Download I. Pain and the Nervous System

I.
Pain and the Nervous System
• All sensory stimulation, including pain, starts with
activation of sensory neurons and proceeds with the
relay of neural impulses toward the brain.
A. Somatosensory System
• The somatosensory system conveys sensory
information from the body through the spinal
cord to the brain.
1.
Afferent Neurons
▫ Afferent (sensory) neurons convey sensory
information from sense organs to the spinal cord
and then to the brain.
▫ Primary afferents are those neurons that have
receptors in the sense organs and that originate
action potentials, the discharge of the neuron’s
electrical message.
▫ The vast number of neurons and their
interconnections makes neural transmission
complex.
2.
Involvement in Pain
▫ Nociceptors are neurons capable of sensing pain
stimuli. Three different types of neurons are
involved with transmitting pain impulses.
▫ The large A-beta fibers and smaller A-delta fibers
are covered with myelin, which speeds neural
transmission.
▫ The smaller and more common C fibers require
high levels of stimulation to fire.
▫ These different fibers with their different thresholds
and transmission speeds may relate to different
types of pain sensation.
B. The Spinal Cord
• Primary afferents from the skin enter the spinal cord
where they synapse with transmission cells
(secondary afferents) in the dorsal horns of the
spinal cord. The dorsal horns contain several
laminae (layers).
• Laminae 1 and laminae 2 form the substantia
gelatinosa, a structure that receives sensory input
from the A and C fibers (see Figure 7.1).
B. The Spinal Cord
• Complex interactions of sensory input occur in the
laminae of the dorsal horns, and these interactions
may affect the perception of sensory input before it
gets to the brain.
C. The Brain
• The thalamus receives sensory input from the
different neural tracts in the spinal cord.
• The skin is mapped in the somatosensory cortex in
the parietal lobe of the cerebral cortex, and the
proportion of cortex devoted to an area of skin is
proportional to that skin’s sensitivity to
stimulation (see Figure 7.2).
C. The Brain
• Sensory information from internal organs are not
mapped as precisely as the skin, leading people to
have the ability to identify stimulation from the
skin but less distinct sensory perceptions of their
internal organs.
D. Neurotransmitters and Pain
• The neurotransmitters that form the basis for neural
transmission also play a role in pain perception. The
discovery of the endogenous opiates—enkephalin,
endorphin, and dynorphin—led to the discovery of
neural receptors specialized for these neurotransmitters
and the conclusion that opiate drugs produce analgesia
because of the brain’s own chemistry.
• The neurotransmitters glutamate and substance P and
the chemicals bradykinin and prostaglandins may
exacerbate pain stimulation.
D. Neurotransmitters and Pain
• Proinflammatory cytokines produced by the
immune system are also involved in pain, possibly
creating chronic pain by sensitizing neurons in the
spinal cord.
E. Modulation of Pain
▫ When the periaqueductal gray, a structure in the
midbrain, is stimulated, pain relief occurs.
▫ The neurons in the periaqueductal gray synapse
with neurons in the nucleus raphé magnus, a
structure in the medulla (see Figure 7.3).
▫ These neurons descend to the spinal cord and may
constitute a descending control system for pain
perception.
II. The Meaning of Pain
• The traditional view of pain focused on the physical
sensations, but about 100 years ago,
• C. A. Strong proposed that pain consists of not only
the sensation but also person’s reaction to that
sensation.
A. Definition of Pain
• Perhaps the most acceptable definition of pain is the
one proposed by the International Association
Subcommittee for the Study of Pain that saw pain as
an unpleasant sensory experience accompanied by an
emotional experience and actual or potential tissue
damage.
A. Definition of Pain
• At least three stages of pain have been identified.
▫ Acute pain is ordinarily adaptive, lasts a relatively
short period of time, and includes pain from cuts,
burns, and other physical trauma.
