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Electromyographic signals were recorded from above a bilateral lumbar erector spinae muscle. This study aimed to reveal the complex associations among the pain site, pain intensity/quality, muscle activity, and muscle activity distribution. If the stimulus intensity is increased still further, so that the small-diameter, slowly conducting C fiber axons are brought into play, then a duller, longer-lasting sensation of pain is experienced. Pain can alter muscle activity, although it is not clear how pain intensity and site location affect muscle activity.
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When the stimulus intensity is raised to a level that activates a subset of Aδ fibers, however, a tingling sensation or, if the stimulation is intense enough, a feeling of sharp pain is reported. Stimulation of the large, rapidly conducting Aα and Aβ axons in peripheral nerves does not elicit the sensation of pain. In general, two categories of pain perception have been described: a sharp first pain and a more delayed (and longer-lasting) sensation that is generally called second pain ( Figure 10.2A). How, then, do these different classes of nociceptors lead to the perception of pain? As mentioned, one way of determining the answer has been to stimulate different nociceptors in human volunteers while noting the sensations reported.