Chapter 012. Pain: Pathophysiology and Management (Part 1)

Harrisons Internal Medicine Chapter 12. Pain: Pathophysiology and ManagementPain: Pathophysiology and Management:IntroductionThe task of medicine is to preserve and restore health and to relieve suffering. Understanding pain is essential to both these goals. Because pain is universally understood as a signal of disease, it is the most common symptom that brings a patient to a physicians attention. The function of the pain sensory system is to protect the body and maintain homeostasis. It does this by detecting, localizing, and identifying tissue-damaging processes. Since different diseasesproduce characteristic patterns of tissue damage, the quality, time course, and location of a...
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Chapter 012. Pain: Pathophysiology and Management (Part 1) Chapter 012. Pain: Pathophysiology and Management (Part 1) Harrisons Internal Medicine > Chapter 12. Pain: Pathophysiology andManagement Pain: Pathophysiology and Management: Introduction The task of medicine is to preserve and restore health and to relievesuffering. Understanding pain is essential to both these goals. Because pain isuniversally understood as a signal of disease, it is the most common symptom thatbrings a patient to a physicians attention. The function of the pain sensory systemis to protect the body and maintain homeostasis. It does this by detecting,localizing, and identifying tissue-damaging processes. Since different diseasesproduce characteristic patterns of tissue damage, the quality, time course, andlocation of a patients pain complaint and the location of tenderness provideimportant diagnostic clues and are used to evaluate the response to treatment.Once this information is obtained, it is the obligation of the physician to providerapid and effective pain relief. The Pain Sensory System Pain is an unpleasant sensation localized to a part of the body. It is oftendescribed in terms of a penetrating or tissue-destructive process (e.g., stabbing,burning, twisting, tearing, squeezing) and/or of a bodily or emotional reaction(e.g., terrifying, nauseating, sickening). Furthermore, any pain of moderate orhigher intensity is accompanied by anxiety and the urge to escape or terminate thefeeling. These properties illustrate the duality of pain: it is both sensation andemotion. When acute, pain is characteristically associated with behavioral arousaland a stress response consisting of increased blood pressure, heart rate, pupildiameter, and plasma cortisol levels. In addition, local muscle contraction (e.g.,limb flexion, abdominal wall rigidity) is often present. Peripheral Mechanisms The Primary Afferent Nociceptor A peripheral nerve consists of the axons of three different types of neurons:primary sensory afferents, motor neurons, and sympathetic postganglionic neurons(Fig. 12-1). The cell bodies of primary sensory afferents are located in the dorsalroot ganglia in the vertebral foramina. The primary afferent axon bifurcates tosend one process into the spinal cord and the other to innervate tissues. Primaryafferents are classified by their diameter, degree of myelination, and conductionvelocity. The largest-diameter fibers, A-beta (Aβ), respond maximally to lighttouch and/or moving stimuli; they are present primarily in nerves that innervatethe skin. In normal individuals, the activity of these fibers does not produce pain.There are two other classes of primary afferents: the small-diameter myelinated A-delta (Aδ) and the unmyelinated (C fiber) axons (Fig. 12-1). These fibers arepresent in nerves to the skin and to deep somatic and visceral structures. Sometissues, such as the cornea, are innervated only by Aδ and C afferents. Most Aδand C afferents respond maximally only to intense (painful) stimuli and producethe subjective experience of pain when they are electrically stimulated; thisdefines them as primary afferent nociceptors (pain receptors). The ability to detectpainful stimuli is completely abolished when Aδ and C axons areblocked. Components of a typical cutaneous nerve. There are two distinct functional categories of axons: primary afferentswith cell bodies in the dorsal root ganglion, and sympathetic postganglionic fiberswith cell bodies in the sympathetic ganglion. Primary afferents include those withlarge-diameter myelinated (Aβ), small-diameter myelinated (Aδ), andunmyelinated (C) axons. All sympathetic postganglionic fibers areunmyelinated.Individual primary afferent nociceptors can respond to severaldifferent types of noxious stimuli. For example, most nociceptors respond toheating, intense cold, intense mechanical stimuli such as a pinch, and applicationof irritating chemicals including ATP, serotonin, bradykinin and histamine.