Pain

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Chapter 3 Sensation and Perception
Sensing Your Body
䉴 Which is the largest organ in my body?
Some senses are not located in a specific organ, such as the eye or the ear. These are
the somatic senses, also called somatosensory systems, which are spread throughout the
body. The somatic senses include the skin senses of touch, temperature, and pain, as
well as a body sense, called kinesthesia, that tells the brain where the parts of the body
are. Kinesthesia is closely related to our sense of balance. Although balance is not strictly
a somatosensory system, we describe it here.
Touch and Temperature
TOUCH AND VISION Just
as
taste and smell interact, so
by
do touch and vision. To experience an example of this interaction, ask
someone sitting across a table from you
to stroke the tabletop while stroking
your knee under the table in exactly the
same way, in exactly the same direction.
If you watch the person’s hand stroking
the table, you will soon experience the
touch sensations coming from the table,
not your knee! If the person’s two hands
do not move in synch, however, the illusion will not occur.
doing
2
learn
People can function and prosper without vision, hearing, or smell. But a person without a sense of touch would have difficulty surviving. Without this sense, you could not
even swallow food, because you could not tell where it was in your mouth and throat.
You receive touch sensations through your skin, which is the body’s largest organ. The
skin covers nearly two square yards of surface area, weighs more than twenty pounds,
and has hair virtually everywhere on it. The hairs on your skin do not sense anything
directly. However, when the hairs are bent, they push against the skin beneath them.
Receptors in, and just below, the skin send the “touch” message to the brain.
The sense of touch codes information about two
aspects of an object contacting the skin: its weight and its location. The intensity of the
stimulus—how heavy it is—is coded both by the firing rate of individual neurons and
by the number of neurons stimulated. A heavy object triggers a higher rate of firing
and stimulates more neurons than a light object. The brain “knows” where the touch
occurs based on the location of the nerves that sense the touch information.
Coding Touch Information
Adapting to Touch Stimuli Continuous input from all your touch neurons would
provide a lot of unnecessary information. Once you get dressed, you do not need to be
constantly reminded that you are wearing clothes. Thanks in part to the process of
adaptation described earlier, you do not continue to feel your clothes against your skin.
Changes in touch (as when your belt or shoe suddenly feels loose) provide the most
important sensory information. The touch sense emphasizes these changes and filters
out the excess information. How? Typically, a touch neuron responds with a burst of firing when a stimulus is applied, then quickly returns to its baseline firing rate, even
though the stimulus may still be in contact with the skin. If the touch pressure increases,
the neuron again responds by increasing its firing rate, then slowing down. A few neurons adapt more slowly, however, continuing to fire as long as pressure is
learn applied. By attending to this input, you can sense a constant stimulus (try
by
doing doing this by focusing on touch sensations from your glasses or shoes).
2
Sensing Temperature Some of the skin’s sensory neurons respond to a change in
temperature but not to simple contact. “Warm fibers” and “cold fibers” respond to specific temperature changes only. However, many fibers that respond to temperature also
respond to touch, so these sensations sometimes interact. For example, if you touch an
object made up of alternating warm and cool sections, you will have the sensation of
intense heat (Thunberg, 1896, cited in Craig & Bushnell, 1994).
Pain
somatic senses Senses including touch,
temperature, pain, and kinesthesia that
are spread throughout the body rather
than located in a specific organ. Also
called somatosensory systems.
Touch can feel pleasurable, but if the intensity of touch stimulation increases too much,
it can turn into a pain sensation. Pain tells you about the impact of the world on your
body. It also has a distinctly negative emotional component that interrupts whatever
you are doing (Eccleston & Crombez, 1999).
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Sensing Your Body
A LIFE WITH NO PAIN Ashlyn
Blocker, shown here at age 5 being
checked for injuries, was born with a rare
genetic disorder that prevented the development of pain receptors. As a result,
she feels no pain if she is cut or bruised,
if she bites her tongue while eating, or
even if she is burned by hot soup or a hot
stove. She only knows she has been injured if she sees herself bleeding, so she
will have to find ways to protect herself
from danger without the vital information provided by the pain system. Ashlyn
doesn’t yet understand the seriousness of
her condition, but her worried mother
says “I would give anything for her to
feel pain” (Associated Press, 2004).
Pain as an Information Sense The information-carrying aspect of pain is very
similar to that of touch and temperature. The receptors for pain are free nerve endings, which come from the spinal cord, enter the skin, and then simply end. Painful
stimuli cause the release of chemicals that fit into these specialized receptors in pain
neurons, causing them to fire. The axons of pain-sensing neurons release neurotransmitters not only near the spinal cord (thus sending pain information to the brain) but
also near the skin (causing inflammation).