▫ Chronic pain endures beyond the time of normal
healing, is relatively constant, is often reinforced by
other people, and may become self-perpetuating.
▫ Prechronic pain is experienced between acute and
chronic pain and is critical because during this time
the pain may either go away or evolve into chronic
pain.
B. The Experience of Pain
▫ Pain is both personal and subjective, but situational and
cultural factors play a role in its experience.
▫ An early example of the importance of situational
factors was chronicled by Beecher, who observed
wounded soldiers during World War II.
▫ Many soldiers with serious battle injuries reported very
little pain and. in fact, were quite cheerful and
optimistic.
B. The Experience of Pain
▫ Beecher reasoned that these men had been removed
from the battlefront and thus from the threat of death or
further injury.
▫ Ten years later Beecher found that injured civilians
experienced more pain and requested more pain-killing
drugs than did the wounded World War II soldiers, even
though the civilians' injuries were less severe.
▫ Later studies have confirmed that pain is a variable
experience affected by the context and culture of the
person.
C. Theories of Pain
• How people experience pain is the subject of a
number of theories.
• Of the several models of pain, two capture the
divergent ways of conceptualizing pain: the
specificity theory and the gate control theory.
1.
Specificity Theory
▫ The specificity theory can be traced to Descartes, who
hypothesized that the body works by mechanistic
principles.
▫ Applied to pain, this theory holds that pain is the result
of transmission of specific signals.
▫ Research has failed to find pain receptors or fibers
specifically devoted to pain transmission.
▫ This theory also fails to integrate the variability of the
pain experience.
2.
The Gate Control Theory
▫ Melzack and Wall formulated the gate control theory
of pain as a way to explain the variability of pain
perception (see Figure 7.4).
▫ They hypothesized that a gating mechanism exists in
the spinal cord and that sensory input is modulated
in the substantia gelatinosa of the dorsal horns of
the spinal cord.
▫ This modulation can change pain perception, as can
brain-level alterations from a hypothesized central
control trigger.
2.
The Gate Control Theory
▫ This theory includes explanations of both
physiological and psychological modulations of the
pain experience.
▫ Melzack has proposed an extension to the gate
control theory, called neuromatrix theory, which
places a stronger emphasis on the brain’s role in
pain perception.
III. The Measurement of Pain
• Tools for measuring pain are important in order to
evaluate the various pain therapies.
• A number of techniques have been used to measure
laboratory and clinical pain, and these fall into three
main categories: self-reports, behavioral assessments,
and physiological measures.
A. Self-Reports
• Self-reports of pain include simple rating scales,
standardized pain inventories, and standardized
personality inventories.
1.
Rating Scales
▫ With self-report rating scales, patients rate the intensity
of their pain on a scale; for example, the scale may
range from 1 to 100.
▫ A similar technique is the Visual Analog in which
patients check severity of pain on a continuum from no
pain to worst pain imaginable.
2.
Pain Questionnaires
▫ Melzack developed the McGill Pain Questionnaire
(MPQ), an inventory that categorized pain into three
dimensions: sensory, affective, and evaluative. The
sensory dimension includes pain described in terms of
its temporal, spatial, pressure, and thermal properties;
the affective dimension defines pain in terms of fear,
tension, and autonomic properties of the pain
experience; and the evaluative dimension includes the
perceived severity of the entire pain experience. The
MPQ has adequate validity, but its vocabulary is
somewhat difficult.
2.
Pain Questionnaires
▫ The West Haven-Yale Multidimensional Pain Inventory
(MPI) is another questionnaire that measures several
aspects of pain.
▫ Despite early promise, research revealed problems with
the classification of pain patients into categories of
dysfunctional, interpersonally distressed, and adaptive
copers. Rather than stable categories, pain patients
change categories, making this inventory less useful in
assessing patients’ pain.
3.
Standardized Psychological Tests
▫ Standardized tests, such as the MMPI and the MMPI-2,
have also been used to assess pain.
