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.