may come from minor irritations to the face that are allowed through the pain gate and are perceived to be pain in the missing limb, because the neural message stimulates the part of the cortex that used to serve input from the arm. The phantom-limb experience is another illustration of the fact that our conscious experience is not always a direct and simple representation of the sensory information that reaches the brain.
Human Diversity
Culture and Pain
In this chapter, we examine the ways in which neural impulses from the sense organs are experienced as sensations and perceptions. Although much of this process is deter- mined by the biological nature of the sense organs and neurons, learning experiences in our cultures apparently can infl uence even basic sensations such as pain.
Let’s consider an example of the impact of culture on the perception of pain.
Members of the Bariba society in Benin, West Africa, appear to be able to tolerate pain more easily than members of most cultures. Bariba folklore includes many examples of honored people who showed strength in the face of pain, and this calm response to pain is seen as an integral part of Bariba pride (Sargent, 1984). For example, pregnant women are expected not to reveal to others that they are experiencing labor pains.
When labor becomes advanced, they leave the company of others to go through labor and childbirth alone, calling for help only with cutting the umbilical cord.
Continued on pg. 143
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The body contains a number of sense organs that provide vital information about the body’s movement and orientation in space and about the world that contacts our skin. Information about posture, movement, and orientation is coded and sent to the somatic sensory area of the cortex by the vestibular organ in the inner ear and by kinesthetic receptors spread throughout the body. Skin receptors send information about temperature, pressure, and pain to the same area of the brain. We experience the pain from cuts and other injuries in two steps: a fi rst sensation of pain tells us what has happened and where it has happened, followed by a second, more emotional pain. These two aspects of the pain experience travel on different neural pathways to different parts of the brain. The phenomenon of pain pro- vides a good example of the lack of direct relationship between physical stimuli and con- scious sensations in that a number of factors increase or decrease the experience of pain.
The phantom limb experience also provides compelling evidence that conscious experiences are constructed in the brain and do not always have a direct relationship to incoming sensa- tions. Even our culture-based learning experiences infl uence our perception of pain.
Review
Continued from pg. 142 To the Bariba, letting other people see that they are in pain is cause for great shame. When discussing pain, many quote a Bariba proverb that trans- lates to “Between death and shame, death has the greater beauty.” According to a Bariba physician, an individual who displays pain lacks courage, and cowardice is the essence of shame. Rather than live in shame, a Bariba would rather die (Sargent, 1984).
In this cultural context, one would do everything possible to avoid displaying signs of pain.
Do Bariba women who are in labor actually experience less pain than women in other cultures do, or have they simply learned not to let the pain show? It is diffi cult to answer such questions, partly because of the diffi culties involved in describing pain to another person. Because pain is a private experience, language must be used to communicate the experience to others, and language is shaped by culture. It is not surprising that there is a more limited vocabulary for describ- ing pain in the Bariba language than in most other languages. When the Bariba discuss the experience of pain, therefore, it is diffi cult to know how much their description is infl uenced by their language.
There is some reason to believe that the cultural emphasis on not show- ing reactions to pain might actually reduce the amount of pain that the Bariba experience. As noted in chapter 11, there is evidence that facial expressions are an important part of the experience of pain (Izard, 1977). Apparently, sensory feedback to the brain from facial muscles supplies part of the neural input for the perception of pain (along with input from the part of the body that is cramped or injured). Indeed, persons who were given electrical shocks reported less pain when they were told to make no facial reactions than when they let their emotions show in their faces (Colby, Lanzetta, & Kleck, 1977). Maybe the calm face of a Bariba woman in labor results in the experience of less pain than does the agonized grimace of women in other cultures.
According to Linda Garro (1990), medical professionals who work with people in pain must understand the impact of culture on the expression of pain. If culture is not taken into account, the physician may overestimate or underestimate the amount of pain patients experienced. However, professionals must also remember that not all members of a culture are the same: as in all other aspects of human diversity, one must be aware of variation within cultures.
What did you learn about pain in your own culture? Were you taught to minimize pain because it is important to be tough? Did you learn that no one will pay attention to your pain unless you exaggerate it? How do you respond when a parent or your friends are in pain? Such questions help you think about cultural infl uences on perception.
Human Diversity
Culture and Pain
cont’d from pg. 142