Box 1: A Life Without Pain To understand the importance of pain for all human beings, consider the case of Andre L., a young boy in Melbourne, Australia. By the time Andre was 1 year old, he had been seen multiple times by pediatricians and dentists because he kept biting hard into his lips and tongue. As he got older, his injuries became more severe: At one point, he self-extracted a tooth, and he broke his leg three times before he was 3 years old. By the time he was 4 years old, his parents bound his hands with gloves because, alarmingly, he had bitten through the ends of three fingers! Due to his frequent hospitalizations with many unwitnessed injuries, child protective services became involved. A team of geneticists and neurologists examined Andre and determined that, although he was of normal intelligence and development, he was completely unresponsive to painful stimuli . All of his injuries appeared to have been self- inflicted or accidental. Andre is one of a very small group of individuals with what is called congenital insensitivity to pain (CIP). This very rare condition is associated with a genetic mutation and means that no painful information is carried by the nerves to the brain for processing. Sadly, most people with CIP do not survive childhood due to recurrent injuries (Amroh, et al. 2020). These cases serve as a reminder that a life without pain is truly maladaptive. The elements of pain
review of the neurophysiology underlying pain is beyond the scope of this course, the following is an overview of its basic principles. Noxious (unpleasant) stimuli are first detected by specialized sensory receptor cells, called nociceptors , in bodily tissues. These nociceptors transmit information to the central nervous system. In general, nociceptors respond to three kinds of stimuli: mechanical (such as pressure); thermal (such as heat or cold); and chemical, in the form of endogenous algesic neurotransmitters (i.e., pain-producing chemicals that are released by damaged cells). Two nociceptor types are most commonly associated with physical pain: ● A-delta : Myelinated A-delta fiber receptors send signals at very high speeds (more than 20 meters per second). Activity of the A-delta nociceptors is associated with quickly transmitted pain sensations and is usually felt as “sharp.” ● C-fiber : Unmyelinated C-fiber receptors deliver their messages relatively slowly (about 0.5 meters per second). Activity by the C-fiber receptors is usually described as “aching” or “dull.” Once a pain signal is picked up by a nociceptor, it is carried along to the brain and spinal cord, where it neurochemically connects with two other types of specialty cells. These specialized cells in the spinal cord are referred to as nociceptive- specific (NS) cells and wide dynamic range (WDR) cells. Their functions can be divided as follows: ● Nociceptive-specific cells respond only to painful signals. ● Wide dynamic range cells receive input from A-delta fibers, C-fibers, and fibers carrying non-nociceptive input. Regardless of whether a pain signal is carried by NS or WDR cells, it travels up the spinal cord and into the brain, via two particular tracts of nerves: ● The spinothalamic tract : Is the most direct pathway for nociceptive information and a key pathway for pain. It ascends through the spinal cord to a part of the brain called the thalamus (which is located deep in the brain), where it is projected onward to various parts of the cortex (the outermost layer of the brain, just beneath the skull). An area of the brain known as the somatosensory cortex processes the sensory dimensions of pain and provides information relating to the quality, location, and intensity of the painful stimulus. This is pain sensation at its most basic: how it feels, where it hurts, and how much it hurts. ● The spinoreticulothalamic tract : Is a less direct, slower pathway that sends projections to many regions of the brain for processing. These brain regions include the hypothalamus (which handles autonomic responses to pain such as increases in blood pressure and irregular breathing), the limbic system (which coordinates the emotional responses to pain, such as a feeling of unpleasantness), and the frontal lobe (which involves higher-level thinking, such as interpreting the meaning of pain). The spinoreticulothalamic tract is related to emotional and motivational responses to pain.
The biopsychosocial model of pain is best explained by using a ring diagram, or “pain onion,” as shown in Figure 1. This model begins at the center ring with biological pain, also called nociception , or the neurological basis of pain. In the second level, or ring, the brain eventually recognizes these neurological signals as unpleasant. This recognition is typically referred to as pain perception , or simply pain . In the third level of pain processing, an individual has a psychological and emotional response to the pain signal (typically, acute anxiety), sometimes referred to as suffering in response to pain. At the next (fourth) level out from the core, the individual engages in a range of simple and complex social behaviors to communicate pain or alleviate it; these are known as pain behaviors . These individual-level factors (nociception, pain, suffering, and behavior) generate a response in the environment, often from friends, family members, or caregivers. This fifth level is referred to as the environmental reaction in response to pain. An individual’s level of impairment and/or disability (sixth level) is then impacted by all of the aforementioned factors (nociception, pain, suffering, behavior, and environmental reaction) and occurs in a broader sociopolitical context (level seven), which includes a range of factors such as social stigma around pain and governmental programs to help with pain-related disability. Each of these aspects of pain will be discussed in turn. Figure 1: The Biopsychosocial Model of Pain
Note. Based on the biopsychosocial model of chronic pain as developed by J. D. Loeser (1980). Perspectives on pain. In P. Turner (Ed.), Clinical pharmacology and therapeutics . London, UK: Macmillan, as further modified in the unpublished work of John G. Cagle. Used with permission of John G. Cagle. Biological pain (Nociception) Nociception refers to pain perception at its most basic, neurological, level. It represents the activation of pain receptors and the resulting transmission of pain signals via the peripheral nervous system to the spinal cord and brain. Although a full
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