pain, and that the pain they report is more likely to be moderate to severe than chronic pain in those who are not obese (Kerver et al., 2021). A large-scale population study found that the likelihood of reporting chronic pain increased proportionately with BMI: Compared with groups of people with a healthy BMI, in people with a BMI of 30–34, the rates of pain were 68%; the relative rates of chronic pain were 136% in those with a BMI of 35–39, and 254% in those with BMI >40. This increased prevalence of chronic pain is seen even after adjusting for the impact of obesity on other medical conditions, which contribute to multimorbidity and are independently associated with pain. There are both environmental and genetic elements to the relationship between pain and obesity. There is, however, limited evidence that weight loss improves chronic pain. However, there is evidence that being underweight is a consideration when managing patients with chronic pain: One study showed a higher chronic pain prevalence in men over 50 who had a BMI of less than 18.5; they also had higher rates of severe depression (Elma et al., 2022). Nutrition There is a connection between nutrition and the incidence of pain in a population, however, this relationship appears to be unclear. This, perhaps, stems from the large volume of papers and research submissions exploring this subject over the years.
The conclusions from these submissions have suggested controversial links between these two. Nutrition management plans may be of benefit to patients with chronic pain by improving pain management and reducing cardiovascular risk factors that are related to chronic pain. There have been calls for patients with chronic pain to be offered personalized nutrition assessment and counseling targeted at improving diet and supplement use, and emerging evidence is that this may improve the quality of life and clinical outcomes in patients with chronic pain. Omega-3 as a diet supplement in preclinical trials did show an improvement in inflammatory pain, while garlic has been suggested to reduce pain severity in overweight women with knee arthritis (Vezza et al., 2021). A recent systematic review and meta-analysis of 23 papers found that interventions based on nutrition, particularly those testing an altered overall diet or a single nutrient, had a significant effect on reducing participants’ reported pain severity and intensity. However, the studies in the field of nutrition and chronic pain, including those included in the meta-analysis, were of low quality, and there is insufficient evidence to make specific dietary recommendations. More rigorous studies examining nutrition with chronic pain as a primary outcome are needed to determine the role of nutrition in chronic pain (Lara-Palomo et al., 2022).
NOCICEPTORS AS SENSORS OF THE PAIN PATHWAY
Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury-related chemicals and transducing these stimuli into long-ranging electrical signals that are relayed to higher brain centers. For decades, attempts have been made to study and classify these specialized sensory neurons. Although we are far from understanding the cell biology of pain perception. Significant insights into the cellular and molecular basis of cutaneous nociception have been realized from studies on conscious humans and surrogate animal models. To a considerable extent, these advances are hampered by the difficulties inherent in studying neuronal processes in humans: cellular changes in nociceptors induced by invasive methods, the inability to record directly from the tiny structures where transduction of noxious stimuli occurs, and the uncertainty in model systems that an animal’s behavior is due to its perception of pain. However, cutaneous nociceptors have been significantly studied in their role in pain conduction, modulation, and transmission. Cutaneous nociceptors are an extremely heterogeneous group of neurons housed in peripheral sensory ganglia located just outside the central nervous system (CNS) that transduce external noxious stimuli in the skin, up to meters away from their cell bodies. Minimally invasive extracellular single-unit recordings from nerve fibers in peripheral nerves (microneurography) and skin-nerve preparations in mammals and microneurography combined with psychophysical measurements in human subjects have revealed the existence of distinct classes of nociceptor activated by noxious stimuli. Adequate stimuli include temperature extremes (> ~40°C–45°C or < ~15°C), intense pressure, and chemicals signaling potential or actual tissue damage. Nociceptors are generally electrically silent and transmit all-or-none action potentials only when stimulated. However, nociceptor activity does not per se lead to the perception of pain. The latter requires peripheral information to reach higher centers and normally depends on the frequency of action potentials in primary afferents, temporal summation of pre-and postsynaptic signals, and central influences (Roza & Bernal, 2022). The speed of stimuli transmission is reportedly correlated to the diameter of axons of sensory neurons and whether or not they are myelinated. Most nociceptors have small diameter unmyelinated axons (C-fibers) bundled in fascicles surrounded by Schwann cells and support conduction velocities of 0.4–1.4 m/s. Initial fast-onset pain is mediated by
A-fiber nociceptors whose axons are myelinated and support conduction velocities of approximately 5–30 m/s (most in the slower Aδ range). Nociceptive fibers have been classified based on their conduction velocity and sensitivity and threshold to noxious mechanical (M), heat (H), and cold (C). Units responding to thermal, mechanical, and chemical stimuli (polymodal) are the most common C-fiber type observed in fiber recordings (C-MH, C-MC, C-MHC). See Figure 2. Figure 2: Anatomy of Nociceptors
From: Khendroud et al., 2022. ©2010 American Society for Clinical Investigation. Recreated under the Creative Commons Attribution License (CC BY 4.0).
Book Code: PYFL4024
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