Two types of nerve fibers carry pain signals from the skin to the spinal cord. A-delta
fibers carry sharp, pricking pain sensations; C-fibers carry continuous, dull aches and
burning sensations. When you stub your toe, for example, that immediate wave of
sharp, intense pain is signaled by A-delta fibers, whereas that slightly delayed wave of
gnawing, dull pain is signaled by C-fibers. When pain impulses reach the spinal cord,
they form synapses with neurons that relay the pain signals to the thalamus and other
parts of the brain. Different pain neurons are activated by different types and degrees
of painful stimulation (Ploner et al., 2002).
There are specific pathways that carry the emotional
component of a painful stimulus to areas of the hindbrain, reticular formation, and
cortex via the thalamus (Johansen, Fields, & Manning, 2001). However, our overall
emotional response to pain depends greatly on how we think about it (Spinhoven
et al., 2005; Wager, 2005). In one study, some participants were told about the kind of
painful stimulus they were to receive and when to expect it. Others were not informed.
Those who knew what to expect objected less to the pain, even though the sensation
was reported to be equally noticeable in both groups (Mayer & Price, 1982). People can
lessen their emotional responses to pain by using pain-reducing strategies (such as distracting thoughts), especially if they expect these strategies to succeed (Bantick et al.,
2002). Scientists are also developing special biofeedback systems that may someday
allow patients to relieve chronic pain by reducing activity in the brain regions involved
in pain perception (deCharms et al., 2005).
Emotional Aspects of Pain
Modulating Pain: The Gate Control Theory Pain is useful because it can pro-
tect you from harm. There are times, though, when enough is enough. Fortunately, the
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Chapter 3 Sensation and Perception
The stress of
athletic exertion causes the release of endorphins, natural painkillers that have
been associated with pleasant feelings
known as “runner’s high.”
NATURAL ANALGESIA
nervous system has several mechanisms for controlling the experience of pain. One theory about how these mechanisms work is called the gate control theory (Melzack &
Wall, 1965). This theory suggests that there is a “gate” in the spinal cord that either allows
pain signals to reach the brain or stops them. Some details of the original theory were
incorrect, but more recent work supports the idea that natural mechanisms can indeed
block pain sensations (Stanton-Hicks & Salamon, 1997; Sufka & Price, 2002).
For example, input from other skin senses can come into the spinal cord at the same
time the pain gets there and “take over” the pathways that the pain impulses would
have used. This appears to be the reason we can temporarily relieve pain by rubbing
the skin around a wound or using creams that produce temperature sensations. It also
helps explain why scratching relieves itching; itchy sensations involve activity in fibers
located close to pain fibers (Andrew & Craig, 2001).
The brain itself can close the gate to pain impulses by sending signals down the
spinal cord. These messages from the brain block incoming pain signals at spinal cord
synapses. The result is analgesia (pronounced “ann-nuhl-JEE-zhah”), a reduction in
pain sensation in the presence of a normally painful stimulus. Drugs that dull pain sensations, such as aspirin, are called analgesics.
Natural Analgesics As described in the chapter on biology and behavior, natural
gate control theory
A theory suggesting the presence of a “gate” in the
spinal cord that either permits or
blocks the passage of pain impulses
to the brain.
analgesia Reduction in the sensation
of pain in the presence of a normally
painful stimulus.
opiates called endorphins play a role in the brain’s ability to block pain signals. Endorphins are natural painkillers that act as neurotransmitters at many levels of the pain
pathway. In the spinal cord, for example, they block the synapses of the fibers that carry
pain signals. Endorphins may also relieve pain when the adrenal and pituitary glands
secrete them into the bloodstream as hormones. The more endorphin receptors a person has inherited, the more pain tolerance that person has (Kest, Wilson, & Mogil, 1999;
Uhl, Sora, & Wang, 1999).
Several conditions can cause the body to ease its own pain. For example, endorphins
are released where inflammation occurs (Cabot, 2001). During the late stages of pregnancy,
a spinal cord endorphin system develops to reduce the mother’s labor pains (DawsonBasoa & Gintzler, 1997). An endorphin system is also activated when people believe they
are receiving a painkiller, even when they are not (Colloca & Benedetti, 2005; Zubieta
et al., 2005); this may be one reason for the placebo effect, discussed in the introductory
chapter (Stewart-Williams, 2004). Interestingly, the resulting pain inhibition is experienced
in the part of the body where it was expected to occur, but not elsewhere (Benedetti,