▫ Both instruments are useful in differentiating among
types of pain patients and have some ability to predict
which patients will respond to medical treatments for
pain.
▫ Other commonly administered tests include the Beck
Depression Inventory and the Symptom Checklist-90.
B. Behavioral Assessment
• People in pain often behave in ways that communicate to
others that they are suffering from pain.
• Spouses and others close to pain patients can be trained to
make observations of pain behaviors.
• These observations are sometimes compared to the
patient's own pain diary.
• Trained observers have also been used to assess pain
behaviors in both clinical and laboratory settings.
• The most frequent behaviors include guarded movement,
bracing, position shifts, partial movement, grimacing,
limitation statements, and emitting pain sounds.
C. Physiological Measures
• Although pain produces an emotional response,
research has failed to identify specific organic states
that are correlated with pain.
• Muscle tension and autonomic responses such as
heart rate and skin temperature show some
relationship to the experience of pain, but neither
type of measurement shows sufficient reliability and
validity to be a good measurement technique.
IV.
Pain Syndromes
• Pain can be classified according to location or
syndrome, symptoms that occur together and
characterize a condition.
• Headache and low back pain are the two most
frequently treated types of pain, but health
psychologists also deal with other pain syndromes.
A. Headache Pain
• Headache pain is the most common of all pains, with
more than 99% of Americans suffering from some
form of headache over their lifetime.
• The most common varieties are migraine, tension,
and cluster headaches, although the symptoms
overlap and clear classification is often not possible.
A. Headache Pain
• Migraine (or vascular) headaches bring about loss
of appetite, nausea, vomiting, and increased
sensitivity to light.
• Tension headaches are muscular in origin and are
characterized by contractions of the muscles of the
neck, shoulders, scalp, and face.
• Cluster headaches produce intense pain localized
in one side of the head and occur frequently over a
period of days, then disappear for weeks or
months.
B. Low Back Pain
• The most frequent causes of low back pain are injury
or stress resulting in musculoskeletal, ligament, or
neurological problems in the lower back.
• In addition, stress and psychological factors may play
roles in back pain.
B. Low Back Pain
• Most of the people who experience back pain do not
progress to chronic pain, but those who do tend to
have persistent pain.
• Only about 20% of back pain patients have an
identified, physical cause for their pain.
C. Arthritis Pain
• A variety of arthritic pains exist, and many involve
inflammation of the joints. Rheumatoid arthritis,
perhaps the most frequent cause of arthritic pain,
is an autoimmune disorder characterized by a dull
ache within or around a joint. Osteoarthritis is a
progressive inflammation of the joints mostly
affecting older people and characterized by a dull
ache in the joint area.
D. Cancer Pain
• Cancer pain is caused by either a malignancy or by
treatment of a malignancy. Pain is present in a
majority of terminal cancer cases, and both
chemotherapy and radiation therapy produce pain.
Bone and cervical cancer patients are quite likely to
suffer pain, but leukemia patients rarely do.
E. Phantom Limb Pain
• Phantom limb pain is the experience of chronic
pain in an amputated part of the body.
• People who have had arms, legs, or breasts
removed nearly always continue to feel some
sensation (frequently pain) despite the removal of
that body part and the nerves that underlie
sensation.
V. Preventing Pain
• Acute pain has adaptive aspect, but it also produces
suffering for those recovering from injuries.
• Chronic pain has no adaptive advantages and creates
widespread misery.
• Most types of acute and some types of chronic pain
can be successfully prevented.
• Several programs have attempted to help people
avoid low back injuries and the subsequent pain.
• Educational programs have shown some promise for
preventing pain, and adding an application
component improves effectiveness.
• Cognitive behavioral interventions help injured
workers to return to work rather than develop chronic
pain.
• In addition, a study conducted in Finland
demonstrated that the most effective treatment for
back injury was no treatment—continuing with daily
activities produced the best outcome.