Afferent pain fibers are classified into two broad categories, visceral and somatic, with their stimulation resulting in distinct pain syndromes. The dermis and parietal pleura are innervated by somatic pain fibers. They enter the spinal cord at specific levels, are arranged in dermatomal patterns, and map to specific areas on the parietal cortex. Visceral pain fibers are found in internal organs such as blood vessels, the esophagus, and the visceral pleura. These fibers enter the spinal cord at multiple levels, along with somatic pain fibers, and map to areas on the parietal cortex corresponding to the cord levels shared with the somatic fibers. Therefore, pain from somatic fibers is usually easily described, precisely located, and experienced as a sharp sensation. Pain from visceral fibers is more difficult to describe and is imprecisely localized. Those experiencing visceral pain are more likely to use terms such as discomfort, heaviness, or aching. Further, patients frequently misinterpret the origin of visceral pain because it is often referred to a different area of the body corresponding to an adjacent somatic nerve. For example, diaphragmatic irritation can present as shoulder pain, and arm pain may actually represent myocardial ischemia.
Many physiologic, psychological, and cultural factors further influence how patients perceive, interpret, and communicate their symptoms. Gender, age, comorbidities, polypharmacy, drugs, and alcohol can all affect perception of pain.
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