forces and pressures upon bending. With the use of these hairs, he could carefully determine the pressure required to elicit a sensation at each of the skin spots identified by Blix and Goldscheider. Later on, in 1967, myelinated primary afferent fibers that respond only to mechanical noxious stimuli were discovered. Soon thereafter, nociceptive, unmyelinated afferent fibers were discovered by Bessou and Pear. These discoveries revolutionized the field of pain research and helped advance and develop many theories of pain. Since Sherrington’s endorsement of the specificity theory of pain, this became the dominant theory at the time. However, its popularity waned with the postulation of the gate control theory of pain by Melzack and Wall in 1965 (Sola & Pulido, 2022). Intensity theory of pain An intensive theory of pain (now referred to as the intensity theory) has been postulated at several different times throughout history. First conceptualized in the fourth century, this theory defines pain not as a unique sensory experience but rather, as an emotion that occurs when a stimulus is stronger than usual. Centuries later, Erasmus Darwin reiterated this concept in Zoonomia . One hundred years after Darwin, Wilhelm Erb also suggested that pain occurred in any sensory system when sufficient intensity was reached rather than being a stimulus modality in its own right. Arthur Goldscheider further advanced the intensity theory, reportedly based on a different experiment performed earlier in 1859. These experiments showed that repeated tactile stimulation (below the threshold for tactile perception) produced pain in patients with syphilis who had degenerating dorsal columns (Marchand, 2021). When this stimulus was presented to patients 60–600 times/s, they rapidly developed what they described as unbearable pain. These results were produced in a series of experiments with distinct types of stimuli, including electrical stimuli. It was concluded that there must be some form of summation that occurs for the subthreshold stimuli to become unbearably painful. Goldscheider suggested a neurophysiological model to describe this summation effect: Repeated subthreshold stimulation or suprathreshold hyper- intensive stimulation could cause pain. Goldscheider further suggested that the increased sensory input would converge and summate in the gray matter of the spinal cord. This theory competed with the specificity theory of pain, which was championed by von Frey. However, the theory lost support with Sherrington’s evolutionary framework for the specificity theory and postulated the existence of sensory receptors that are specialized to respond to noxious stimuli, for which he coined the term nociceptor . Pattern theory of pain In an attempt to overhaul theories of somaesthesis (including pain), J. P. Nafe postulated a “quantitative theory of feeling” in 1929. This theory ignored findings of specialized nerve endings and many of the observations supporting the specificity and/or intensive theories of pain.
The theory stated that any somaesthetic sensation occurred by a specific pattern of neural firing and that the spatial and temporal profile of firing of the peripheral nerves encoded the stimulus type and intensity championed this theory and added that cutaneous sensory nerve fibers, except those innervating hair cells, are the same. Primarily this claim was supported by earlier works that had shown that distorting a nerve fiber would cause action potentials to discharge in any nerve fiber, whether encapsulated or not. Furthermore, intense stimulation of any of these nerve fibers would cause the percept of pain. Gate control theory of pain The gate theory of pain was proposed in 1965 by Ronald Melzack and Charles Patrick. Not only did this theory revolutionize earlier pain research, it also significantly shaped the current understanding of pain transduction in the central nervous system. This theory recognized the experimental evidence that supported the specificity and pattern theories and provided a model that could explain these seemingly opposed findings. Drawing inspiration from different research models on pain, Melzack and Wall carefully re-examined the shortcoming of the dominant theories of the pain of the era. This re-examination was focused on bridging the gap between these theories with a framework based on the aspects of each theory that had been corroborated by physiological data. Specifically, Melzack and Wall accepted that there are nociceptors (pain fibers) and touch fibers and proposed that these fibers synapse in two different regions within the dorsal horn of the spinal cord: cells in the substantia gelatinosa and the “transmission” cells. The model proposed that signals produced in primary afferents from stimulation of the skin were transmitted to three regions within the spinal cord (Campbell et al., 2020). These include: 1. Substantia gelatinosa. 2. Dorsal column. 3. Group of cells that they called transmission cells . According to Melzack and Wall’s proposal, the gate in the spinal cord is the substantia gelatinosa in the dorsal horn, which modulates the transmission of sensory information from the primary afferent neurons to transmission cells in the spinal cord. This gating mechanism is controlled by the activity in the large and small fibers. Large-fiber activity inhibits (or closes) the gate, whereas small-fiber activity facilitates (or opens) the gate. Activity from descending fibers that originate in supraspinal regions and project to the dorsal horn could also modulate this gate. When nociceptive information reaches a threshold that exceeds the inhibition elicited, it “opens the gate” and activates pathways that lead to the experience of pain and its related behaviors. Therefore, the gate control theory of pain provided a neural basis for the findings that supported and helped to reconcile the apparent differences between the pattern and specificity theories of pain.
INCIDENCE AND PREVALENCE OF PAIN IN THE GLOBAL POPULATION—A SYSTEMATIC REVIEW Prevalence describes the proportion of the at-risk
Although difficult to accurately determine because of a lack of longitudinal studies, the incidence of chronic pain in one region of the UK has been estimated at 8% per year (Denche-Zamorano et al., 2022). Chronic pain of moderate to severe intensity has been estimated to occur in 19% of adult Europeans, seriously affecting their daily activities and social and working lives. Most had not received pain specialist treatment and 40% reported inadequate management of their pain (Ghafouri
population affected by a condition. Population estimates for the prevalence of chronic pain vary widely according to case definition and ascertainment methods, time place, and population. Research suggests that chronic pain affects 13–50% of adults in the UK. Of those who live with chronic pain, 10.4–14.3% were found to have moderate to severe disabling chronic pain. Incidence is the number of new cases of a disease developing during a particular time in a population at risk of developing the disease.
Book Code: PYCA2725
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